Astrophysics 08 - Stellar Spectral Classes

Explain how you can use the light output of a star to get information it.

The light has passed through cooler gas in the form of the star’s atmosphere. By using an absorbtion spectrum, and comparing the absorbtion lines against those for different elements, you can see what the star is fusing.

Fusing of different elements happens at different temperatures, so you can determine its temperature as a result.

Explain the Hydrogen Balmer absorbtion lines.

The wavelengths corresponding to the visble part of hydrogen’s line-absorbtion spectrum, caused by electrons moving between the first exitation level (n = 2) and higher energy levels.

How does the intensity of the absorbtion lines depend on temperature.

For a given absorbtion spectrum, if the temperature is too low, then not many photons have enough energy to be absorbed. If the temperature is too high, then most electrons are in too high an energy level to absorb the photons corresponding to the absorbtion spectrum.

What affects the spectral class of a star?

Only its absorbtion spectrum, and by extension temperature.

Give the seven main spectral classes, along with a mnemonic, range of surface temperatures and the colour a star appears.

Spectral Class	Mnemonic	Surface Temperature (103 K)	Colour
O	Obese	25 - 30	Blue
B	Boys	11 - 25	Blue
A	And	7.5 - 11	Blue / White
F	Fat	6-7.5	White
G	Girls	5 - 6	Yellow / White
K	Kill	3.5 - 5	Orange
M	MacDonalds	< 3.5	Red

Give the colour, temperature and prominent absorption lines for an O-class star.

• Colour: Blue

• Temperature: > 25000 K

• Absorption Lines: He and He+ ions, with weak Hydrogen Balmer lines

Give the colour, temperature and prominent absorption lines for a B-class star.

• Colour: Blue

• Temperature: 11000 - 25000 K

• Absorption Lines: He and Hydrogen Balmer lines

Give the colour, temperature and prominent absorption lines for an A-class star.

• Colour: Blue-white

• Temperature: 7500 - 11000 K

• Absorption Lines: Hydrogen Balmer lines

Give the colour, temperature and prominent absorption lines for an F-class star.

• Colour: White

• Temperature: 6000 - 7500 K

• Absorption Lines: Hydrogen Balmer lines, Ca+ and Fe+ lines

Give the colour, temperature and prominent absorption lines for a G-class star.

• Colour: Yellow-white

• Temperature: 5000 - 6000 K

• Absorption Lines: Strong Ca+ and Fe+ lines, weaker Hydrogen Balmer lines and Fe atom lines

Give the colour, temperature and prominent absorption lines for a K-class star.

• Colour: Orange

• Temperature: 3500 - 5000 K

• Absorption Lines: Strong Ca+ lines, weaker Fe and Ca lines

Give the colour, temperature and prominent absorption lines for an M-class star.

• Colour: Red

• Temperature: < 3500 K

• Absorption Lines: Ca+, Ca and Fe lines, and TiO lines