My son wants to know how long it would take us to see a star explode; meaning how long after it really exploded would the light of it exploding be seen on earth?
Depends on how far away the star is. If it’s 1000 light years away, then the answer is 1000 years.
Zev Steinhardt
If the star in question was Alpha Centauri, we’d know about it after a little more than four years; if the star in question was more distant, then it would take longer; the distance between stars is commonly expressed in light-years; however many of these there are between you and the star is how long it would take for you to see it.
Some of the stars you see when you look up at the night sky aren’t there anymore (actually most of them aren’t there anymore because they will have moved as the galaxy swirls round, but you’re seeing the light that left them thousands or millions of years ago).
If the sun blew up (not likely apparently) we would know nothing about it (and notice nothing unusual for 8 or 9 minutes.
No star that you see in the sky is a million light years away. I don’t know what the most distant star you can see as a star is, but the Milky Way is only about 100,000 light years across. You can, of course, see the Andromeda Galaxy (2.2 million light years away), but you can’t see any individual stars that make it up.
This site has the following to say on what the most distant star you can see with an un-aided eye is.
The most distant thing period that can be seen un-aided is, as Achernar correctly pointed out, the Andromeda Galaxy at 2+ million light years away.
I guess I should note that you can only see an individual star that is shining normally out to a thousand light years or so (give or take). However, an exploding star (ala a supernova) could probably be seen unaided all the way across the galaxy. IIRC a supernova can briefly outshine the entire galaxy it is in. Just a WAG on my part but I am certain you could see an exploding star further than 1,000 light years away.
That’s right, Whack-a-Mole, in fact Supernova 1987a was visible to the naked eye and was tens of thousands of lightyears away.
For example, Supernova 1987A was about 160,000 light years away. It was visible to the naked eye within hours of its onset.
About 1.4 billion hours, by my calculations. I’m sure by “onset”, you mean the time the explosion was first noticeable at Earth but it took the light from the event 160,000 years to reach us.
Just out of curiosity if the Milky Way is roughly 100,000 light years across how is it possible to get a supernova 160,000 light years away? I thought Andromeda was the next closest galaxy and it’s over 2 million light years away. Did it come from a globular cluster or something (for instance M54 which is the closest…so close in fact that I think it is actually inside of the Milky Way itself…essentially colliding with it).
IIRC, Supernova 1987A was in the Large Magellanic Cloud, a galaxy much closer to us than the Andromeda Galaxy.
Hmmm…gotcha. Thanks. I always thought Andromeda was our nearest galaxy but it seems it is our nearest large galaxy (ala on par with the Milky Way). The Large Magellanic Cloud is regarded as a dwarf galaxy and is, as you said, at roughly the right distance for Supernova 1987A.
The Andromeda Galaxy (note: It’s not just “Andromeda”, that’s the name of the constellation. If you want to shorten the name, call it M31) is the nearest major galaxy to our own, but both the Milky Way and M31 have dozens of smaller galaxies orbiting them. The two largest galaxies orbiting the Milky Way are the Large and Small Magellanic Clouds, and 1987a was in one of them.
Globular clusters are the next step down from the smallest galaxies, and are found by the hundreds in halos around galaxies. All of the globs on the Messier list are part of our own galaxy, but they’ve been discovered around other galaxies, as well (you can see a few of M31’s globs in a decent-sized amateur scope).
And for the record, while we would still see a star for some time after it died, while the light is still travelling to us, it’s probable that all of the stars we see in the night sky are still there. The one possible exception is Betelgeuse, which is getting on well into old age, and is (as we currently see it) probably within a millenium of going supernova. Since it’s 800 lightyears away, it might actually have blown by now.