NATIONAL OPTICAL ASTRONOMY OBSERVATORIES / SCIENCE PHOTO LIBRARY NATIONAL OPTICAL ASTRONOMY OBSERVATORIES / SCIENCE PHOTO LIBRARY
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Spectral star classification. Comparison of the visible light spectra (400-700 nanometres, rainbow bands) of 13 types of stars. The types (from top to bottom) are O6.5, B0, B6, A1, A5, F0, F5, G0, G5, K0, K5, M0, M5. Our Sun is a G2-type star. The order OBAFGKM ranks the stellar spectral types in order of decreasing surface temperature, while the numbers 0-9 provide sub-types. An O-type would be hotter than 25,000 Kelvin (K) and appear blue. An M-type would appear red and be cooler than 3500K. The dark spectral bands show stellar composition.
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