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PHY121 Ch 9 Notes - Astronomy Chapter 9 The Formation and...

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Astronomy Chapter 9: The Formation and Structure of Stars The space between the stars is not empty but is filled with low-density gas and dust called the interstellar medium it is made up of 75% hydrogen, 25% helium, and various other elements 1 percent of the mass is made up of interstellar dust (microscopic dust particles). We are interested in the interstellar medium because: Dense interstellar clouds are the birthplace of stars Dark clouds alter and absorb the light from the stars behind them Astronomers know there is an interstellar medium because they can see nebulae : emission nebulae , also called HII regions – produced where very hot stars excite clouds of low-density gas to emit light (by being brought back to ground state) reflection nebulae – produced when starlight is scatters from a dusty nebula; the reflection nebula appears blue because blue light is scattered more efficiently than red light (reason our sky appears blue) dark nebulae – produced when dense clouds of gas and dust absorb the light from the stars behind; appear dark in front of brighter background Also, you know there is an interstellar medium because they can detect interstellar absorption lines in the spectra of distant stars. These can be distinguished from stellar absorption lines though: Absorption from wrong ionization states (due to the low temperature of the cloud); For example, gas atoms of elements such as calcium and sodium absorb photons of certain wavelengths, producing narrow absorption lines. You can be sure the lines originated in the ISM because they appear in the spectra of O and B stars (stars too hot to form calcium and sodium absorption lines in their own atmospheres) Small line width (due to density); indicates that they could not have been formed in the hot atmospheres of stars, instead very cold. multiple interstellar lines appear with slightly different Doppler shifts because the light from the star is passed through a number of gas clouds on its way to Earth. In addition, the interstellar medium makes distant stars look fainter, and the interstellar dust makes distant stars look redder than expected, an effect called interstellar reddening .
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