Spectral Classes

Question 1

How do you get absorption lines?

Answer 1

Stars are approximately black bodies: They emit a continues spectrum of EM radiation.
You get absorption lines in the spectrum when radiation from the star passes though a cooler gas e.g in the stars atmosphere.

Question 2

What are dark lines caused by in spectra

Answer 2

Question 3

What shows the amount of radiation of a particular wavelength being absorbed

Answer 3

Question 4

What is the Balmer Series?

Answer 4

A set of lines that represent the wavelengths corresponding to the visible part of the emission/absorption spectrum of hydrogen

Question 5

For a hydrogen absorption line to occur in the visible part of a star’s spectrum, what excitation level must electrons in the hydrogen atom already be in?

Answer 5

n = 2

Question 6

These lines are seen in Stella spectra. What is this?

Answer 6

Light emitted by the star has been absorbed by hydrogen atoms in the Stella atmosphere as light passes through it.

Question 7

How do hydrogen atoms in the n = 2 excitation level occur?

Answer 7

High temperatures, where collisions between atoms give the electrons extra energy

Question 8

What happens to the excitation level if the temperature is too high?

Answer 8

The majority of the electrons will reach the n = 3 level

So there are won’t be many Balmer transitions

Question 9

What does the intensity of the Balmer lines dependent on?

Answer 9

The temperature of the star.

Question 10

What do astronomers do for the particular intensity of the Balmer lines for which two temperatures are possible?

Answer 10

Look at the absorption lines of other atoms and molecules

Question 11

What spectrum is used for the Balmer series?

Answer 11

Spectrum of Hydrogen

Question 12

What spectral class is the sun?

Answer 12

G

Question 13

What is the order of the spectral classes?

Answer 13

O, B, A, F, G, K, M

Question 14

What class are the strongest hydrogen Balmer lines in?

Answer 14

A

Question 15

What is the diagram of absolute magnitude against temperature called?

Answer 15

Hertzsprung-Russell

Question 16

What are the three categories of stars?

Answer 16

White Dwarfs, Main Sequence, Red Giants/Supergiants

Question 17

What spectral classes do White Dwarfs occupy?

Answer 17

half of O
all of B
a bit of A

Question 18

What spectral classes do Main Sequence occupy?

Answer 18

half of O → all of M

Question 19

What spectral classes do Red Giants/Red Supergiants occupy?

Answer 19

a bit of G → all of M

Question 20

What is the absolute magnitude of White Dwarfs?

Answer 20

5 → 15

Question 21

What is the absolute magnitude of the Main Sequence?

Answer 21

-10 → 15

Question 22

What is the absolute magnitude of the Red Giants/Red Supergiants?

Answer 22

-10 → -4

Question 23

What happens inside a main sequence star?

Answer 23

Hydrogen fuses to helium

Question 24

What are the properties of a red giant star? Which law does this relate to?

Answer 24

High luminosity, low surface temperature. Stefan’s law states it must have a huge surface area.

Question 25

What happens inside a red giant?

Answer 25

Fusion reactions other than Hydrogen to Helium are also happening in them

Question 26

What are the properties of a white dwarf star?

Answer 26

Low luminosity, high temperature, Stefan’s law states they must be very small - about the size of the Earth.

Question 27

What happens inside a White Dwarf?

Answer 27

End of their life

They are slowly cooling

Question 28

What is unusual about the Hertzsprung-Russell Diagram y-axis?

Answer 28

Absolute magnitude goes from positive to negative (15 → -10),
because a lower number is a greater brightness

Question 29

What is unusual about the Hertzsprung-Russell Diagram x-axis?

Answer 29

Temperature is decreasing (50,000k → 2,500k), because it is in order of spectral class

Question 30

What do astronomers do for the particular intensity of the Balmer lines for which two temperatures are possible?What

Answer 30

Look at the absorption lines of other atoms and molecules