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View Full Version : Betelgeuse, Betelgeuse, Be.. When the star goes supernova, should we worry?


GIGObuster
02-11-2008, 11:36 AM
As if Google Earth was not already a great waste of time. :)

Google Earth now has the Space view (available in the latest download) just press the space icon and then see the night sky in unprecedented detail, I decided to take a stroll around Orion an did a close up on Betelgeuse.

A few years ago, I asked in the Straight Dope if there was something to worry if this star did go supernova. Even The Bad Astronomer mentioned that it was going to be a spectacular sight but because the star was about 600 light years away it was determined that Earth would barely be affected by the explosion. Then on Google Space I notice the link to an astronomy site andů did something change? Now even Wikipedia is reporting that the latest SIMBAD satellite query showed that Betelgeuse is actually 427 light-years away from Earth.

Mmm.

Should we worry now? :dubious:

(Not that it will be likely go supernova now, but should our descendants worry?)

Squink
02-11-2008, 12:00 PM
About the only way I can see the impending supernova causing us trouble is if one of Betelgeuse's poles is lined up on us, and we receive the brunt of a gamma ray burst (http://en.wikipedia.org/wiki/Gamma_ray_burst).
Still, I keep an extra jar of bug juice (http://www.turtlewax.com/main.taf?p=2,5,3,1) in the garage, just in case.

Anne Neville
02-11-2008, 12:29 PM
About the only way I can see the impending supernova causing us trouble is if one of Betelgeuse's poles is lined up on us, and we receive the brunt of a gamma ray burst (http://en.wikipedia.org/wiki/Gamma_ray_burst).

If what we think is one of Betelgeuse's poles in fact is, it looks like we might be safe from that. (http://en.wikipedia.org/wiki/Betelgeuse#Observation)

A few years ago, I asked in the Straight Dope if there was something to worry if this star did go supernova. Even The Bad Astronomer mentioned that it was going to be a spectacular sight but because the star was about 600 light years away it was determined that Earth would barely be affected by the explosion. Then on Google Space I notice the link to an astronomy site andů did something change? Now even Wikipedia is reporting that the latest SIMBAD satellite query showed that Betelgeuse is actually 427 light-years away from Earth.

Determining distances to stars is a tricky business. But we think we're safe if a supernova of the type that Betelgeuse could become happens more than 100 light-years away (unless we're in the path of a gamma ray burst from it), so either way, we think we're still OK.

tdn
02-11-2008, 12:55 PM
Here's something that I've been wondering about for a while.

Let's suppose that a nearby star went supernova and we were caught in the blast radius. Would we even know about it? Would the light from the event reach us before the blast material does?

RickJay
02-11-2008, 01:59 PM
1. No. 427 LY is still far enough.

2. When you see distances to stars printed - no matter where - bear in mind they're not quite wild-ass guesses, but they're pretty close. Except for the very nearby stars, those distances are extremely rough estimates.

3. If I recall correctly, there is, in fact, NO star close enough to Earth that can possibly go supernova that could harm life here.

Q.E.D.
02-11-2008, 02:00 PM
Let's suppose that a nearby star went supernova and we were caught in the blast radius. Would we even know about it? Would the light from the event reach us before the blast material does?
Well, yes. Light being what it is, travels at the speed of, well, light. Other stuff, not so fast.

Anne Neville
02-11-2008, 02:10 PM
Light being what it is, travels at the speed of, well, light. Other stuff, not so fast.

Supernova remnant stuff travels at about 1% of the speed of light.

Mangetout
02-11-2008, 02:11 PM
I certainly don't think we should worry. If it's a significant and serious risk, then maybe you should try to finish that book you've only been dipping into, but worrying isn't going to stop it happening, or alter its severity if it does happen.

Squink
02-11-2008, 02:18 PM
Other stuff, not so fast.Neutrinos are no slouches: (http://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions) Approximately three hours before the visible light from SN 1987A reached the Earth, a burst of neutrinos was observed at three separate neutrino observatories. This is due to the neutrino emission (which occurs simultaneously with core collapse) preceding the emission of visible light (which occurs only after the shock wave reaches the stellar surface).
...
the distance to SN1987A, which is about 168,000 light-years.

tdn
02-11-2008, 02:18 PM
Supernova remnant stuff travels at about 1% of the speed of light.
Really, that slow?

I guess when I think about it the matter couldn't possibly travel at the speed of light. It would have to have basically no mass in order to do so.

Anne Neville
02-11-2008, 02:36 PM
I guess when I think about it the matter couldn't possibly travel at the speed of light. It would have to have basically no mass in order to do so.

It would have to have exactly no mass to travel at the speed of light. (There's a short story by Isaac Asimov where this is a plot point)

It would have to have imaginary mass (ie, involving the square root of -1) to travel faster than light.

Neutrinos are no slouches

Neutrinos have mass (at least that's the current scientific opinion), so they travel slower than light. They managed to reach us from Supernova 1987A before the light did because the process in the supernova that emits them happens before the process that emits the light. They had a head start, in other words. This isn't the case for the supernova remnant stuff, at least not AFAIK (IANAsupernova expert).

Sean Factotum
02-11-2008, 02:54 PM
3. If I recall correctly, there is, in fact, NO star close enough to Earth that can possibly go supernova that could harm life here.
Not even that big yellow one that shines in my eyes every morning as I drive to work? ;)

GIGObuster
02-11-2008, 02:58 PM
I certainly don't think we should worry. If it's a significant and serious risk, then maybe you should try to finish that book you've only been dipping into,

How did you know? Oh wait, you mean reading it, not writing it. ;)

but worrying isn't going to stop it happening, or alter its severity if it does happen.
IIRC on the last thread there was talk that if the distance was less than 500 light years away then we have to worry not only about gamma rays but also x-rays. However, at that distance the damage would be limited to satellites and some roasted astronauts. So it is not that I'm worrying much for humanity, but the problems some could encounter when it happens.

As I noticed that Betelgeuse is closer than 500 light years, then I wanted to investigate how much more then would Betelgeuse going supernova could affect us.

Thanks to Anne Neville for the explanations.

Voyager
02-11-2008, 03:15 PM
Not even that big yellow one that shines in my eyes every morning as I drive to work? ;)
Nope. The Sun's too small. It will kill us in other ways, just as effectively.

I'm too lazy to compute what percent of Betelgeuse's mass would reach us as a part of the surface of a sphere with a radius of 427 LY. Would we even be able to detect it?

Anne Neville
02-11-2008, 03:23 PM
Not even that big yellow one that shines in my eyes every morning as I drive to work? ;)

It can't go supernova. To be able to go supernova, a star has to either be at least eight or nine times as massive as the Sun, or be a white dwarf and have a mass of at least 1.4 solar masses.

White dwarfs generally get a mass of more than 1.4 solar masses from a companion star that has expanded to a red giant. The outer layers of the red giant aren't held very tightly, and if the white dwarf is close enough, it might be able to pull those layers onto itself. When the white dwarf gets a mass greater than about 1.4 solar masses (depending somewhat on its rotation), it collapses and creates a Type Ia supernova.

There actually is a star within 10 light years that may eventually become a Type Ia supernova- Sirius B, the white dwarf companion of Sirius. But Sirius B isn't gaining mass now, as far as we can tell- it won't until Sirius A becomes a red giant. (A stellar collision could probably do it, but that's incredibly unlikely) We think that, by the time Sirius B could go supernova, it will have moved far enough away from us so as not to be a problem.

I'm too lazy to compute what percent of Betelgeuse's mass would reach us as a part of the surface of a sphere with a radius of 427 LY. Would we even be able to detect it?

Me too, but my guess is that we wouldn't.

tdn
02-11-2008, 03:48 PM
(IANAsupernova expert).
Really?

Squink
02-11-2008, 03:53 PM
Neutrinos have mass (at least that's the current scientific opinion), so they travel slower than light. They managed to reach us from Supernova 1987A before the light did because the process in the supernova that emits them happens before the process that emits the light. They had a head start, in other words.Sure, a few hours, or maybe a day. But to precede the photon front after a run of 168,000 light years, they'd still have to be ripping right along.

Polycarp
02-11-2008, 03:53 PM
So supernovas in the immediate stellar neighborhood are no concern right now, but given the potential at Alpha Canis Majoris, in astronomical time scales, it could be a Sirius problem?

Astroboy14
02-11-2008, 04:36 PM
So supernovas in the immediate stellar neighborhood are no concern right now, but given the potential at Alpha Canis Majoris, in astronomical time scales, it could be a Sirius problem?

When Cthulhu comes, you will be among the last consumed.

IAmNotSpartacus
02-11-2008, 07:39 PM
Why are we so worried about the gamma ray burst? What will happen if we take one head on?

Q.E.D.
02-11-2008, 07:41 PM
Why are we so worried about the gamma ray burst? What will happen if we take one head on?
Poof.

Mangetout
02-11-2008, 08:00 PM
Why are we so worried about the gamma ray burst? What will happen if we take one head on?
It will apply directly to our foreheads.

Squink
02-11-2008, 08:22 PM
Poof.Just to increase the tension, here's the Gamma-ray Burst Real-time Sky Map. (http://grb.sonoma.edu/)
There've been four sighted in the last week alone.

IAmNotSpartacus
02-11-2008, 09:32 PM
Poof.

I'm sorry, fight my ignorance for me.

Does the earth get vaporized or something? Can a supernova from across the universe cause this, or only within a certain proximate range?

Wikipedia is not helping much here :smack:

Squink
02-11-2008, 09:49 PM
Can a supernova from across the universe cause this, or only within a certain proximate range?No one is sure how far away you need to be for safety, but statements like this are common: (http://news.bbc.co.uk/2/hi/science/nature/4433963.stm) "A gamma ray burst originating within 6,000 light-years from Earth would have a devastating effect on life," said co-author Dr Adrian Melott, an astronomer at the University of Kansas, US.Some astronomers claim that even 90,000 light years, across the galaxy, might be too close for comfort. I suppose it depends on the size of the burst.
No one worries much about gamma ray bursts that originate in other galaxies.
Of course, there's not much point in worrying about a nearby burst either.

Q.E.D.
02-11-2008, 09:49 PM
Does the earth get vaporized or something? Can a supernova from across the universe cause this, or only within a certain proximate range?
I wouldn't worry too much. It has to happen fairly close to us (in cosmic terms, at least)--say, several hundred to a few thousand light years, and one of the axes of rotation of the star must be pointed more or less directly at us. Simplistically stated, the intense magnetic field that is created during a supernova essentially funnels electrically charged material towards the poles, resulting in twin jets of gamma rays radiating along the axis of rotation. Clearly, however, there is considerable beam spread since we observe many of these GRBs each year with no ill effects; we would not be able to observe them unless they were pointed in our general direction. But, if one were close enough, and was aimed our way, yeah, we'd be toast. The atmosphere would be blown away, the oceans would boil off and everything on the surface of the Earth would be instantaneously charred. There's just that much energy in a GRB.

Jragon
02-11-2008, 09:56 PM
I wouldn't worry too much. It has to happen fairly close to us (in cosmic terms, at least)--say, several hundred to a few thousand light years, and one of the axes of rotation of the star must be pointed more or less directly at us. Simplistically stated, the intense magnetic field that is created during a supernova essentially funnels electrically charged material towards the poles, resulting in twin jets of gamma rays radiating along the axis of rotation. Clearly, however, there is considerable beam spread since we observe many of these GRBs each year with no ill effects; we would not be able to observe them unless they were pointed in our general direction. But, if one were close enough, and was aimed our way, yeah, we'd be toast. The atmosphere would be blown away, the oceans would boil off and everything on the surface of the Earth would be instantaneously charred. There's just that much energy in a GRB.

So THAT'S what happened to the martians...

IAmNotSpartacus
02-11-2008, 10:47 PM
I wouldn't worry too much. It has to happen fairly close to us (in cosmic terms, at least)--say, several hundred to a few thousand light years, and one of the axes of rotation of the star must be pointed more or less directly at us. Simplistically stated, the intense magnetic field that is created during a supernova essentially funnels electrically charged material towards the poles, resulting in twin jets of gamma rays radiating along the axis of rotation. Clearly, however, there is considerable beam spread since we observe many of these GRBs each year with no ill effects; we would not be able to observe them unless they were pointed in our general direction. But, if one were close enough, and was aimed our way, yeah, we'd be toast. The atmosphere would be blown away, the oceans would boil off and everything on the surface of the Earth would be instantaneously charred. There's just that much energy in a GRB.
:eek:

Thank you!

MikeS
02-11-2008, 10:52 PM
I'm too lazy to compute what percent of Betelgeuse's mass would reach us as a part of the surface of a sphere with a radius of 427 LY. Would we even be able to detect it?I get a figure of about 17.4 million kilograms, assuming that the entire mass of Betelguese was thrown off in a uniform spherical shell. This actually isn't too far off of some estimates I've seen of the yearly meteorite mass gained by the Earth each year, although of course it's still a tiny fraction of the entire mass of the Earth.

Pasta
02-12-2008, 12:17 AM
Sure, a few hours, or maybe a day. But to precede the photon front after a run of 168,000 light years, they'd still have to be ripping right along.They'd be moving at something like (1-10-16)c, or for the fun of it: 0.9999999999999999c. Indeed, the light hasn't got a snowball's chance of catching up for any supernova we could see.

Bootis
02-12-2008, 12:49 AM
one of the axes of rotation of the star must be pointed more or less directly at us.

What are the odds of this happening? I guess the closer to Earth, the better the chances, but, say for a star 500 light years away, for all the possible allignments of it's pole, what % of them would put the burst on a course to hit Earth?

(I'm not sure how hard of a problem this is to figure out, I'm sure it requires knowing arc's and radians and whatnot. And you'd need to know how much the pulse dissipates outward from the line over time, and how close to the actual plumb line of the pulse the Earth needs to be to be effected from that distance)

Pushkin
02-12-2008, 03:07 AM
I certainly don't think we should worry. If it's a significant and serious risk, then maybe you should try to finish that book you've only been dipping into, but worrying isn't going to stop it happening, or alter its severity if it does happen.

Did you ever think of contributing horoscopes to The Onion ;)

Anne Neville
02-12-2008, 07:25 AM
(IANAsupernova expert).
Really?

Nope, when I was in astronomy, I was a planets person. Some of my information might be a bit out of date, too- I haven't been keeping up with the latest in the field since I left in 2000.

So THAT'S what happened to the martians...

There is a theory that that's what happened to cause the mass extinction at the end of the Ordovician period, one of the worst mass extinctions in Earth's history (http://en.wikipedia.org/wiki/Ordovician-Silurian_extinction_events).

One gamma-ray burst can ruin your whole day.

What are the odds of this happening? I guess the closer to Earth, the better the chances, but, say for a star 500 light years away, for all the possible allignments of it's pole, what % of them would put the burst on a course to hit Earth?

These astronomers seem to think it's not very likely that a gamma ray burst will get us. (http://www.space.com/scienceastronomy/060424_mm_star_deathray.html) Seems that our galaxy isn't optimal for producing gamma ray bursts. It's not impossible, but not terribly likely, either.

What will happen if we take one head on?

If any of you are fans of the History Channel show Mega Disasters (http://www.history.com/minisites/megadisasters/), they had a show about what might happen if a gamma ray burst (one not close enough to wipe out all life on Earth) hit us.

Does the earth get vaporized or something?...

Wikipedia is not helping much here

For stuff like this, you need to go to the How To Destroy The Earth page (http://qntm.org/?destroy#8).

According to Steve Thorsett, quoted on that site (and from whom I took a class in grad school at UC Santa Cruz, but not one about gamma ray bursts), no, it won't vaporize the Earth. Wipe out all life on Earth, maybe, but not vaporize it.

Anne Neville
02-12-2008, 10:09 AM
Betelgeuse may not be big enough to create a gamma ray burst when it does go supernova. According to current thinking, you need a supernova that is big enough to produce a black hole. Not all supernovae do this- some leave neutron stars as a remnant. We're not sure just how big a star has to be to produce a black hole, but we think it has to be at least 20 times the mass of the sun. We think Betelgeuse is only 14 times the mass of the sun (though determining the mass of a star that isn't a binary is, like determining distances to stars, a tricky business).

I've also seen estimates that, according to our currently accepted model of gamma ray bursts from supernovae, a star has to be extremely massive- 40 times the mass of the sun or more- to produce a gamma ray burst. There are stars that massive out there, but they're extremely rare. Stars as large as our Sun are relatively rare, too, for that matter- something like 80% of the stars in our galaxy are red dwarfs, which are smaller than the Sun. (http://www.spacedaily.com/reports/Study_Shows_Most_Milky_Way_Stars_Are_Single_Red_Dwarfs.html)

Cerowyn
02-12-2008, 01:43 PM
IIRC, the binary system IK Pegasi is the nearest supernova risk at ~150 LY. It's a type 1A candidate, which means that almost all of its mass (~2.8 solar masses) would be ejected by the explosion.

RiverRunner
02-12-2008, 02:20 PM
It will apply directly to our foreheads.

I'll get you for this. I don't know how or when, but I'll get you.


RR

Yllaria
02-12-2008, 04:38 PM
It will apply directly to our foreheads.Is this a Hulk reference? I don't remember the name of the character that got smarter instead of stronger, but reading your post makes me think of him.

Was it refering to something else?

Jragon
02-12-2008, 05:17 PM
Is this a Hulk reference? I don't remember the name of the character that got smarter instead of stronger, but reading your post makes me think of him.

Was it refering to something else?

There's a product called head on for headaches. It's a stick of... something (reminds me of oversized chapstick)...

The commercial says "head on, apply directly to forehead. Head on, apply directly where it hurts" etc etc. He was making a joke since the quote said "head on" in it.

Anne Neville
02-13-2008, 09:58 AM
IIRC, the binary system IK Pegasi is the nearest supernova risk at ~150 LY. It's a type 1A candidate, which means that almost all of its mass (~2.8 solar masses) would be ejected by the explosion.

But AIUI, it won't be a type Ia supernova risk until IK Pegasi A evolves off the Main Sequence. It hasn't done that yet.

Even when it does, the red giant will have to be close enough to the white dwarf that the white dwarf can suck material out of the red giant's atmosphere. Sirius A and B may be too far apart for that to happen (http://www.astronomycafe.net/qadir/ask/a11207.html) (I had read an article, years ago, saying that Sirius B would probably eventually produce a type Ia supernova, but now that I think about it, it makes sense that it wouldn't- Sirius B is in an orbit with a 50-year period, which clearly isn't that close to Sirius A). The white dwarf in the IK Pegasi system is much closer to the other star- its orbital period is 21.7 days. So it probably is a more plausible type Ia supernova candidate than Sirius B.

AtomicDog
02-13-2008, 10:27 AM
Is this a Hulk reference? I don't remember the name of the character that got smarter instead of stronger, but reading your post makes me think of him.

Was it refering to something else?

The Leader (http://en.wikipedia.org/wiki/Leader_%28comics%29) buys it by the case.


http://www.leaderslair.com/

squeegee
02-13-2008, 10:42 AM
Just to add to the supernova database in the thread: there's also Eta Carinae (http://en.wikipedia.org/wiki/Eta_carinae), a hypergiant star of 100+ solar masses about 7500 LY away. Eta Car actually had a failed supernova event in 1730. If it finally blows, which could be any time from 1 minute from now to a zillion years, it will likely be visible in the daytime on Earth. If one of its poles were pointed toward us, pretty bad stuff would likely happen. Luckily, the poles are not pointed our way, so all we expect is a nice visible bang-bang.

Anne Neville
02-13-2008, 11:21 AM
Just to add to the supernova database in the thread: there's also Eta Carinae (http://en.wikipedia.org/wiki/Eta_carinae), a hypergiant star of 100+ solar masses about 7500 LY away... If it finally blows, which could be any time from 1 minute from now to a zillion years, it will likely be visible in the daytime on Earth. If one of its poles were pointed toward us, pretty bad stuff would likely happen. Luckily, the poles are not pointed our way, so all we expect is a nice visible bang-bang.

But we're more excited about Betelgeuse, because Eta Carinae is in the far southern sky. A supernova there wouldn't be visible to most observers in the northern hemisphere, no matter how bright it is. Betelgeuse is near the celestial equator, so if and when it goes supernova, it will be visible throughout most of the world.

Sunspace
02-13-2008, 12:01 PM
But, if one were close enough, and was aimed our way, yeah, we'd be toast. The atmosphere would be blown away, the oceans would boil off and everything on the surface of the Earth would be instantaneously charred. There's just that much energy in a GRB.:eek:

I wonder what colour it would be....

Anne Neville
02-13-2008, 12:14 PM
I wonder what colour it would be....

I asked Mr. Neville this very same question when he got his name put in as one of the authors on a paper about gamma-ray bursts (although he doesn't work on GRBs). He didn't know. I would still like very much to find out (though preferably not by seeing an actual dangerous gamma-ray burst go off).

Polycarp
02-13-2008, 03:29 PM
:eek:

I wonder what colour it would be....

Here's your answer. (http://www.dagonbytes.com/thelibrary/lovecraft/thecolouroutofspace.htm) ;)

squeegee
02-14-2008, 09:46 AM
But we're more excited about Betelgeuse, because Eta Carinae is in the far southern sky. A supernova there wouldn't be visible to most observers in the northern hemisphere, no matter how bright it is. Betelgeuse is near the celestial equator, so if and when it goes supernova, it will be visible throughout most of the world.
Where's your motivation? I would certainly take a trip to SA to see a hypernova!

Chronos
02-14-2008, 03:44 PM
A GRB would have to be very close indeed to blow the atmosphere away, and short of that, it's not going to wipe out life on Earth (or probably even human life). Although GRBs are very energetic, they're also very short-duration: One that lasts ten seconds would be considered very long duration indeed. So the Earth isn't going to rotate appreciably under the burst, and so only half of the planet would be flash-sterilized (a few thousand kilometers of rock makes one heck of a radiation shield).

However, what it could do is photodissociate the oxygen and nitrogen in the atmosphere, turning it (for a while, at least) into a soup of opaque brownish NOx compounds. Exactly how much impact this would have would depend on the latitude of the burst: Air doesn't easily cross the equator, so a burst at high latitude in one hemisphere wouldn't have much effect on the other, but a burst near the equator would effect both. This opacity of the atmosphere would result in a "nuclear winter" effect over the affected hemisphere(s). Oh, and the whole planet would smell like Los Angeles, so maybe we'd be glad to go extinct.

And while neutrinos do have mass, they're miniscule at best (most experimental upper bounds are in the vicinity of one or two eV, compared to half a million eV for the next lightest known particle, the electron). And there is no known lower bound for the mass of the lightest neutrino. So while they don't quite travel at the speed of light, they come awfully darned close. They might even get closer to the speed of light than light does itself: The interstellar medium is not quite a perfect vacuum, and therefore has a very slight index of refraction, and so it's possible that neutrinos might travel through the ISM faster than light does.

Regardless, though, the neutrinos from a supernova explosion are nothing to worry about. Even the neutrino flux at the surface of the star wouldn't be a problem, with how weakly-interacting neutrinos are (though a lot of other things would, of course, be a problem there). The same is also true, but even more so, of the gravitational waves produced in a supernova (which travel at exactly c).

Mindfield
02-14-2008, 10:06 PM
2. When you see distances to stars printed - no matter where - bear in mind they're not quite wild-ass guesses, but they're pretty close. Except for the very nearby stars, those distances are extremely rough estimates.
What about using Cephid-type variable stars as a means of more accurate measurement? A recent NASA PotD did a piece on RS Pup and explained its usefulness in improving the accuracy of measuring stellar distances:

Using new measurements of the time delay and angular size of the nebula, the known speed of light allows astronomers to geometrically determine the distance to RS Pup to be 6,500 light-years, with a remarkably small error of plus or minus 90 light-years. An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup, and by extension other Cepheid stars, improving the knowledge of distances to galaxies beyond the Milky Way.
From here (http://antwrp.gsfc.nasa.gov/apod/ap080212.html), plus a page on Cephid-type variable stars (http://www.aavso.org/vstar/vsots/0900.shtml).

Squink
02-14-2008, 10:35 PM
From the Hipparcos FAQ: (http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=FAQ#item26) Q. How far away (PC or LY) was, or what was the smallest parallax for, the most distant star measured by the Hipparcos satellite? I have seen varying numbers - one ESA publication stated that it was 1000 LY while I've seen various astronomy professors say it was on the order of 3000 LY. For whatever maximum distance is correct, can I assume that Hipparcos pretty much mapped all the stars within that distance from the Earth - i.e., the # in the Hipparcos catalog plus the # in the Tycho catalog? I suppose if I knew how to search the Hipparcos and Tycho catalogs, I could determine this answer, but I thought I'd ask anyway.

A. Let's talk in terms of angles (a parallax of 1 milliarcsec means a distance of 1000 pc). You will find very small values of the parallax in the catalogue, e.g. 0.1 milliarcsec which formally would mean a distance of 10,000 pc. There are even negative parallaxes, which physically means nothing. The important point is also the accuracy of these estimates. If we measure 0.1 milliarcsec, and the accuracy is 1 milliarcsec, it doesn't mean that we have really determined a distance of 10,000 pc. So there is not really a simply answer to your question "what is the max distance measured"... it depends on the accuracy you are happy to accept. Many workers say that only want to work with distances accurate to say 10%. For a typical measurement accuracy of 1 milliarcsec, this will mean stars with parallaxes greater than 10 milliarcsec, i.e. stars within just 100 pc of the Sun. If you're happy with distance accuracies of 5%, our measurements extend out to around 200 pc. But not every star out to these distances was measured by Hipparcos, so the catalogue is certainly not complete to these distances.

Anne Neville
02-15-2008, 09:04 AM
Where's your motivation? I would certainly take a trip to SA to see a hypernova!

I'd want to, too. But if it happened to a star that was visible from the northern hemisphere, it would be like having a bright comet around- everybody gets excited about it. That's a lot of fun.

Plus, if the supernova were a star that is only visible from the southern hemisphere, it wouldn't quite be the same- it wouldn't be a change in the stars I've known all my life.

What about using Cephid-type variable stars as a means of more accurate measurement?

The problem with that is that a lot of the stars we want to know distances to are not Cepheid variables.

However, what it could do is photodissociate the oxygen and nitrogen in the atmosphere, turning it (for a while, at least) into a soup of opaque brownish NOx compounds.

The Mega Disasters show about gamma ray bursts depicted this.

Oh, and the whole planet would smell like Los Angeles, so maybe we'd be glad to go extinct.

It might smell even worse than that, if whoever they consulted for Mega Disasters is right. On the Mega Disasters show about a gamma ray burst, they said it might cause red tides, which would add a lovely rotting fish element to the smog smell (http://www.nctimes.com/articles/2005/07/07/news/coastal/21_49_187_6_05.txt).

Polycarp
02-15-2008, 10:05 AM
Anne (or whoever cares to respond):

1. I think Mindfield's point is that stars in association with Cepheids could be reliably distanced by that means -- just as is done with galaxies containing them. I suppose the problem is that open clusters containing Cepheids are still relatively rare.

2. Would someone kindly explain in simple terms what provokes the different kinds of supernova? I understand about the iron crisis at the cores of irregular variables; I do not understand the mechanism for a white dwarf going supernova. And what's the other type -- if they're Type Ia and II respectively, what's Type Ib?

3. SN 1987a was, so I'm told, odd because it happened to a blue giant rather than a red giant -- and IIRC the former is the term for O and B main sequence stars, which supposedly aren't ready to go supernova yet. Anyone want to sort my confusion here, and explain what we actually learned from 1987a?

Anne Neville
02-15-2008, 11:09 AM
2. Would someone kindly explain in simple terms what provokes the different kinds of supernova? I understand about the iron crisis at the cores of irregular variables; I do not understand the mechanism for a white dwarf going supernova.

White dwarfs go supernova because you can't have two electrons with the same spin in the same state.

After a low-mass red giant throws off its outer layers, there's a core of material left over. That core isn't going to be able to generate energy by fusion, so it's going to collapse.

In a normal star, the size of the star is determined by a balance between gravity, which wants to pull all the material into the center of the star, and thermal pressure, which wants to make the star expand. The heat for the thermal pressure is, of course, generated by fusion. Normal stars stay in equilibrium because the rate of fusion goes up rapidly when gravity squeezes the star inward, raising the temperature and making the star expand. This keeps the thermal pressure and gravity in equilibrium.

The upshot of this is that, if you don't have fusion going on, a star is going to collapse inward until something else stops it. To form a white dwarf, the leftover core of a star collapses inward until it is so dense that all its available electron states are occupied.

Matter in a state like this is said to be supported by electron degeneracy pressure. In addition to being very dense (a white dwarf about the same mass as the Sun is about the size of Earth), it's also rather weird- if you add mass to a white dwarf, its radius will actually decrease (the radius is inversely proportional to the cube root of the mass). There's a limiting mass at which point the radius would have to shrink to zero, called the Chandrasekhar limit (probably spelled wrong- this and Schwarzschild are some of the hardest names in astronomy to spell...). The limiting mass is about 1.4 solar masses, assuming the white dwarf is not rotating (the limiting mass is higher if it is).

The current model for Type Ia supernovae says that white dwarfs never actually reach this limiting mass. Instead, they get massive enough and hot enough that they manage to restart fusion, this time of the carbon that was formed as a result of helium fusion during the red giant phase. But there's a difference here- a red giant or Main Sequence star is supported by thermal pressure, which is (obviously) dependent on temperature. If fusion tries to go faster in one of those stars, the thermal pressure pushes the material further apart, which slows down the fusion. A white dwarf is supported by electron degeneracy pressure, which is independent of temperature. When the fusion speeds up, the white dwarf doesn't get bigger. You can get runaway fusion here, which goes until the white dwarf is so hot it blows itself up.

And what's the other type -- if they're Type Ia and II respectively, what's Type Ib?

Supernovae are classed as Type I or II based on whether they have significant hydrogen lines in their spectra. There are also Type Ib and Ic supernovae, which differ in some other details of their spectra. We think they come from massive stars, and differ from Type II supernovae in that the massive stars have thrown off most of their hydrogen before the supernova. This happens either because the progenitor stars of Type Ib/c supernovae have stronger stellar winds than those of Type II, or because they have a nearby companion that has sucked away a lot of their hydrogen.

3. SN 1987a was, so I'm told, odd because it happened to a blue giant rather than a red giant -- and IIRC the former is the term for O and B main sequence stars, which supposedly aren't ready to go supernova yet.

The fact that SN 1987a came from a blue giant and not a red giant was (and remains) puzzling and goes against our understanding of what causes supernovae. The current theory is that the progenitor of this supernova was an unstable blue star created by the merger of a binary pair of stars about 20,000 years before the supernova.

Anyone want to sort my confusion here, and explain what we actually learned from 1987a?

One thing we learned is that this was the first supernova from which we detected neutrinos. This gives some support to supernova models where neutrinos play a major role.

We learned that the neutrinos are given off in a supernova before the light is, since the neutrinos were detected before we saw the supernova.

We actually detected both neutrinos and antineutrinos from SN 1987a. They both arrived at the same time, which tells us that antimatter and matter interact with gravity in the same way.