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If we parked Betelgeuse just outside the Solar System, how big would it look from Earth?

ProfRob
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Jordan
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    It was just recently discovered that Betelgeuse only shifted from yellow to red a couple thousand years ago. So it won't be going supernova for thousands of years. Sadly. – Julie Gaskill Nov 15 '22 at 23:53
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    "It was just recently discovered that Betelgeuse only shifted from yellow to red a couple thousand years ago." I did not know that, and I'm (pleasantly) astonished! Can you supply a source I can look at? – AJM Nov 16 '22 at 15:11

2 Answers2

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The distance to Betelegeuse is not precisely known for reasons you can read about here and here. But let's assume a likely distance of 200 pc. The angular diameter of the star has been measured with optical and IR interferometry to be about 0.05 arcseconds (see the relevant section of the wikipedia page on Betelgeuse), though this is uncertain by about 10%.

These two numbers translate into a linear photospheric diameter for the star of 10 astronomical units (au).

We then have to interpret what you mean by just outside the Solar System. If you mean half way to the next star - i.e. around 0.6 pc, then a star with this diameter would have an angular diameter of 6 arcseconds - so similar in size to Mars viewed from Earth. However, if you meant just beyond the edge of the Kuiper belt at say 100 au from the Sun, the angular diameter would be about 6 degrees! This is more than ten times the size of our Sun, which would indeed look very impressive if it were at all possible for us to view such an event.

ProfRob
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    Now I'm wondering how long an orbit at 100au would be, and how much energy Betelgeuse would irradiate the Earth with compared to what we are already receiving... – Michael Nov 15 '22 at 18:50
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    @Michael the orbit would be around ~230 years and the irradiation would be around ~12x that of the Sun. – 2012rcampion Nov 15 '22 at 20:33
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    @Michael WikipediAlists the masof Betelguese as 16.5 to 19 times the mass of the Sun, so the Sun would orbit around Betelguese. More accurately both would orbit around the center of gravity which would be a lot closer to the Sun than to Betelguese.. That was not specified in your comment or 2012campion's so I thought that it was worth mentioning for the sake of anyone who reads the comments. – M. A. Golding Nov 15 '22 at 21:04
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    @M.A.Golding: If, as you say, Betelgeuse is more massive than the sun, the barycenter (center of gravity) would I think be a lot closer to Betelgeuse than the Sun. It would be interesting to know what vaguely habitable orbits the earth could have (if any, give the increased irradiation mentioned in other comments). – Seth Robertson Nov 15 '22 at 21:24
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    in regard to habitability, please, pretty please, keep Betelgeuse where it is. First, it is impressively variable (e.g by a magnitude or two), second it is impressively sooty and third it is expected to blow soon. – fraxinus Nov 15 '22 at 21:39
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    @fraxinus Thanks, I briefly considered moving it closer but your comment dissuaded me. ;) – Michael Nov 15 '22 at 21:49
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    @Michael Since humanity seems to be intent on destroying the environment, maybe we should just pull it in and get it over with. – Barmar Nov 16 '22 at 15:55
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Betelgeuse is 7000-14000 times brighter than the sun, so if it were 100AU away, the inverse square law means that we would be hit with 10000 times less radiation, so it would be 7 to 1.4 times brighter than the sun.

bandybabboon
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