The space between stellar systems in a galaxy is far from empty. Let's take a look at what the Interstellar Medium (ISM) contains.

A fascinating type of star is the Wolf-Rayet star: supermassive, super-energetic, and surrounded by a super-aesthetic Wolf-Rayet Nebula.

We’ve looked at individual galaxies’ properties, here we consider their ordering at the cosmic scale.

Although all stars are giant gas balls conducting nuclear fusion in their core, there are different types of them, for which we observe a “signature” spectrum.

To study spectra, it is useful to keep the three Kirchhoff laws in mind:

1. A hot gas at high pressure has a continuous spectrum
2. A gas at low pressure and high temperature will produce emission lines
3. A gas at low pressure in front of a hot continuum causes absorption lines.

In What is a star cluster, you mostly learned about how to observe a star cluster. Here I will tell you more about their formation and evolution.

We’ve touched upon the idea of nebulae as “recycling stations” for stars in What is a Nebula and talked about that a bit more in What is a Dark Nebula, but let’s look more precisely at how stars form.

Theories about star formation have to include, evidently, the different stages in the birth of the star: from a molecular cloud to a young stellar object, but they also need to consider the formation of binary star systems, star clusters, planets…

We’ve seen that a galaxy is a structured collection of star systems, dust and gas in space, bound together by gravity. Some galaxies have extremely luminous cores – Active Galactic Nuclei (AGN).

You might be familiar with the different types of nebulae (Nebulae, Dark nebulae, Planetary nebulae), but let’s learn more about what happens in emission nebulae dominated by fully ionized hydrogen.

“What is a Supernova” and “What is a Supernova Remnant” described the explosive death of massive stars and what they leave over. The less chaotic end-of-life of stars with lower masses produces equally aesthetic residues: Planetary Nebulae.

In “What is a Nebula”, we considered emission nebulae, now we are going to look at nebulae where the opposite process absorption happens.