# An astronomy question: regarding Betelgeuse going supernova.

**URL:** <https://boards.straightdope.com/t/an-astronomy-question-regarding-betelgeuse-going-supernova/144991>\
**Category:** Factual Questions\
**Created:** [December 26, 2002, 5:46pm UTC](https://boards.straightdope.com/t/an-astronomy-question-regarding-betelgeuse-going-supernova/144991 "2002-12-26T17:46:29Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\
**Post date:** [December 27, 2002, 7:45am UTC](https://boards.straightdope.com/t/an-astronomy-question-regarding-betelgeuse-going-supernova/144991/21 "2002-12-27T07:45:59Z")

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Well, after 900 years the crab nebula is about 10 light years in diameter.

Given those kinds of numbers, the exploding Supernova would start out as a point source when it exploded, and gradually growing to be maybe the size of the moon in the sky over a couple of thousand years.

What would determine its brightness? I guess nearby neighbors could make it a reflection nebula. Is the nebula itself going to be bright enough to be visible from Earth? I dunno.

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [December 27, 2002, 2:40pm UTC](https://boards.straightdope.com/t/an-astronomy-question-regarding-betelgeuse-going-supernova/144991/22 "2002-12-27T14:40:03Z")

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“Is the nebula itself going to be bright enough to be visible from Earth?”  
Well, the Crab nebula has a magnitude of 8 to 8.4. Using the same brightness ratio as before gives a magnitude of 2-2.4 for a betelgeuese nebula; and that’s after it’s cooled for nearly a thousand years. By way of comparison, M31, the galaxy in Andromeda which is visible to the unaided eye, has a magnitude of only 3.4.

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**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\
**Post date:** [December 27, 2002, 4:07pm UTC](https://boards.straightdope.com/t/an-astronomy-question-regarding-betelgeuse-going-supernova/144991/23 "2002-12-27T16:07:32Z")

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> [@](#):
>
> \*Originally posted by scr4 \*  
> I definitely agree. And every telescope on earth will be pointed at the supernova anyway, so it doesn’t matter if you can see other objects or not.
> 
> However I think many instruments will have problems observing such a bright target. **Bad Astronomer,** since you’re familiar with the Hubble, do you think it’ll be able to see the SN?

Actually, while the supernova is at its brightest, most telescopes would be massive overkill. You don’t need a 10 meter telescope to observe a -18.5 magnitude object! In fact, you’d have to have a ridiculously short exposure time to do anything–even take spectra.

Similarly, neither world-class telescopes nor Hubble are routinely used to observe the Moon because it’s so damn bright. IIRC, there have been only a few observations of the Moon with HST–one I know was a spectrum of earthshine off dark side of the Moon.

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