Classification

13.5 - 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
Type Colour Temperature Spectral lines
O Violet -white 30,000° K+
Helium with Hydrogen
B Blue-white 12,000 -
30,000° K
Hydrogen with Helium
A White 8,000 -
12,000° K
Hydrogen Rich
F Yellow-White 6,000 -
8,000° K
Calcium
G Yellow 5,000 -
6,000° K
Some Iron lines
K Orange 3,000 -
5,000 ° K
Many Metallic lines
M Red <3,000 ° K

Metallic and Carbon lines

 

Main Sequence Stars
Questions

Give an example of how a star's colour is related to its temperature

Mix & Match
Top Tip

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.

Links
20 BRIGHTEST STARS
Proper name Spectral class Apparent magnitude Distance (ly)
Sun G2 V −26.74 -
Sirius A0mA1 Va −1.46 8.6
Canopus A9 II −0.74 310
Rigil Kentaurus G2 V −0.27* 4.4
Arcturus K0 III −0.05 37
Vega A0 Va 0.03* 25
Capella G1 III 0.08* 42
Rigel B8 Ia 0.13* 860
Procyon F5 IV-V 0.34 11
Achernar B6 Vep 0.46* 140
Betelgeuse M1-M2 Ia-ab 0.50* 640
Hadar B1 III 0.61 350
Altair A7 V 0.76 17
Acrux B0.5 IV 0.76* 320
Aldebaran K5 III 0.86* 65
Antares M1.5 0.96* 600
Spica B1V 0.97* 260
Pollux K0 III 1.14 34
Fomalhaut A3 V 1.16* 25
Deneb A2 Ia 1.25 2,600
https://en.wikipedia.org/wiki/List_of_brightest_stars
* 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.