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http://learningdesigner.org/viewer.php?uri=/shared/fid/feb2982ec29f08516ad6dcbbdc7e812bef855689b8d5b47fa6efe8bdb076b925
http://learningdesigner.org/viewer.php?uri=/shared/fid/77515f287253b6cd830d374cd597d2783eb01180bedad8b4f3969a6a32a753cc

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Red giants and red supergiants

As we mentioned earlier, when stars burn hydrogen in their core, which is the better part of their lives, they are located in the main sequence of the H-R diagram. When hydrogen stock runs out, the star compresses its core in order to increase its temperature and continue producing energy by fusing helium into carbon. This compression is so abrupt that the outer layers of the star get detached, they start expanding and lose energy fast. This causes the star to get a lot bigger and red. This is the phase when a small star becomes a “Red Giant”. More massive stars, manage to get their core temperature high enough to fuse carbon and heavier elements all the way to iron. More massive stars become “Red supergiants” instead. Red giants can be 10 to 150 times bigger than the initial star. A star like our Sun, when it becomes a red giant will become about 2000 times brighter.

Red supergiants, that get to the point of fusing silicon to iron reach a core temperature of about 3 billion degrees. Their outer layers get so swollen, they can reach a diameter of a billion kilometres.

White dwarfs

Small mass stars (up to 8 solar masses) after going through the phase of red giants and their outer layers get blown away from the core of the star, what remains is basically their naked core. That naked core is what we call a “White Dwarf” which eventually cools down over billions of years and eventually becomes black. The core of the original star is now super dense (about a million  times the density of the sun), hot (about 100.000 degrees) pile of carbon ash roughly the size of the Earth, with incredible gravity. Imagine that a cough drop of white dwarf material equals the weight of a car on Earth! Normally, a white dwarf has such tremendous gravity, that it would collapse. However, this is not what happens thanks to quantum mechanics. Pauli’s exclusion principle and Heisenberg’s uncertainty principle keeps the mass from becoming any denser and thus the white dwarfs survives. Due to the quantum phenomena, the matter of white dwarfs is called “degenerate” which means it has some very peculiar properties. For example, the more massive the white dwarf, the smaller its size!

Planetary Nebulae

Depending on the mass of the star (as usual!) in some cases, when the outer shell of a star gets blown away, it will form something like a gaseous cloud around the white dwarf that remains. That is what we call a “Planetary Nebula”. Radiation emitted by the white dwarf will interact with the gases of the nebula causing it to emit light at different wavelengths.

What’s odd about nebulae is that, since they come from the outer shell of a star, one would expect them to be spherical. And yet they are not. They come in many different shapes.

Image Source (from left to right): NASA, NASA, Evangelos Souglakos

The reasons behind these shapes can be different, binary stars (two stars orbiting around each other) or planets orbiting the star could be a couple of these reasons. The gases of the nebulae vary; there is hydrogen but there can also be oxygen, sulfur and nitrogen. These gases emit in different colors of the spectrum. This is also why nebulae have such different colors. As nebulae came from the blowing away of the stars outer shell, they don’t remain still, they keep expanding. After a few thousands of years, they become very faint to observe. Their short life time is the reason why there are not so many of them on the sky. However, the size, color and shape of a nebula can tell us a lot about the life and death of the original star.

Neutron stars

When stars between 8 and 20 solar masses eventually stop producing energy through fusion, they don’t simply have their core exposed after their outer shell is detached. The phenomena taking place here are so extreme, they cause the star to collapse under its own gravity and explode. What remains is a core that has undergone so many extreme changes that is truly extraordinary. The gravity in the remains of the core of such stars is so huge that it overcomes electron degeneracy (which occurs in white dwarfs) and literally force electrons to smash into protons thus creating neutrons. Very few of them survive. So, what really remains, is a supermassive object made of neutrons with some protons, electrons and element left here and there. These objects also have a thin crust of heavier elements nuclei, like iron and electrons. Imagine something like a gigantic atomic nucleus. These are the “Neutron Stars”. Neutron stars are among the most bizarre objects of the Universe. They have a radius of about 20 km and the mass of an entire star. To understand how insanely massive these objects are, imagine that a cough drop of neutron star material would weigh as much as all humans on Earth combined.

Neutron stars (also called pulsars) also spin very rapidly (a few times per second) and they emit very accurate pulses.  The pulses they emit are so accurate and consistent that when first detected scientists believed it was not coming from a natural object but from extra-terrestrial life.

Think about introducing neutron stars to your students when you talk to them about atomic nuclei, the strong and weak interaction!

Black holes

For stars beyond 20 solar masses, the fate of a star is even more extreme. The gravity in such dying stars is so massive that nothing can stop it. After the star explodes it leaves behind what we call a “Black Hole”. A black hole, has such tremendous gravity that nothing can escape, it not even light. That’s why it’s black. In other words, the escape velocity* of a black hole is greater than the speed of light. Since not even light can escape a black hole, we can never really see it. The area around the black hole were the escape velocity is equal to the speed of light is called the “Event Horizon”. Nothing escapes once it passes that horizon. Black holes come in different sizes. The smaller can have as much as 3 solar masses. Unlike what some might thing they don’t suck all mass around it. Depending on the distance of an object from it, the object can actually orbit a black hole without being sucked in. However, if the object gets close enough, then it can indeed be pulled in by the black hole. It is believed that galactic centres are occupied by black holes and they could be playing a significant role in the evolution of galaxies.

*Escape velocity is the velocity an object needs in order to escape permanently the gravitational pull of another object. The escape velocity for an object in order to escape Earth’s gravity  for example is 11 km/sec.

As you may imagine, such staggering objects as black holes, also have very peculiar properties. For example, remember tides? In an earlier module we talked about how the Moon’s gravity affects the Earth and how tidal forces tend to stretch objects. Well, imagine what happens near an object with gravity so massive as a black hole’s! If an object like a tree was to fall in a black hole roots first, the gravity at the roots would be millions of times stronger than at the top of the tree, stretching it out to a thin line. And let’s not also forget about space-time! Gravity can affect space-time. The greater the gravity the slower time passes by. In fact, it slows down so much that at the event horizon it stops completely. So if we were standing near a black hole watching our tree pass the event horizon, time would freeze! We would watch the tree slowing down more and more taking infinite time to reach the horizon.

Image Source: NASA

Our students’ perception of gravity is usually limited up to the point of gravity holding planets together around their parent star. Black holes, like white dwarfs and neutron stars are excellent examples, to demonstrate to your students how powerful gravity can be in extreme situations.

Supernovae

When stars bigger than 8 solar masses come to a point when they can no longer produce energy through fusion the go out with a tremendous explosion and become either neutron stars or black holes. The explosions of these stars are called “Supernova”. In fact this is only one type of supernova. A white dwarf can also cause a supernova when it’s part of a binary system of stars. The white dwarf will take mass from its companion and when it has accumulated enough mass it will collapse under its own gravity causing a supernova. Supernovae release unforeseen amounts of energy in a very short amount of time, at speeds close to the speed of light. They enrich the interstellar medium with heavy elements. In fact, we wouldn’t be here if it wasn’t for supernovae. Perhaps you noticed earlier that even the largest of stars can fuse elements only reaching up to iron. Iron is the ultimate form of ash in the Universe. That’s because fusion beyond iron is not producing any energy, it requires more energy than it emits. Thus stars can’t fuse iron to produce energy. Particles beyond iron can only produce energy through fission. So if it wasn’t in the interest of stars to make all the other elements, how did they come to exist? The calcium in our bones? Gold? Platinum?

During supernovae, conditions are so extreme that fusion takes place even if it’s not in the benefit of the exploding star thus creating heavier elements. These heavier elements are also scattered through the interstellar medium during the blast of a supernova. Eventually, over long periods of time, gas and dust that make up new stars and their planets are enriched by these heavier elements that were created during supernovae explosions.  Bottom line? We are all made of stardust!

When introducing the periodic table, binding energy, fusion and fission in class, talking about supernovae and how these elements came to exist can be very inspiring for your students and give them an idea of how tightly our lives are bound to the Universe and the stars.

More on the life of stars:

Imagine the Universe

You can use the “Star in a box” simulation in your class to teach your students about the H-R diagram and the fate of stars. Among many other interesting aspects students will also see how fast stars evolve once they leave the main sequence compared to the era when they fuse hydrogen.

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The life of stars

Going back to stars, we’ve talked about how they produce energy and how that energy is emitted. But how did stars get to start producing energy in the first place? What happens when they run out of fuel? Although stars may seem eternal compared to our lifetime they do have a finite lifespan. Massive stars, about 150 times bigger than our Sun live a few million years while smaller stars, about 1/10 of our star, for example, are believed to live around 10 trillion years or more. So the first of these objects that were created are still around given that our Universe is ‘only’ 13.8 billion years old. Such small stars are known as “Red dwarfs”. Overall, the bigger the star the shorter it will leave.

The bright dust areas of galaxies, like the spirals of a spiral galaxy are star nurseries. These areas have enough gas and dust so that stars can begin to form. Stars start to form when gravity pulls together gases and dust into bulges. As more matter is added to that bulge the greater the gravity. Increased gravity will attract even more dust and gas but it will also increase the density and the temperature of the main bulge even more. Eventually temperature will be high enough, to allow the object to start producing energy through hydrogen fusion. Sometimes, the process doesn’t end exactly like that. Sometimes, that bulge does not have enough mass and not enough temperature to start fusing hydrogen. Such objects are hot enough to emit some radiation but eventually they cool down and become very dark. This “almost stars” are called “Brown dwarfs”.
https://www.youtube.com/watch?v=mkktE_fs4NA
Once a star is formed, its lifetime depends on each initial mass. Big stars are much brighter and they require greater amounts of energy. They use up their fuel much faster than smaller stars and thus they have shorter lifespans. Astronomers classify stars based on their temperature and their spectra.  There are seven basic categories which are divided into subcategories.  The brightest of all (yet very rare) belong to categories O and B. The dimmest of all are category M. In between there are types A, F,G and K. Thus in total we have O,B,A,F,G,K,M. You could come up this a mnemonic for these categories with your students. For example: “Oh Be A Fine Girl/Guy Kiss Me”. Astronomers also use what we call the “Hertzsprung-Russell diagram” (also called H-R diagram) a plot of effective temperature (the temperature on a star’s surface) vs. luminosity to map the evolution of stars.
https://www.youtube.com/watch?v=Zt8Sw79ObBs

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Constellations

Constellations can be very helpful to astronomers for mapping the sky and locating objects, but we also need to know what is the actual connection between the stars of a constellation in order to have a complete understanding of them. Some people may think that constellations are neighbourhoods of stars. In fact they are not, they only appear to be on our celestial sphere. Constellations are groups of stars that appear not to be moving relatively to each other on the night sky as we see it and they make up a particular shape. When looking at the night sky we often get the misconception that all the stars we see at night are on the same plane (the celestial sphere). Thus many people think that the stars of a constellation are also on the same plane. But this is not the case. In fact, stars of a constellation can be moving relatively to each other, but they are too far away for us to see that change in their relative position. They are also not on the same plane. They can be millions of light years away from each other. To help your students understand this, try doing a small activity with them looking at the same stars from different angles, like in the following image.

So constellations may be neighbourhoods of stars when it came to the celestial sphere, but their stars are not really connected to each other. So what are the true neighbourhoods of stars? Ah! Galaxies!

Galaxies

Galaxies are systems of hundreds of billions of stars, with enough gas and dust to make a lot more, all bound together thanks to gravity. Ever since ancient times people, believed that the milky bright band that appears on the night sky, could consist of stars. Today we know that that is the galactic disk of our own Galaxy, the “Milky Way”. Over the years, more and more astronomers from many different places around the planet supported this idea and the idea that there could be more such ‘bands’ in the Universe. Observations of nearby galaxies like Andromeda and the Magellanic clouds came to support these ideas. Ever since Galileo’s time, the first to point his telescope at the night sky, people have been observing galaxies on the night sky using telescopes.

After Galileo’s historic observations, another important milestone for astronomical observations was when in the 1920s Edwin Hubble using the largest telescope of that time discovered many galaxies beyond our own. Among his major contributions, is the “Hubble’s law” which suggests that the Universe is expanding and “Hubble’s tuning fork” which is the first major effort of astronomers to categorize galaxies based on their shapes. His diagram may be relatively simple but to this day, it constitutes the main scheme of galaxy categorization. He categorized galaxies into three main categories; ellipticals, spirals and barred spirals.

Image Source: Palomar Observatory/California Institute of Technology (first image), Hubble Space Telescope

In more recent times, the Hubble telescope, named to honour Edwin Hubble has given us an unprecedented view of the Universe beyond our own Galaxy. With more than 1.3 million observations, Hubble has shown us about 100 billion galaxies beyond the Milky Way. As technology advances however, this number is likely to increase as we will get more and more accurate observations of deep space.
http://www.space-awareness.org/en/activities/1607/what-is-a-constellation/
https://www.youtube.com/watch?v=O57DyNMRGY8
Many observations indicate that some galaxies are close enough to interact through the gravitational force. Of course, it takes a tremendous amount of time to see any changes, so to us they seem as if they are standing still. Scientists have developed simulations to view such interactions in “fast forward”. Similar, simplified simulations are also created for educational purposes and can be a lot of fun for students. Here you can find two nice tools for your students and a quick activity that combines them:

You can use the “Observing with NASA” robotic telescope network to take an astronomical observation of different celestial objects. It can be used to take an observation of two interacting galaxies. Then use this “Galaxy Crash” simulation and try to tell the story of these galaxies. Based on the simulation students can understand how these two galaxies where before they started to interact, how much time has it been since they started interacting and what will happen to them in future. Here you can find the answer keys for playing with M51 and its companion and the antennae galaxy in a galaxy crash.

More about galaxies

Related theory on the classification of Galaxies

Galaxies

Types of Galaxies

Clusters of Galaxies

Discovery of Galaxies

Active Galactic Nuclei

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Galaxies are the neighbourhoods of stars. They are systems of hundreds of billions of stars but how many stars are there in the Universe? How can we count them? Can we see them all? Counting the stars is not an easy task for sure! First, we need to clarify that we can only speak about the stars that we can observe. This is why we often talk about the “Observable Universe”. That is the part of the Universe we can observe. Why can’t we see the entire Universe? Because light has a finite speed! A first estimation is that the observable Universe is as big as light could travel since the beginning of the Universe, which would be around 13.8 billion years ago. That means that the observable Universe could be around 13.8 billion years in radius. Well… not exactly. There are other factors to take into consideration, like the expansion of the Universe.  A photon that was born during the big bang and has travelled 13.8 billion years to reach us, by the time we detect it, it has travelled much further away from us than where it originally was, making the observable Universe much bigger than 13.8 billion light years. So how big is the observable Universe? Around 45.7 billion light years.
https://www.youtube.com/watch?v=QXfhGxZFcVE
https://www.youtube.com/watch?v=DmUiCweDic4

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https://www.youtube.com/watch?v=GVfCI0GR4Jg

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Basic information about the Universe” didactic course

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Basic information about the Universe” didactic course

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https://www.youtube.com/watch?v=RxPucjYMOME

Other Activities for students

Universe in a box

Earth, Moon and Sun interactives

Astronomy and astrology

Babies and the Moon

Calculation of the Sun's effective Temperature

Seasons around the world

Sun, Earth and Moon Model

Measure the Solar Diameter

There are many interesting activities you can do with your students using data, images and videos from the ISS.

STEM Lessons From Space

International Space Station Activity Book

Astro Academy: Principia

ESA’s ISS Education kit for primary education

“Our own Golder Record”:

One activity that can be very inspiring for students is to ask them to create their own Golden record for a future mission. In other words, ask them to send a message to aliens about Earth. Students can choose images, videos and sounds. Such an activity could help them increase their sense of global citizenship. Students can compare their own disk to the real one and see if their line of thought was similar to the people that created the golden record. Depending on their age and the time you have at your disposal, you can extend this activity further. For example, you can talk about the idea of establishing a language of communication. How would we give instructions to the unknown civilization about how to play the record?

Here are some links for such an activity:

Voyager Record Photograph Index

Music From Earth

Sounds of Earth

Spacecraft Overview

Set the Controls for the Murmurs of Earth

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Rosetta is among the most famous contemporary missions. It set out in 2004 to chase and catch a comet, 67P Churyumov-Gerasimenko. And it did it! After a long 10-year trip, Rosetta and Philae reached their destination during the summer of 2014. Rosetta orbited the comet and Philae landed on it. They both took invaluable data for scientists. To grasp the magnitude of this achievement, think that humans launched a spacecraft which travelled 500 billion km and managed to land on an object about 5km long with an accuracy of a few hundred meters.
http://www.esa.int/Our_Activities/Space_Science/Rosetta
Voyager Mission

The Voyager mission is one of humankind’s greatest achievements. The twin voyager spacecraft was launched in 1977 and its primary objective was to study Saturn and Jupiter as well as the larger moons of these two planets. After completing this mission, they were assigned with a new one. The Voyagers would continue their trip in outer space. In August 2012, they went past the limits of our solar system and entered interstellar space. They are the first human-made objects to ever reach this far into space. Today, forty years later, they continue to function.

One special thing about the Voyagers is their “Golden Record”. Each spacecraft has on board a phonograph record-a 12-inch gold-plated copper disk which includes information about the Earth, including different sounds and images from the planet. Thus, in case one of these spacecraft is found by an alien civilization they would be able to get information about our existence. The disk is accompanied by instructions in symbolic language, that aim to explain how the record can be played and where the spacecraft came from.
https://www.youtube.com/watch?v=NN3ppVemVhQ
https://voyager.jpl.nasa.gov/spacecraft/goldenrec.html
The International Space Station

The International Space Station (ISS) is a space laboratory in orbit around Earth. It is the product of collaboration between the United States, Russia, Japan Canada, and Europe and for many it is one of the greatest international project of all time. The first components of the station were launched in 1998 and its construction was completed two years later when it was finally ready for people to visit it. The ISS is meant for research that cannot be done on Earth. For example, studying the effects on people from living in space. You can take a tour inside the ISS here.

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Tides :)
https://www.youtube.com/watch?v=KlWpFLfLFBI
The Moon and life on Earth

The Moon appears to have played a crucial role in the existence and evolution of life on Earth. Back during the era when life was created on Earth, days where much shorter and the Moon was closer to Earth. Thus tides where more frequent and much more intense. These intense tides seem to have allowed for the complex chemistry required to create the building blocks of life to occur. In addition to tides, the Moon also helped slow down Earth’s rotation and stabilize to a great degree the axial tilt of our planet.

Here are two videos about the role the Moon possibly played in creating life on Earth.

More on the Moon and life one Earth:

Without the Moon, Would There Be Life on Earth?

With or without you: The role of the moon on life

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Solar and Lunar Eclipses

Coming back to the Earth’s and the Moon’s orbits- as we mentioned earlier, there are only two points where the Moon’s orbit crosses the ecliptic, called the lunar nodes. Only when the Moon is on one of those two points of its orbit are the Sun, the Moon and the Earth on the same plane. This happens twice in every full rotation (every sidereal month). When the Moon passes between the Earth and the Sun at a lunar node we have a solar eclipse.

So, in order for a solar eclipse to occur, two things must happen at the same time.

The Moon has to be passing between the Earth and Sun.

The Moon has to be passing by a lunar node.

These two conditions can give us a solar eclipse, but not necessarily a total solar eclipse. The Moon’s orbit is not a perfect circle, which means that the lunar disks’ angular size as seen from Earth is not always the same and it is not always the same to the solar disk’s angular size as we see it from Earth. So the three objects may be perfectly aligned but the lunar disk’s apparent angular size and be smaller than that of the solar disk’s so it won’t cover it fully. In that case we have an annular solar eclipse. If the three objects are perfectly aligned, and the Moon is at a point of its orbit where its disk’s apparent angular size is as that of the Sun’s, only then do we have a total solar eclipse. If the lunar disk covers only part of the solar disk than we have a partial solar eclipse. As you have probably realized already, total solar eclipses are very rare!

Images Credit: NASA

When a solar eclipse is about to occur you may have heard people talk about where it can be visible from. A question that may come from your students is “Why can’t we see an eclipse from anywhere on Earth?”

To understand this we need to thing a little bit how shadows work. When the Moon passes between the Earth and the Sun, it will cast a shadow only at small part of the Earth. Hence, only people on the area where the Moon’s shadow appears will be able to see the total eclipse.
https://www.youtube.com/watch?v=asO2wBT9W9M
Lunar eclipses happen pretty much due to the same reasons. When the Moon passes behind Earth while at a lunar node we have a lunar eclipse instead of a solar eclipse. This means that the Earth blocks the light from the Sun so instead of a full Moon we get a lunar eclipse. The difference here is that it is the Earth that casts its shadow on the Moon. Thus lunar eclipses are visible from the entire part of the planet that can see the Moon. Because the Earth is much bigger than the Moon, lunar eclipses are not as rare as the solar eclipses. However, even during a full lunar eclipse, some sunlight will still pass through the Earth’s atmosphere and reach the Moon. As a result, during a lunar eclipse, we are still able to see the Moon, but it appears red as the scattered light reaching the Moon is towards the red part of the spectrum.
https://www.youtube.com/watch?v=kgbK2FZFFdw

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https://www.youtube.com/watch?v=wz01pTvuMa0
The Moon makes a complete clockwise rotation around Earth with respect to the fixed stars every 27.32 days (sidereal month). However, it takes on average 29.53 days to reach the same phase (new moon for example) which is referred to as the synodic month. Although the sidereal months are fixed, synodic months are not. Synodic months are also longer than sidereal months. That is because sidereal moths are only related to the rotation of the Moon around Earth. Synodic months however have to do with Moon phases, so they are also related to the relative position of the Moon and the Sun as well. Thus as the Earth-Moon system also orbits around the Sun the relative positions change every month.

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https://www.youtube.com/watch?v=jxm_GBuxVpI

Moon phases

One of the most prominent characteristics when we look at the Moon from Earth is that we don’t always see the entire lunar disc. We’ve learned why we always see the same side of the Moon so now we’ll see why we don’t always see the entire lunar disk. Before talking about the Moon’s phases to our students, we first need to clarify a couple of things and make sure there are no misconceptions:

1. The Moon does not have its own light. We only see the Moon due to the light coming from the Sun and being reflected on the Moon’s surface.

2. The Moon is not orbiting the Earth on the same plane that Earth orbits around the Sun. There is an approximately 5o tilt of the Moon’s orbital plane with respect to the ecliptic (Earth’s orbiting plane around the Sun).

Image Credit: Universe in a box

3. Moon phases do not occur because the Earth casts its shadow on the Moon. That only happens during lunar eclipses.

As one would expect, like Earth, only half of the Moon is being lit by the Sun at any given moment. Depending on the relative position of the Sun, the Moon and the Earth, we only see part of its lit disk here on Earth. The reason why we have Moon phases is the relative position of the sun, the Moon and the Earth.

We can see the fully lit part of the Moon when the Moon is behind Earth (so Earth is between the Sun and the Moon) and we cannot see any portion of the Moon’s lit part when the Moon is between Earth and the Sun (New moon). This can be a little confusing to some of the students. Here are two question that might come up:

“If the Earth is between the Sun and the Moon, doesn’t that mean that Earth is hiding the Moon?”

“When the Moon is between the Sun and the Earth, isn’t that when we have solar eclipses?”

The answer is the same to both questions. Remember that the Moon’s tilted orbital plane we talked about a little earlier? The Moon can be behind Earth or between the Earth and the Sun but that does not mean that the three objects are on the same line. The Moon can be between the Sun and the Earth but located lower or higher than the line between the Sun and the Earth. In fact, as the Moon orbits around the Earth, there are only two points (called lunar nodes) when all three objects are aligned; when the lunar orbit crosses the ecliptic (points 1 and 3 on the image below).

Image Credit: Royal Museums of Greenwich

Let’s have a look at the phases of the Moon and their names:

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The Sun’s electromagnetic radiation

The Sun emits energy that is crucial for life on Earth. The amount of energy received on Earth affects our climate and all subsystems of our planet. Let’s see a little bit about solar radiation itself.

To many people the term ‘electromagnetic radiation’ is identified with some kind of dangerous radiation that is harmful to living organisms. However, this is not exactly the case. Electromagnetic radiation is in fact light. Depending on the wavelength (or frequency) of the radiation, we have different bands - from gamma rays to visual light and radio waves. Visible light is the only type of electromagnetic radiation that is visible to humans. Some animals and insects have the ability to see colors beyond the visual spectrum. Before talking about solar radiation let’s see a few things about electromagnetic radiation in general, to clarify some misconceptions.
https://www.youtube.com/watch?v=zI2vRwFKnHQ
Our Sun emits all types of electromagnetic radiation from gamma rays to radio waves but not in equal amounts.

Image Credit: UC Regents

Gamma rays are produced at the core of the Sun due to nuclear fission processes. However the high densities in the Sun’s inner part (remember the photons’ random walk?) and the constant interactions of the produced photons with the plasma in the Sun, make these photons lose a big part of their energy, so from gamma ray photons they become photons of lower energy radiation. However, some gamma rays are emitted from the Sun because of solar flares. The solar corona is also responsible for the emission of ultraviolet radiation and X-rays. However, these make up only a small fraction of the total energy and radiation emitted. Ultraviolet radiation (in the form of UV-A, B, and C radiation) is mostly absorbed at higher altitudes. Extreme ultraviolet and soft X-ray radiation is also mostly absorbed by the atmosphere at high altitudes (around 96 km). Although soft X-ray radiation is absorbed by the atmosphere, it can be quite dangerous to living organisms. Such electromagnetic radiation with short wavelengths, can indeed be harmful to people as they can interact with molecules on our skin and give us sunburns or effect our DNA. Visible, infrared light and radiation of lower frequencies ( longer wavelengths) on the other hand, do not harm humans. Instead, this radiation reaches the Earth’s surface, keeping it warm and habitable. Here is a graph about the amount of energy emitted from the Sun at different wavelengths:

Image Credit: Wikimedia

The Sun emits electromagnetic radiation of many different frequencies. Some types of radiation (those with high frequency) can be harmful to humans and we need to be protected from them as they can interact with our skin giving us sunburns or effect our DNA causing more serious problems. Luckily for us a total amount of this kind of radiation that reaches the Earth’s surface is relatively small. The greatest amount of electromagnetic radiation coming from the Sun is for radiation between 100 and 1000nm, so between near ultraviolet to far infrared. Its pick is around the wavelengths of visible light. It is no coincidence that human eyes have evolved to detect the electromagnetic radiation that is emitted the most from the Sun!
https://www.youtube.com/watch?v=J23odHiEC-k

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The Earth’s tilt and the Seasons

Many people think that the seasons are a phenomenon due to the Earth’s orbit around the Sun. The argument is that the Earth’s orbit is elliptic with the Sun located at one of the ellipse’s focal points.  For this reason when the Earth is located at the point nearest to the Sun we have summer and when the Earth is located to the farthest from the Sun we have winter.

Image source: Wikimedia

Wait… if that was the case, wouldn’t the entire Earth have the same season at any given time? This can’t be right… we know for a fact that when the northern hemisphere has winter the southern hemisphere has summer and vice versa. Therefore, the Earth’s orbit cannot be the explanation of why we have seasons. Let’s look elsewhere.

One important factor when it comes to sunlight is the angle at which light reaches the ground. The secret is in the tilt of the Earth’s axis. The more vertical the light rays, the less loss we have and the warmer the light rays.

Image credit: Siyavula Education

The Earth’s axis is not perpendicular to the Earth’s plane of orbit. It has an approximate tilt of 23.44°. That means that at any given time, one part of Earth is closer to the Sun and sunlight hits the ground at a different angle
https://www.youtube.com/watch?v=DD_8Jm5pTLk
http://d3tt741pwxqwm0.cloudfront.net/WGBH/npls13/npls13_int_seasons/index.html#
https://earthobservatory.nasa.gov/Features/Milankovitch/

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The time of the year

So far, we’ve learned that the Sun’s apparent motion on the celestial sphere is due to Earth rotation around its axis and that our latitude effects the inclination of that apparent motion. Let’s see now, how the time of the year affects the Sun’s apparent motion. All students have experienced the days getting longer as we move towards summer and smaller as we move towards winter.
https://www.youtube.com/watch?v=nCp_1iN8eQo

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In order to understand the role of the coordinates on the Sun’s daily path, we first need to make sure that our students are aware of certain concepts like “horizon”, “celestial sphere” or “Polaris”. In this simulation, follow the steps in the left side of the screen starting from “Horizon” up to “Summer Solstice” to get an excellent introduction to these key concepts.
http://edson.tw/earth/sunrise/sunrise.html
https://www.youtube.com/watch?v=bIPBKfqYzJc
To sum up, the latitude, affects the inclination of the Sun’s path on the sky. Longitude on the other hand, has no effect on the Sun’s path. Latitude also affects the points where the Sun rises and sets. As the inclination of the trajectory changes, the arc visible above the horizon also changes and consequently the points of sunrise and sunset. However, latitude in not the only factor that affects the location of sunrise and sunset over a year.

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The Sun’s daily trajectory on the sky

The Sun’s daily trajectory on the sky is related to the Earth’s rotation around its axis. Although this is something we experience every day, talking to our students about the Sun’s trajectory on the sky can be harder than it sounds. Why are days longer during summer? Why does the Sun change position on the sky? Why does the Sun follow a different path depending on the day of the year? Why isn’t sunrise and sunset always at the same time or place? These are only a few questions your students may ask you on the subject.

To help you answer all  these questions we need to go step by step. There are three different components we need to discuss and help students with understanding of basic concepts:

1. Apparent motion

2. Coordinates

3. The time of the year

Try to introduce these components gradually to help your students get a deeper understanding of trajectories on the sky.

Understanding apparent motion

When we look at the Sun and the Earth from far away, it is easy to tell how they move. However, when we are here on Earth, things look very different. It is also quite difficult for our students to understand these motions based just on their everyday experience. One key concept students need to understand before getting into discussions about trajectories with them, is the concept of apparent motion.

In other words, the fact that things are not always the way they look. Apparent motion has to do with the real motion of an object compared to the one seen by an observer. The observation point affects the motion we see. In order to see the real motion of objects we need to be standing at an observation point that is stable and not affected by the motions under investigation. If we place our observer on a moving object against an object that is not moving, the observer may think that it is the other object that is moving. So, an object may appear to be moving in relation to another but in fact it may not be moving. Apparent is the motion an observer sees, but not necessarily the one that is really taking place. This is exactly the case when we see celestial objects move on the celestial sphere.

One way to help the students understand apparent motions is by giving them examples based on experiences. Like for example when they are in a moving car and they see everything around them pass by.

Apparent motions can explain the Sun’s and the Moon’s trajectories on the sky, sunrise, sunset and more. During sunrise and sunset for example, we think that it is the Sun that is moving, whereas in fact the Sun stands still and the Earth is the one moving. Once you talk to your students about apparent motion in general, then you can start discussing the Sun’s trajectory on the sky.

Start by doing a little representation again where one student plays the role of the Sun and another the role of the Earth. In a day’s course, the Earth has moved little around the Sun so it is ok to ignore that motion for this particular representation. Have a look at the following video to see an example of the representation.
https://www.youtube.com/watch?v=sDf5__hPKW8

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https://www.youtube.com/watch?v=OHd8UyarcMw
The Ecliptic and the zodiac

When we look at the sky, we see many celestial objects -  the Sun and the Moon during day time, the Moon and many other celestial bodies like other stars and the planets at night. All these objects are in different distances from the Earth. Stars can be billions of light years away from each other. However, we have the illusion that they are all on the same plane. That is because we only see a projection of these objects on the sky. So, the idea that all celestial objects are on the same plane, is an illusion. Before we talk to our students about motions on the sky, it is good to start by clarifying this point, as it will be very important for anything we do next.

As Earth rotates around its axis and around the Sun, celestial objects change position on the sky.  They seem like they are moving. Although we now know it is the Earth that is moving and not them, we get the impression that the Earth stands still and everything else moves around it. This illusionary motions of celestial objects are in fact apparent motions, for which we will discuss more a little bit later. When we look at the sky, it looks like celestial objects move on specific orbits. We can view the Sun’s and the Moon’s path on the sky and we can also see the position and motion of other stars, constellations and planets.

However, false as it may be, an illusion that all celestial objects are on the same plane, can be very helpful to astronomers. It allows them to make a map of the sky and to study celestial orbits. Imagine that there is a bubble around our planet, on which all celestial objects are projected. We call this imaginary bubble the “Celestial Sphere” and it is used as a guide to the sky. On this celestial sphere, we can locate the Sun’s and the Moon’s orbits as well as that of other stars, constellations and planets.

Image Source: http://cosmic-watch.com/how-it-works/

As the Earth goes around the Sun, what we see from Earth is the Sun changing location compared to the other stars on the sky. Throughout the year, the Sun’s apparent path on the celestial sphere forms a circle which we call “Ecliptic”. As the orbits of the other planets of the Solar System are on the same plane as Earth, the motions of the other planets throughout the year are also near the ecliptic.  According to theories, during the time of planet formation in the Solar System a disc composed of dust surrounded the Sun, and all planets were "born" there, on the same plane.

Be careful, the ecliptic is not to be confused with the Sun’s daily path on the sky! That is a different motion and is related to Earth’s rotation around its axis. We will discuss this in our next section. The ecliptic on the other hand, has to do with the Sun’s position on the sky, compared to the position of other stars due to Earth’s rotation around the Sun. Let’s try to see where exactly the ecliptic is located.

The night sky is full of constellations. Constellations are groups of stars that are completely unrelated to each other and very far away from each other. Yet they appear to move as a group on our celestial sphere. On the celestial sphere, the positions between the stars of a constellation are fixed and these stars seem to move all together. There are 88 identified constellations on the night sky, and astronomers have defined regions on the sky based on the location of these constellations (think of them as landmarks on the night sky). Some constellations are bigger than the other ones, so their regions are not equal on the night sky. Below, you see the night sky for Athens on June 1st, 2017. The white lines mark the boundaries of the constellations.

Out of the 88 constellations that we know of, the Sun crosses only 13 of them as it moves on the ecliptic. These 13 constellations are known as the “Zodiac”. Depending on the time of year, the Sun points to a different constellation on the sky.

Image source: http://cnx.org/contents/48KLj4my@1/-The-Sky-Above

Here is a map of the regions of the constellations and the path of the ecliptic:
https://www.youtube.com/watch?v=R4y0fwJ8t5Y

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Didactic Course

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Heliophysics, space weather and Lunar Science

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Heliophysics, space weather and Lunar Science

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https://science.nasa.gov/heliophysics
One of the most important phenomena that take place on the Sun is the gases streams that the Sun ejects which can pass past the Earth at incredible speed (about 500km/sec). We refer to these streams as solar wind. Solar wind can have an effect on the Earth’s magnetic field. It can affect or damage satellites, threaten the life of astronauts and cause problems to power stations and power lines on Earth.  Space weather studies how solar wind and the phenomena that take place on the Sun affect the Earth and other planets. As technology progresses, space weather starts to play an increasingly important role. Scientists study in great detail phenomena that are related to space weather like CMEs, solar flares and sunspots in order to understand them and protect human infrastructures.  Here is a video on space weather.
https://www.youtube.com/watch?v=CMhOREPx-Rg
Space Weather:
https://sohowww.nascom.nasa.gov/spaceweather/
Selenology

Aside from its own story, the Moon can tell us a lot about the history and formation of our Solar system and our own planet. This is why there are scientists who focus their research entirely on the Moon. “Selenology” or “Lunar Science” is a branch of science that focuses on studying the origin, the nature and the physical features of our Moon. Among others, Selenology includes Selenography which includes the mapping and studying of the Moon’s surface features.  Geologists can obtain an abundance of information about the Moon’s composition, studying rocks brought to Earth by lunar missions and thus understand much about the Moon’s history.

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Apollo missions

Apollo is not one but a series of 17 missions. The Apollo missions’ primary objective (but not only) was to land astronauts to the Moon and get them back to Earth safely. After the disastrous Apollo 1 mission, which claimed the life of three astronauts in 1967 during a preflight test, there was a series of additional unmanned Apollo test missions and Apollo 7 was the first manned mission to be launched. Manned Apollo missions continued, testing several aspects that were critical for landing on the Moon and it wasn’t until Apollo 11 in 1969 when Neil Armstrong said the magic words "That's one small step for a man. One giant leap for mankind." as he was stepping on the lunar surface for the first time. Six more missions followed the Apollo 11 and all of them except for Apollo 13 landed safely on the Moon. After Apollo 17, there were 3 more missions planned but they were cancelled due to budget constraints.
https://www.space.com/12771-nasa-apollo-missions-photo-countdown.html
Documentary:
https://www.youtube.com/watch?v=GNJpoP642wc
Observing the Sun:
SOHO - Solar and Heliospheric Observation
https://sohowww.nascom.nasa.gov/
SOHO:
https://www.youtube.com/watch?v=qZe5D3MSjOI
https://www.youtube.com/watch?v=4gjnn9kSwDk
https://www.youtube.com/watch?v=LJAcrorHx8I

Observing the Moon:
Luna programme and future lunar missions

NASA wasn’t the only one aiming at the Moon. Luna was a robotic spacecraft programme designed by the Soviet Union which launched 15 successful missions (orbiters or landers) between 1959 and 1976. Three of these missions (Luna 16, 20 and 24) managed to collect 300 grams of lunar soil from three different sites and bring it back to Earth.

Since then, missions have been limited to lower orbit missions (less than 500km from the Surface of the Earth).  However, lunar missions did not come to an end in the 1970’s. The Moon is once again among the destinations of future missions.

https://www.youtube.com/watch?v=Xe_nuRMH30c

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https://www.youtube.com/watch?v=mCzchPx3yF8

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https://www.youtube.com/watch?v=GUlcUppw5QQ

https://en.wikipedia.org/wiki/Lunar_mare
https://www.youtube.com/watch?v=kJkVegBsNyE

http://lunarexploration.esa.int/#/explore/science/223?ha=279
Classroom activity suggestion: You can easily explain this to your students, by doing a little representation. Ask a student to play the role of the Earth and another the role of the Moon. You can also ask them to wear masks to make this a more vivid experience. Than ask you student to simulate the motions of the Moon and the Earth. While the Earth-student rotates around his/her axis, ask the Moon-student to perform:

1. One rotation around the Earth-student without spinning around his/her axis;

2. One rotation around the Earth-student while spinning fast;

3. One rotation around the Earth-student while spinning very slow;

4. One rotation so that the rotation around the Earth-student is equal to the time it takes to rotate once around his/her own axis.

During the process ask the rest of the class to observe the faces of the students and see in which case the face of the Moon-student always faces the Earth-student.

When introducing the question to your students ask them to make a guess (hypothesis), but don’t tell them the correct explanation. Then, perform the representation as a small experiment and then ask them to decide what is the correct explanation and compare it to their initial hypothesis.

https://www.youtube.com/watch?v=6jUpX7J7ySo
Here are some more myths and misconceptions about the Moon:

Five myths about the Moon

Mythbusting Moons - Moons

Activities for students:

Lunar craters

Making craters #1

Making Crates #2

The Late Heavy Bombardment Era

Intuitive drawing and personal the geography activities using Moon:

Intuitive drawing (or painting) and personal geography are two widely known art techniques through which the artists try to develop their imagination and reflect on themselves or different situations. These are fun activities you could also do with young students. The Moon is an excellent starting point for both.

If you have young students, you can stir up their imagination by giving them a picture of the full Moon and ask them to draw anything that the areas of the Moon reminds them of. This is a little intuitive drawing activity, similar to the game we often do with cloud shapes when trying to imagine what they could depict.

Alternatively, or in addition to their drawing, students can engage in a little personal geography activity. Ask your students to create their own map of the Moon, by naming the Maria, highlands and craters using names of their choosing. The names the students could think of using could be the names of their family members, their favorite heroes, their favorite football team, words that remind them of happy memories, they can be anything the students come up with!

https://www.youtube.com/watch?v=mCzchPx3yF8

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The Sun is the star that supports life on Earth. Many people think that it is a ‘quite ball of light’ but in reality it is anything but that. Our Sun is a medium sized, ‘middle-aged’ (4.6 million year old) star where massive large-scale phenomena occur on a constant basis. Solar activity varies over the years and it usually follows an 11 years cycle that also has an effect on our planet.
https://www.youtube.com/watch?v=N6Rp1XGWnKc
Every approximately 11 years, the Sun’s poles flip. North becomes South and vice-versa. The Sun behaves more or less the same regardless which pole is North and which South. During this 11 year-cycle solar activity varies. As we go towards the peak of a solar cycle, the Sun is much more active, meaning that there are a lot more sunspots (appearing mostly around the equator) many more flares, Coronal Mass Ejections and other related features.
https://www.nasa.gov/sun
https://www.youtube.com/watch?v=hHC1YwYB3IU - 11 cycle

Fun Fact: When looking at an eclipse, one can tell if the Sun is going through an active or quite period by looking at the solar corona. The material there and the streams they follow can ‘show’ us what the Sun’s magnetic field looks like during that period. When the Sun is in a low activity period, we can "see" indirectly the lines of its magnetic field (Image A). When the Sun is in a high activity period, there are many sunspots and flares, which involve very intense magnetic fields. Thus, the lines of the magnetic field are not so clear (Image B).

Image A: Solar eclipse 2008 (low Solar activity)

Image B: Solar eclipse 1999 (high

Activities for students:

Tracking Sunspot using real data

Solar Rotation

Calculation of the Sun's effective temperature

Safe Sun Viewer

Make your own Sunflower

Observing the sun for yourself

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http://archive.oapd.inaf.it/othersites/stelle/laluna/english/moon2.htm

https://www.livescience.com/20415-folklore-5-sun-myths.html

http://www.unawe.org/resources/education/activity_spacescoop_storytelling/

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http://www.space-awareness.org/en/skills/ - materijali
https://www.youtube.com/watch?v=hqpYc_wSTYw - introduction

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https://www.youtube.com/watch?v=FykVDQY7kk4

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http://www.esa.int/ESA/Our_Missions
https://www.nasa.gov/missions/
https://www.missionjuno.swri.edu/news/

https://www.nasa.gov/mission_pages/juno/main/index.html
https://www.jpl.nasa.gov/images/pdf/Juno_Spacecraft_Paper_Model_FC.pdf

Juno and mythology: You may want to link the Juno mission and the myth behind Juno the ancient roman goddess. Juno’s Greek equivalent is Hera and in Etruscan mythology is Uni. Talk to your students about how different ancient myths still inspire people to this day.

Juno and arts/crafts: The images received from Juno are absolutely stunning (check NASA’s website photo gallery). You can use these images as art inspiration for your students or work with your students to recreated some of them, exploring the colors and the shapes. Additionally, you may want to use the Spacecraft paper model to have some fun with your students while exploring the spacecraft’s instruments and structure.

Juno and science: NASA has designed a many activities for students around the Juno mission. You may want to have a look at the "Explore Jupiter's Family Secrets" guide for students between 8 and 13. For older students, you can explore the Juno's "SpaceMath" resources.

Participate in the mission with JunoCam: NASA invites amateur astronomers to send their pictures and help scientists decided what images to take during the spacecraft’s flybys using the JunoCam.

Cassini

Saturn activities from NASA: NASA has a number of activities designed around Saturn. You can explore some of them here.

Explore Saturn and Titan at School with ESA: ESA offers a cartoon-style teaching aid with fact sheets for teachers to use in the classroom as well as a series of videos that can be used in class.

Cassini-Huygens paper model

SpaceScoop article “Countdown to Cassini’s grand finale”

Exomars: https://www.youtube.com/watch?v=zm1t69fpJj0
Activities for students

Mars activity book

Mars for educators

Return to Mars activities

Role model - Meet Jorge Vago, the 2020 Exomars mission Project Scientist

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https://www.youtube.com/watch?v=0mr61XECXic

https://www.youtube.com/playlist?list=PLL23tiBntRtJVj4D6KKCR8HvUPV4GTuQT

https://www.youtube.com/watch?v=ZPYmND_w_sE
http://www.space-awareness.org/en/activities/1615/know-your-planets/
http://esamultimedia.esa.int/docs/edu/PR01_Our_Solar_System_teacher_guide_and_pupil_activities.pdf

https://eyes.nasa.gov/mobile-apps.html

I use facts about Solar System when talking about ratio in Mathematics, and on a lot of topics in physics (gravity, energy, heat, etc.)

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https://www.youtube.com/watch?v=SmyMQ_SdpNc

https://www.esa.int/esaKIDSen/SEMF8WVLWFE_OurUniverse_0.html
Because it is fascinating :) Students really get better perspective of life on Earth when they learn about other celestial bodies, especially those that can be seen by the naked eye.
https://www.youtube.com/watch?v=0mr61XECXic

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https://docs.google.com/spreadsheets/d/1nyZWL8jc7-S0a41MdeihRRGCTilMBmVBQAiiI6T78m0/edit#gid=1374083031