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In this video, between 4 and 6 minutes, the lecturer describes M type stars as being less massive, smaller, and redder than G type stars.

He then gives Betelgeuse as an example of an M type star and mentions that the sun is a G type star.

However, Betelgeuse is more massive than our sun, and larger than our sun.

How can I reconcile these facts?

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If you take a look at the Hertzsprung-Russell Diagram that helps us to classify stars

HR-Diagram
(c) Wikimedia commons

(while this one uses data gathered from the Hipparcos satellice, roughly 100.000 stars) we see, that there are 2 main types of red stars: Giants and M-dwarves. The latter are stars on the main sqeuence (they have stable hydrogen-burning in their cores), while the giants are stars that have left the main sequence due to aging. They appear red as they bloat up to bigger radii compared to their main-sequence life which also makes them cooler, and thus redder.

Betelgeuse is now one of those giants, and he's actually more massive than the sun (somewhere between 7 and 20 solar masses, according to wikipedia), so there is no contradiction with M-Dwarves also being red.

Please let me know if you desire to know more details on this.

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  • $\begingroup$ The lecturer said M will be smaller than G. Then he gives an example (Betelgeuse) that is larger than the Sun. That's the contradiction that I don't understand. Are main-sequence M's all smaller, less massive, and redder than G? Or are some (Betelgeuse) exceptions to this? $\endgroup$
    – what_is_it
    Dec 4, 2015 at 17:30
  • $\begingroup$ @what_is_it: Yes, all main sequence M's are smaller, less massive and redder than G. Just look at the Diagram. That's how they're defined. But Betelgeuse is not a Main sequence star. It's a red Giant. $\endgroup$
    – AtmosphericPrisonEscape
    Dec 4, 2015 at 17:36
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Type M is a spectral class: that is a star colour. Type M stars are red, and relatively cool.

There are, roughly, two types of stars that are Type M:

  1. Main sequence stars that are small, have relatively low rates of hydrogen fusion, and cool exteriors. These are called "red dwarfs" and account for the majority of stars. All red dwarfs are too dim to be visible with the naked eye, but the closest star, Proxima centuri, is an example.

  2. Stars near the end of their life swell up, and the outer layers cool and become cool and red. Very massive stars like Betelgeuse have a similar colour to red dwarfs but are much larger. Betelgeuse is a Red Giant; it is not on the Main Sequence.

The video shows the Hertzsprung-Russell diagram which shows the two groups. It correctly describes Main Sequence type M stars as smaller, redder and dimmer than the sun. It then gives Betelgeuse as an example of a Red Giant. Which is larger and brighter than the sun, yet still cooler on the outside.

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  • $\begingroup$ So weird that the lecturer would use a non-main-sequence star as an example in a lecture titled "Main Sequence Stars". It doesn't mention at all that Betelgeuse is not a main sequence star... I'm at 6 minutes into the video so far. $\endgroup$
    – what_is_it
    Dec 4, 2015 at 17:45
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    $\begingroup$ There are a few issues I'd have with the video: He mentions "Green stars" (of which there are none) and I don't like the "Beetle-juice" pronunciation, and it is confusing that he mentions Betelgeuse (a red giant) when he has been talking about main sequence stars. He does clarify this last point later on. I guess it is because there are no naked eye visible main sequence red stars. $\endgroup$
    – James K
    Dec 4, 2015 at 18:07

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