Stellar Spectral Classes and the H-R Diagram

Question 1

What are energy levels in atoms?

Answer 1

Certain well-defined levels which electrons exist in.

Question 2

What is line absorption spectra?

Answer 2

When you split the light from a star after it has passed through a cooler gas, you get absorption lines in the continuous spectrum.

Question 3

What happens when electrons absorb certain wavelengths of photons?

Answer 3

They are excited to higher energy levels and then when the de-excite, the same wavelength of light is emitted in all directions, meaning the intensity of this wavelength is reduced hence the dark lines in the continuous spectrum.

Question 4

How can you tell the intensity of an absorption line?

Answer 4

How dark it is - the more intense the absorption line, the more radiation of that wavelength has been absorbed.

Question 5

What are hydrogen balmer absorption lines?

Answer 5

Wavelengths corresponding to the visible part of hydrogen’s line absorption spectrum are caused by the electrons in atomic hydrogen moving between the first excitation levels (n=2) and higher energy levels.

Question 6

Why are hydrogen balmer lines visible for star light?

Answer 6

Because the star light has been absorbed by hydrogen atoms in the stellar atmosphere.

Question 7

What does the intensity of the hydrogen balmer lines depend on?

Answer 7

The temperature of the star.

Question 8

What are the spectral classes in order and what colour and temperature do they correspond to?

Answer 8

O - Blue - 50000 - 25000 K
B - Blue - 25000 - 11000 K
A - Blue-white - 11000 - 7500 K
F - White - 7500 - 6000 K
G - Yellow-white - 6000 - 5000 K
K - Orange - 5000 - 3500 K
M - Red - less than 3500 K

Question 9

Outline the absorption lines for each spectral class.

Answer 9

O - Helium-plus ion and weak hydrogen balmer lines
B - strong helium atom and hydrogen absorptions
A - strong hydrogen balmer lines and some metal ion
F - strong metal ion absorptions
G - both metal ion and metal atom absorptions
K - mostly neutral metal atoms
M - neutral atoms and molecular band absorptions like titanium oxide

Question 10

Outline the H-R diagram.

Answer 10

• X-axis is spectral type (temperature) from high to low
• Y-axis is absolute magnitude from 15 to -10
• White dwarfs in bottom left
• Main sequence in long area through middle
• Red Giants and Supergiants in top right