A nebula is a cloud of gas, dust, and plasma, floating around in the vacuum of space. They're mostly made up of Hydrogen and Helium, as these are the lightest and most abundant elements in the universe. Some come from dying stars, and others are the places where new stars are born. The word nebula literally means "cloud" or "fog" in Latin, and in years past, the term was used to describe any celestial object larger than a star or planet, including galaxies. It's now pretty standard that it's only used to describe the large clouds of interstellar gas we see with our telescopes. These are easily the most beautiful of celestial objects, and they come in many shapes, sizes, and colors.

Clouds above the sky

Astronomers have been observing nebulae for centuries, and it's said that Arabic and Chinese astronomers would have seen the birth of the Crab Nebula almost 1000 years ago. As early as 150 AD, Ptolemy wrote in his first treatise about five stars that "appeared nebulous", as well as a "little cloud" between the constellations Ursa Major and Leo, that didn't result from a star. Turns out, that little cloud is actually the Andromeda Galaxy.

The 17th century saw the discovery and study of the Orion Nebula, and the 18th century saw a boom in the discovery of thousands of nebulae, though it's pertinent to note that this was a time when anything bigger than a star was called a nebula. The telescopic technology just wasn't able to provide the clear images we get today from satellites like Hubble. By the 1920's, it was understood that many of these "nebulae" where actually whole galaxies outside our own.

The zoo of gas clouds

Nebulae are formed in multiple ways, and each has its own unique story. Some exist because of the cooling and condensing of heavier gases into Hydrogen and Helium, while others are formed when a star sheds matter during its cycle of birth and death.

Molecular Clouds

These are some of the youngest, coolest, and most dense of all nebulae, and are the only place you'll find new stars being born. These clouds get their name because the gas contained within is so dense that molecules are able to form within, namely molecular Hydrogen (H2). When this molecular hydrogen starts to clump together due to gravity, the process starts to increase in speed, until enough hydrogen has clumped together to trigger a fusion reaction at its core. The image on the left is of a molecular cloud called the Pillars of Creation, embedded within the Eagle nebula. The image was taken with the Hubble Telescope in visible light. This means if you were able to peer that far into the cosmos, this is exactly what you would see... The pillar on the left is approximately 4 light years tall, and the light shining from within comes from baby protostars that have yet to emerge from their "EGGs".

Diffuse nebulae

A diffuse nebula is one that doesn't seem to have a rigid outline or structure. They are mostly made up of ionized Hydrogen, and most have already gone through the process of creating new stars and having their matter spread thin throughout the Interstellar Medium. Diffuse nebulae come in a few different forms:

Emission Nebulae are highly energized from the ultraviolet light of nearby stars. When the particles fall back to a lower state of energy, they will emit radiation that can be seen as mostly red light.

Reflection Nebulae don't emit their own light, but instead reflect the light coming from a nearby star(s). Whereas an emission nebula will glow red because of the hydrogen within, reflection nebula are typically blue, for the same reason our sky is. Blue light scatters much more efficiently, so it sorta blocks out the other colors.

Dark Nebula
These are almost the same thing as reflection nebulae. The only differences being that the light shining on a dark nebula is either coming from an angle that doesn't reflect light toward us, or it is simply dense enough to obscure light that is shone on it.

Planetary Nebulae

Here's where we start getting into the fun ones. Planetary nebulae actually get their name because when they were first observed by William Herschel in the late 18th century, they appeared to be planets through his telescope.

Planetary nebulae are formed when a red giant star begins to die, expelling its outer layers of matter into space, and leaving behind a white dwarf. This matter is ionized by the remaining white dwarf, making it shine just like an emission nebula. These nebulae only last for around 10,000 years, which is the blink of an eye on the grand scale of the universe.

Supernovae Remnants

When stars much, much larger than our own reach the end of their lives, fusion in their core stops, and they begin to collapse in on themselves. When the outer layers fall inward, they are super-heated, and expand at a rapid pace. This causes the star to explode, and the result is nothing short of breathtaking.

The gif to the right is multiple images taken by the Chandra X-Ray Observatory from 2000 to 2015. It shows a rapidly expanding outer core of gas and dust, and the star that created it all is hidden deep in the core of this beautiful glowing bubble. When the star first exploded back in the 1570's, the Danish astronomer Tycho Brahe wrote in-depth about the event, stating that it was so bright it could be seen during the day! This seems incredible considering we now realize this supernova remnant is some 10,000 light years away, but in fact, many supernovae will produce so much energy that they can briefly outshine the entire galaxy that they reside within.

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Sometimes they're nurseries, and sometimes they're cemeteries, though one thing remains true for them all; Nebulae are an integral part of the life cycle of the universe, and without them, the sky would certainly be much more boring.

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References

• Catalogue of one thousand new nebulae and clusters of stars
• https://www.nasa.gov/subject/6893/nebulae/
• https://arxiv.org/pdf/astro-ph/9902246.pdf
• http://astroa.physics.metu.edu.tr/twn/types.html

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