Spectral analysis of starlight can tell us about: composition (by matching spectra). temperature (compare to blackbody curve). (line-of-sight) velocity (Doppler shift or “red shift”). object’s rotation rate (broadening of spectral lines). pressure of the gas (broadening of spectral lines). magnetic field (splitting of lines into two lines). Let’s go over the first three topics:

Spectral analysis of visible light: The visible spectrum is seen in the familiar rainbow colors that appear due to refraction in falling droplets of rain. The spectrum of light emitted from atoms has only a small number of wavelengths of light compared to the so-called Blackbody Spectrum, which has all the wavelengths, depending on temperature.

Spectrum of a “blackbody” is continuous. All objects emit electromagnetic radiation, called blackbody radiation. As the temperature increases, the wavelength of the peak of the emission shifts to shorter wavelength (which has higher frequency). At several hundred degrees, objects emit some visible light, and the emission spreads across the spectrum toward the blue, as the temperature increases.

The spectrum of a “blackbody” is continuous, and at high temperature all the colors will be seen:

Spectral analysis of starlight can tell us about: composition (by matching spectra). temperature (compare to blackbody curve). (line-of-sight) velocity (Doppler shift or “red shift”). object’s rotation rate (broadening of spectral lines). pressure of the gas (broadening of spectral lines). magnetic field (splitting of lines into two lines).

The Solar Spectrum has dark absorption lines that can be compared to known lines from gases in the laboratory. This can be used to determine the composition of the Sun.

The Doppler Effect occurs when a source of sound (or light) is in motion relative to an observer

Doppler Effect for a moving source Doppler Effect for a moving source. Simulations show this more clearly (link). If the source moves faster than the speed of sound, shock waves are created. (link2)

We use spectrometers to separate the wavelengths. Doppler Shift of spectral lines can be used to measure a star’s velocity toward or away from us We use spectrometers to separate the wavelengths. Look in detail at the red line on the next slide.

Doppler Shift of H-alpha spectral line for hydrogen in a cloud of hydrogen gas that is receding from us at 320 km/s. This is called a “red shift” since the line moves more to the red end of the spectrum (longer wavelength).

Red shift Red shift is the movement of a spectral line toward the red end of the spectrum. In other words, the wavelength gets longer, or the frequency gets lower. This is due to a star moving AWAY from us, and the Doppler effect changes the frequency of the light we detect.