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The Real Meaning of E equals Em Cee Squared

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The Real Meaning of E equals Em Cee Squared

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Overview and examples of Kepler's Three Laws of Planetary Motion; Law of Orbits, Law of Areas, and Law of Periods. "Chipper Doodle" Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 3.0 http://creativecommons.org/licenses/by/3.0/

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NASA brings you images, videos and features from the unique perspective of America's space agency. Get updates on missions, watch NASA TV, read blogs, view the latest discoveries, and more.

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A light-year is how astronomers measure distance in space. It's defined by how far a beam of light travels in one year - a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong. If you like to keep up with what's going on in astronomy, it's worth spending a little bit of time understanding what the deal is with this funny unit of measurement.

Have you ever been on your way to meet some friends and, while en route, called to tell them you're running late? If you're like me, you might tell them "I'm about ten minutes away". You're using how long it will take you to get there as a substitute for how far away you are. Astronomers do the same thing when figuring out the distance to a star or galaxy, but instead of relying on how fast they could drive or walk, they use a beam of light as their reference.

Why do things this way? First, light is convenient. Throughout the universe, all light travels at exactly the same speed: about 670 million miles per hour. We don't usually think about light traveling anywhere because when we flick on a light switch - there it is! We don't have to wait for the room to light up. It happens instantaneously. Except that it's not instantaneous, just insanely fast. In fact, let's pause for a second and reflect on how ludicrously quick the speed of light is.

Traveling at that speed, you would encircle the globe of Earth almost eight times in one second.

Impressive! Certainly faster than traveling by airplane. So if you were to travel off the Earth in a straight line at light speed, you'd get pretty far in the same amount of time, right? Actually, since space is so vast, you wouldn't have even made it to the moon. Travel to the moon takes about a second-and-a-half, at light speed. Travel to the sun at light speed takes about eight minutes. Can you even imagine? Traveling at a speed where you cross nearly 200,000 miles every second for eight full minutes would only get you to the center of our solar system.

The fact that light takes time to get anywhere has an interesting side effect. If light from the sun takes eight minutes to get to us, then we're actually seeing the sun as it was eight minutes ago. We're looking into the past!

While this sounds a little crazy, it's actually something with which you're already familiar. If you've ever seen fireworks, for example, you know that you see the explosion and then a few seconds later you hear it. If you close your eyes during the fireworks show, you'd only have your ears to know when things were happening. Since it takes some time for the sound to get to you, you'd always be hearing things a few seconds after they happened. The same happens with light: we only see something once the light from that event actually gets to our eyes. When we're looking across a room, the time delay is only a few billionths of a second. But when we start looking across large enough distances, the light becomes noticeably delayed like the sounds are from exploding fireworks.

The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to miles or kilometers we quickly run into unwieldy numbers just measuring the distance to the nearest star: a dim red dwarf called Proxima Centauri that sits a mere 24,000,000,000,000 miles away! Using a longer yard stick, so to speak, helps keep the numbers at least manageable. To an astronomer, the distance to Proxima Centauri is only four light years. Put another way, it takes the light from our stellar next door neighbor four years to get to Earth.

The Milky Way galaxy in which our sun and all the stars we see at night reside spans 100,000 light-years from one end to the other. Putting that into perspective, the duration of recorded human history is roughly 5,000 years. So light from a star at one end of our galaxy takes 20 times longer than all of recorded history to get to the other end.

Moving beyond our galaxy, it's just over two million light years to our nearest galactic neighbor, the Andromeda galaxy. The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools.

Our last stop takes us to the edge of the visible universe. It's also here where the trickiness of measuring distance in an ever-expanding universe becomes apparent. The light we see coming from the farthest depths of the universe has been traveling across the cosmos for almost three times longer than our planet has existed: nearly 14 billion years! But there's a catch: we cannot say that the edge of the visible universe is 14 billion light-years away.

Why? Because the universe has grown larger in that time! A galaxy whose light took 14 billion years to reach our little planet has, in the intervening aeons, moved even further away. The current physical distance to that remote beacon, if we stopped the universe from expanding and stretched out a really long tape measure, is just over 46 billion light years! Even in light years, measuring distances across the universe becomes unwieldy. But measuring in something familiar, like miles, is truly humbling. From here to the edge of our vision spans a distance of approximately 276,000,000,000,000,000,000,000 miles.

And it's getting bigger every day.

Bottom line: How far is a light-year? It's the distance a beam of light travels in one year - a distance of six trillion miles.

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distance to sun and moon in light-minutes

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A light-year is how astronomers measure distance in space. It's defined by how far a beam of light travels in one year - a distance of six trillion miles. Think of it as the bigger, badder cousin of the inch, the mile, the kilometer, and the furlong.

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We are a globally renowned resource that provides assistance with English to students, teachers, professionals, and organizations across the world. Our goal is to assist clients in their development as writers - no matter their skill level.

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Purdue OWL® - Purdue OWL® - Purdue University

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We are a globally renowned resource that provides assistance with English to students, teachers, professionals, and organizations across the world. Our goal is to assist clients in their development as writers - no matter their skill level.

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We are a globally renowned resource that provides assistance with English to students, teachers, professionals, and organizations across the world. Our goal is to assist clients in their development as writers - no matter their skill level.

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Purdue OWL® - Purdue OWL® - Purdue University

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We are a globally renowned resource that provides assistance with English to students, teachers, professionals, and organizations across the world. Our goal is to assist clients in their development as writers - no matter their skill level.

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Although we still can't travel to Mars as easily as commuting to work, the very idea of being able to go on a trip to another planet and live there someday has long piqued the interest of scientists and outer space fans alike.

A group of scientists in the future far, far away may be able to design and develop a real-life Millennium Falcon with a hyperdrive that could take us beyond the limits of our solar system - one that would probably cost us a hundred years of research and more than trillions of dollars.

Engines with hyperdrives only exist in the realm of science fiction. Nevertheless, that doesn't stop humans from creating hypotheses or theories.

Just like in sci-fi books or movies, the human race would have to travel in space and vacate the Earth for pure conquest because it has finally managed to successfully break the bounds of science.

The reason, however, could be far worse. In the far away future, the world might fall ill to climate change or perhaps be the site of a zombie apocalypse.

Nonetheless, astrophysicists from different parts of the world have been searching for planets where humankind could possibly survive. Over the years, the list of potentially habitable planets has increased.
Potentially Livable Planets
NASA's Kepler has already discovered at least 1,000 planets since 2009, 12 of which are less than twice the size of our own planet. These 12 planets orbit their host stars at the right distance to make them neither too cold nor too hot to potentially contain water.

The official authority to list down habitable exoplanets is the Planetary Habitability Laboratory. The agency's Habitable Exoplanets Catalog contains 29 exoplanets that are considered to be livable, and the list will be updated soon because of the discovery of a new exoplanet.
1. Wolf 1061c
In December last year, astronomers in Australia discovered a possibly habitable super-earth called Wolf 1061c, a planet 14 million light years away from Earth, located in what scientists call the "Goldilocks" zone. In this zone, temperatures are just right for water to exist.

"It is fascinating to look out at the vastness of space and think a star so very close to us - a near neighbour - could host a habitable planet," said Dr. Duncan Wright, the lead author of the research. The exoplanet Wolf 1061c will soon be added into the catalog.
2. Kepler-452b
On July 23, NASA confirmed the discovery of Kepler-452b, an exoplanet nearly the size of the Earth that is found in the Goldilocks zone.

This exoplanet orbits a Sun-like host star. Its diameter is 60 percent larger than that of Earth, and scientists theorize that this exoplanet is likely to be rocky.

Scientists also found that this exoplanet orbits around its star for 385 days. Hence, calling it the bigger and older cousin of Earth.
3. Kepler-186f
The exoplanet Kepler-186f is about 500 million light-years away from our planet, and is 10 percent bigger than it. Scientists also believe that this exoplanet is mostly rocky.

Like Kepler-452b, Kepler-186f is another one of Earth's distant cousins. "Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth," said scientist Thomas Barclay.
4. Kepler-22b
Orbiting the habitable zone of its host star is Kepler-22b. This exoplanet is about 600 million light years away from Earth, and has a radius that is 2.4 times greater than our planet's.

Scientists noted that Kepler-22b's planetary temperature is almost the same as the Earth's, and that its surface temperature is a life-friendly 72 degrees Fahrenheit.
5. Gliese 581d
In 2009, scientists discovered that the exoplanet Gliese 581d had the potential to support water on its rocky surface.

The super-earth's mass is about 7 times greater than that of our planet, and it would be two times bigger in mass. If possible, the exoplanet could contain an atmosphere and a liquid ocean.

Below is the official list of habitable planets from PHL.

(Photo : Planetary Habitability Laboratory)
How Do Scientists Determine Which Planets Can Host Life?
Because stars and planets beyond our solar system are too distant and unreachable, a team of scientists created a technique that could determine which exoplanets have the potential to accommodate human life. For them, finding out which planets are livable is highly dependent on the properties of the host star.

"If you don't know the star, you don't know the planet," said Jaymie Matthews, Astrophysics Professor at the University of British-Columbia and co-author of the study featured in the journal Science Advances.

Matthews and his colleagues took advantage of the stars' gravitational pull and looked into the differences in each star's brightness levels.

Why do brightness levels in stars vary? Imagine a pot of boiling soup. Its surface is hot and unstable, and if you hover your hand on top of the pot, you will feel the heat from the boiling soup rise onto the palm of your hand.

This happens because of convection--where the heated air or liquid is transferred away from the source, carrying energy and surface turbulence.

"Substitute the chicken broth held in a pot by Earth's gravity with hydrogen gas held together by a star's gravity," said Thomas Kallinger, Matthews' co-author. "Convection in the gas can trigger vibrations in the star."
'The Boy Who Cried Earth'
Gravity is extremely crucial to this study as it is to Earth. It will aid scientists in finding the properties of host stars, their radii and masses, as well as in identifying the specific properties that make surrounding exoplanets habitable.

For instance, to know the size of the exoplanet, one must know the size of its host star. Matthews said that not knowing the properties of the host star may bring confusion.

He explained that scientists may possibly find a planet around a Sun-like star, but they might mistake it for a livable Earth-sized world even when its star is actually a giant.

"Remember the fable 'The Boy Who Cried Wolf?' No astronomer wants to be The Boy Who Cried Earth and later has to say 'Never mind. My bad,'" said Matthews.

Matthews and his team based their new technique on data provided by Canada's Microvariability and Oscillations of Stars (MOST) telescope and Kepler satellites.

Matthews said whichever the distance of the first habitable planet is, it will be a footnote in the history books. "The headline will be 'We found life!'"

© 2017 Tech Times, All rights reserved. Do not reproduce without permission.

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Researchers Believe These Are The Planets Where Humankind Could Possibly Survive

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Although we still can't travel to Mars as easily as commuting to work, the very idea of being able to go on a trip to another planet and live there someday has long piqued the interest of scientists and outer space fans alike.

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News, images and videos from the Cassini mission, which explored Saturn and its moons from 2004 to 2017.

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NASA.gov brings you the latest images, videos and news from America's space agency. Get the latest updates on NASA missions, watch NASA TV live, and learn about our quest to reveal the unknown and benefit all humankind.

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Top 10 STRANGEST Planets In The Known Universe

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From an ice planet that's on fire to a strange planet made of solid diamond, we look at 10 of the strangest planets in the known universe! 10. J1407 b 9. HD 106906 b 8. TrEs-2b 7. 55 Cancri E 6. Gliese 436b 5. WASP-12b 4. Gliese 581c 3.

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Introduction to the Solar System: Crash Course Astronomy #9

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The Big Crunch, the Big Freeze and the Big Rip - The Big Bang and the Big Crunch - The Physics of the Universe

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The Physics of the Universe - The Big Bang and the Big Crunch - The Big Crunch, the Big Freeze and the Big Rip

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Wormholes - Black Holes and Wormholes - The Physics of the Universe

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The Physics of the Universe - Black Holes and Wormholes - Wormholes

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The dying star, known as a red giant, in its final stages blows out its outer layers, leaving clouds of gas and dust, which is called a planetary nebula.

NASA and the European Space Agency released the photo of the Calabash Nebula. The gas, seen in yellow, was ejected as fast as 621,371 miles per hour, according to the ESA.

Astronomers rarely get to see this kind transition because it occurs "within the blink of an eye -- in astronomical terms," the ESA said.

The nebula is also known as the Rotten Egg Nebula, because it contains a lot of sulfur, which smells like rotten eggs. Luckily, this is happening about 5,000 light-years away in the constellation of Puppis, according to the ESA.

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hubble captures explosive death of a star  - cnn.com

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Hubble captures explosive death of a star

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The dying star, known as a red giant, in its final stages blows out its outer layers, leaving clouds of gas and dust, which is called a planetary nebula. NASA and the European Space Agency released the photo of the Calabash Nebula.

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Halley's Comet has been know since at least 240 BC and possibly since 1059 BC. Its most famous appearance was in 1066 AD when it was seen right before the Battle of Hastings.

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Astrobiology (other life forms in space)

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Astrobiology (other life forms in space)

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The Search for Life in the Universe -- NASA Astrobiology Magazine

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This essay first appeared in 2003 in NASA's Astrobiology magazine.

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Consequently, scientists in and out of the SETI community have conjured up other arguments to deal with the conflict between the idea that aliens should be everywhere and our failure (so far) to find them. In the 1980s, dozens of papers were published to address the Fermi Paradox.

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This article is for students grades K-4.

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

How Big Are Black Holes?

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.

Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center.

The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

How Do Black Holes Form?

Scientists think the smallest black holes formed when the universe began.

Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

Scientists think supermassive black holes were made at the same time as the galaxy they are in.

If Black Holes Are "Black," How Do Scientists Know They Are There?

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Could a Black Hole Destroy Earth?

Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.

Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.

How Is NASA Studying Black Holes?

NASA is using satellites and telescopes that are traveling in space to learn more about black holes. These spacecraft help scientists answer questions about the universe.

More About Black Holes

Space Place in a Snap: What Is a Black Hole?
Black Hole Rescue
Fall Into a Black Hole

Read What Is a Black Hole? (Grades 5-8)

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What Is a Black Hole? (Grades K - 4) - NASA

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A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space.

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From Mr. P:  Earth Science Astronomy Independent Research Project

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From Mr. P:  Earth Science Astronomy Independent Research Project

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Come up with a question you have about something astronomical or some topic in astronomy that you would like to research.

Write out an idea of what you would like to research and have your teacher approve the question/topic.

You will have two to three days in the computer lab to research your topic and create a brief (3 - 10 slides) presentation of the results of your research.  You may also choose to write a paper.

Grading will be based on the quality of the information in your presentation and its adherence to the question/topic proposed.  If the content you present doesn't relate to your question/topic, then your grade will not be happy.  As always, you MUST CITE YOUR SOURCES.  For internet resources, a URL that takes me to the exact web page you are using as an information source will suffice.  Always use quotation marks if/when you copy and paste...

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Space Is Cold, Vast, and Deadly. Humans Will Explore It Anyway

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From space madness to crash landings, here’s how we’ll beat the 13 most difficult challenges to space exploration.

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Actual Planet Found!

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What Does The Inside Of A Black Hole Look Like?

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NASA.gov brings you the latest images, videos and news from America's space agency. Get the latest updates on NASA missions, watch NASA TV live, and learn about our quest to reveal the unknown and benefit all humankind.

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Meteors, or shooting stars as they are more commonly known, are the streaks of light produced when a meteoroid burns up in the Earth's atmosphere. It looks like a star falling towards us as it momentarily flashes above us. The meteoroids, which produce the meteors, are dust and rocks in space.

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Here's a short explanation of star colors to help you better appreciate what you see when you look at the stars in the night sky. In just a couple of minutes, you'll understand more about stars than 99% of people who've ever lived.

Basics

* Stars radiate light a little like glowing coals in a campfire. Just as a glowing red-hot coal is cooler than a white-hot coal, for example, so a red star is cooler than a white star, and a white star is cooler than a blue star. This was a major scientific discovery... simply by measuring the color of light coming from a star, and applying a little physics, it was possible to estimate a star's surface temperature.

* Like most scientists, astronomers love to classify things. In the late 19th century, Harvard astronomers developed a system to classify stars not according to color, but by the strength by which hydrogen gas absorbed light at particular wavelengths. The star classes were labeled A to N in order of decreasing hydrogen absorption strength. After a time, the classes were simplified to O, B, A, F, G, K, and M. This is the Harvard spectral classification, which is still used today. So what does this have to do with star color? Keep reading...!
* * * New from One-Minute Astronomer * * *
Good reading for a cloudy night! New e-Book gives you the best 111 articles from the first three years of One-Minute Astronomer. Tips, tales, and tours of the solar system and deep sky. Ideal for experienced and armchair stargazers. Support One-Minute Astronomer! Click here to learn more.
* * * * * * * * *
* As astronomers and physicists learned more about atomic structure and the spectra of light from stars, they discovered the Harvard classification system really described the temperature of a star's atmosphere. They discovered the type-O stars are hotter than type-B stars, and type-B stars are hotter than type-A stars, and so on. But hot stars are blue, and medium-hot stars are white, and cool stars are red.

* Here's a summary of the dominant color and temperatures of the main classes of stars, along with examples of stars that belong to each class:

Deeper Look

* The upshot is this: the color of a star depends on its surface temperature. But a blue star doesn't emit only blue light, nor does a red star emit only red light. They emit visible light of all colors to some degree. It's just that their spectrum peaks at a particular color.

* So why are there blue stars, yellow stars, red stars, but no green stars? As it turns out, there are green stars, that is, stars that radiate much of their light in the green part of the spectrum. But the total combination of the full range of colors of a "green" star appears white to our eyes. If you pass the color from a whitish star through a prism, you'll see all the colors, including green, spread out in a continuum.

* Astronomers came to understand that bluer stars are intrinsically brighter because they are more massive than white or red stars, and more massive stars burn much faster and hotter than less massive stars. The bluish type-O stars, for example, are only 30-50 times more massive than yellow-white stars like our sun. But O stars burn a million times brighter, so they have far shorter lifetimes. O and B stars only last a few million years before they die in spectacular supernova explosions, while cooler and less massive K and M stars burn steadily for billions of years.

* Some 88% of stars in the universe seem to be the cooler type K and M. Only 1 in 3,000,000 stars are type O. Even middle-weights like our type-G Sun comprise only 8% of all known stars.

Good To Know

This relationship between star mass, luminosity, and color holds only for stars burning hydrogen in the core during the prime of their lives. For example, young and middle-aged M-type stars are small, faint and long-lived. But as stars age and start burning heavier elements in the core, bluish O and B stars, for example, evolve briefly into immensely bright M-type red stars known as red supergiants. We'll explain this in later issues. If it sounds complicated, fear not. Even astronomy majors wrestle over this for some time before they understand how stars live and evolve.

Personal View

To remember star types O-B-A-F-G-K-M in order of hotter to cooler, you can use the mnemonic "Oh Be A Fine Girl (or Guy), Kiss Me". My personal favorite alternative: Only Bored Astronomers Find Gratification Knowing Mnemonics

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Here's a short explanation of star colors to help you better appreciate what you see when you look at the stars in the night sky. In just a couple of minutes, you'll understand more about stars than 99% of people who've ever lived. * Stars radiate light a little like glowing coals in a campfire.

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Planet-sized moons orbiting huge gas giants could provide havens for life around other stars, but in order to be habitable these moons would need to be larger and more massive than Mars, according to new research by René Heller and Ralph Pudritz of McMaster University in Canada.

There are 181 currently known moons around planets and dwarf planets in the Solar System (this figure may grow if New Horizons finds more moons around Pluto), but thus far no moon around an exoplanet - a planet orbiting a star other than the Sun - has ever been found. However, astronomers consider it a safe bet that exoplanets will be joined by exomoons and that so far we simply lack the sensitivity to detect them. Just as there are extreme exoplanets several times more massive than Jupiter, astronomers suspect that there could also be moons many times more massive than the moons in our Solar System.

These giant exoplanets are gaseous worlds without a surface and certainly could not bear life as we know it. Their solid moons, however, might potentially be habitable, especially if the planets have migrated inwards towards the habitable zone. This zone is just at the right distance from a star for temperatures to be just right for worlds wrapped in an atmosphere to have liquid water on their surface. In our Solar System Earth resides inside the Sun's habitable zone, with Venus just beyond the inside edge of the zone and Mars on the outer edge.

In a pair of papers, published respectively in the journals Astronomy and Astrophysics and The Astrophysical Journal, Heller and Pudritz explored what it would take for a moon that has tagged along with its migrating planet to be able to potentially support life.

Chiefly, a moon has to be able to retain its liquid water, and massive moons have two advantages in doing so. One, is that the extra gravity is able to hold onto a water-rich atmosphere better. Two, is that the more massive a moon, the longer it can retain heat in its core to drive an internal dynamo that generates a magnetic field. This heat can be added to by the friction it feels inside from the gravitational tides wielded by its parent gas giant. The magnetic field forms a protective bubble around the moon, called a magnetosphere, deflecting the solar wind and preventing the atmosphere and its water vapour from being stripped away. This is crucial, particularly as in 2013, Heller and Jorge Zuluaga of the University of Antioquia in Colombia discovered that the parent planet's magnetic field would not extend far enough to protect moons beyond the ice line.

So Heller and Pudritz set about exploring the formation of massive moons, using our own Jupiter and its four Galilean moons as ideal test subjects. They found that, in any moon-forming disc that settles around a growing planet, the point where temperatures drop low enough for water-ice to condense and snow out as solid ice is the crucial factor in controlling the mass of the moons. The reason is that the addition of the ice increases the overall density of the disc beyond the ice line, meaning that more massive moons can form out there. If we look towards Jupiter, its two largest moons - Ganymede and Callisto - both formed beyond the ice line.

Scaling this up to gas giants several times the mass of Jupiter, Heller and Pudritz calculated that it was feasible for ice-rich moons larger than Mars to form beyond the ice line. Then, when gas giants migrate inwards towards their star, the massive icy moons they bring with them warm up, melting the ice - and they would have a lot of ice.

"I've been wondering for years now what would happen to those formerly icy moons that take a piggyback ride with their planets to the habitable zone around a Sun-like star," Heller tells Astronomy Now. "I'd assume they would end up as water worlds, but the persistence of the ocean would crucially depend upon the moon's surface gravity, which determines whether the moon is capable of holding onto a substantial atmosphere."

So, had Jupiter migrated further into the inner Solar System than it did, and settled there, Ganymede would have warmed up but would have been incapable of holding onto its water because it is not big enough, despite being larger than Mercury. Heller reckons that moons at least two or three times more massive than Mars would be required.

Furthermore, it is likely that habitable exomoons will only be found around Sun-like stars. Red dwarf stars, which are smaller and cooler but far more common in the Universe, tend not to produce particularly massive gas giant planets. Yet there are still plenty of stars like the Sun and many super gas giants that have migrated inwards. So even if we find a planetary system where a marauding world has barged through and kicked out all the smaller terrestrial planets, we should not rule that system out as a place to look for life. With the next generation of telescopes such as the European Extremely Large Telescope and the Thirty Meter Telescope under planning and construction, and new exoplanet missions such as TESS and PLATO soon to be launched, the exomoons might not be able to hide for much longer.

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Habitable exomoons will need to be bigger than Mars

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Planet-sized moons orbiting huge gas giants could provide havens for life around other stars, but in order to be habitable these moons would need to be larger and more massive than Mars, according to new research by René Heller and Ralph Pudritz of McMaster University in Canada.

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No, there's no science behind an astrologer's prediction for 2016, but that doesn't mean it can't be accurate

smithsonian.com

Astrology is either an ancient and valuable system of understanding the natural world and our place in it with roots in early Mesopotamia, China, Egypt and Greece, or complete rubbish, depending on whom you ask.

But newspaper and magazine horoscopes? The ones advising you to not "fight against changes" today, or to "go with the flow", whatever that means, or to "keep things light and breezy with that new hottie today"? They get even less respect, from both skeptics and true believers. So it's a bit surprising, then, that they remain so popular with everyone in between.

The first real newspaper horoscope column is widely credited to R.H. Naylor, a prominent British astrologer of the first half of the 20th century. Naylor was an assistant to high-society neo-shaman, Cheiro (born William Warner, a decidedly less shamanistic name), who'd read the palms of Mark Twain, Grover Cleveland, and Winston Churchill, and who was routinely tapped to do celebrity star charts. Cheiro, however, wasn't available in August 1930 to do the horoscope for the recently born Princess Margaret, so Britain's Sunday Express newspaper asked Naylor.

Like most astrologers of the day, Naylor used what's called a natal star chart. Astrology posits that the natural world and we human beings in it are affected by the movements of the sun, moon and stars through the heavens, and that who we are is shaped by the exact position of these celestial bodies at the time of our birth. A natal star chart, therefore, presents the sky on the date and exact time of birth, from which the astrologer extrapolates character traits and predictions.

On August 24, 1930, three days after the Princess's birth, Naylor's published report predicted that her life would be "eventful", an accurate if not entirely inspired forecast given that she was, after all, a princess (he didn't, it appears, foresee the Princess's later star-crossed romances and lifelong love affair with alcohol and cigarettes). He also noted that "events of tremendous importance to the Royal Family and the nation will come about near her seventh year", a prediction that was somewhat more precise - and seemed to ring true right around the time that her uncle, King Edward VIII, abdicated the throne to her father.

Celebrity natal star charts weren't a particularly novel idea; American and British newspapers routinely trotted astrologers out to find out what the stars had in store for society pagers like Helen Gould and "Baby Astor's Half Brother". Even the venerable New York Times wasn't above consulting the stars: In 1908, a headline declared that President Theodore Roosevelt, a Sagittarius, "might have been different with another birthday", according to "expert astrologer" Mme. Humphrey.

But though it wasn't the first of its kind, Naylor's article was a tipping point for the popular consumption of horoscopes. Following the interest the public showed in the Princess Margaret horoscope, the paper decided to run several more forecasts from Naylor. One of his next articles included a prediction that "a British aircraft will be in danger" between October 8 and 15. When British airship R101 crashed outside Paris on October 5, killing 48 of the 54 people on board, the tragedy was taken as eerie evidence of Naylor's predictive skill. Suddenly, a lot more people were paying attention to the star column. The then-editor of the paper offered Naylor a weekly column - on the caveat that he make it a bit less dry and bit more the kind of thing that lots of people would want to read - and "What the Stars Foretell", the first real newspaper horoscope column, was born.

The column offered advice to people whose birthdays fell that the week, but within a few years, Naylor (or a clever editor) determined that he needed to come up with something that could apply to larger volumes of readers. By 1937, he'd hit upon the idea using "star signs", also known as "sun signs", the familiar zodiac signs that we see today. "Sun sign" refers to the period of the year when the sun is passing through one of 12 30-degree celestial zones as visible from earth and named after nearby constellations; for example, if you're born in the period when the sun is passing through the constellation Capricornus (the "horned goat", often represented as a half-fish, half-goat), roughly December 22 to January 19, then that makes your sun sign Capricorn.

"The only phenomenon in astrology allowing you make a wild generalizations about everybody born in this period to that period every year without fail is the sun sign," explained Jonathan Cainer, prominent astrologer who writes one of Britain's most-read horoscope columns for The Daily Mail.

"[The column] was embraced by an enthusiastic public with open arms and it spawned a thousand imitations. Before we knew it tabloid astrology was born... this vast over-simplification of a noble, ancient art," Cainer says. Cainer pointed out that even as newspaper and magazine horoscope writing became more and more popular - which it did and quickly, on both sides of the Atlantic - the practice was largely disregarded by the "proper" astrological community. The accusation, he says, was bolstered by the fact that historically, a lot of horoscope columns weren't written by actual astrologers, but by writers told to read a book on astrology and get cracking.

Astrologers' consternation notwithstanding, the popularity of newspaper and magazine horoscope has never really died down; they became, along with standards like the crossword, newspaper "furniture", as Cainer put it (and people hate it when the furniture is moved, Cainer says). Cainer also noted that there are few places in newspapers and, to some extent magazines, that address the reader directly: "It's an unusual form of language and form of relationship and as such, it lends itself well to a kind of attachment."

Tiffanie Darke, editor of The Sunday Times Style section, which runs astrologer Shelley von Strunckel's column, confirmed that via email, saying, "There is a significant readership who buy the paper particularly for Shelley's column, and there is a very considerable readership who you will see on Sundays in the pub, round the kitchen table, across a table at a cafe, reading out her forecasts to each other."

This fits with what newspapers really are and have virtually always been - not just vehicles for hard news and so-called important stories, but also distributors of entertainment gossip and sports scores, advice on love matters and how to get gravy stains out of clothing, practical information about stock prices and TV schedules, recipes and knitting patterns, comics and humor, even games and puzzles. Whether those features are the spoonful of sugar to help the hard news medicine go down or whether people just pick up the paper for the horoscope makes little difference to the bottom line.

So as to why newspapers run horoscopes, the answer is simple: Readers like them.

But the figures on how many readers actually like horoscopes aren't entirely clear. A National Science Foundation survey from 1999 found that just 12 percent of Americans read their horoscope every day or often, while 32 percent read them occasionally. More recently, the American Federation of Astrologers put the number of Americans who read their horoscope every day as high as 70 million, about 23 percent of the population. Anecdotally, enough people read horoscopes to be angry when they're not in their usual place in the paper - Cainer says that he has a clause in his contract allowing him to take holidays, making him a rarity in the business: "The reading public is gloriously unsympathetic to an astrologer's need for time off."

Other evidence indicates that significant numbers of people do read their horoscopes if not daily, then regularly: When in 2011, astronomers claimed that the Earth's naturally occurring orbital "wobble" could change star signs, many people promptly freaked out. (Astrologers, meanwhile, were far more sanguine - your sign is still your sign, they counseled; some, Cainer included, sighed that the wobble story was just another salvo in the fiercely pitched battle between astronomers and astrologers.)

At the same time, a significant portion of the population believe in the underpinnings of newspapers horoscopes. According to a 2009 Harris poll, 26 percent of Americans believe in astrology; that's more people than believe in witches (23 percent), but less than believe in UFOs (32 percent), Creationism (40 percent) and ghosts (42 percent). Respect for astrology itself may be on the rise: A more recent survey from the National Science Foundation, published in 2014, found that fewer Americans rejected astrology as "not scientific" in 2012 than they did in 2010 - 55 percent as compared to 62 percent. The figure hasn't been that low since 1983.

People who read their horoscopes also pay attention to what they say. In 2009, an iVillage poll - to mark the launch of the women-focused entertainment site's dedicated astrology site, Astrology.com - found that of female horoscope readers, 33 percent check their horoscopes before job interviews; 35 percent before starting a new relationship; and 34 percent before buying a lottery ticket. More recent research, published in the October 2013 issue of the Journal of Consumer Research, found that people who read a negative horoscope were more likely to indulge in impulsive or self-indulgent behavior soon after.

So what's going on? Why are people willing to re-order their love lives, buy a lottery ticket, or a take a new job based on the advice of someone who knows nothing more about them than their birthdate?

One reason we can rule out is scientific validity. Of all the empirical tests that have been done on astrology, in all fields, says Dr. Chris French, a professor of psychology at London's Goldsmith College who studies belief in the paranormal, "They are pretty uniformly bad news for astrologers."

There's very little scientific proof that astrology is an accurate predictor of personality traits, future destinies, love lives, or anything else that mass-market astrology claims to know. For example, in a 1985 study published in the journal Nature, Dr. Shawn Carlson of University of California, Berkeley's Physics department found that seasoned astrologers were unable to match individual's star chart with the results of a personality test any better than random chance; in a second test, individuals were unable to choose their own star charts, detailing their astrologically divined personality and character traits, any better than chance.

A smaller 1990 study conducted by John McGrew and Richard McFall of Indiana University's Psychology department and designed with a group of astrologers, found that astrologers were no better at matching star charts to the corresponding comprehensive case file of a volunteer than a non-astrologer control subject or random chance, and moreover, didn't even agree with each other. A study out in 2003, conducted by former astrologer Dr. Geoffrey Dean and psychologist Dr. Ivan Kelly, tracked the lives of 2,000 subjects who were all born within minutes of one another over several decades. The theory was that if astrological claims about star position and birthdates were true, then the individuals would have shared similar traits; they did not.

Studies that support the claims of astrology have been largely dismissed by the wider scientific community for a "self-attribution" bias - subjects had a prior knowledge of their sign's supposed characteristics and therefore could not be reliable - or because they could not be replicated. Astrologers are, unsurprisingly, not impressed by scientific efforts to prove or disprove astrology, claiming that scientists are going about it all wrong - astrology is not empirical in the way that, say, physics is: "Experiments are set up by people who don't have any context for this, even if they were attempting to do something constructive," says Shelley von Strunckel, American astrologer and horoscope writer whose column appears in The Sunday Times, London Evening Standard, Chinese Vogue, Tatler and other major publications. "It's like, 'I'm going to cook this great French meal, I've got this great cook book in French - but I don't speak French.'"

But despite a preponderance of scientific evidence to suggest that the stars do not influence our lives - and even personally demonstrable evidence such as that financial windfall your horoscope told you to expect on the eighth of the month failed to materialize - people continue to believe. (It's important to note, however, that some astrologers balk at the notion of "belief" in astrology: "It's not something you believe in," says Strunckel. "It's kind of like believing in dinner. The planets are there, the cycles of nature are there, the full moons are there, nature relates to all of that, it's not something to believe in.")

The "why" people continue to read and credence their horoscopes is most often explained by psychologist Bertram Forer's classic 1948 "self-validation" study. Forer gave his students a personality test, followed by a description of their personality that was supposedly based on the results of the test. In reality, there was only ever one description, cobbled together from newspaper horoscopes, and everyone received the same one. Forer then asked them to rate, on a scale of 0 (very poor) to 5 (excellent), the description's accuracy; the average score was 4.26 - pretty remarkable, unless all the students really were exactly the same. Forer's observation was quickly dubbed the Forer effect and has often been replicated in other settings.

Part of what was happening was that the descriptions were positive enough, without being unbelievably positive:

You have a great deal of unused capacity which you have not turned to your advantage. While you have some personality weaknesses, you are generally able to compensate for them.

and, importantly, vague enough to be applicable to a wide audience:

At times you have serious doubts as to whether you have made the right decision or done the right thing.

At times you are extroverted, affable, sociable, while at other times you are introverted, wary, reserved.

Even horoscope writers admit that some of their success rests in not saying too much. Says Cainer, "The art of writing a successful horoscope column probably confirms what all too many skeptics and cynics eagerly clutch to their bosoms as charlatanry. Because it's writing ability that makes a horoscope column believable... ultimately a successful column will avoid specifics wherever possible. You develop the art of being vague."

The other element of the Forer effect is that the individual readers did most of the work, shaping the descriptions to fit themselves - not for nothing is the Forer effect also called the Barnum effect, after the famous showman's claim that his shows "had something for everyone". French, the Goldsmith psychologist, notes that people who read horoscopes are often invested in making their horoscope right for them. "If you buy into the system and the belief, it's you that's kind of making the reading appear to be more specific than it actually is," he explains. "Most days for most people is a mix of good things and bad things, and depending on how you buy into the system... if you're told to expect something good that day, then anything good that happens that day is read as confirmation."

Astrologer Cainer has another, more practical explanation for why people read horoscopes: "It's because they're there." There's very much a "can't hurt" and "might help" perception of horoscopes; at the same time, newspaper horoscopes, he says, also allow casual horoscope readers "a glorious sense of detachment: 'I don't believe in this rubbish but I'll have a look.'" This resonates with what Julian Baggini, a British philosopher and writer for The Guardian, says about why people read horoscopes: "No matter how much the evidence is staring someone in the face there's nothing in this, there's that 'Well, you never know.'" (Even if you do know.)

But "you never know" and even the Forer effect doesn't entirely explain the longevity of a form that many critics complain has no business being in a newspaper - so maybe there's something else going on. When French taught a course with a section on astrological beliefs, he'd sometimes ask on exams: "Does astrology work?" "Basically, the good answers would be the ones that took part the word 'work,'" he says. On the one hand, the straightforward answer is that, according to a host of scientific studies, astrology does not work. "But you've then got the other question... 'Does astrology provide any psychological benefit, does it have an psychology function?'" he said. "The answer to that is, sometimes, yes."

Psychologists see people on a scale between those who have what's called an external locus of control, where they feel that they are being acted upon by forces out of their influence, and people with an internal locus of control, who believe that they are the actors. "Not so surprisingly, people who believe in astrology tend to have an external locus of control," says French. That observation tallies with what other psychologists say: Margaret Hamilton, a psychologist at the University of Wisconsin who found that people are more likely to believe favorable horoscopes, noted that people who are believers in astrology also tend to be more anxious or neurotic.

Newspaper horoscopes, she said, offer a bit of comfort, a sort of seeing through the veil on a casual level. French agrees: astrology and newspaper horoscopes can give people "some kind of sense of control and some kind of framework to help them understand what's going on in their lives." It's telling that in times of uncertainty, whether on a global, national or personal level, he notes, astrologers, psychics, and others who claim to be able to offer guidance do a pretty brisk business; that belief in astrology is apparently on the rise in America, according to the NSF survey published in 2014, may have something to do with recent financial uncertainty. Cainer agreed that people take horoscopes more seriously when they're in distress: "If they're going through a time of disruption, they suddenly start to take what's written about their sign much more seriously.... If you're worried and somebody tells you not to worry, you take that to heart." (On whether astrologers are taking advantage of people, French is clear: "I am not saying that astrologers are deliberate con artists, I'm pretty sure they're not. They've convinced themselves that this system works.")

Philosophically, there is something about reading horoscopes that does imply a placing of oneself. As Hamilton notes, "It allows you to see yourself as part of the world: 'Here's where I fit in, oh, I'm Pisces.'" Looking deeper, Baggini, the philosopher, explains, "Human beings are pattern seekers. We have a very, very strong predisposition to notice regularities in nature and the world, to the extent that we see more than there are. There are good evolutionary reasons for this, in short a false positive is less risky than failure to observe a truth." But, more to the point, "We also tend to think things happen for a reason and we tend to leap upon whatever reasons available to us, even if they're not entirely credible."

Horoscopes walk a fine line, and, for many people, an appealing one. "On the one hand, people do want to feel they have some agency or control over the future, but on the other, it's rather frightening to think they have too much," explained Baggini. "So a rather attractive world view is that there is some sense of unfolding benign purpose in the universe, in which you weren't fundamentally responsible for everything, but were given some kind of control... and astrology gives us a bit of both, a balance."

Astrologers might agree. "I'm a great believer in freewill," says Cainer. "There's a lovely old Latin phrase that astrologers like to quote to each other: Astra inclinant non necessitant. The stars suggest, but they don't force... I like to think that astrology is about a way of fighting planetary influences, it's not entirely about accepting them."

But really, at the end of the day, are horoscopes doing more harm than good, or more good than harm? It all depends on whom you ask (and, of course, on the appropriateness of the advice being given). Strunckel and Cainer, obviously, see what they do as helping people, although both acknowledge that, as Strunckel says, "Astrology isn't everybody's cup of tea."

Richard Dawkins, the outspoken humanist and militant atheist, came out strongly against astrology and horoscopes in a 1995 Independent article published on New Years' Eve, declaring, "Astrology not only demeans astronomy, shrivelling and cheapening the universe with its pre-Copernican dabblings. It is also an insult to the science of psychology and the richness of human personality." Dawkins also took newspapers to task for even entertaining such "dabblings". More recently, in 2011, British rockstar physicist Brian Cox came under fire from astrologers for calling astrology a "load of rubbish" on his Wonders of the Solar System program on BBC. After the BBC fielded a bunch of complaints, Cox offered a statement, which the broadcaster probably wisely chose not to release: "I apologize to the astrology community for not making myself clear. I should have said that this new age drivel is undermining the very fabric of our civilization."

What Dawkins and Cox may not want to acknowledge is that humans don't tend to make decisions based on a logical, rational understanding of facts (there's a reason why "cognitive dissonance" is a thing) - and horoscope reading might be just as good a system of action as any. "Most people don't base their views and opinions the best empirical evidence," French says. "There are all kinds of reasons for believing what you believe, not least of which is believing stuff because it just kind of feels good."

At their heart, horoscopes are a way to offset the uncertainty of daily life. "If the best prediction you've got is still completely rubbish or baseless, it's better than no prediction at all," says Baggini. "If you have no way of controlling the weather, you'll continue to do incantations and dances, because the alternative is doing nothing. And people hate doing nothing."

About Linda Rodriguez McRobbie

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smithsonian.com Astrology is either an ancient and valuable system of understanding the natural world and our place in it with roots in early Mesopotamia, China, Egypt and Greece, or complete rubbish, depending on whom you ask. But newspaper and magazine horoscopes?

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Unlike previously thought, many supernovae result from stellar collisions

On the evening of November 11, 1572, twenty-six-year-old astronomer Tycho Brahe was about to make a discovery that would change his life and consequentially boost the scientific revolution significantly. While casually staring at the night sky, he suddenly noticed a very bright unfamiliar star in the Cassiopeia constellation. The star, which was as bright as Venus, was located in the inner parts of the famous W-shaped constellation, which was well known to many common people, let alone astronomers. What Tycho saw looked like the appearance of a new star ( nova stella). He was so astonished that he sought the confirmation of others to assure himself that he was not hallucinating.

Unknown to Tycho, such new stars had appeared during the previous centuries ("guest stars" in Chinese records), with a much brighter star reported in 1006. While these events were very important to astrologers, they had no lasting effect on astronomical thinking at the time. Tycho, however, realized that such an event was revolutionary. By accurately and repeatedly measuring the position of the "nova," Tycho showed that it was much further than the moon. In one night, Tycho managed to scientifically falsify the millennia-old Aristotelian belief that anything beyond the sphere of the moon cannot change. This convinced Tycho that the "known" cosmology was wrong and motivated him to devote his life to performing measurements of stars and planets to study the "true" cosmology. His hard, lifelong work paid off. His careful measurements of the positions of the planets enabled the discovery of the law of gravity by Johannes Kepler and Isaac Newton. Kepler would later say that if Tycho's star did nothing else, it produced a great astronomer. Yet, even Tycho and Kepler could not have appreciated that what seemed like a new star was actually an explosion of unimaginable power and that such explosions are crucial for our existence.

Tycho's "nova" faded from sight about eighteen months after its discovery. However, modern instruments reveal a spectacular sight at the position recorded by Tycho. Observations of the same spot in the sky by NASA's Chandra X-ray observatory, nearly five centuries later, show material expanding with velocities of thousands of kilometers per second with a mass of a few billion billion billion tons, comparable to that of the Sun. Tycho had evidenced a powerful explosion, which we now term "supernova." The huge amount of energy released in a few months is equivalent to the output of a few billion billion billion H-bombs. At the current rate of emission, it would take our Sun several billion years, of order of its lifetime, to release the same amount of energy. This phenomenon turned out to be very rare, occurring roughly once per century within a galaxy like our own. While supernovae have not been detected in the Milky Way during the last three centuries, modern technology has allowed thousands of supernovae to be detected in other galaxies. Supernovae are not just beautiful cosmic fireworks-many of the elements that are essential for life, such as calcium and iron, are believed to have been produced by supernovae.

But what causes these explosions? There are currently two explanations involving theoretical mechanisms that are related to the two ways in which stars die. Stars cannot shine forever for the simple reason that their energy supply-nuclear burning-is finite. What happens to stars once they exhaust their nuclear fuel (mainly hydrogen) is believed to depend crucially on their mass. One of the most important theoretical discoveries in astrophysics is that a critical mass exists above which stars cannot sustain themselves against their own gravitational pull without a continuous supply of energy. The two types of star endings depend on whether their mass is above or below this critical mass, which is called the Chandrasekhar mass limit, named after Subrahmanyan Chandrasekhar (Member, 1941, 1976), one of its discoverers. If, by the time a star exhausts its fuel, it has a mass greater than this limit, the core of the star cannot sustain itself and collapses. A huge amount of energy is released when the core collapses to the tiny size of a few kilometers, becoming a black hole or a neutron star. While most of this energy is emitted in invisible neutrinos, a small fraction of this energy ejects the outer parts of the star, creating an explosion sufficient to produce a supernova. Such a theoretical event is called a core-collapse supernova.

Stars that are less massive than the Chandrasekhar mass limit, or lose enough mass during their life to become so, are able to resist gravity once their nuclear fuel is exhausted. Gravity does manage to shrink them considerably, however, and they settle at a radius of a few thousand kilometers. Such dense stars, with a mass comparable to that of the Sun (one million times the mass of Earth) and a size comparable to that of Earth, are called white dwarfs. These stars are abundant and have been extensively studied observationally and theoretically. Without their previous nuclear energy supply, white dwarfs slowly become dimmer and eventually become unobservable. While white dwarfs are stable, their exceptional high density make them very powerful thermonuclear bombs. If properly ignited, white dwarfs are capable of powerful thermonuclear explosions with a sufficient energy release to account for a supernova. Such theoretical events are called thermonuclear supernovae. An appealing aspect of theories involving nuclear energy is that they naturally explain why the energy-per-unit mass released by supernovae is comparable to that of H-bombs and our Sun. Yet neither core collapse in massive stars nor thermonuclear explosions of white dwarfs have been theoretically established to account for the supernovae that we see. While core collapse is likely inevitable, it has not been shown that the outer parts can be ejected successfully. While white dwarfs can explode if ignited, a robust ignition mechanism has not been identified. Yet the two explosion mechanisms are widely believed to occur, based on several successes for explaining supernovae observations and given that we simply do not have better ideas.

Perhaps the best clues for identifying an explosion mechanism have come from a few nearby supernovae where a star, located in pre-explosion images at the precise position of the supernovae, had disappeared in images taken after the supernova had faded away. In all of these cases, the exploding star was massive and at a late stage of its life in accordance with the core-collapse theory. This scenario was confirmed for the famous supernova that exploded in the Large Magellanic Cloud galaxy in 1987-the closest among these supernovae-in which the predicted neutrino burst was detected.

However, core collapse of massive stars cannot account for all of the supernovae. A significant fraction of supernovae occur in elliptical galaxies where massive stars simply do not exist. These massive stars have lifetimes of only millions of years and have long died in elliptical galaxies where stars have ceased forming for billions of years. Interestingly, it turns out that all supernovae that occur in such old stellar environments seem to share many distinctive emission properties, belonging to a separable class called type Ia. These supernovae are distinct by not showing any trace of hydrogen and helium, the most abundant elements in the universe, but exhibiting significant abundance of much heavier elements such as silicon, nickel, and iron. These features are naturally explained if these supernovae are the thermonuclear explosions of white dwarfs, which are abundant in all stellar environments, including elliptical galaxies, and are predominantly made of carbon and oxygen rather than hydrogen and helium. Perhaps the strongest piece of evidence in this direction is the huge amount-a considerable fraction of the ejected materials-of the unstable isotope Ni ⁵⁶ detected in these supernovae, which is an expected outcome of a thermonuclear explosion. In fact, given the short lifetime of a few days, this isotope must have formed in the explosion itself, favoring a nuclear origin. While type Ia are the only supernovae known to occur in elliptical galaxies, they are observed to occur in all stellar environments, including spiral galaxies like our own. In particular, the supernova observed by Tycho, which occurred in our own galaxy, was a type Ia supernova. Although observations of type Ia supernovae are compatible with explosions of white dwarfs, they do not provide any hint as to why some white dwarfs suddenly explode.

Before we address the mystery behind the sudden explosion of white dwarfs, it is worth mentioning the importance of the resulting type Ia supernovae. The large amounts of Ni ⁵⁶ that are observed in these supernovae decay to Co56 and then to iron. Type Ia supernovae are the most efficient known production sites of iron. A significant amount of the iron in the universe, including a large fraction of Earth's mass and the iron in your blood, was produced in such supernovae. In addition, type Ia supernovae have turned out to be very useful tools for measuring fundamental properties of the observable universe. Among the wide variety of supernovae, type Ia are the brightest and comprise a relatively homogeneous group. Their high brightness allows them to be detected clearly across the observable universe. Their homogeneity allows a calibration of their intrinsic luminosity, which enables their distance to be measured. An object with a given intrinsic luminosity appears dimmer when it is more distant and brighter when it is closer, allowing a determination of its distance based on the measurement of its apparent brightness. Astrophysical objects with known intrinsic luminosity are rare and precious, and type Ia supernovae are one of the few such "standard candles" that can be seen from cosmological distances. In recent years, distances across the observable universe that were measured using type Ia supernovae led to the surprising conclusion that the expansion of the universe is accelerating, prompting fundamental modifications to the Big Bang cosmological model. The 2011 Nobel Prize in Physics was awarded to three astronomers due to this application of type Ia supernovae. The growing importance of the use of type Ia as accurate rulers for cosmological distance has strengthened the need for a firm theoretical understanding of these explosions.

This brings us back to the pressing issue of why some white dwarfs explode (about 1 percent of white dwarfs account for type Ia supernovae). During the last several decades, the standard lore has centered on increasing the mass of these white dwarfs toward the unstable Chandrasekhar limit where an explosion is assumed to occur. In the most popular scenarios, the mass of some white dwarfs grows with time, due to material deposited by a nearby companion. Despite its popularity, this scenario has never been convincingly shown to lead to an explosion, and there is observational evidence that there are not enough relevant systems to account for the rate of type Ia supernovae even if they were to explode. Also lacking an explanation is the amount of Ni ⁵⁶, which varies by about an order of magnitude between the faintest and brightest type Ia supernovae. While the shortcomings of this Chandrasekhar model have been presented in many recent papers and reviews, it remained popular mostly due to the fact that there were no other good alternative suggestions.

We now believe we have found the solution to this problem: type Ia supernovae occur when two white dwarfs directly collide with each other. The explosion is triggered by the shock waves that result from the collision, which occurs at velocities of thousands of kilometers per second due to the gravitational pull of the white dwarfs. Such collisions lead to successful explosions over a wide range of white dwarf masses and produce the right range of Ni ⁵⁶ masses seen in type Ia supernovae. They have nothing to do with the Chandrasekhar mass. While it was known that such collisions could lead to explosions, it was never regarded as a possible explanation for type Ia supernovae since it was believed that such collisions were very rare. A very small rate was thought to occur in dense stellar systems, with less than 1 percent of all type Ia supernovae possibly resulting from such collisions.

In a paper published last year by Members Boaz Katz and Subo Dong, it was argued that the rate of collisions actually may be as high as the total rate of type Ia supernovae. The argument was based on the fact that most stars are observed to start their life in small groups of three or more. It was shown that collisions are likely in triple systems, consisting of two white dwarfs orbited by a third star, due to the three-body gravitational interaction between them. The rate of such collisions can reach the rate of Ia supernovae if a few tens of percent of stars remain in triple systems when they become white dwarfs, a quite reasonable assumption. That type Ia supernovae are the result of common collisions in triple systems seems like a highly unlikely explanation at first thought. Given the small size of white dwarfs, the chance of a collision at any given passage is roughly one in a million. But numerical calculations by Katz and Dong showed that a few percent of these systems lead to a collision. The key is that the systems have billions of years to spare, during which the white dwarfs experience millions of passages. Due to the subtle yet persistent pulls from the distant third star, each passage of the two white dwarfs occurs at a slightly different configuration and, after a million passages, the chance for a collision becomes significant.

In a combined effort led by Member Doron Kushnir with Katz, Dong, Member Rodrigo Fernandez, and Eli Livne of Hebrew University, the detailed process of white dwarf collisions was calculated and shown to reproduce several features of the broad distribution of type Ia supernovae. For the first time since their discovery by Tycho more than four centuries ago, a detailed scenario for a significant fraction of supernovae was studied in which the explosions could be numerically calculated from first principles based on the physically well understood processes of Newtonian dynamics, hydrodynamics, and thermonuclear burning. In a paper published by our group, the model was shown to successfully pass three independent and robust observational tests, including the successful recovery of the wide distribution of Ni ⁵⁶ masses. It is thus very likely that what Tycho thought was the sudden birth of a new star was actually the violent death of two. He might have been gratified to learn that the two stars collided due to the application of the same law of gravity that would later be deduced based on his dedicated observations.

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On the evening of November 11, 1572, twenty-six-year-old astronomer Tycho Brahe was about to make a discovery that would change his life and consequentially boost the scientific revolution significantly. While casually staring at the night sky, he suddenly noticed a very bright unfamiliar star in the Cassiopeia con­stellation.