Stars - Spectra and atoms
Andrew Conway
Line spectra
- We've looked at the overall shape and peak of a star's spectrum.
- But now we'll look at departures from that shape.
- Mostly these are spectral lines.
- Spectral lines can tell us:
- what elements are present in a star.
- densities and temperatures in different outer layers
- clues to how the star rotates
Emission spectra
- Heat up a gas sample.
- Split the light into different wavelengths, using a prism or diffraction grating.
- You will see that light is only emitted at very specific wavelengths - emission lines.
Hydrogen emission spectrum
Source: Merikanto Public Domain
| Red |
656 |
| Cyan |
486 |
| Blue |
434 |
| Violet |
410 |
Absorption spectra
- Put a cool gas sample in front of a white light source.
- White light emits at all wavelengths across the visible.
- Dark lines will be seen due to the gas sample - absorption lines.
- This is what we saw in the star spectra:
- The star's photosphere emits a continuum of light
- A layer of gas above it absorbs to make the absorption lines
The atom
- To understand line spectra, we must understand atoms.
- The word derives from the Greek notion of the smallest indivisible bit of matter.
- But we now know that the atom has internal structure.
- This structure explains line spectra.
Protons, neutrons and electrons
- The nucleus is at the centre of an atom.
- It has most of the mass.
- It is made of protons (positive charge) and neutrons (zero charge).
- Around the nucleus are electrons (negative charge).
Atomic energy levels
- Electrons can have different energies.
- But not any energy; their energy can only take on certain values (quantised)
- When an electron changes energy level, a photon is emitted.
- The photon is the particle associated with electromagnetic radiation.
Emission
- When an electron falls from a higher level to a lower one, it loses energy.
- The lost energy comes out as light.
- Bigger falls in energy give higher energy photons.
- High energy photons correspond to short wavelengths of light.
- The possible energy levels in an atom give a variety of possible emission lines.
Absorption
- When a photon arrives, it can bump an electron up in energy, if it has just the right energy.
- That photon is then absorbed, and disappears.
- If the photon doesn't have exactly the right energy, nothing happens.
- So if you shine white light on a sample of gas, then only lines corresponding to differences in energy levels will be removed.
- These lines will be at exactly the same wavelengths as the emission lines for that gas.
Atomic number
- The type of atom is determined by the number of protons in the nucleus.
- For example:
- hydrogen (H) has 1 proton
- helium (He) has 2 protons
- carbon (C) has 6 protons
- The atom is neutral, because it has the same number of electrons as protons.
Chemical composition from line spectra
- In a lab, measure the wavelengths of spectral lines of a sample of gas containing certain atoms.
- Then, look for that same set of spectral lines in the light from a star.
- If you find them, you've confirmed that the star also contains those atoms.
Helium and the Sun
- A set of unknown spectral lines was found in the spectrum of sunlight.
- Helios is the Greek word for the Sun, so this new element was called helium.
- It was subsequently found in the Earth.
- It is rare on Earth, but makes up almost 25% of the mass of the Sun.
The Doppler effect
Waves emitted by a moving source will have their wavelengths increased or decreased:
- If it is moving away, the spectrum will be red-shifted.
- If moving towards the observer, the spectrum will be blue-shifted.
- The shift in wavelength is proportional to the speed.
What the Doppler effect can tell us
- Is the star orbiting with something else? Another star, a planet?
- Rapidly rotating stars will have broadened lines.
- High temperatures can broaden lines, because atoms are moving fast.
- Distant galaxies are moving away from us - Hubble's Law, see later in the course.
