The awe and wonder of a starry sky is something we have all experienced at least once in our lives. We’ve all stood watch while constellations danced across the night sky. Some stars are bright and vibrant, while others appear just barely visible. It can be a cold reminder of how small we are and how big the universe really is. From small red dwarfs to massive hypergiants, these celestial beings come in a vast array of different sizes. So, how many types of stars are there and what do we really know about them?
Before we get into star types, let’s talk about Spectral Class. A star is categorized into one of seven Spectral Classes (O, B, A, F, G, K, M) based on its surface temperature (K) and the elements that are absorbed. O and B are the hottest, brightest, and rarest of the Spectral Classes. While on the opposite end of the spectrum, M has the coolest surface temperatures (K) and is the most common. However, this class is also the most dim.
Spectral Class is just one of the variables that astronomers use to plot stars on the Hertzsprung-Russell Diagram (H-R Diagram). Using this diagram, astronomers are able to see a stars’ surface temperature(K), luminosity, size, color, and even the current stage of its evolution. Within this plot are stars that are classified as “Main Sequence’’ stars. They are the most common and are in the most stable part of their lifespan. Stars that are fed by nuclear fusion are classified as such. Our own Sun is a Main Sequence star. The hotter they burn, the brighter they appear.
Measure of Brightness
Absolute magnitude(Mv) is the absolute visual magnitude of a star. It measures a star’s brightness. This graph works backwards, in that the smaller the absolute magnitude(Mv) the brighter the star. In fact, the ones that are the most luminous have a negative rating. We can see that blue stars at -5 absolute magnitude(Mv) have recorded surface temperatures of up to 40,000(K). Stars at this temperature are some of the hottest and most unstable star types. Even though blue stars have a massive temperature range, most don’t seem to burn hotter than 25,000(K). There is a small cluster of blue stars that are between the absolute magnitude(Mv) of 10 and 15. However, most are within the -10 and 0 absolute magnitude(Mv) range. The majority of the red stars have surface temperatures within the cooler 5,000(K) scale. It appears that most of these stars are really dim, but there is a smaller group that is really bright with an absolute magnitude(Mv) of -10.
That brings us to star types. As we can see, dwarf stars have some of the coolest surface temperatures. Brown Dwarf “Stars” have the coolest out of all of the star types. In fact, their surface temperatures are so low that they are not technically classified as stars. They do not burn hot enough to perform nuclear fusion. Red Dwarf Stars, however, burn at a minimum surface temperature that is just hot enough for it to perform nuclear fusion, thus making it the smallest star type. White Dwarfs burn hot, but dim and are considered below the Main Sequence. These stars are not classified as Main Sequence because they do not perform nuclear fusion. This is due to being so near to the end of their lifespan. Supergiants and hypergiants are the largest of the stars. These massive stars do not typically burn as hot as the others. The hottest giants are the Blue Giants. They are extremely unstable. All giants are classified as being above the Main Sequence. Due to their massive size, Supergiants and Hypergiants do not live very long. They will eventually turn into White Dwarfs and then burn out into cold Black Dwarfs, marking the end of their lifecycle.
Stars are categorized into different classes based on their many characteristics. This information is just the beginning of what astronomers already know and have yet to discover. There are even more categories and classes that take a closer look into the makeup of different star types. Scientists have been studying stars for a long time and there is still so much to learn about them. It will be interesting to see what kind of findings the future holds for astronomy.
For the curious: Click HERE to check out the exploratory data analysis conducted for this blog!