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.   :) :3

 

A nebula is a truly wondrous thing to behold. Named after the Latin word for “cloud”, nebulae are not only massive clouds of dust, hydrogen and helium gas, and plasma; they are also often “stellar nurseries” – i.e. the place where stars are born. And for centuries, distant galaxies were often mistaken for these massive clouds.

Alas, such descriptions barely scratch the surface of what nebulae are and what there significance is. Between their formation process, their role in stellar and planetary formation, and their diversity, nebulae have provided humanity with endless intrigue and discovery.

For some time now, scientists and astronomers have been aware that outer space is not really a total vacuum. In fact, it is made up of gas and dust particles known collectively as the Interstellar Medium (ISM). Approximately 99% of the ISM is composed of gas, while about 75% of its mass takes the form of hydrogen and the remaining 25% as helium. 

 

  a celestial object of very small radius (typically 18 miles/30 km) and very high density, composed predominantly of closely packed neutrons. Neutron stars are thought to form by the gravitational collapse of the remnant of a massive star after a supernova explosion, provided that the star is insufficiently massive to produce a black hole. 
 Neutron stars are city-size stellar objects with a mass about 1.4 times that of the sun. Born from the explosive death of another, larger stars, these tiny objects pack quite a punch. Let's take a look at what they are, how they form, and how they vary.

When stars four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often-spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons, yielding the name "neutron star                       Neutron stars pack their mass inside a 20-kilometer (12.4 miles) diameter. They are so dense that a single teaspoon would weigh a billion tons — assuming you somehow managed to snag a sample without being captured by the body's strong gravitational pull.  On average, gravity on a neutron star is 2 billion times stronger than gravity on Earth. In fact, it's strong enough to significantly bend radiation from the star in a process known as gravitational lensing, allowing astronomers to see some of the back side of the star.
The power from the supernova that birthed it gives the star an extremely quick rotation, causing it to spin several times in a second. Neutron stars can spin as fast as 43,000 times per minute, gradually slowing over time. 

 

There is no absolute agreement on what is an average star, but we'll define it like this: An average star, or more properly, an intermediate-mass star, ranges from approximately 0.5 to 8 solar masses during the beginning of its life. An average star proceeds through three major life stages, which include:

 

Stars come in a wide variety of sizes, colors, and temperatures, ranging from tiny brown dwarfs with too little mass to give off visible light, to enormous red supergiants with 30 to 50 times the sun's diameter and 30,000 to 100,000 times the sun's brightness. However, more than 90% of stars fit into neither of these extremes, and instead, form a population of stars, and not the kind you might see on the Hollywood Walk of Fame, called main sequence stars. Main sequence stars range in mass from 0.1 to 200 times the mass of our sun. Intermediate-mass stars form the bulk of the main sequence, ranging from orange stars like Arcturus to blue stars like Spice

A supernova happens where there is a change in the core, or center, of a star. A change can occur in two different ways, with both resulting in a supernova.

The first type of supernova happens in binary star systems. Binary stars are two stars that orbit the same point. One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova.

The second type of supernova occurs at the end of a single star’s lifetime. As the star runs out of nuclear fuel, some of its mass flows into its core. Eventually, the core is so heavy that it cannot withstand its own gravitational force. The core collapses, which results in the giant explosion of a supernova. The sun is a single star, but it does not have enough mass to become a supernova.