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Stellar Radiation Page 11

Signature of a star

By comparing line spectra with the spectra generated in the laboratory, astronomers can make out which elements are present in the outer layers of the star or in the inter-stellar gas. It is as if these chemical elements produce their particular barcode in the star’s spectrum, identifying them to the observer.

The absorption spectrum of a star shows the sum of different “barcodes” of different elements. It does not matter how far away the star is, as long as we can receive the starlight and separate it through a spectrograph, we can get this information.

Based on spectral image from
www.weasner.com

If a certain element is very abundant, the absorption that takes place will be more intense. Therefore this abundance can be related to the strength of various absorption lines.

To produce certain spectral lines, a star must be hot enough to excite the electrons out of a particular state but not too hot to ionise a significant fraction of the atoms. In ionised atoms, the electrons are removed, so there are no electrons to excite, and therefore no spectral lines. Thus astronomers can also relate a star’s temperature to the strength of its various spectral lines.

High Resolution Sun spectrum

1

This image was created from data observed with the Spectrometer at the McMath-Pierce Solar Facility at Kitt Peak National Observatory, near Tucson, Arizona, USA.

It must be read like a book: from left to right,
top to bottom.

Each of the 50 slices covers
6 nm, for a complete spectrum across the visual range from 400 to 700 nm.

Click on the image to download a high resolution version of this image (2.8 MB).

Image credit: N A Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF