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Let's say there is a star about 3000 light-years away from earth visible in our night sky. If this star were to go supernova tomorrow(not relative to earth's night sky), we would know about it 3000 years later as all the information would take 3000 years to arrive at the least. At first, I thought detecting the supernova would be easy as we could use X-ray telescopes or other indirect methods to find out the composition of the stars and know if a supernova occurred or not.

Still, unfortunately, this information would also travel at the speed of light, causing a delay in our process.My question is, how can we know for far-away stars whether they have gone supernova?

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    $\begingroup$ As I mentioned here, if we could get good neutrino data from the core of a large star we could estimate how much time it has left before it goes supernova. $\endgroup$
    – PM 2Ring
    Sep 16 at 0:18
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    $\begingroup$ Looking far away is looking into the past. $\endgroup$ Sep 16 at 16:40
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    $\begingroup$ Not only can we not know if it’s gone supernova, we also can’t unambiguously say that it went supernova 3,000 years ago even when the light has reached us. The time between the supernova and now that’s perceived in the Earth’s frame of reference will be different from the time difference perceived by someone moving at a significant fraction of the speed of light relative to the Earth. $\endgroup$
    – Mike Scott
    Sep 16 at 18:40
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It is not possible to know. The speed of light is the speed of information. The information "the star has exploded" cannot travel faster than the speed of light, so there is no way to know that a star has gone supernova before that information reaches us. Usually the first particles to reach us from a supernova are actually neutrinos, which can escape from the core of the exploding star a little time before the shock wave of the exploding star reaches the surface and the supernova becomes visible.

It may be possible to forecast a supernova, if (as PM2 ring comments) we could measure the neutrinos from its core before it explodes. But this would be a prediction, not an observation of an explosion. And we can't get that data with the kind of neutrino detectors on Earth.

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    $\begingroup$ How much warning would the neutrinos actually give us (assuming we had the ability to detect them) before the supernova became visible? Minutes, hours, days? How much of a head start do they get? $\endgroup$ Sep 16 at 7:53
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    $\begingroup$ @RogerLipscombe See astronomy.stackexchange.com/a/18424/7411 $\endgroup$ Sep 16 at 8:16
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    $\begingroup$ @James K “we can't get that data with the kind of neutrino detectors on Earth.” Actually, we probably can, if it’s within a kpc or so. This paper estimates that the Japanese KamLAND detector could get advance warning of Betelgeuse going SN several hours (maybe several dozen hours) in advance, depending on things like the star’s exact mass and distance and how many nuclear reactors are running in Japan. $\endgroup$ Sep 16 at 8:28
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    $\begingroup$ Note that looking for increasing neutrino emissions as a forecast of an imminent supernova won't work for Type Ia supernovae, since they're not core-collapse objects. $\endgroup$ Sep 16 at 11:05
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    $\begingroup$ @PeterErwin We can get the "hours warning" of a supernova. What we can't get is the 3000 year warning, by noting that neutrino emmision are consistent with fusion of (perhaps) neon and magnesium. $\endgroup$
    – James K
    Sep 16 at 17:27
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There are very few stars visible to the unaided eye in the night sky of Earth that are 3,000 light years (LY) from Earth.

Wikipedia has a list of "brightest stars" which includes the Sun and 92 other stars which have the greatest apparent brightness as seen from Earth.

When the list is sorted by distance I find that only six are more than one thousand light years from Earth, and even the farthest star listed, Deneb, is "only" about 2,615 light years from Earth.

https://en.wikipedia.org/wiki/List_of_brightest_stars

They also have a "list of stars more luminous than than any closer star". Each star on the list is more luminous than any star which is closer to Earth than it.

WR 24 is listed as being 5,000 light years from Earth, and Eta Carinae is listed as being 7,500 light years from Earth. All the other stars on the list farther than 2,000 lightyears from Earth are not naked eye stars.

https://en.wikipedia.org/wiki/List_of_stars_more_luminous_than_any_closer_star#:~:text=This%20is%20a%20list%20of%20stars%20which%20are,luminous%20star%20within%205%20light-years%20of%20the%20Sun.

The list of most luminous stars known has only three over 3,000 lightyears from Earth which are visible to the naked eye from Earth, including Eta Carinae at 7,500 light years (LY), WR 24 8,200 LY, and WR 82A at 8,200 LY.

https://en.wikipedia.org/wiki/List_of_most_luminous_stars

It also has a secondary list of notable stars which are very luminous but less than the 1,000,000 times the luminoisty of the Sun required for the main list.

All of those stars are naked eye stars as seen from Earth and 14 of them are at least 3,000 light years from Earth.

The five that are closest to 3,000 LY from Earth are:

Omicron 2 Canis Majoris 2,800 LY, Upsilon Orionis 2,900 LY, Lambda Cephei 3,100 LY, Mu Normae 3,260 LY, and Sigma Cygni 3,260 LY.

Type II supernovas are caused by core collapse in massive stars. Some subtypes occur in stars in the mass range of 140 to 250 times the mass of the Sun. Other subtypes can occur in stars with lower mass ranges, down to 9 to 10 times the mass of the Sun.

So all or almost all stars with 10 times the mass of the Sun should become supernovas eventually.

Most main sequence (luminsity class V) spectral class B stars have less than 10 times the mass of the Sun, but spectral class B0V and B1V stars have over 10 times the mass of the Sun.

Main sequence (luminsity class V) spectral class O stars have masses between 15 and 90 times the mass of the Sun. They are very rare with an estimated 20,000 in the entire Mikly Way Galaxy.

Wolf-Rayet stars have masses between about 10 and 200 times the mass of the Sun.

Giant (luminosity class III) stars usually have masses in the range of 0.3 to 8 times the mass of the Sun and so would not become type II supernovae.

Supergiant (Luminosity class I) stars usually have masses over 10 tiemes the mass of the Sun and become type II supernovas.

Hypergiant (luminosity class 0) stars have masses of over 25 times the mass of the Sun and become type II supernovas.

The five stars mentioned above closet to bein 3,000 lightyears from Earth:

Omicron 2 Canis Majoris 2,800 LY, Upsilon Orionis 2,900 LY, Lambda Cephei 3,100 LY, Mu Normae 3,260 LY, and Sigma Cygni 3,260 LY.

Should all become type II supernovas someday.

Type Ia supernovas happen in binary or multiple star systems where at least one of the stars is a white dwarf star. If the two stars are close enough, the white dwarf can acquire matter from the other star which might eventually result in a supernova explosion.

So astronomers can classify which stars should become supernovas and which star systems have a chance of becoming supernovas.

Astronomers predict that Betelguese, for example, is about to become a supernova, sometime in the next million years or so.

So eventually, as better and better obsevations and mesurements are made, and as theories of stellar evolution become more accurate, astronomers should be able to make better and better predictions about when a specific supernova candidate star will become a supernova.

But current laws of physics show that it is impossible for any signal that a star has become a supernova to arrive on Earth more than minutes, hours, or days before the light of the supernova arrives.

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