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lmxbftw t1_j5km6bo wrote

Are you talking about volume here, or mass? If volume, then the biggest ones we know ARE indeed at the end of their "life", up at the top-right corner of the Hertzsprung-Russell diagram. To know the radius of a star, you need to know its distance, which there are different ways of working out, some more precise than others. In the case of VY Canis Majoris, one of the largest stars known, there's a very precise radio parallax distance measure from the VLBI. Once you know distance, how bright it appears to be, and the temperature (these second two are relatively straight-forward and easy to measure from Earth) you can work out how physically large the star has to be to produce the observed amount of light from the Stephan-Boltzmann law.

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CrackpipeStickman999 t1_j5kp7jv wrote

Do the brightness and temp give a clue to if the star is about to "die" or that it's just a large star?

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Any-Broccoli-3911 t1_j5kqon3 wrote

Yes, we can put the stars in the diagram based on their brightness and their temperature.

If they are in the main diagonal, they burn core hydrogen and will last a while.

If they are brighter than the main diagonal, they burn shell hydrogen and either core or shell helium, they are close to their death.

If they are less bright than the main diagonal, they don't burn anything and are just slowly cooling down.

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GreenRangers t1_j5lmw0h wrote

Have we(humans) ever seen a star die?

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Any-Broccoli-3911 t1_j5lnuk0 wrote

Yes, each supernova is a star dying. Scientists see some with telescope every year. Humans have seen a few with just their eyes, SN 1604: Kepler's Supernova is the latest one in 1604.

Most stars die without supernovae, but we don't see those.

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jlittlenz t1_j5lrnsh wrote

You don't count SN 1987A? From wikipedia, Ian Shelton "went outside to look with the naked eye, and saw that the bright light was indeed present".

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9dnguy t1_j5okm8t wrote

Are black holes considered stars? If they are, I was just reading a few days ago about the largest black hole/quaser/blazer ever found with a mass of 40 billion suns. How big or what is the size of a black hole of that mass can be? Is it bigger than the solar system?

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lmxbftw t1_j5oqq0c wrote

No, black holes aren't considered stars. Those super massive ones are enormous, though - the radius is linear to mass, and a 1 solar mass black hole has a radius of 3 km. So a 40 billion solar mass black hole would have a radius of about 120 billion km! Which is about 20 times further out than Pluto is, on average.

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Krail t1_j61yhi4 wrote

How does that compare to the further most reaches of the solar system? (Like, to the Heliopause? Is that the considered the edge of the solar system?)

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lmxbftw t1_j623uu9 wrote

It's about 6-7 times further out than the heliopause. (The heliopause is far from spherical, since the Sun is moving relative to the ISM, but from the close part of it.)

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