A nebula is mostly a cloud of gas and dust. If you have more than one, they are called nebulae. Less than 100 years ago people believed our galaxy was the whole universe, and before the 1920s people used to believe that galaxies (other than our own) were actually local nebulae within the galaxy. They called them “spiral nebulae”.
It wasn’t until astronomer Edwin Hubble decided to tackle that particular subject that he managed to resolve individual stars in a spiral nebulae. The Andromeda Nebulae turned out to be the Andromeda Galaxy! These weren’t mere gas clouds, but actual galaxies just like our own.
Our understanding of the size of the Universe jumped many orders of magnitude that day! We now know there are at least 100 billion other galaxies, each with hundreds of billions of stars; we know that most stars have planets, if we understand stellar formation correctly. That’s a lot of planets where life might exist. And all that came from studying nebulae.
Space Is Not A Vacuum
Space may only have an atom or two in a cubic metre, or just a few per cubic kilometre, but it’s not an absolute vacuum. A nebula begins to form when a few atoms get close enough together to start to clump up. The more that come together, the stronger their gravitation influence; they start to draw more particles towards them. After eons of time you get a large gaseous cloud. Maybe some places in the cloud are even denser. More and more matter accumulates until the material (mostly hydrogen – 99 % of the universe is hydrogen) starts to collapse upon itself, crushing itself together. At some critical point the sheer mass and high gravity start a nuclear reaction and hydrogen starts fusing to make helium. A sun is born.
Stars Are Born
The light from that new star starts pushing extra gas away with its solar wind. More concentrations develop and planets form. Most of the rest of the gas travels off and starts new cascades of accumulation and mores stars form, and more planets, and then it repeats over and over again. Nebulae are actually stellar nurseries; they’re the places where stars are born. Sometimes it’s even the place where stars die.
Every once in a while a star blows up. It runs out of hydrogen or helium fuel, it gets too big, collapses and then goes boom – don’t worry our Sun is stable for about another 5 billion years. The resulting explosion sends out material and shock waves that can disturb the nebulae causing it to form more stars. It’s an ongoing process.
Four Main Types Of Nebulae
Planetary Nebulae: These are called such because they look like a planet when viewed with a small telescope. One of the best examples of a planetary nebula is the Ring Nebula (M57) in Lyra.
Reflection Nebulae: A reflection nebula does not emit any visible light of its own, and shines only because the light from an embedded source illuminates its dust. The Trifid Nebula (M20) in Sagittarius is a good example of a reflection nebula illuminated by a group of stars.
Emission Nebulae: In the Crab Nebula, the Orion Nebula and other similar emission nebulae, ultraviolet light from young, hot stars strip electrons from the surrounding hydrogen atoms. As they recombine they emit longer wavelengths in the red part of the spectrum.
Absorption Nebulae: This type nebula absorbs or obscures the light coming from sources behind them, including stars and bright nebulae, a famous example of which is the Horsehead Nebula. The darkness of the Horsehead is caused mostly by thick dust, although the lower area of the Horsehead’s neck casts a shadow to the left.
Stellar Nurseries and Stellar Graveyards – nebulae have it all. They mark the beginning and end of stars in the universe. They can be hundreds of light-years from side to side, but are still so tenuous that if you scooped out a chunk the size of our home planet, it would only mass a very few kilograms. Whatever kind they are, they’re fascinating to look at. Go grab a telescope and take a peek! You might like to check out the Orion Nebula (M42), for instance, which is situated 1,344 light years distant in Orion’s Belt and is visible to the naked eye.