Classification13.5 - Understand how stars can be classified according to spectral type
13.6 - Understand how a star’s colour and spectral type are related to its surface temperature
Each star is different and there are many different types of stars. Astronomers categorise these by temperature and their chemical composition that they obtain from a star’s stellar spectrum.
By analysing their spectral lines, we can understand what they are made of and their temperature. This determines their colour and we can plot them on the main sequence.
Stars are divided into 7 main categories and then given a number between 0 to 9 within each to denote temperature within the category. Distinguishing letters after that you might see such as ‘III’, ‘V’, ‘VII’ etc tell us if the star is a regular giant, main sequence or white dwarf etc. So you can have an ‘M’ class star of under 3,000° K and depending on the information after it, this could then be a red dwarf or super red giant.
There are further categories that include rarer stars but these are not used often. There are several different classification systems. The most widely used is the 'Morgan–Keenan'(MK) classification .
The Sun is classed as a G2 star.
Features of different types of stars
|O||Violet -white||30,000° K+
||Helium with Hydrogen|
|Hydrogen with Helium|
|Some Iron lines|
5,000 ° K
|Many Metallic lines|
|M||Red||<3,000 ° K
||Metallic and Carbon lines|
Give an example of how a star's colour is related to its temperature
Mix & Match
Remember the types of stars from hottest to cooles using a helpful mnemonic:
e.g. Oh Be A Fine Girl Kiss Me
Er... some are better than others. Check out the link to other mnemonics below.
20 BRIGHTEST STARS
|Proper name||Spectral class||Apparent magnitude||Distance (ly)|
|Rigil Kentaurus||G2 V||−0.27*||4.4|
* Variable Stars
Did you notice?
Polaris, the north star, incorrectly thought by some to be the brightest star, isn't in the list above.
At apparent magnitude 1.98, it is the 49th brightest star from Earth.