Spectral Classes Flashcards
How do you get absorption lines?
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.
What are dark lines caused by in spectra
What shows the amount of radiation of a particular wavelength being absorbed
What is the Balmer Series?
A set of lines that represent the wavelengths corresponding to the visible part of the emission/absorption spectrum of hydrogen
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?
n = 2
These lines are seen in Stella spectra. What is this?
Light emitted by the star has been absorbed by hydrogen atoms in the Stella atmosphere as light passes through it.
How do hydrogen atoms in the n = 2 excitation level occur?
High temperatures, where collisions between atoms give the electrons extra energy
What happens to the excitation level if the temperature is too high?
The majority of the electrons will reach the n = 3 level
So there are won’t be many Balmer transitions
What does the intensity of the Balmer lines dependent on?
The temperature of the star.
What do astronomers do for the particular intensity of the Balmer lines for which two temperatures are possible?
Look at the absorption lines of other atoms and molecules
What spectrum is used for the Balmer series?
Spectrum of Hydrogen
What spectral class is the sun?
G
What is the order of the spectral classes?
O, B, A, F, G, K, M
What class are the strongest hydrogen Balmer lines in?
A
What is the diagram of absolute magnitude against temperature called?
Hertzsprung-Russell
What are the three categories of stars?
White Dwarfs, Main Sequence, Red Giants/Supergiants
What spectral classes do White Dwarfs occupy?
half of O
all of B
a bit of A
What spectral classes do Main Sequence occupy?
half of O → all of M
What spectral classes do Red Giants/Red Supergiants occupy?
a bit of G → all of M
What is the absolute magnitude of White Dwarfs?
5 → 15
What is the absolute magnitude of the Main Sequence?
-10 → 15
What is the absolute magnitude of the Red Giants/Red Supergiants?
-10 → -4
What happens inside a main sequence star?
Hydrogen fuses to helium
What are the properties of a red giant star? Which law does this relate to?
High luminosity, low surface temperature. Stefan’s law states it must have a huge surface area.
What happens inside a red giant?
Fusion reactions other than Hydrogen to Helium are also happening in them
What are the properties of a white dwarf star?
Low luminosity, high temperature, Stefan’s law states they must be very small - about the size of the Earth.
What happens inside a White Dwarf?
End of their life
They are slowly cooling
What is unusual about the Hertzsprung-Russell Diagram y-axis?
Absolute magnitude goes from positive to negative (15 → -10),
because a lower number is a greater brightness
What is unusual about the Hertzsprung-Russell Diagram x-axis?
Temperature is decreasing (50,000k → 2,500k), because it is in order of spectral class
What do astronomers do for the particular intensity of the Balmer lines for which two temperatures are possible?What
Look at the absorption lines of other atoms and molecules
