Universe

 The life cycle of stars starts from a bunch of gas and dust that form a protostar. Then nuclear fusion happens, turning them into main sequence stars, this stage lasts the longest. Then they start to burn out and become red giants, with bigger stars being super red giants. This stage makes the star flare up more. Then they turn into planetary nebulas and shed their skin, becoming white dwarfs. They then turn into brown dwarfs. Big stars explode into a supernova and either turn into a neuron star or black hole. They then turn into nebulae to recycle and start the process again.

The size of a star determines their life. Bigger stars use up more energy, so they divert from the regular cycle. Smaller stars are the average ones, they will still go through the normal cycle.

Stars form from nebulae. Gravity pulls gas and dust together, and nuclear fusion happens. When helium is formed, it releases energy, which creates a star.

The purpose of the HR Diagram is to show the relationship between the temperature and brightness of stars.  The x-axis is the temperature. The y-axis is where the brightness/luminosity is.

The 4 groups seen on the diagram are white dwarfs, main sequence, supergiants, and giants. Each group have specific spots on the diagram. Our Sun belongs to the main sequence group.

The hotter the star is, the brighter it is. This relationship only applies to the main sequence stars.  The unique part about the x-axis is that it uses spectral classes instead of the temperature, but it's still the same thing.

The top left of the diagram is hot and bright. The top right of the diagram is cooler, but still as bright as the top left corner. The bottom left corner is hot and dim. The bottom right is cool and dim.

 A galaxy is a system of stars held together by gravity. There are 3 different types of galaxies. These types are based on what the galaxy looks like and the kind of celestial bodies that are in it.

We classify galaxies by their appearance and what kind of bodies in it. The types are spiral galaxies, that look like a spiral. There's the elliptical galaxy, that look like a circle. Lastly, there's the irregular galaxy, it has no defined shape.

 Spiral galaxies look like spirals, they often have a black hole in the center of their bodies. Elliptical galaxies have a circular shape, often being built of old stars. Irregular galaxies have no defined shape, often being full of young and new stars.