You probably know that there are different kinds of stars up there. Even if you didn’t, you’ve probably looked up at the clear night sky and noticed that not all of them look alike.
The classification of stars is based on multiple parameters and you’ll see what some of the important ones tell us about these dangling celestial objects.
Ready? Let’s get started…
Stellar Classification: Short Summary
If you look at temperature, there are seven categories of stars that are designated with the letters OBAFGKM. There are also O and B categories, but they are quite uncommon along with being incredibly bright and hot.
If you consider their luminosity, there are six main types of stars designated with Roman numerals Ia, Ib, II, III, IV and V.
And you don’t need to be an astronomy buff to know these things because look up at the sky on a clear night and you will see that some stars have a warm color temperature and some are not as bright.
Now, we can also classify stars based on mass, the elements they tend to absorb called their spectra and their apparent magnitude. Let’s take a look at all that.
Stellar Classification Based on Spectral Type
There is a simple way to remember the classification of stars based on their spectral type. Work on this started only at the beginning of the 20th century at the Harvard Observatory. There are seven letters and the mnemonic Only Boring Astronomers Fight Green Killer Martians is a popular choice for OBAFGKM. Here’s how the classification goes.
- O: Stars with a temperature above 30,000 Kelvin and are blue in color
- B: Stars with temperatures in the 10k-30k Kelvin range and are blue-white
- A: Stars with temperatures in the 7.5k-10k Kelvin range and are white
- F: Stars with temperatures in the 6k-7.5k Kelvin range and are yellow-white
- G: Stars with temperatures in the 5.2k-6k Kelvin range and are yellow
- K: Stars with temperatures in the 3.7k-5.2k Kelvin range and are orange
- M: Stars with temperatures in the 2.4k-3.7k Kelvin range and are red in color
So, M is the coolest category of stars with a temperature range starting at 2,400 Kelvin or 3,860.33 degrees Fahrenheit. The category O, on the other hand, is on the other side of the spectrum with temperatures that could be as high as 30,000 Kelvin or 53,540.33 degrees Fahrenheit.
Interestingly, the Sun, which is yellow in color, is classified as G-type.
Now, you shouldn’t be surprised to learn that there are sub-categories in each of these types that are also based on temperature and they are categorized from 0 to 9 in increasing order of heat.
In case you were wondering, the Sun is classified as G2.
Stellar Classification Based on Luminosity
Luminosity is the total amount of brightness of the star, which is calculated based on the energy it radiates per second. Each of them, from bright supergiants to dwarfs, is divided into the six categories mentioned below.
They are all assigned Roman numerals as follows.
- Ia: Most Luminous Supergiants or Bright Supergiants
- Ib: Less Luminous Supergiants or Supergiants
- II: Luminous Giants or Bright Giants
- III: Normal Giants or Giants
- IV: Subgiants
- V: Main Sequence Stars or Dwarfs
The Sun, which has a G2 classification as we understand so far, is further classified as G2V. that’s because it is yellow, falls in a certain temperature range and is a main sequence star or a dwarf.
You could keep going down the rabbit hole of the many other parameters to classify stars but these are the main categories.
Frequently Asked Questions
Q: Do All Known Stars Fall into These Categories?
A: Actually, no. Some stars don’t really fall into any of these categories as they approach the end of their lifespan. They are classified as asymptotic giants with S, N and R types.
Q: How Does Stellar Classification Work?
A: This is a classification created based on the spectral characteristics of every star. The electromagnetic radiation coming from the star is analyzed for information on the star’s spectral lines.
Q: Which Classification of Star Has the Most Energy?
A: That would be supergiants which are very rare.
Conclusion & Summary
When divided based on spectral type (temperature and hence the color), there are seven types of stars and when it’s luminosity, there are six. Each parameter adds to our understanding of the millions of stars that exist in our solar system alone. And now you know where to begin that journey.