On point 3) though, you'll have a big problem. The diffraction limit of an apertures defines the smallest angular detail you can see, and for any appreciable distance from the earth, you rapidly lose any interesting information. You also have the problem that planets which are illuminated by their parent stars, which are up to ten billion times brighter than the light reflected from the planet's surface towards you.
This is what the Terrestrial Planet Finder mission is trying to do - it is trying to see the light of other earth-like planets around other stars, and the diffraction effect for finite sized mirrors means that the light of a planet is buried within the diffraction halo from the parent star, by a few million times. Two proposed techniques to improve detection of planet light include nulling interferometry, and coronagraph optics.
Interferometry takes the light from two widely separated telescopes and combines them such that the parent star light is nulled out whilst the planet light passes through (essentially a fantastically accurate spatial filter) and the coronagraph has a black disk flying in front of the telescope blocking the light from the central star.
For some great educational sources for the non-astro-physicist, see The Elegant Universe excellent program
I watched this program and I thought it was awful. Beautiful graphics, yes, but almost completely void of any informed content whatsoever. String theory is an almost completely untestable theory. It's elegant, I'll grant you that, and it's completely exciting, in that it explains everything whilst unburdened by anything resembling reality, but it is just a very abstract theory. I remember sitting down to watch this program, and I was tremendously disappointed with it. A real shame.
When I was a young astronomer in Britain, they had a series of programs called "Horizon" which were the American equivalent of the Nova series, and they were excellent, informative programs. I remember being mesmerised by the episodes on the Voyager flybys. Now that was cool!
In the end, if it inspired your kids to look up at the stars, then it has served it's purpose, and I hope your kids keep their enthusiasm and interest, so I can't argue with that. But why did PBS have to do it with something as useless as bloody String theory?!?
Feh. I need my coffee and to stop being a grumpy bastard. Sorry.
Yes, that's definitely scattered light in the optical halo. My feeling is that it is scattered light from the corrective lenses, and not the primary and secondary mirrors, because if it was due to mirror aberrations, it would be achromatic, and not the bright blue colour it is in the photo.
Working at the diffraction limit you CAN see that the diffraction due to the spider arms is rainbow coloured, which is expected for a diffraction limited telescope with secondary spiders.
The stellar cores are also wildly overexposed, so I suspect that the blue light scattered halo is only a percent or so of the total flux from each star. Nice image for an example!
I think we're talking at slightly different odds here. After re-reading the parent thread, I see your point about the end of HST being portrayed as the end of space-based science, and your comment is that this is not true, and I agree with you.
However, I disagree with you about the return. Hubble provides an observing window that will not be relaced by Webb (notably UV and blue end on visible) and even so I don't believe Webb will be up in 2011. I suspect 2020 and a reduced capability. The only analogy I can think of is designing, building, and initially servicing a car, then skipping on an oil change and letting the car go to ruin. Yes, it's a hell of an expensive oil change, but then ALL space science is expensive.
And as for the comments on the optics - I presume you mean the deep images of galaxies that have a foreground star in them? After the camera was modified to correct the spherical aberration, the point spread function of the telescope is damn stable and you can do science that no amount of AO and ground based observing can correct for, notably faint substellar companions and dust disk work. The speckles you're referring to in addition to the diffraction spikes of the secondary mirror are seen on ground based mirrors too, and even AO systems cannot remove them to the same degree that flying above the atmosphere can do.
Actually, if you have a link to an image you're thinking about, I can have a look at it and comment on the psf structure, if you want.
FYI, I work on an AO system for a large telescope, so you may think I'd be cheering for no servicing of Hubble. Absolutely not at all! It's doing great stuff at a good rate, along with all the other Great Observatories of NASA.
It's a good thought, but ozone holes are really ozone depletions, and they still rule out sensitive UV measurement. That, and the holes are at the poles, and so you're limited to the amount of sky you can look at.
You are on the money, but there really isn't a gap. Look at SIM - launch in 2009 - BEFORE JWST and perhaps TPF, perhaps 2014. Both of these guys will make Hubble look like an out-of-date toy.
Both of these are interferometers, designed for ultra-high precision astrometry and photometry of star systems, they are not wide-field imagers. Hubble is a (reasonably) wide field optical imaging telescope.
Spitzer, Chandra, GALEX, FUSE, INTEGRAL, RXTE, WMAP and XMM-Newton - all flying now.
...and NONE of them work in the optical. They complement the Hubble, not replace them. My bet is that the JWST will get downsized and postponed. Look toward 2020 for a smaller version of JWST.
Because thanks to adaptive optics, it is now possible to get very close to hubble's resolution with Earth-based telescopes. Thus, it is much, much cheaper to use those ground-based scopes.
...but you cannot do UV work from the ground, as the atmosphere almost absorbs all UV flux of astrophysical interest. Also, AO is limited to about an arcminute around bright guide stars, and cannot provide good correction for the Earth's atmosphere beyond this radius. Laser projection systems are being developed to provide all-sky coverage, but they're a hassle to run consistently.
You're completely right about the point in time beyond which we wouldn't see any more, and you're right about the expansion of the universe being faster than the speed of light. So it means that the whole universe may be many trillions of light years (or even infinite) in size, but the oldest light we can possibly see is that from less than 13 billion light years away (rough age of the universe).
General Relativity *does* allow separate parcels of space to move apart from each other faster than the speed of light, in that GR is only valid for local inertial reference frames. Space *itself* is doing the expanding, and not the interaction of mass and energy sitting through it, which is what is described by GR. Sorry, I'm not a GR physicist, so I can't give a good explanation for what seem like a cheat in GR, but it is apparently valid.
Your last line is partly true, in that the universe is thought to have had an era of insanely rapid expansion - one doubling of its effective size every 10**-34 seconds or so - for many tens of thousands of doublings, back when the universe was about 10**-30 seconds old. It's called Inflation theory, and it was created to explain why the effective temperature of seemingly separate parts of the universe is very very similiar. The argument is that the universe we can see (and anything beyond 13 billion light years away) was, at one time, in thermal equilibrium with itself, and then this inflation era blew up the universe faster than the local speed of light in any causal region.
Erm. Just had a few beers, so this may not help, but go check out "The inflationary Universe" by Alan Guth, who explains it a lot clearer than me at the moment:)
They're basing it on a single emission line detection - that the identified emission line is highly redshifted Lyman alpha. Normally, you'd like at least one or two other emission lines to pin down the redshift uniquely. I can see that they're also arguing that its blackbody color leads to a photometic redshift from z of 9 to 11, but those error bars look mighty big to me, and they're relying on the non-detections short of 1 micron. Any quasar savvy astronomers care to comment? I know you're out there in/. land...
Even if it doesn't turn out to be a z~10 quasar, this is an excellent piece of detective work. Big kudos to the authors on this.
Greg Egan wrote "Diaspora" which deals with the atmospheric effects of a setrilizing blast of radiation from two neutron stars coaelescing nearby. Egan writes excellent hard sci-fi, and the rest of the stories in this novel (set in the same timeline, with characters appearing and disappesring from the stories) are all linked together - very good indeed.
I've read the Elegant Universe, and I'm pretty sure that he never claims that distant frames of reference cannot move away from each other faster than the speed of light.
If you are after a more professional book on the subject, try gravitation, the classic text on the subject. I hope this qualifies as enough time for you to do some research yourself;)
> It is an accepted scientific "fact" that nothing > (and I mean nothing, in NO circumstances), can > travel faster than light. It HAS been suggested > that the speed of light is changing, however. And > THAT is the real answer.
Not quite right. General relativity still applies, in that local parcels of space must have particles that cannot exceed the speed of light, but distant parcels of space *can* move apart faster than the speed of light.
For stars within our galaxy, the largest difference in line of sight velocity between a star with an ET laser system and our Solar system is about 400km/s, not enough to cause a red/blue shift from the visible to the infra-red or radio.
Red-shifts *could* be important for looking at more distant galaxies, but looking for ET signals from other galaxies is ruled out, partially because the inverse-square law makes the laser beem too dilute over those distances, and the intergalactic medium would cause very slight smearing of the beam signal, and you'd have to use a much lower transmission rate to make a digital signal keep intact over the distance.
In addition, even though a laser beam is 'beamed', optical diffraction causes a fundamental expansion of the beam that, over long enough distances, dilute the light down to noise levels - we couldn't see a laser signal for the light of other stars in the galaxy.
ET laser searches are restricted to stars in our galaxy.
This is an interesting method, because it really irons out systematic effects due to the local patch of atmosphere above any one telescope.
The atmospheric turbulence causes 'scintillation' of starlight (a rapid, small variation in stellar brightness), and for the very short exposures they're proposing, it'd be difficult with just one telescope to pull out an ET laser modulated signal from the atmospheric generated scintillation.
Distributed telescopes with accurate positions would pull out a laser signal very easily.
> What happens to light that loses all its energy to > red shift, as would happen during attempted escape > from a black hole?
Hmm, good question, a real bugger to answer though. General relativity doesn't have good solutions within an event horizon of a black hole, but I *think* what happens is that photons within the black hole are on closed loops - they can't cross the event horizon because there is no available path from inside the hole to outside the hole. It's complicated by the fact that time and space are both distorted significantly within a few event horizon diameters of the hole, but I think I'm roughly on the mark.
However, if some matter emits a photon just above the event horizon, I can tell you what happens. The photon travels at the speed of light away from the hole, but it's energy is redshifted tremendously to very long radio wavelengths - but it still travels at the speed of light.
As for the light passing by a mass, yes, the photon's energy does increase, and then decrease, as it passes the point of closest approach to a massive object. The photon is also slightly deflected by the gravitational field, and this leads to the gravitational lensing as seen in distant galaxy clusters.
Light is red-shifted climbing out of the gravity well.
Can you expand on this? I've never heard of this, and I can't think of anything in my 40+ years of layman's reading on physics that could be expressed this way.
Sorry if I've borked up the details, haven't had coffee yet!
Dr. Fish
yes, it does affect luminosity of the star
on
Non-Spherical Stars
·
· Score: 3, Informative
In the more recent surveys of bright stars in a cluster, they've seen that faster rotating stars (seen indirectly by the rotational broadening of spectral lines of the star) of the same spectral type have a wider scatter of observed brightnesses. The explanation for this is that:
(i) Faster rotating stars are brighter at their poles than their equators (because of centripetal force slightly expanding the distance of the equator from the core of the star), and:
(ii) The spin axes of stars are randomly oriented with respect to telescopes on Earth, so:
(iii) For a large sample of fast rotating stars, you sample all the brightnesses from the equator to the poles, hence a large scatter in measured brightness. You can assume that all stars are effectively at the same distance if they are in a distant cluster.
The Happy Face motif is seen all throught the graphic novel/comic book "Watchmen" written by Alan Moore. One scene, set on Mars, has the reader's point of view pull back from a Martian crater to reveal a Happy Face.... all it needs is the splash of blood on one corner near the left eye...
If you've not read "Watchmen", I'd highly recommend it, along with "From Hell". Both are excellent stories and the art in them is superb.
I think you're confusing what the article is saying.
Yes, it is true that there is no known astrophysical process that can make a sub-stellar mass black hole. But in theory, there is no reason why you can't make a arbitrarily SMALL black hole. You could make one out of a mountain, if you compressed it up to the right density.
The problem is that IF there was a way to make a molecular sized black hole via some novel particle physics, you would then have a black hole that would fall through your particle accelerator and then start vacuuming up, one atom at a time admittedly, as it passes through the earth. Put simply, there's no way to 'hold' the hole in place because the damn thing is small enough and dense enough to slice through atomic matter like a bullet through fog.
The other problem is that the damn thing will GROW all the time... so eventually it'll be eating kilograms a second, then tonnes, then... no earth left! Black holes have to follow conservation of momentum as well, so as it falls back and forth through the earth, it's velocity will decrease until it settles at the core.
Hopefully these things would evaporate almost instantaneously due to Hawking radiation, which goes as the inverse of the surface area of the hole, but AFAIK this is only known to apply to 'naturally' formed holes, and it may not hold true for some sort of artificially created/stabilised hole.
As for the strangelet quarkball, theoretically a quarkball has a lower energy state than an equivalent mass of atomic matter, but in our universe, naturally occurring quark balls don't exist. The idea is that if you make a small quarkball, normal matter will be converted to quarkball as soon as it comes into contact with it, and the process is exponential, converting all atomic matter touching it.
Sorry I don't have references handy, this is from my grad physics and astro classes..
I think we may be both right, according to this reply on the Cornell ask an astronomer site, the 'self peturbation' theory can explain the fuzzier spiral arm galaxies, but not the larger grand design galaxies, which do need an external companion. It's not clear cut at all, though.
As for your second question, it's to do with the way the stars orbit in a galaxy. If you could draw all the orbits of the stars in a spiral galaxy, you would see a badly stacked set of hula-hoops, - most orbits are nearly circular, and they all go around the galaxy in the same direction. With an elliptical galaxy, the orbits would resemble a tangled hairnet. There's no perferred axis of rotation, and there are many very eccentric orbits.
You tend to see giant elliptical galaxies sitting in the middle of groups of smaller spiral galaxies, and it is thought that collisions between spiral galaxies lead to formation of giant elliptical galaxies. The interaction of the two spiral galaxies' gravitational potentials then scrambled the orbits of the stars together.
Spiral structure NEEDS ordered co-rotating material in which to form, so that's why ellipticals don't form them -the orbits of the stars are too randomised to form them.
I thought that standard opinion on spiral forms (e.g. galaxies) was that they were created by interaction with massive companions.
Spirals in galaxies and these spirals in protoplanetary disks have different origins, and in the galactic spirals case, you don't need a binary companion to cause spiral structure.
Who has ever proposed that internal bodies can cause a spiral form?
OK, this is probably a gross simplification, so if there are any disk formation astronomers out there (you know who you are!), they'll give a much better description than this one!
It partially depends on the viscosity of the material in the disk, and where most of the mass resides. If the mass of the disk is much smaller than the mass of the central star, the disk structure is dominated by the gravitational field of the central star and this tends to smooth out any spiral structure in the disk, and then you need a binary companion to stir up spiral modes in the disk.
If the disk itself is massive enough, and the viscocity of the material is low enough, the disk's gravitational field can amplify up any spiral patterns that occasionally appear. So no, you don't need a binary compantion if the disk is massive enough. In this specific case, though, the disk mass is small, and so there's probably a binary companion acting as a swizzle stick.
For galaxies, nearly all the mass resides in the disk of the galaxy and not in the centre (the mass of the black hole in the centre of the galaxy is tiny compared to the rest of the mass in our galaxy, and there's a honking huge halo of dark matter, I know, I know...) and so spiral modes tend to be self-reinforcing as they sweep around the galaxy.
This event goes contrary to everything what is known about the star life cycle so far.
New physics just for this star? I doubt it.
One reasonable suggestion without reaching for mysterious new physics is that it is part of a binary system, with a compact object (neutron star, white dwarf or possibly black hole) in a highly eccentric orbit around this main sequence star.
Every x number of years, the compact object skids in on its highly eccentric orbit, and slams through the upper layers of the visible star. Material falls onto compact object, flares up and heats the outer layer of the main sequence star, compact object whizzes out again and is not seen again for another few hundred years or so. The fraction of total mass ejected from the main star is miniscule, so this process does not significantly alter the main star's evolution on the timescale of a few million years, hence the repeated shells of dust seen in these light echoes.
Binary systems of this types have been known about for quite a while (well, on the order of 20 years). The trick is looking for the signature of the compact object, which is a difficult detection.
The collapse of the core of a star as it is about to go supernova takes less than an hour or so, and it is in the shock wave of the exploding star that the really heavy elements are formed from nucleosynthesis. Not everything in astronomy works on gigayear timescales!
Slashdot Mirror
Solar power, wind turbines, geothermal power...
Getting the energy to make antimatter is trivial. The trick is making an efficient production and separation process.
Dr Fish
Heh. A cute idea.
On point 3) though, you'll have a big problem. The diffraction limit of an apertures defines the smallest angular detail you can see, and for any appreciable distance from the earth, you rapidly lose any interesting information. You also have the problem that planets which are illuminated by their parent stars, which are up to ten billion times brighter than the light reflected from the planet's surface towards you.
This is what the Terrestrial Planet Finder mission is trying to do - it is trying to see the light of other earth-like planets around other stars, and the diffraction effect for finite sized mirrors means that the light of a planet is buried within the diffraction halo from the parent star, by a few million times. Two proposed techniques to improve detection of planet light include nulling interferometry, and coronagraph optics.
Interferometry takes the light from two widely separated telescopes and combines them such that the parent star light is nulled out whilst the planet light passes through (essentially a fantastically accurate spatial filter) and the coronagraph has a black disk flying in front of the telescope blocking the light from the central star.
Dr Fish
I watched this program and I thought it was awful. Beautiful graphics, yes, but almost completely void of any informed content whatsoever. String theory is an almost completely untestable theory. It's elegant, I'll grant you that, and it's completely exciting, in that it explains everything whilst unburdened by anything resembling reality, but it is just a very abstract theory. I remember sitting down to watch this program, and I was tremendously disappointed with it. A real shame.
When I was a young astronomer in Britain, they had a series of programs called "Horizon" which were the American equivalent of the Nova series, and they were excellent, informative programs. I remember being mesmerised by the episodes on the Voyager flybys. Now that was cool!
In the end, if it inspired your kids to look up at the stars, then it has served it's purpose, and I hope your kids keep their enthusiasm and interest, so I can't argue with that. But why did PBS have to do it with something as useless as bloody String theory?!?
Feh. I need my coffee and to stop being a grumpy bastard. Sorry.
Dr Fish
Thanks for the link, I now see what you mean!
Yes, that's definitely scattered light in the optical halo. My feeling is that it is scattered light from the corrective lenses, and not the primary and secondary mirrors, because if it was due to mirror aberrations, it would be achromatic, and not the bright blue colour it is in the photo.
Working at the diffraction limit you CAN see that the diffraction due to the spider arms is rainbow coloured, which is expected for a diffraction limited telescope with secondary spiders.
The stellar cores are also wildly overexposed, so I suspect that the blue light scattered halo is only a percent or so of the total flux from each star. Nice image for an example!
Cheers,
Dr Fish
I think we're talking at slightly different odds here. After re-reading the parent thread, I see your point about the end of HST being portrayed as the end of space-based science, and your comment is that this is not true, and I agree with you.
However, I disagree with you about the return. Hubble provides an observing window that will not be relaced by Webb (notably UV and blue end on visible) and even so I don't believe Webb will be up in 2011. I suspect 2020 and a reduced capability. The only analogy I can think of is designing, building, and initially servicing a car, then skipping on an oil change and letting the car go to ruin. Yes, it's a hell of an expensive oil change, but then ALL space science is expensive.
And as for the comments on the optics - I presume you mean the deep images of galaxies that have a foreground star in them? After the camera was modified to correct the spherical aberration, the point spread function of the telescope is damn stable and you can do science that no amount of AO and ground based observing can correct for, notably faint substellar companions and dust disk work. The speckles you're referring to in addition to the diffraction spikes of the secondary mirror are seen on ground based mirrors too, and even AO systems cannot remove them to the same degree that flying above the atmosphere can do.
Actually, if you have a link to an image you're thinking about, I can have a look at it and comment on the psf structure, if you want.
FYI, I work on an AO system for a large telescope, so you may think I'd be cheering for no servicing of Hubble. Absolutely not at all! It's doing great stuff at a good rate, along with all the other Great Observatories of NASA.
Dr Fish
It's a good thought, but ozone holes are really ozone depletions, and they still rule out sensitive UV measurement. That, and the holes are at the poles, and so you're limited to the amount of sky you can look at.
Dr Fish
Both of these are interferometers, designed for ultra-high precision astrometry and photometry of star systems, they are not wide-field imagers. Hubble is a (reasonably) wide field optical imaging telescope.
Spitzer, Chandra, GALEX, FUSE, INTEGRAL, RXTE, WMAP and XMM-Newton - all flying now....and NONE of them work in the optical. They complement the Hubble, not replace them. My bet is that the JWST will get downsized and postponed. Look toward 2020 for a smaller version of JWST.
Dr Fish
...but you cannot do UV work from the ground, as the atmosphere almost absorbs all UV flux of astrophysical interest. Also, AO is limited to about an arcminute around bright guide stars, and cannot provide good correction for the Earth's atmosphere beyond this radius. Laser projection systems are being developed to provide all-sky coverage, but they're a hassle to run consistently.
Dr Fish
You're completely right about the point in time beyond which we wouldn't see any more, and you're right about the expansion of the universe being faster than the speed of light. So it means that the whole universe may be many trillions of light years (or even infinite) in size, but the oldest light we can possibly see is that from less than 13 billion light years away (rough age of the universe).
:)
General Relativity *does* allow separate parcels of space to move apart from each other faster than the speed of light, in that GR is only valid for local inertial reference frames. Space *itself* is doing the expanding, and not the interaction of mass and energy sitting through it, which is what is described by GR. Sorry, I'm not a GR physicist, so I can't give a good explanation for what seem like a cheat in GR, but it is apparently valid.
Your last line is partly true, in that the universe is thought to have had an era of insanely rapid expansion - one doubling of its effective size every 10**-34 seconds or so - for many tens of thousands of doublings, back when the universe was about 10**-30 seconds old. It's called Inflation theory, and it was created to explain why the effective temperature of seemingly separate parts of the universe is very very similiar. The argument is that the universe we can see (and anything beyond 13 billion light years away) was, at one time, in thermal equilibrium with itself, and then this inflation era blew up the universe faster than the local speed of light in any causal region.
Erm. Just had a few beers, so this may not help, but go check out "The inflationary Universe" by Alan Guth, who explains it a lot clearer than me at the moment
Cheers,
Dr Fish
Even if it doesn't turn out to be a z~10 quasar, this is an excellent piece of detective work. Big kudos to the authors on this.
Dr Fish
The detailed detection images from one of the authors.
Greg Egan wrote "Diaspora" which deals with the atmospheric effects of a setrilizing blast of radiation from two neutron stars coaelescing nearby. Egan writes excellent hard sci-fi, and the rest of the stories in this novel (set in the same timeline, with characters appearing and disappesring from the stories) are all linked together - very good indeed.
Dr Fish
"can space expand faster than the speed of light"
How can the universe expand faster than the speed of light during inflation?
I've read the Elegant Universe, and I'm pretty sure that he never claims that distant frames of reference cannot move away from each other faster than the speed of light.
If you are after a more professional book on the subject, try gravitation, the classic text on the subject. I hope this qualifies as enough time for you to do some research yourself ;)
HTH,
Dr. Fish
> It is an accepted scientific "fact" that nothing
> (and I mean nothing, in NO circumstances), can
> travel faster than light. It HAS been suggested
> that the speed of light is changing, however. And
> THAT is the real answer.
Not quite right. General relativity still applies, in that local parcels of space must have particles that cannot exceed the speed of light, but distant parcels of space *can* move apart faster than the speed of light.
HTH,
Dr Fish
For stars within our galaxy, the largest difference in line of sight velocity between a star with an ET laser system and our Solar system is about 400km/s, not enough to cause a red/blue shift from the visible to the infra-red or radio.
Red-shifts *could* be important for looking at more distant galaxies, but looking for ET signals from other galaxies is ruled out, partially because the inverse-square law makes the laser beem too dilute over those distances, and the intergalactic medium would cause very slight smearing of the beam signal, and you'd have to use a much lower transmission rate to make a digital signal keep intact over the distance.
In addition, even though a laser beam is 'beamed', optical diffraction causes a fundamental expansion of the beam that, over long enough distances, dilute the light down to noise levels - we couldn't see a laser signal for the light of other stars in the galaxy.
ET laser searches are restricted to stars in our galaxy.
Dr Fish
This is an interesting method, because it really irons out systematic effects due to the local patch of atmosphere above any one telescope.
The atmospheric turbulence causes 'scintillation' of starlight (a rapid, small variation in stellar brightness), and for the very short exposures they're proposing, it'd be difficult with just one telescope to pull out an ET laser modulated signal from the atmospheric generated scintillation.
Distributed telescopes with accurate positions would pull out a laser signal very easily.
Cute trick.
Dr Fish
> What happens to light that loses all its energy to
> red shift, as would happen during attempted escape
> from a black hole?
Hmm, good question, a real bugger to answer though. General relativity doesn't have good solutions within an event horizon of a black hole, but I *think* what happens is that photons within the black hole are on closed loops - they can't cross the event horizon because there is no available path from inside the hole to outside the hole. It's complicated by the fact that time and space are both distorted significantly within a few event horizon diameters of the hole, but I think I'm roughly on the mark.
However, if some matter emits a photon just above the event horizon, I can tell you what happens. The photon travels at the speed of light away from the hole, but it's energy is redshifted tremendously to very long radio wavelengths - but it still travels at the speed of light.
As for the light passing by a mass, yes, the photon's energy does increase, and then decrease, as it passes the point of closest approach to a massive object. The photon is also slightly deflected by the gravitational field, and this leads to the gravitational lensing as seen in distant galaxy clusters.
Urgh, Makes my head spin! HTH,
Dr Fish
The diffusion time for photons from the core of a star such as the sun is about 3 million years, not 25 years.
:)
And yes, I'd like to see a SNe go off, but at a reasonable distance
Dr Fish
Can you expand on this? I've never heard of this, and I can't think of anything in my 40+ years of layman's reading on physics that could be expressed this way.
It's a well-known effect in General Relativity (well, to General Relativists!) and it is called the gravitational redshift effect. In fact, GPS software has to take in effect the gravitational time dilation of radio photons 'falling' from the satellites to the receivers, amongst some other relativistic corrections, in order to get a triangulation down to a few meters.
Sorry if I've borked up the details, haven't had coffee yet!
Dr. Fish
In the more recent surveys of bright stars in a cluster, they've seen that faster rotating stars (seen indirectly by the rotational broadening of spectral lines of the star) of the same spectral type have a wider scatter of observed brightnesses. The explanation for this is that:
(i) Faster rotating stars are brighter at their poles than their equators (because of centripetal force slightly expanding the distance of the equator from the core of the star), and:
(ii) The spin axes of stars are randomly oriented with respect to telescopes on Earth, so:
(iii) For a large sample of fast rotating stars, you sample all the brightnesses from the equator to the poles, hence a large scatter in measured brightness. You can assume that all stars are effectively at the same distance if they are in a distant cluster.
Hope that's reasonably clear,
Dr Fish
The Happy Face motif is seen all throught the graphic novel/comic book "Watchmen" written by Alan Moore. One scene, set on Mars, has the reader's point of view pull back from a Martian crater to reveal a Happy Face.... all it needs is the splash of blood on one corner near the left eye...
If you've not read "Watchmen", I'd highly recommend it, along with "From Hell". Both are excellent stories and the art in them is superb.
Dr Fish
I think you're confusing what the article is saying.
Yes, it is true that there is no known astrophysical process that can make a sub-stellar mass black hole. But in theory, there is no reason why you can't make a arbitrarily SMALL black hole. You could make one out of a mountain, if you compressed it up to the right density.
The problem is that IF there was a way to make a molecular sized black hole via some novel particle physics, you would then have a black hole that would fall through your particle accelerator and then start vacuuming up, one atom at a time admittedly, as it passes through the earth. Put simply, there's no way to 'hold' the hole in place because the damn thing is small enough and dense enough to slice through atomic matter like a bullet through fog.
The other problem is that the damn thing will GROW all the time... so eventually it'll be eating kilograms a second, then tonnes, then... no earth left! Black holes have to follow conservation of momentum as well, so as it falls back and forth through the earth, it's velocity will decrease until it settles at the core.
Hopefully these things would evaporate almost instantaneously due to Hawking radiation, which goes as the inverse of the surface area of the hole, but AFAIK this is only known to apply to 'naturally' formed holes, and it may not hold true for some sort of artificially created/stabilised hole.
As for the strangelet quarkball, theoretically a quarkball has a lower energy state than an equivalent mass of atomic matter, but in our universe, naturally occurring quark balls don't exist. The idea is that if you make a small quarkball, normal matter will be converted to quarkball as soon as it comes into contact with it, and the process is exponential, converting all atomic matter touching it.
Sorry I don't have references handy, this is from my grad physics and astro classes..
Dr Fish
As for your second question, it's to do with the way the stars orbit in a galaxy. If you could draw all the orbits of the stars in a spiral galaxy, you would see a badly stacked set of hula-hoops, - most orbits are nearly circular, and they all go around the galaxy in the same direction. With an elliptical galaxy, the orbits would resemble a tangled hairnet. There's no perferred axis of rotation, and there are many very eccentric orbits.
You tend to see giant elliptical galaxies sitting in the middle of groups of smaller spiral galaxies, and it is thought that collisions between spiral galaxies lead to formation of giant elliptical galaxies. The interaction of the two spiral galaxies' gravitational potentials then scrambled the orbits of the stars together.
Spiral structure NEEDS ordered co-rotating material in which to form, so that's why ellipticals don't form them -the orbits of the stars are too randomised to form them.
I hope this makes some sense!
Dr Fish
Spirals in galaxies and these spirals in protoplanetary disks have different origins, and in the galactic spirals case, you don't need a binary companion to cause spiral structure.
Who has ever proposed that internal bodies can cause a spiral form?
OK, this is probably a gross simplification, so if there are any disk formation astronomers out there (you know who you are!), they'll give a much better description than this one!
It partially depends on the viscosity of the material in the disk, and where most of the mass resides. If the mass of the disk is much smaller than the mass of the central star, the disk structure is dominated by the gravitational field of the central star and this tends to smooth out any spiral structure in the disk, and then you need a binary companion to stir up spiral modes in the disk.
If the disk itself is massive enough, and the viscocity of the material is low enough, the disk's gravitational field can amplify up any spiral patterns that occasionally appear. So no, you don't need a binary compantion if the disk is massive enough. In this specific case, though, the disk mass is small, and so there's probably a binary companion acting as a swizzle stick.
For galaxies, nearly all the mass resides in the disk of the galaxy and not in the centre (the mass of the black hole in the centre of the galaxy is tiny compared to the rest of the mass in our galaxy, and there's a honking huge halo of dark matter, I know, I know...) and so spiral modes tend to be self-reinforcing as they sweep around the galaxy.
Blurgh, too early on Saturday morning...
Dr Fish
This event goes contrary to everything what is known about the star life cycle so far.
New physics just for this star? I doubt it.
One reasonable suggestion without reaching for mysterious new physics is that it is part of a binary system, with a compact object (neutron star, white dwarf or possibly black hole) in a highly eccentric orbit around this main sequence star.
Every x number of years, the compact object skids in on its highly eccentric orbit, and slams through the upper layers of the visible star. Material falls onto compact object, flares up and heats the outer layer of the main sequence star, compact object whizzes out again and is not seen again for another few hundred years or so. The fraction of total mass ejected from the main star is miniscule, so this process does not significantly alter the main star's evolution on the timescale of a few million years, hence the repeated shells of dust seen in these light echoes.
Binary systems of this types have been known about for quite a while (well, on the order of 20 years). The trick is looking for the signature of the compact object, which is a difficult detection.
Dr Fish
The collapse of the core of a star as it is about to go supernova takes less than an hour or so, and it is in the shock wave of the exploding star that the really heavy elements are formed from nucleosynthesis. Not everything in astronomy works on gigayear timescales!
Dr Fish