17// nebulae

Nebulae are one of the most fascinating objects in the universe, and everyone can agree on their beauty. What exactly are nebulae, though? And are there any differences between some? Are they in any way useful? I will answer these questions as best as I possibly can!

Firstly, what are nebulae?

Nebulae are clouds of dust and gas in space. Some emit light, which we can observe, and others don't - we can sort them into categories based on this.

What are these categories?

We divide nebulae into 3 types, with two of them having the ability to emit light. We call these emission and reflection nebulae. What's the difference between them?

Emission nebulae emit their own light, with the help of nearby stars around them. The ultraviolet light from these nearby stars gives the electrons in the gas atoms of a nebula more energy, therefore making the atoms emit light as their electrons lose that energy.

Picture: NGC 7822, emission nebula

Reflection nebulae, however, do not emit their own light. They only reflect the light from their nearby stars, as you can make out from their name. These nebulae commonly shine in a blue color. 

Picture: Stars in the Pleiades Cluster, reflection nebula

The third type is dark nebulae, made mostly of dust, also known as molecular clouds. These nebulae do not emit light, they rather block it. We notice these thanks to the light around them, making them appear as huge, dark spots in space. Despite this, we can use infrared cameras to observe the stars behind these nebulae.

Picture: Horsehead nebula, dark nebula

How are nebulae created?

A nebula can either be a huge cloud of gas/dust that has accumulated over time, or there could be a more interesting cause for its existence - the death of a star.

When a medium or small-sized star dies, it ejects its outer layers from the core, leaving behind not only a white dwarf but also a planetary nebula. A planetary nebula is mostly made of hydrogen and helium, since it was the outer layers of a star once. The white dwarf ionizes the surrounding nebula, making it an emission nebula.

Picutre: Ring nebula, planetary nebula

But this isn't the only way a star can die. With the death of a bigger, high-mass star, there occurs a supernova - layers of the previous star get scattered around, leaving behind a nebula, surrounding a neutron star.

Picture: Cassiopeia A, supernova remnant

What are nebulae useful for?

Nebulae are star factories. A star is created in what we call a protoplanetary disk - these disks are formed from their nebulae. And, when that star dies, it leaves behind a nebula, which will create yet another star - stars get recycled.

Picture: protoplanetary disks

Nebulae will slowly disappear, though, if they don't create a star or more.

Is there a reason for a nebula's color?

Different elements shine with different colors. For example, hydrogen glows red, along with nitrogen and sulfur, while oxygen glows green or sometimes blue.

That's about all I have to say about nebulae. If you have any corrections or things to add, please comment!

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