Stars as Observed in the Infrared

The figure below shows what stellar spectra are like in this wavelength range. All stars hotter than about 3000 K effective temperature radiate like a hot blackbody at these long wavelengths, possibly with a very few atomic lines (of Hydrogen, in the A-type stars) or molecular bands (the CO band overtone near 2.25 microns for example, in K-type stars and G-type stars). The continuum is steeply decreasing with wavelength, roughly as wavelength to the -3 power in the type of plots that I use (lambda*F_lambda as a function of lambda), so a log-linear plot is used to be able to see the spectra over the 2.4 to 45 micron range of the Infrared Space Observatory Short Wavelength Spectrograph instrument. Most the spectra shown here are from ISO, with a few also from the Spitzer Space Telescope Infrared Spectrograph instrument.

In the upper left panel is show spectra of three stars, alpha CMa (A1V), alpha Cen A/B (G0V + K2V) and alpha Boo (K1.5III), with some scaling of the other two spectra to roughly match the brightness of alpha CMa at these wavelengths. The two spectra of the hotter stars are very close to the same in shape while the cooler K-type giant star has a slightly less steep shape but the difference is somewhat subtle when looked at on a linear plot.

The lower left panel shows a hot star, P Cyg, which has a strong stellar wind. The wind produces emission limes of H and He and the wind also produces an excess in emission the the longer wavelengths. This is a "hypergiant" star, a very massive somewhat evolved star. If it were not hidden behind about 10 magnitudes of interstellar extinction it would be a second magnitude star in Cygnus. (The ISO spectrum of P Cyg is poor beyond 27 microns and that part should be ignored.)

The right-hand panels show the spectra of much cooler stars without dust. At top is the average ISO spectrum of 7 M-type giants, which show two weak molecular bands due to SiO but otherwise have a continuum shape similar to those of the hot stars. At lower right is a spectrum of a bare carbon star, showing strong HCN, CO, and C₂H₂ molecular bands. (The T Lyr spectrum is poor beyond 27 microns, and that part should be ignored.)

The carbon stars and M-type giants/supergiants generally have dust around them. That is the subject of the next few pages.

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the most common features: silicate and SiC dust

less common features: extreme carbon stars, the AlO/silicate complex, the "unidentified infrared" features, unusual silicate features

dusty HII regions, planetary nebulae, and related objects

unusual features: mixed chemistry sources, Wolf-Rayet stars, ice bands, featureless carbon-star spectra