Caption: The spectra of seven stars ordered by spectral type ranging from the hottest (O-type) at the top to the coolest (M-type at the bottom). The x-axis shows the wavelength of light while the brightness or darkness at each wavelength corresponds to the flux of light received from the star at that wavelength with darker patches having less flux and brighter patches more. Each spectrum is normalized (the flux at each wavelength is divided by the maximum flux for that spectrum) so that the maximum flux should appear with the same brightness for all the spectra. The colour plotted between 400 nm and 700 nm roughly corresponds to the color the human eye would see light of that wavelength. Below 400 nm and above 700 nm, where the human eye can see little to no light, the lines are coloured blue and red respectively.

The hotter stars have more of their flux at the bluer end of the spectrum and the cooler stars have more of their flux at the redder end. However the total amount of flux a star emits depends on its size and temperature. Due to this, a hot star will emit more red light than a cool star of the same size even if the cool star emits almost all its light in red light but this is not visible in this plot due to the normalization mentioned above. The dark, narrow patches in the spectra are absorption lines caused by atoms and ions in the stars’ atmospheres. The strength of a spectral line depends on the temperature of a star’s atmosphere. Take the hydrogen line at 656.5 nm as an example. All of the stars in this plot are primarily made of hydrogen, but the 656.5 nm hydrogen line is weak for the hottest and coolest stars but strongest for spectral types A and F. This is because hydrogen absorbs more light at 656.5 nm at the temperatures of A and F stars’ atmospheres than in hotter or cooler stars.

The coolest star here, the M-type star, has wide absorption bands in its spectra. This is because this star is cool enough to have compounds such as titanium oxide in its atmosphere. These compounds, often called molecules in astronomy, produce wider spectral absorption features than atoms or ions.

Credit: IAU OAE/SDSS/Niall Deacon.

Related glossary terms: Spectral Type , Spectrum , Wavelength
Categories: Stars

Created with support from: OAE Main Office Main Office

License: Creative Commons Attribution 4.0 International (CC BY 4.0) Creative Commons Attribution 4.0 International (CC BY 4.0) icons

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