Why are there so many nebulas out there?
A nebula is defined as a low-density interstellar cloud of dust and gas, mostly hydrogen and helium that often extends out for several light years. They're all over our galaxy, the Milky Way, but what are they doing out there? That's a question that baffled early astronomers. They had no idea what they were looking at and lumped them in with a potpourri of astronomical objects that included galaxies, nebula and star clusters. It wasn't until Edwin Hubble resolved this dilemma by discovering that many of these so-called nebulae were in fact galaxies.
However, this discussion is about the giant gas clouds in space.
Where the heck did these nebulae come from? The answer is that they are the debris left over from a collapsing or exploding star. Some stars, like our sin, will die by collapsing into a white dwarf and blasting the rest of its stuff out as a planetary nebula. The reason why planetary nebulae are visible is because the white dwarf that remains excites the gases by emitting radiation. It's much like how a neon tube works. Energetic electrons excite the neon gas to glow.
Many nebulae are the result of a large star exploding in a supernova, a violent event that blasts massive amounts of dust and gas out for millions of kilometers. If the supernova results in a neutron star, or its variations, such as pulsar or magnestar, the gases in the remnant nebula glow because they are bombarded with ultraviolet light or gamma rays from the neutron star.
A dark nebula is what its name suggests; it's a cloud of gas that is not illuminated by a nearby star or excited to glow by a source of ultraviolet light or gamma rays. Dark nebulae obscure things like stars and appear as a dark spot in the night sky.
Nebulae take on many interesting forms that suggest familiar objects. The Crab Nebula looks like a bunch of filaments that resemble crab legs. The Flame Nebula looks like a burning flame. The Horsehead Nebula looks like a . . . you get my drift here. Planetary nebulae often look like rings of illuminated gases.
Nebulae, especially those that result from supernova explosions are often the sites for the formation of new stars. One good example of this is the 'Pillars of Creation' in the Eagle Nebula, a famous photograph taken with the Hubble telescope. Here, in the ends of the clouds, new stars are being made. This is how our solar system was created. A supernova nebula condensed into our sun and the debris left over formed the planets, including the Earth.
Some nebulae are diffuse. This means that they are too spread out and have no defined boundaries. In most cases, these nebulae are visible by reflected light from nearby stars. A good example of this is the Carina Nebula near the star Eta Carinae, a hypergiant star over a hundred times the mass of our sun.
To answer the question, there are hundreds of visible nebulae in our Milky Way galaxy, and probably thousands that we can't see. They are the remnants of stars that have died over the age of the galaxy. Many stars have cone and gone over the thirteen billion years that our galaxy has been around and they have spawned plenty of nebulae for us to marvel at.
Thanks for reading.