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How are O-type stars different from M-type stars?
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Star
Stars in the open cluster NGC 290, as seen by the Hubble Space Telescope.
In the stellar classification system, O-type and M-type stars represent opposite ends of the temperature spectrum. While they share essentially the same chemical composition, they differ significantly in their physical characteristics and spectral features.1
Key Differences
- Temperature and Color: O-type stars are the hottest and bluest, with surface temperatures typically ranging from 25,000 K to 50,000 K. In contrast, M-type stars are the coolest and reddest, with surface temperatures around 3,000 K.21
- Spectral Features:
- O-type stars: Their spectra are characterized by the presence of ionized helium, as well as lines of doubly ionized nitrogen, oxygen, and carbon, and triply ionized silicon, all of which require high energy to produce.1
- M-type stars: Their spectra are dominated by the presence of familiar metals and titanium oxide molecules, which are prominent in the red and green parts of the spectrum.21
These differences in temperature and atmospheric pressure dictate the ionization states of the elements observed in each star's spectrum.1