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Stellar Spectral Classes
Energy levels in atoms
- Electrons in an atom exist in energy levels.
- N=1 is ground state – lowest energy level
- Atom is excited when or more electron is in an energy level higher than the ground state
- Electrons excited to higher energy levels by absorbing photons of specific wavelengths and frequencies, which is a concept of AS/A-level physics exam
- E=hf
Line absorption spectra
- Spectrum got by splitting light by prism/ diffraction grating
- Absorption lines got in spectrum when radiation from black body passes through a cooler gas
- Low temperatures – electrons in gas atoms at n=1.
- Photons of particular wavelengths excite them
- When electrons de excite,same wavelength of light is emitted and radiated in all directions = intensity of radiation with this wavelength reaching earth reduced ( dark lines in absorption spectrum)
- The more intense the absorption line at a particular wavelength, the more radiation of that wavelength has been absorbed.
Hydrogen Balmer absorption lines
- orbit the nucleus in shells. The difference in energy between shells is always exact. An electron can change from energy level 1 to 2 (which is further away), if it absorbs a photon with energy of exactly that difference.
- The higher shell is always more unstable, and so the electron radiates the energy away as a photon and falls back down to level 1 again.
- The specific energy of the photon means it has specific wavelength. The Balmer Series is used to describe the transitions of electrons from shells 3,4,5, etc to shell 2. The wavelength of emitted radiated (as photons) lies mostly in the visible spectrum. It is usually always to talk about hydrogen transitions and spectra.
- Lower wavelength produced = more energy released = bigger de excitation to n=2
- Light emitted by star has been absorbed by hydrogen atoms in stellar atmosphere as light passes through it.
Determining stellar temperatures
- If temperature is too high, electrons will reach n=3 or above = not many Balmer transitions
- Intensity of Balmer lines depends on temperature of star
Spectral Classes
- Stars classified into groups – spectral classes
- Spectral class of star depends on relative strength of certain absorption lines
- In order of decreasing temperature, peak wavelength increasing:
O,B,A,F,G,K,M
- O – hottest stars – blue+ M – coolest stars – red
- Sun – G star
- A – strong absorption lines
Spectral Class | Colour | Temp/K | Absorption lines |
O | Blue | 25000-50000 | Strongest spectral lines are He+/He absorptions – they need high temp and have weak H Balmer lines |
B | Blue | 11000-25000 | Spectra Has strong He + H absorptions |
A | Blue-white | 7500-11000 | Visible spectra – strong H Balmer lines + metal ion absorption |
F | White | 6000-7500 | Spectra has strong metal ion absorptions |
G | Yellow – white | 5000-6000 | Metal ion + metal absorptions |
K | Orange | 3500-5000 | Spectral lines mostly from neutral metal atoms |
M | Red | Under 3500 | Spectral lines are from neutral atoms and TiO absorptions which need cool temp to form. |
That's the end of the topic!
Drafted by Kin (Physics)