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Fiery Phoenix
2009-Feb-11, 05:45 PM
If the red supergiant Betelgeuse were put at the same distance as Alpha Centauri from our Solar System, which is no more than 4.5 light-years, how would it look in terms of size and brightness? Would it look significantly different from how it does at its current approximate distance of 430-600 light-years?

I'm just curious.

NEOWatcher
2009-Feb-11, 05:55 PM
It sounds to me like a pretty simple calculation that I would struggle getting the variables for, but I'll try.
Basically; with Betelgeuse being magnitude -5.14 then being about 130 times closer would bring it up somewhere around -11. (5 magnitudes being about 100x per H-A (http://www.heavens-above.com/glossary.aspx?&term=magnitude))

So a lot brighter than an iridium flare, but dimmer than the full moon.
I'm sure the angular size wouldn't be more than a planet though.

or does that magnitude scale with the square?

Fiery Phoenix
2009-Feb-11, 06:14 PM
It sounds to me like a pretty simple calculation that I would struggle getting the variables for, but I'll try.
Basically; with Betelgeuse being magnitude -5.14 then being about 130 times closer would bring it up somewhere around -11. (5 magnitudes being about 100x per H-A (http://www.heavens-above.com/glossary.aspx?&term=magnitude))

So a lot brighter than an iridium flare, but dimmer than the full moon.
I'm sure the angular size wouldn't be more than a planet though.

or does that magnitude scale with the square?

Are you sure Betelgeuse has a magnitude of -5.14? :confused:

EDG
2009-Feb-11, 06:19 PM
Assuming a luminosity of 135,000 Sols for Betelgeuse and a distance of 640 ly, I get its Absolute Magnitude (10 pc) to be -8.00, and the apparent magnitude at 4.3 lightyears to be -12.40. So it'd be about as bright as the full moon at that distance.

(absolute magnitude is M = 4.83 - (2.5*log L), right? I have reason to be a little suspicious of that though, so I'm not sure)

NEOWatcher
2009-Feb-11, 06:20 PM
Are you sure Betelgeuse has a magnitude of -5.14? :confused:

Oops, sorry. I pulled that off of wiki without much context or thinking about it. I thought that sounded rather bright.
Apparent magnitude is 0.58. So; at 4.5LY we are talking about -5 (again, if I'm not wrong in not considering the square)

astromark
2009-Feb-11, 06:33 PM
So its as bright as a full moon and may be resolved into a disk. visually as big as Venus. How lovely that might look.... there's a ...but.
But that is not the problem... The possibility of a catastrophic Nova event from Betelgeuse. We would be toast if that were to happen. Lets be thankful that at 640 L/Y we are safe.

Fiery Phoenix
2009-Feb-11, 06:33 PM
Oops, sorry. I pulled that off of wiki without much context or thinking about it. I thought that sounded rather bright.
Apparent magnitude is 0.58. So; at 4.5LY we are talking about -5 (again, if I'm not wrong in not considering the square)

EDG says it would have a magnitude of -12.40, and you say -5. Sorry, but I'm starting to get confused at this point.

Edit: I'm only asking about the apparent magnitude here, which is how the star would look if viewed from Earth, not from space.

NEOWatcher
2009-Feb-11, 06:35 PM
EDG says it would have a magnitude of -12.40, and you say -5. Sorry, but I'm starting to get confused at this point.
I'm with you there; but, I thought I was clear in conveying that uncertainty.
I was hoping to get the ball rolling...

Fiery Phoenix
2009-Feb-11, 06:39 PM
I'm with you there; but, I thought I was clear in conveying that uncertainty.
I was hoping to get the ball rolling...

It's OK, thanks anyway. I'm sure someone like Jeff or Grant will fix me up. :)

redshifter
2009-Feb-11, 06:41 PM
So its as bright as a full moon and may be resolved into a disk. visually as big as Venus. How lovely that might look.... there's a ...but.
But that is not the problem... The possibility of a catastrophic Nova event from Betelgeuse. We would be toast if that were to happen. Lets be thankful that at 640 L/Y we are safe.

Is Betelgeuse the type of star that will go nova or supernova, or have a catastrophic nova event? Even so, at Alpha Centauri's distance that might be bad for us, but I'd think a star like Rigel or Deneb would be a lot worse at 4 ly's once they go nova.

Fiery Phoenix
2009-Feb-11, 06:45 PM
Is Betelgeuse the type of star that will go nova or supernova, or have a catastrophic nova event? Even so, at Alpha Centauri's distance that might be bad for us, but I'd think a star like Rigel or Deneb would be a lot worse at 4 ly's once they go nova.

It will go supernova, and within a 1000-year period I believe, although it might have already gone supernova but we just don't know yet.

Nick Theodorakis
2009-Feb-11, 06:47 PM
Is Betelgeuse the type of star that will go nova or supernova, or have a catastrophic nova event? ...

Yes (http://www.astro.uiuc.edu/~kaler/sow/betelgeuse.html).

Nick

tony873004
2009-Feb-11, 06:51 PM
I get a visual size of 7 arcseconds. tan-1(1500000000000/(4.5*9.4605284E+15)). If I remember correctly, Venus pushes about 50 arcseconds at its largest. But it would be about 5 times the angular size of Ganymede. I can't resolve shape in Ganymede in any telescope I've used, but I've seen pictures taken by others who have.

tracer
2009-Feb-11, 06:54 PM
It will go supernova, and within a 1000-year period I believe, although it might have already gone supernova but we just don't know yet.

Good thing we're not as close to it as we are to Alpha Centauri, then! :eek:

tony873004
2009-Feb-11, 06:55 PM
I get mag -9.4 using an absolute magnitude of -5.14 and this formula: http://orbitsimulator.com/formulas/vmag9.html

NEOWatcher
2009-Feb-11, 07:16 PM
I get mag -9.4 using an absolute magnitude of -5.14 and this formula: http://orbitsimulator.com/formulas/vmag9.html
Is the visual magnitude the same as apparent magnitude?

Plugging in the numbers for betelgeuse on wiki, I get a different apparent magnitude where it is. Wiki=0.58(+/-), formula=1.36.

So; I'm starting to wonder if my WAG of -10 was close enough.

StupendousMan
2009-Feb-11, 07:22 PM
... because

a) Betelgeuse is variable in brightness, by a few tens of
percent

b) the "magnitude" of Betelgeuse varies wildly depending
on the passband through which one measures
its light, because Betelgeuse is a VERY red star

Don't bother arguing once you're within a factor of two of
another value.

EDG
2009-Feb-11, 07:25 PM
Don't bother arguing once you're within a factor of two of another value.

Do you know if the equation I gave was the correct one though? I hammered that out with Grant and others on the Celestia forum ages ago, so I *hope* it's correct... (and I put it on this PDF on my website: http://evildrganymede.net/rpg/world/magnitudes.pdf ).

EDG
2009-Feb-11, 07:27 PM
It will go supernova, and within a 1000-year period I believe, although it might have already gone supernova but we just don't know yet.

It'll go supernova at some point, but nobody's pinned down when (it's unlikely within 1000 years. Within a million years though, much more likely).

At 640ly, I think it's close enough to cause problems on Earth when it does. If it was at Alpha Centauri's distance, I'm pretty sure that at the very least the radiation would destroy the ozone layer (and it'd be ridiculously bright).

Fiery Phoenix
2009-Feb-11, 07:45 PM
It'll go supernova at some point, but nobody's pinned down when (it's unlikely within 1000 years. Within a million years though, much more likely).

At 640ly, I think it's close enough to cause problems on Earth when it does. If it was at Alpha Centauri's distance, I'm pretty sure that at the very least the radiation would destroy the ozone layer (and it'd be ridiculously bright).

Yes, but just imagine the sight when it goes supernova (at its original distance, that is). I hope I witness that event before I die! LOL!

By the way, at 640 light-years, it won't cause any problems.

EDG
2009-Feb-11, 08:00 PM
By the way, at 640 light-years, it won't cause any problems.

On what do you base that assertion? We had a recent Gamma Ray Burst at 50,000 ly that affected the Earth's upper atmosphere. A Supernova much closer than that should have an effect.

Fiery Phoenix
2009-Feb-11, 08:09 PM
On what do you base that assertion? We had a recent Gamma Ray Burst at 50,000 ly that affected the Earth's upper atmosphere. A Supernova much closer than that should have an effect.

That's what Alan Longstaff said in answer to a related question in the Ask Alan section of Astronomy Now.

NEOWatcher
2009-Feb-11, 08:11 PM
On what do you base that assertion? We had a recent Gamma Ray Burst at 50,000 ly that affected the Earth's upper atmosphere. A Supernova much closer than that should have an effect.
Is it fair to compare the two? I was under the impression that a GRB is much more dangerous due to the fact that the power is directed. I would think an (undirected) Supernova would dissapate at a much greater rate.

eburacum45
2009-Feb-11, 08:19 PM
I've just created a fake Betelgeuse at Alpha Centauri's distance using Celestia; the star has an apparent magnitude of -9.51 according to that program.

I do note, however, that Celestia uses a distance of 427 light years for the 'original' Betelgeuse; this is much closer than most estimates I've read, and is presumably unreliable because Hipparchos wasn't too good at measuring distances to certain stars.

Fiery Phoenix
2009-Feb-11, 08:25 PM
I've just created a fake Betelgeuse at Alpha Centauri's distance using Celestia; the star has an apparent magnitude of -9.51 according to that program.

I do note, however, that Celestia uses a distance of 427 light years for the 'original' Betelgeuse; this is much closer than most estimates I've read, and is presumably unreliable because Hipparchos wasn't too good at measuring distances to certain stars.

Yes, that was probably a decade or so ago. Now the star is at some 640 light-years away from us.

tony873004
2009-Feb-11, 08:29 PM
Are you sure Betelgeuse has a magnitude of -5.14? :confused:

This is what wiki lists as its "absolute" magnitude (i.e. what would its visual magnitude be if it were 10 parsecs away). From Earth, its visual magnitude (or appearant magnitude) is 0.58 (0.3 to 1.2) according to Wiki. So if you plug -5.14 into M (absolute magnitude) and 600 LY into d (distance) you get its visual (appearant) magnitude from Earth of 1.8, which is at the high end of the range wiki gives. Change 600 LY into 4.5 LY and you get -9.4.

Fiery Phoenix
2009-Feb-11, 08:36 PM
This is what wiki lists as its "absolute" magnitude (i.e. what would its visual magnitude be if it were 10 parsecs away). From Earth, its visual magnitude (or appearant magnitude) is 0.58 (0.3 to 1.2) according to Wiki. So if you plug -5.14 into M (absolute magnitude) and 600 LY into d (distance) you get its visual (appearant) magnitude from Earth of 1.8, which is at the high end of the range wiki gives. Change 600 LY into 4.5 LY and you get -9.4.

That's real close to what others have suggested. I guess that's it, then. But what about the size? In other words, how "big" would it look at such a relatively close distance from us?

Thank you.

NEOWatcher
2009-Feb-11, 08:42 PM
That's real close to what others have suggested. I guess that's it, then. But what about the size?
I think tony's answer in post 13 has a good answer.


In other words, how "big" would it look at such a relatively close distance from us?
It would still be a dot, but it would be so bright, that resolving it without magnification or filtering would be futile.

Have you ever seen an Iridium flare at above a -8 magnitude? It's pretty impressive, but it's also basically an indistinguished flash of light. I would imagine it would be a very similar effect.

Fiery Phoenix
2009-Feb-11, 08:51 PM
I think tony's answer in post 13 has a good answer.

Oops! I never saw that post! No wonder this thread got so many posts in a very short period of time. (not that that's a bad thing or anything, just saying :lol:)


Have you ever seen an Iridium flare at above a -8 magnitude? It's pretty impressive, but it's also basically an indistinguished flash of light. I would imagine it would be a very similar effect.

No, never. But I think I know what you're talking about.

Thanks a lot everyone. You've been very helpful. :)

George
2009-Feb-11, 09:29 PM
So if you plug -5.14 into M (absolute magnitude)... You may have a typo there as I get -5.74 (-5.9 if from 640 lyr.).
Oh, I see. You are using Wiki's VM value of -5.14. This would be true only by using the 1.2 apparent mag. value at 600 lyrs.

George
2009-Feb-11, 09:35 PM
Do you know if the equation I gave was the correct one though? I hammered that out with Grant and others on the Celestia forum ages ago, so I *hope* it's correct... (and I put it on this PDF on my website: http://evildrganymede.net/rpg/world/magnitudes.pdf ).
It sure looks nice. Do you need it reviewed?

It would also be nice to see the combination magnitude formula included because it is a little harder to find and often lacks your more explanatory style.

EDG
2009-Feb-11, 09:45 PM
It sure looks nice. Do you need it reviewed?

It would also be nice to see the combination magnitude formula included because it is a little harder to find and often lacks your more explanatory style.

Thanks! It should be correct because (as I said) it's what we hammered out on the Celestia forums with grant hutchison and a bunch of other knowledgeable people. But now I'm not so certain, so if anyone can take a look and verify it or point out any errors then I'd appreciate it!

And which "combination magnitude formula" are you referring to? I didn't discuss the UBV stuff there because that was too complicated for what I was talking about (I just wanted to provide the basic equations).

dodecahedron
2009-Feb-11, 10:57 PM
Yes (http://www.astro.uiuc.edu/~kaler/sow/betelgeuse.html).

Nick

I'm not the sharpest knife in the deck but I just want to see if I'm correct here.

If a star gets to the point where it fuses iron and elements beyond that then it's a candidate for a supernova?

Also what is the axis of rotation for Betelgeuse? Are its poles directed towards us or will we just get a sidelong blast?

EDG
2009-Feb-11, 11:09 PM
I'm not the sharpest knife in the deck but I just want to see if I'm correct here.

If a star gets to the point where it fuses iron and elements beyond that then it's a candidate for a supernova?

I think technically it's the point beyond which it's fusing elements heavier than carbon that makes it a potential candidate for a supernova. If it fuses heavy enough elements and gets to the point where it has to fuse iron, that's when it's doomed because (IIRC) fusing iron requires energy instead of producing it. At that point, the star collapses and then explodes as a supernova (heavier elements than iron are produced during the explosion, but through a different process - in this case, everything gets pummelled by loads of fast neutrons which make the nuclei heavier and transforms them to other elements).

grant hutchison
2009-Feb-11, 11:55 PM
(absolute magnitude is M = 4.83 - (2.5*log L), right? I have reason to be a little suspicious of that though, so I'm not sure)
It's pretty straightforward. 2.5 log10(LΘ), with LΘ the luminosity in solar units, is just the absolute magnitude difference between your chosen star and the Sun. Subtract that from the absolute magnitude of the Sun, and you have the absolute magnitude of your chosen star.

But we can skate right past that step by looking up a reasonably authoritative absolute visual magnitude for Betelgeuse: it's given as -5.1 by Kaler in Extreme Stars. So that's the brightness at 10pc. Moving it in from 10pc to 1.3pc will further brighten it by a factor of 59, or 4.4 magnitudes. So that's an apparent magnitude of -9.5, if we accept Kaler's estimate of MV.
(Tony's there ahead of me, I see. :))

Grant Hutchison

grant hutchison
2009-Feb-12, 12:24 AM
2.5 log10(LΘ), with LΘ the luminosity in solar units, is just the absolute magnitude difference between your chosen star and the Sun.Just in case anyone's fretting about where the factor of 2.5 comes from ...

It's all Norman Pogson's fault, for making the magnitude scale operate in logarithmic units to the base 1001/5, or ~2.511886... He did that so five magnitude units represent a 100-fold difference in luminosity.
Call Pogson's number NP. By definition we know that logNP(100) = 5, where logNP indicates the logarithm to the base NP.

So what we want to calculate above is:

logNP(LΘ)

But since most calculators don't do bespoke logs, we need to come at it like this:

logNP(LΘ) = logNP(10) log10(LΘ) = 2.5 log10(LΘ)

Presto!


As a general relationship, we have:

m2 − m1 = −2.5 log10(L2/L1)

which gives:

m2 = m1 − 2.5 log10(L2/L1)

which is the origin of EDG_'s original equation.

Grant Hutchison

EDG
2009-Feb-12, 12:34 AM
OK... so I wonder what's going on then. I used the newly updated luminosity of 135,000 Sols, and the updated distance of 640 ly. (see http://www.astro.uiuc.edu/~kaler/sow/betelgeuse.html )... and using that equation that you verified, I got an absolute magnitude of -8.00, not -5.1.

If I determine the apparent magnitude at 640ly based on that luminosity I get... -1.5? I'm using this: -8.00 + 5.log10(196.3/10)

So is the luminosity value too high?

grant hutchison
2009-Feb-12, 12:43 AM
OK... so I wonder what's going on then. I used the newly updated luminosity of 135,000 Sols, and the updated distance of 640 ly. (see http://www.astro.uiuc.edu/~kaler/sow/betelgeuse.html (http://www.astro.uiuc.edu/%7Ekaler/sow/betelgeuse.html) )... and using that equation that you verified, I got an absolute magnitude of -8.00, not -5.1.

If I determine the apparent magnitude at 640ly based on that luminosity I get... -1.5? I'm using this: -8.00 + 5.log10(196.3/10)

So is the luminosity value too high?No, it's just bolometric, and you should be using the visual luminosity.
(Betelgeuse has a large bolometric correction, not all of accounted for by simple black-body calculations.)

Visual luminosity was estimated at 9400 for 425ly, so it'll be about 21000 at 640ly. That pushes the absolute visual magnitude up to -6.0, and the brightness at the distance of Alf Cen to -10.4.

Grant Hutchison

Nick Theodorakis
2009-Feb-12, 02:14 AM
I'm not the sharpest knife in the deck but I just want to see if I'm correct here.

If a star gets to the point where it fuses iron and elements beyond that then it's a candidate for a supernova?

Also what is the axis of rotation for Betelgeuse? Are its poles directed towards us or will we just get a sidelong blast?

I don't know which way Betel is pointed.

As to core-collapse type supernovae, there is a summary in the wiki article on Type II Supernovae (http://en.wikipedia.org/wiki/Type_II_supernova).

Nick