The life cycle of stars starts as stars being born as clouds of dust and gas, it eventually becomes a main sequence star. After millions and billions of years, it becomes a red giant or supergiant before ending as a white dwarf or others. Its initial mass determines what route it takes in its life cycle. Stars are born in nebulae. In 5 billion years, the sun will expand into a red giant, and finally cool down into a white dwarf.

The purpose of an HR diagram is to organize stars. The variable found on the X axis is the stars temperature, and the variable on the Y axis is the stars luminosity. The four main groups of stars on a HR diagram are main sequence, giants, supergiants, and white dwarfs. The sun is a main sequence star. Hotter stars on the HR diagram are significantly brighter, and cooler stars are dimmer. What's unique about the temperature on the X axis is that its reversed. On the HR diagram, its hotter on the left and cooler on the right, and its brighter on the top and dimmer on the bottom.

A galaxy is a massive system containing stars and other matter. We classify galaxies by their shape, structure, and content. The three main types of galaxies are spiral, elliptical, and irregular. Spiral galaxies have spiral arm extending from their centers and are made of mostly new stars. Elliptical galaxies are made of old stars. An irregular galaxy doesn't have a distinct shape, lacking a center.