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X-ray Spectroscopy of Young SNR

Almost all the elements in your body and on the Earth except for Hydrogen and Helium come from stars.

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 occurring 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 occurred 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.

spectral analysis of Cas A

Spectrum of the supernova remnant Cas A showing prominent lines

Things We Can Learn About Stars from SNR

Astronomers are able to use SNR spectra to get all kinds of information about the stars that exploded long ago and made the remnant. By looking at the narrow lines and comparing to the lines of known elements we can begin to figure out what the star was made of. By using the line strengths of different lines, it is possible to estimate properties such as temperature and pressure and density in the gas, as well as how much the layers of gas in the star are mixed. We can compare the observed values to stellar models to learn what kind of star the progenitor was.

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Thank you to Una Hwang for contributing to this article.

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Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Project Leader: Dr. Barbara Mattson
Curator: Meredith Gibb
Responsible NASA Official: Phil Newman
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This page last updated: Monday, 27-Sep-2004 11:26:09 EDT