3.9.2 Classification of stars; the better version Flashcards
Define black body radiation
A body that absorbs all wavelengths of electromagnetic radiation and can emit all wavelengths of electromagnetic radiation.
Stars are assumed to behave as black bodies and their black body radiation produces their continuous spectrum.
Describe Wien’s displacement law
For each temperature, there is a peak in the black body curve at a wavelength called the peak wavelength.
Peak wavelength is inversely proportional to the temperature of the body.
λ_max*T = constant
State Wien’s constant
2.9*10^-3 mK (‘metre-Kelvin’)
State Stefan’s law
The power output/luminosity of a star is directly proportional to its surface area and its temperature to the power of four.
P=σAT^4
State Stefan’s constant
5.67*10^-8
What is the equation for intensity?
I = P/4πd^2
I = L/4πd^2
where d is the distance to the star, NOT the radius
Why is it difficult to get accurate measurements for certain wavelengths?
- The atmosphere only lets certain wavelengths through (like, visible light, most radio waves, near infrared, bit UV), and opaque to the rest. Pollution and dust also interfere with results.
- Measuring devices since their sensitivity depends on wavelength. E.g. glass absorbs UV but is transparent to visible light.
How can we reduce the effect of the atmosphere on measurements?
- Locate observatories at high altitudes, away from cities with light pollution and in low humidity. (Best solution, however, is to send satellites).
- Calibrate instruments to the wavelength you want to measure.
What are Balmer lines?
Lines in emission and absorption spectra occur because electrons in an atom can only exist at well-defined energy levels.
In atomic hydrogen, the electron is usually in its ground state (n=1), but there are lots of energy levels (excitation levels) that the electron could exist in.
The wavelengths corresponding to the visible bit of hydrogen’s spectrum are caused by electrons moving from higher energy levels to the first excitation level (n=2).
This leads to a series of lines called the Balmer series.
What do the strengths of the spectral lines show?
The temperature of a star.
For a hydrogen absorption line to occur in the visible part of the star’s spectrum, electrons in the hydrogen atoms already need to be in the n=2 state.
This happens at high temperatures, where collisions between the atoms give electrons extra energy.
If the temperature is too high, the majority of electrons will be in the n=3 level or above instead, which means there won’t be so many Balmer transitions.
Hence the intensity of the Balmer lines depends on the temperature of the star.
For a particular intensity of Balmer lines, two temperatures are possible.
How can we get around getting 2 corresponding temperatures from hydrogen Balmer lines?
We can use the absorption lines of other atoms and molecules.
What are the visible spectrum characteristics of the different spectral classes?
O - He+ and H, weak hydrogen Balmer lines
B - He and H
A - H, strongest hydrogen Balmer lines
F - Metal ion absorptions e.g. Fe+
G - Metal ion and metal atom absorptions e.g. Fe+, Fe
K - Neutral metal atom absorptions e.g. Fe, Ca
M - Molecular band absorptions e.g. TiO2
Describe the Hertzsprung-Russel diagram
Absolute magnitude on the y-axis (decreasing going up)
Temperature on the x-axis (decreasing going right).
Long, diagonal band in the middle called the main sequence (main sequence stars are in their long-lived stable phase where they are fusing hydrogen and helium).
Stars with a high luminosity and a low surface temperature must have a high surface area due to Stefan’s law (red giants and supergiants) and are located in the top right.
Stars with a low luminosity and a high surface temperature must have a low surface area due to Stefan’s law (white dwarfs). They are located on the bottom left. These stars are towards the end of their lives and are cooling down, they have stopped fusion reactions.
