Putting it all together: spectra and astronomy

In a star, there are many elements present. The way we can tell which ones are there is by looking at the spectra of the star. The science of spectroscopy is quite sophisticated. From spectral lines astronomers can determine not only the element, but the temperature and density of that element in the star. Emission lines can also tell us about the magnetic field of the star. The width of the line can tell us how fast the material is moving, giving us information about stellar wind. If the lines shift back and forth, it means that the star may be orbiting another star – the spectrum will give the information necessary to estimating the mass and size of the star system and the companion star. If the lines grow and fade in strength we can learn about the physical changes in the star.

Spectral information, particularly from energies of light other than optical, can tell us about material around stars. This material may have been pulled from a companion star by a black hole or a neutron star, where it will form an orbiting disk. Around a compact object (black hole, neutron star), the material in this accretion disk is heated to the point that it gives off X-rays, and the material eventually falls onto the black hole or neutron star. It is by looking at the spectrum of X-rays being emitted by that object and its surrounding disk, that we can learn about the nature of these objects.

Let’s look at the SNR spectrum:

X-ray Spectrum of supernova remnant Cas A
X-ray spectrum of the Cassiopeia A supernova remnant

Note the emission lines, which are characteristic lines from neon, iron, magnesium, silicon, sulfur, argon and calcium (check the names against a periodic table to be certain). The origin of the continuum emission is still being debated by astronomers – it could be from synchrotron radiation or Bremsstrahlung or some combination of the two. Astronomers would need more detailed spectra, and from different parts of the supernova remnant, to pin down the processes at work.

For more detailed information and exercises relating to the spectra of supernovae, please visit our Unit on X-ray Spectroscopy and the Chemistry of Supernova Remnants! Or, to try your own hand at anaylzing spectra, check out Student Hera on Imagine the Universe!.