X-ray Spectroscopy of Young SNR

by Allie Cliffe

Some are generated by stars by nuclear fusion, releasing the light that makes them shine in the process. Other, heavier elements are created in the powerful throes of an exploding star, or supernova explosion and hurled into space. When a star explodes, it releases a tremendous amount of energy and can shine more brightly than an entire galaxy.

Supernova explosions come in two different types, depending on the mass of the stars that exploded. Type II SNe are the result of the gravitational collapse of massive stars. Type I SNe arise from runaway nuclear fusion on the surface of a white dwarf in a binary orbit.

The explosive force blasts the stellar material out into space. The expanding material, and the gas it sweeps up as it expands, is called a supernova remnant(SNR). A SNR is composed of stellar material and interstellar material (ISM), heated to millions of degrees by the shock of the supernova explosion. As the remnant expands, the stellar material and the ISM mix. Eventually the remnant will disappear, dispersing into space, enriching the ISM with heavier elements such as carbon and oxygen (and heavier, up to and beyond iron). All of the naturally occuring heavy elements we observe were distributed by SNes and many heavy elements are only created in the extreme conditions of a supernova explosion.

Most supernovae occured hundreds or even thousands of years ago, so we could not have observed the stars that exploded to form them. However, by looking carefully at the supernova remnant, we can get many clues about what the stars were made of and how they evolved.

Tools of the Trade

One of the most direct and most useful tools for learning about what a star is made of is its spectrum. A spectrum is a measure of the light received as a function of the light's wavelength, or energy. Gas that is very hot emits radiation in a range of wavelengths. The hotter the gas, the higher energy the thermal emission. Gas heated to millions of degrees by the passing of a powerful SNR shock emits strongly in the X-rays. What is particularly useful is the fact that a hot gas emits in narrow energy bands or lines at very particular energies depending on what gas it is. Looking at a spectrum and identifying the lines in the spectrum allows us to identify elements that are present in the emitting gas. For a young SNR which has not yet swept up much of the gas of space around it, the composition of the progenitor star shows up very strongly in its spectrum.

Spectrum of the supernova remnant Cas A showing prominent lines

Things We Can Learn About Stars from SNR

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