There’s something about the stars in the sky that makes us feel magical. From a young age, we are told that we can wish upon stars to make our dreams come true. And then as we get older, we learn that we can rely on some stars for navigation, just like early explorers and travelers once did. But how much do you know about the stars themselves? Whether we’re talking about our sun or Betelgeuse, it turns out that we can categorize the stars into types.

What is a star?

Photo of stars in deep space
Credit: ESA/Hubble & NASA; the LEGUS team, B. Tully, D. Calzetti; Acknowledgment: Judy Schmidt (Geckzilla)

Contrary to how we often depict stars like the sun or Polaris (also known as the North Star) in children’s books, stars are gaseous-based bodies — not circles with triangle rays or five-sided structures with smiley faces. Specifically, they emit light, are comprised of primarily helium and hydrogen, and usually experience regular nuclear fusion reactions. Common byproducts of those reactions include heat and heavy metals. We classify stars by the amount of heat they produce, which also impacts the hue of the light they emit.

How do we classify stars?

Photo of bright stars in deep purple space
Credit: NASA

There are multiple classification systems in use today, but one of the easiest ones for amateur stargazers to understand is the Morgan-Keenan system.

With the Morgan-Keenan system, stars are assigned one of the following letters: O, B, A, F, G, K, and M. O stars are the brightest and hottest in the universe while an M star is the coolest and dullest. But within each letter, the star is then assigned a number between zero to nine, ranking how hot or cool it is within its star type. Zero is the hottest and nine is the coolest.

However, the Morgan-Keenan system focuses solely on brightness. There are descriptive names given to stars depending on where they are within their life cycle. The Morgan-Keenan system compliments these official types.

What are the types of stars?

Photo of a star shining vibrantly
Credit: ESA/NASA

Star types and classifications are rather complex and technical, so we will walk you through the basics of star classifications:

  • Protostars: As the name implies, these are “pre” stars that haven’t fully formed into a star and have no nuclear fusion reactions. These are gaseous clouds that still need more time for gravity to turn into a gaseous ball. Stars will spend roughly 100,000 years in this stage.
  • T Tauri stars: When a protostar is almost at the point of becoming a fully-fledged star, it’s called a T Tauri star. These “almost” stars look more in-line with the physical characteristics of a star, meaning they are very bright, but still haven’t begun to experience nuclear fusion reactions. This stage can take roughly 100 million years to complete.
  • Main sequence stars: These are the most common stars in the universe as the average layperson would know them. Main sequence stars include our Sun as well as other well-known stars like Alpha Centauri and Sirius. These stars have the traditional spherical shape, experience nuclear fusion reactions where hydrogen is converted to helium, and cover a wide range of temperature and brightness levels including blue stars and yellow, orange, and red dwarfs.
  • Red giant stars: When hydrogen has been almost completely converted to helium, the nuclear fusion reactions begin converting that helium. That final hydrogen ignition causes red giant stars to expand in size from when they were main sequence stars, and they can spend, on average, several hundred million years in this stage.
  • White dwarf stars: At this stage, there’s no hydrogen left inside the star, causing nuclear fusion reactions to come to a halt. While a white dwarf star looks bright, the star is no longer considered active. Now the star will slowly cool off, which can take billions of years to complete.
  • Neutron stars: Some stars are very special because they’re composed entirely of neutrons. These stars are formed as a result of a larger white dwarf star that doesn’t slowly cool down but instead is destroyed in a supernova explosion. And some massive white dwarf stars can become black holes instead of a slowly cooling temperature change.
  • Supergiant stars: As the name implies, these stars are massive. And because of that size, they tend to move through their life cycle quickly — only a few million years — burning through hydrogen at a rapid rate. Since these stars are living the fast life, they tend to expire in supernovas rather than enter the white dwarf stage.

The unique binary star

Photo of a binary star system in space
Credit: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S. Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA

For the most part, stars are solitary celestial beings. But there is an exception, the binary star. Although self-explanatory, a binary star is an arrangement of two stars that share a center of mass. Within this category, there are two types: wide binaries where the stars are orbiting each other from a great distance and close binaries where sometimes the stars are so close that the gravitational pull of the bigger star pulls the smaller one into it.