Stellar Classification

I asked my roommate if there was anything she wanted to know about astronomy and I'd find out and write a blog about it. Her question was "what types of stars are there?"

First of all I'll start by star classification and how this is done. Stars are classified by their spectra and their temperature. There are seven main types of stars O B A F G K M in order from hottest to coolest. M stars are common but dim and O an B stars are uncommon but bright. Our sun is a G2 star. The numbers behind the letter go from zero to nine, zero being the hottest. Brighter stars have a shorter lifetime, about 10 million years. Our sun has a lifetime of 10 billion years.

The Hertzsprung-Russell (HR) Diagram is a graph that plots star color (which is spectral type or surface temperature) versus its luminosity (absolute magnitude). In the diagram three different types of stars can be seen: main sequence stars, supergiants, and white dwarfs.

Main sequence stars: are fueled by nuclear fusion and as they get hotter they become brighter. These stars are in the most stable part of their existence; this stage generally lasts for about 5 billion years. Our sun is considered a main sequence star. About 90% of stars are in the band of main sequence stars.

Giants: are stars that are beginning to die. They become Blue Giants, Red Giants and Red Supergiants as well as many variations such a Brightgiant, Subgiant etc. The core of these stars contract as the outer layer expands creating an explosion.

White Dwarfs: are smaller stars about the size of our sun that eventually become dim white dwarfs. These shrinking stars have given off all of their nuclear fuels and will become cold, dark black dwarfs.

Something that really intrigues me and I find hard to imagine, is the temperature of stars. To imagine something that is 40 000 degrees Kelvin is just hard to believe! Before I learned about stars, I always assumed incorrectly that our Sun was the brightest star. A reason for this probably being it's apparent magnitude and it's close distance to Earth.