Starship Asterisk*

by **zloq** » Fri Dec 23, 2011 5:01 pm

Ann wrote: A third reason for the claim that the Sun is an average star is that its mass is indeed "midway" between the most massive and the least massive of the stars in our galaxy.

Ann

I think that is all that is meant by the sun being "average" - just that it falls in the middle of the range of parameters of known stars. This means it is not meant as an average over the known population of stars which, as you say, is ill-defined because the small, faint ones are hard to see and count. Plus - the relative population depends on where you are in a galaxy - or in other galaxies. So it is just in the middle of the range, and the actual average over stars in our galaxy would be heavily skewed by the vastly more numerous small, faint ones.

zloq

by **All4vols** » Fri Dec 23, 2011 3:40 pm

Chris Peterson wrote: The background galaxy is highly redshifted (z = 2.379; object is 18.8 billion ly away, light was emitted 10.9 billion years ago). Just because something is redshifted, that doesn't mean it will necessarily appear red. Keep in mind that while blue light is shifted towards red, red light disappears into the IR, and ultraviolet is shifted into the blue. So the overall color can shift in unexpected ways. In addition, this is not a "true color" image. It was collected through three wideband photometric filters (B, centered on blue, V centered on green, and I, centered in the near-IR). These filters are mapped to the blue, green, and red of your monitor, but certainly don't produce colors as you'd see them with your eye.

Thanks Chris, I didn't catch the false color bit and really should have just assumed such. I guess the article discussing old stars and their reddish color threw me off. Merry Christmas all.

by **geckzilla** » Fri Dec 23, 2011 12:26 pm

I think the idea that we have an average star is simply a matter of probability. Most people don't study stars so the logical conclusion is that we are probably "normal" in every way possible until otherwise noted.

by **Ann** » Fri Dec 23, 2011 11:42 am

Czerno wrote:

one wonders where that popular idea that our Sun is just an ordinary, less than average even, star came from. Maybe as a corollary of scientific vulgarization trying hard to combat traditional beliefs that Man, his Earth, Sun and Sky (or Heaven) are the center of the universe, the alpha and omega.

I find this interesting, too. Personally I would guess that there are two main reasons for this misconception about the Sun. The first reason is that it is relatively easy to detect the bright stars, but much harder to detect the really faint stars. The idea that the Sun is an average star may have become "set in stone" before really large numbers of small stars were discovered. After all, 99% of all the stars in the sky that are clearly visible to the naked eye are brighter than the Sun! Of the really bright-looking stars, all are intrinsically brighter than the Sun, even Alpha Centauri, which is only a little brighter than the Sun. Actually, Alpha Centauri B, a K-type star, would have been clearly visible to the naked eye if it had been a single star, and this star is only about 40% as bright as the Sun. But Alpha Centauri A+B+Proxima is the nearest known extrasolar star or star system of them all, which is why Centauri A+B look bright. But Proxima, the nearest extrasolar star of them all, is so faint that it can't be seen without a telescope! Its brightness in visual light is only about one part in 10,000 of the brightness of the Sun!

The second reason why it is so popular to think of the Sun as an average star is, or so I believe anyway, the fervent hope to find life among the stars. If the Sun is really an average star, then the Earth might be an average planet, and then there might be life, even advanced life, in orbit around most stars in the galaxy. That is an attractive thought. If the Sun is non-average, then the Earth may be non-average, and then it might be much harder to find life among the stars.

A third reason for the claim that the Sun is an average star is that its mass is indeed "midway" between the most massive and the least massive of the stars in our galaxy. The most massive stars are typically about 100 times more massive than the Sun, but such enormous stars are incredibly rare. There are probably less than ten of them in the entire Milky Way. The smallest stars, on the other hand, may contain only one per cent of the mass of the Sun, although at such puny mass they are very close to being brown dwarfs. On the other hand, such small stars are abundant, although it isn't certain that they are more numerous than more "normal" red dwarfs, whose mass is typically between 10% and 50% of the mass of the Sun.

Ann

by **Czerno 1** » Thu Dec 22, 2011 5:50 pm

Ann wrote:Czerno1 wrote:
Ann, I ever enjoy to read your enthusiastic, informative comments!
Thanks!

It was sincere; and the new piece of information you are bringing from Ken Croswell's book is even more fascinating.

In his book Planet Quest, Ken Croswell claims that our own Sun is unusual as stars go, because it is more massive than about 90% of the stars in our galaxy. (...)
In fact, according to Croswell, the Sun may beat 95% of the stars in our galaxy when it comes to mass. So contrary to belief, our Sun is not an average star.

On the other hand, according to Croswell, the red M-type stars are indeed average! Croswell claims that 80% of the stars in the Milky Way are red dwarfs of spectral class M.

Really fascinating, quite a reversed perspective from what I had assumed.
If this is confirmed, one wonders where that popular idea that our Sun is just an ordinary, less than average even, star came from. Maybe as a corollary of scientific vulgarization trying hard to combat traditional beliefs that Man, his Earth, Sun and Sky (or Heaven) are the center of the universe, the alpha and omega.

The researchers' computer models based on these findings suggest that red dwarfs are far more common than expected, with these galaxies each possessing roughly 20 times more red dwarfs on average than the Milky Way, said researcher Charlie Conroy, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

Ann wrote: And that's why I think that there may actually be trillions of red dwarfs in a large elliptical galaxy.
Ann

And closes my mouth ;=) But I don't regret asking, you taught me something totally unexpected.

--
Czerno

by **zloq** » Thu Dec 22, 2011 11:32 am

Here is a pretty good write up that I think answers many of the questions people have been asking here:

http://blogs.discovermagazine.com/cosmi ... e-amoebas/

The key point is that - yes - gravitational lenses can be used as magnifying "telescopes" to reveal enhanced detail in small galaxies. The idea is to build a model for both the lensing system and its mass distribution, along with a model for the true appearance of the galaxy being lensed. The magnification could be small or big depending on the geometry - so the lensed galaxy could be tiny or large and still show this effect - but for distant galaxies it can reveal structure that would otherwise be unresolvable.

The earliest idea of gravitational lensing dates to 1804 because it can happen in a Newtonian model also, but the effect is half as strong.

One subtle point is that the gravitational lens does behave like a normal imaging system in many ways - including its inability to make objects "brighter." Just like a normal lens, it can only make things bigger while retaining the same surface luminosity. This can still be a huge effect for spectroscopy since you get many more photons falling on the slit if the tiny galaxy patch is magnified. It can also invert images, as seen in the example here.

As for light following geodesics according to general relativity - GR is not a quantum theory, so it has no concept of how light would interact with gravity at a quantum level. Although the observed lensing is consistent with GR, there are also models that do include an interaction of light with gravity to perturb the lensing behavior slightly and cause light not to follow spacetime geodesics exactly. It's not clear if these effects will be measurable any time soon - but it is an active area of research and adds yet another dimension to the importance of gravitational lensing as a tool - because it might show departure from GR that would boost support for newer models. If people modelled observed lensing based on Newtonian gravity, everything might work pretty well - but they similarly might realize something isn't quite right due to a consistent factor of two error (if they can notice it) - pointing them to GR.

Dark matter and dark energy are considered quite "real" in the sense that measurements show "something else" acting in the universe that is not understood. What they are exactly is not known, but models don't fit the observed behavior of the universe without them - so a new model is needed that includes them and mimics their observed behavior - and those added terms are given suggestive names. One of them is "mass like" and the other is more "energy like" - but both of them are poorly understood and not yet observed directly - hence dark.

zloq

by **r.w.b** » Thu Dec 22, 2011 9:19 am

Thanks, Chris; I'll go away and think about all of those responses.

This one set me thinking, however:

Chris Peterson wrote:
So what is happening to the Hubble Constant, as we progress further and further away from here, or back in time, or back along the graph of the perceived change in the speed of light due to changes in mass density as the universe developed?
The Hubble constant isn't constant... not for the reason you suggest, but because the expansion rate of the Universe has not been constant. For the first half of the Universe's existence, its expansion rate was dominated by gravity (so its expansion rate was decreasing). Since then, it has been dominated by dark energy (so now, its expansion rate is increasing).

I'm not at all convinced about 'dark energy' or 'dark matter' - unless the phrase simply means 'all of the black stuff and dust out there'. Granted, it's providing answers which fit most of the facts, so far, and it helps us to find a working explanation for some things, but let's hope that it does not become another version of 'the ether'.

On the other hand, the concept of 'ether' did lead on to better explanations, and so 'dark energy' cannot, for now, be entirely dismissed.

by **Chris Peterson** » Thu Dec 22, 2011 6:05 am

monarchist wrote:Ann and Chris, thanks for probably answering the question of why in such images, the foreground galaxy is always (?) rather red and the lensed galaxie(s) always (?) very blue. However it would be nice to if someone could provide a really definitive explanation of why this appears to be so.

As Ann pointed out, it isn't always so. The apparent colors of lensing and lensed galaxies depend on many factors, both physical (the age and composition of each, and their relative redshifts) as well as instrumental (the choice of filters and processing).

If it is just as Ann suggests a question of filters, then the images we are seeing are pretty misleading as to true color and it would be interesting to see what the "really" look like.

I don't like the word "misleading" here, because it carries some sense of deception. Most scientific astronomical images are not made with filters that lend themselves to producing an image with "accurate" color- such filters are just not that useful. So when we see these images, they are either in a totally artificial color space, or they have been processed in an attempt to approximate "true" color- but those approximations are often rather poor.

Is there a possibility that there is some sort of "gravitational slingshot" affecting light from the background galaxy, resulting in more energy and net blueing?

No. The photons are blueshifted as they approach the lensing galaxy, and then redshifted as they recede from it. There is such a thing as gravitational redshift (or blueshift), but it depends only on the difference in gravitational potential between the point where the photon is emitted, and the point where it is recorded. Nothing that happens along the way matters. And in any case, the magnitude of gravitational redshift is extremely small when compared to cosmological redshift.

Also are we missing any nice cases of lensing because we're depending on color contrast, enhanced or not?

No... because we don't depend on color contrast. Lensing is first detected by morphology, and then the lensing and lensed bodies are examined spectroscopically. They could both have the same exact color to our eyes (or the same color ratios through a few filters), but be very different spectroscopically, due to their very different redshifts.

by **Chris Peterson** » Thu Dec 22, 2011 5:55 am

zbvhs wrote:The way I visualize it, gravity along any axis through a spherical body rises to a maximum at the center and declines to zero at infinity in either direction.

The force of gravity around a sphere of uniform density is proportional to the radius below the surface, and inversely proportional to the square of the radius above the surface. So the force is zero at the center, increases linearly to a maximum at the surface, and then decreases to zero again at infinity. In the case of the Earth, which is not of uniform density, the maximum force of gravity is at about half its radius, at the top of the outer core.

by **Ann** » Thu Dec 22, 2011 5:51 am

monarchist wrote:Ann and Chris, thanks for probably answering the question of why in such images, the foreground galaxy is always (?) rather red and the lensed galaxie(s) always (?) very blue. However it would be nice to if someone could provide a really definitive explanation of why this appears to be so.

If it is just as Ann suggests a question of filters, then the images we are seeing are pretty misleading as to true color and it would be interesting to see what the "really" look like.

Is there a possibility that there is some sort of "gravitational slingshot" affecting light from the background galaxy, resulting in more energy and net blueing?

Also are we missing any nice cases of lensing because we're depending on color contrast, enhanced or not?

It is not true that the background galaxies are always very blue. Here you can see an example of a lensed background galaxy that is (mostly) red. In the picture, you can also see the massive elliptical galaxy that undoubtedly provides much of the mass needed for the lensing effect. But you can also see a spiral galaxy at lower left. This spiral looks undistorted, which means that it is probably at more or less the same distance from us as the elliptical galaxy. But the lensed red galaxy is much farther away.

So why is the lensed red galaxy red and not blue?

Why does the lensed red galaxy contain such intensely blue spots?

Why doesn't the undistorted spiral galaxy contain such bright blue spots?

The reason why the lensed red galaxy looks red overall is because it contains a large and bright population of old yellow stars. And because the lensed galaxy is more distant than the elliptical galaxy and the undistorted spiral, the light from the old yellow population of the lensed galaxy has been redshifted and "stretched" by the expansion of the universe so that it looks red. But the elliptical galaxy and the spiral are not so far away, so their light has been less redshifted and "stretched". Therefore their overall color is less red than the overall color than the lensed galaxy. (But the intrinsic color of the elliptical galaxy may well be redder than the color of the old population of the lensed galaxy. In other words, if the elliptical galaxy and the lensed galaxy had been at the same distance from us, the elliptical galaxy would probably have looked yellower.)

: Click to view full size image

The reason why the lensed red galaxy contains such bright blue spots is because it contains clusters of very hot stars. The hotter a star is, the more ultraviolet light it will emit, and the shorter the wavelengths of the ultraviolet light will be. The blue clusters in the lensed red galaxy contain very hot stars whose "ultraviolet peaks" have been shifted into the blue part of the spectrum due to the expansion of the universe.

The undistorted spiral galaxy doesn't contain brilliant blue spots for two reasons. First, it is possible that the spiral doesn't contain the same kind of clusters as the distant galaxy. In other words, there may be fewer very hot stars in the undistorted spiral than in the lensed galaxy. Second, and probably more important, because the undistorted spiral isn't as far away as the lensed galaxy, not all of its ultraviolet light has been shifted into the blue part of the spectrum. A blue filter will therefore detect most or maybe all of the ultraviolet light from the background galaxy, but it will miss much of the ultraviolet light from the more nearby spiral.

By the way, it is typical for "ultraviolet clusters" to be very concentrated and to show up as "intense spots" through an ultraviolet filter. But a "merely blue" population may be much more spread out. After all, the most intense "ultraviolet knots" will be dominated by small groups of intensely bright O- and early B-type stars, whereas a merely blue population may be made up of large numbers of widely distributed modest A- and F-type stars.

Finally, star formation has generally declined over time in the universe. Therefore, more distant galaxies will, on average, contain more star formation and more blue and ultraviolet stars than nearby galaxies. That is why lensed background galaxies are often blue, and their blue color is exaggerated by the fact that a blue filter will pick up a lot of ultraviolet light that has been redshifted into the blue part of the spectrum.

However, to have a good "lensing effect" you need a very massive foreground galaxy, or better yet, a massive foreground galaxy cluster. But the most massive galaxies are almost always ellipticals, and these galaxies are almost always yellow in the nearby or moderately nearby universe. That is why you can expect to see a massive yellow elliptical star lensing a background starforming galaxy, whose ultraviolet light has been redshifted into the blue part of the spectrum, so that the galaxy looks blue.

Ann

by **Chris Peterson** » Thu Dec 22, 2011 5:45 am

r.w.b wrote:Is the light from the distant galaxy being 'inverted', i.e., light from that part of the distant galaxy which comes from 'above' the lensing galaxy is seen by us as being 'below' the lensing galaxy? Much in the same way as one can invert an image with a glass lens, excepting for the fact that, in the case of a gravitational lens, space itself is being deformed.

The ring isn't really an "image" in the optical sense. It is just the energy of a point, redistributed into a ring.

In other words, in which directions is space being deformed; towards the lensing galaxy, or way from it?

Spacetime is less flat (more distorted) along the optical axis, and increasingly flatter as you get farther from the foreground galaxy.

If it is towards the lens, then this implies that either light is being slowed down,from our viewpoint, as it passes by the large mass, or else time is being speeded up by it. So now we have the perceived fact that light does not have a constant speed as it travels through a vacuum; the more mass there is in a system, the slower light travels through it, from the viewpoint of an outside observer.

Why do you conclude that the light is traveling slower? It takes a longer path (from our frame of reference), and it also takes a longer time. That's just what you'd expect for the light having a constant speed.

Either way, we now run into a big problem when we try to estimate accurately the distances and ages of visible objects. Maybe the understanding of perceived red shift of distant objects is on shaky ground, as a result of this light/time distortion?

Cosmological redshift is orders of magnitude greater than gravitational redshift, so there is really no problem here.

So what is happening to the Hubble Constant, as we progress further and further away from here, or back in time, or back along the graph of the perceived change in the speed of light due to changes in mass density as the universe developed?

The Hubble constant isn't constant... not for the reason you suggest, but because the expansion rate of the Universe has not been constant. For the first half of the Universe's existence, its expansion rate was dominated by gravity (so its expansion rate was decreasing). Since then, it has been dominated by dark energy (so now, its expansion rate is increasing).

by **neptunium** » Thu Dec 22, 2011 4:43 am

zbvhs wrote:The way I visualize it, gravity along any axis through a spherical body rises to a maximum at the center and declines to zero at infinity in either direction. Presumably, space-time curvature, however it's expressed, would do the same. The higher the mass of the body, the higher and sharper the peak in gravity/space-time curvature. If a black hole has finite mass, space-time curvature and gravity should likewise be finite. Therefore, the dimpled-space-time depiction for a black hole would be very deep but should still have a bottom - whatever the bottom means.

But quantum physics can't explain black holes and their effects on space-time. We don't know where the material in a black hole goes to. For all we know, it goes to another universe or an alternate dimension.

And how does zero at infinity equal each other?

by **monarchist** » Thu Dec 22, 2011 4:37 am

Ann and Chris, thanks for probably answering the question of why in such images, the foreground galaxy is always (?) rather red and the lensed galaxie(s) always (?) very blue. However it would be nice to if someone could provide a really definitive explanation of why this appears to be so.

If it is just as Ann suggests a question of filters, then the images we are seeing are pretty misleading as to true color and it would be interesting to see what the "really" look like.

Is there a possibility that there is some sort of "gravitational slingshot" affecting light from the background galaxy, resulting in more energy and net blueing?

Also are we missing any nice cases of lensing because we're depending on color contrast, enhanced or not?

by **zbvhs** » Thu Dec 22, 2011 4:24 am

The way I visualize it, gravity along any axis through a spherical body rises to a maximum at the center and declines to zero at infinity in either direction. Presumably, space-time curvature, however it's expressed, would do the same. The higher the mass of the body, the higher and sharper the peak in gravity/space-time curvature. If a black hole has finite mass, space-time curvature and gravity should likewise be finite. Therefore, the dimpled-space-time depiction for a black hole would be very deep but should still have a bottom - whatever the bottom means.

by **r.w.b** » Thu Dec 22, 2011 12:44 am

Some thoughts, a call for clarification, and perhaps more research, please:

Is the light from the distant galaxy being 'inverted', i.e., light from that part of the distant galaxy which comes from 'above' the lensing galaxy is seen by us as being 'below' the lensing galaxy? Much in the same way as one can invert an image with a glass lens, excepting for the fact that, in the case of a gravitational lens, space itself is being deformed.

Or are we seeing the opposite effect to this; for instance,light from the distant galaxy which is slightly 'above' the lensing galaxy is apparently being moved further 'above' it, from our viewpoint? I.e., the light from the more distant galaxy appears to be 'splayed' apart by the lensing galaxy.

Which of these two is actually happening here, please? Logic tells me that space should be being bent towards the massive foreground galaxy, in which case we are seeing an 'inverted ' image of the lensed galaxy.

In other words, in which directions is space being deformed; towards the lensing galaxy, or way from it?

If it is towards the lens, then this implies that either light is being slowed down,from our viewpoint, as it passes by the large mass, or else time is being speeded up by it. So now we have the perceived fact that light does not have a constant speed as it travels through a vacuum; the more mass there is in a system, the slower light travels through it, from the viewpoint of an outside observer.

Would not that fact add doubt to the accuracy of estimates of distances within the universe? An extreme example of concentrated mass would occur at the Big Bang, or shortly after it. Light would be almost at a standstill, under that condition, and it's velocity would rapidly increase as time progressed.

Or does time come to a near standstill at that point? That would, in the limit, and as I see it, allow for a perceived infinite time for the age of the universe. And, logically, an infinite time for it's future. [Plus no future collapse, then? But that's another line of thought.] Thus pointing to an infinite size for the universe, at least from our point of view. So, any observer, from anywhere in the universe would see exactly the same processes that we are seeing. All could be forgiven for thinking that they are at the centre of the universe.

Either way, we now run into a big problem when we try to estimate accurately the distances and ages of visible objects. Maybe the understanding of perceived red shift of distant objects is on shaky ground, as a result of this light/time distortion?

So what is happening to the Hubble Constant, as we progress further and further away from here, or back in time, or back along the graph of the perceived change in the speed of light due to changes in mass density as the universe developed?

Has anybody else been thinking along these lines? If so, then what are your / their conclusions so far?

Apologies for being completely qualitative here; it's over 30 years since I last studied wave theory and the like, and I've forgotten more than I can remember. But these questions have been niggling me for all of that time, and longer. This is an excellent opportunity to field them.

by **craigmechengr** » Wed Dec 21, 2011 8:18 pm

I think this is proof of intelligent life in the Universe... looks like HAL "I'm sorry dave......"
c

by **Chris Peterson** » Wed Dec 21, 2011 7:42 pm

zbvhs wrote:Ok, I see what gravity does, but I still don't see what gravity is.

Join the crowd. Personally, I think the best answer is that gravity is nothing more than a behavior caused by the way mass and spacetime interact. In that sense, it isn't any thing at all.

Gravity, as I understand it, is related to space-time curvature. Gravity is maximum at the center of a body, therefore space-time curvature is maximum there. Space-time curvature is maximum because the body's mass is concentrated at its center, i.e., the body is a point mass. Is this the correct interpretation?

I think so. Although, you need to be a little careful when considering the gravity of an extended body, and how you define it. If you were at the center of the Earth, you'd have no gravitational force acting on you. But from a distance, the gravitational field of the Earth can be treated as originating at a single point, in the center.

Bodies in space are typically depicted as riding in dimples in space-time with a bottom under the body. Strictly-speaking this is not correct. The bottom shape implies a reversal in space-time curvature where curvature - and therefore gravity - is zero. In fact, the dimples should be depicted as bottomless.

The dimples are only bottomless for black holes. The problem is that this model is trying to show what happens in four dimensions using only three. Instead of a dimple, imagine the body floating in a liquid whose density changes with distance.

I saw a thing on tv recently about an experiment performed by one of the astronauts aboard ISS in which it was discovered that fine particles clump together in zero-g - much to everyone's surprise, apparently. Well, yes. If all bodies possess mass, they should bend space-time and thus possess their own gravity. Even the tiniest particles should be expected to gravitate or clump together in zero-g. What exactly was the mystery that was solved here?

The mass of the particles was too small to allow for gravity to bring them together in the observed time, given the initial volume. So it is assumed that the clumping was an electostatic phenomenon. The significance of the discovery is that it provides a plausible mechanism for the initial clumping of dust presumed to occur early in the process of stellar system formation.

What I'm still missing here is why mass bends space-time. How are we going to be able to generate gravity so that future astronauts will be able to walk the decks of their spacecraft as depicted in Star Trek or Star Wars?

"Why" is a philosophical question <g>. I'd replace it with "how", and the answer to that isn't known. People are trying to answer that using Higgs bosons, gravitons, and other force mediated carriers. But so far, not much luck there, outside lots of elegant math that may have no connection at all with reality.

by **zbvhs** » Wed Dec 21, 2011 7:25 pm

Ok, I see what gravity does, but I still don't see what gravity is.

I understand how lensing works. In this case light from the Distant Galaxy traveling on a specific conical angle from it is deflected by the LRG's space-time curvature to what we see. We would see similar images on very-slightly larger or smaller cones if we were a few light-years further away or nearer along the line of sight. On larger cones, the lensing effect would disappear altogether. On sufficiently smaller cones, the DG's light would converge to a point. What would we see then, a blue blob obscuring the LRG altogether?

Gravity, as I understand it, is related to space-time curvature. Gravity is maximum at the center of a body, therefore space-time curvature is maximum there. Space-time curvature is maximum because the body's mass is concentrated at its center, i.e., the body is a point mass. Is this the correct interpretation?

Bodies in space are typically depicted as riding in dimples in space-time with a bottom under the body. Strictly-speaking this is not correct. The bottom shape implies a reversal in space-time curvature where curvature - and therefore gravity - is zero. In fact, the dimples should be depicted as bottomless.

I saw a thing on tv recently about an experiment performed by one of the astronauts aboard ISS in which it was discovered that fine particles clump together in zero-g - much to everyone's surprise, apparently. Well, yes. If all bodies possess mass, they should bend space-time and thus possess their own gravity. Even the tiniest particles should be expected to gravitate or clump together in zero-g. What exactly was the mystery that was solved here?

What I'm still missing here is why mass bends space-time. How are we going to be able to generate gravity so that future astronauts will be able to walk the decks of their spacecraft as depicted in Star Trek or Star Wars?

by **Chris Peterson** » Wed Dec 21, 2011 7:20 pm

Donnageddon wrote:I understand that the photons "curved" with gravitational lensing are not actually curved, but just following a straight line in a "warped" space time fabric. But is this also the case with a glass lens? Are the photons passing through a glass prism still following a straight line, is the glass prism altering the space-time fabric?

The lensing mechanisms themselves are completely different. In the case of a glass lens, the light changes direction where it encounters a change in index of refraction (i.e. a change in the speed of light in that medium). This change of direction, called refraction, is described by Snell's law. It has nothing to do with any distortion of spacetime, but is a scattering phenomenon.

Gravitational lensing, of course, does involve the distortion of spacetime, and the motion of photons along geodesics. (Actually, geodesics can be used to analyze conventional optics, as well, but only from a mathematical standpoint; the physics behind GR geodesics and classical optics geodesics are different.)

by **Ann** » Wed Dec 21, 2011 7:04 pm

Czerno1 wrote:

Ann, I ever enjoy to read your entusiastic, informative comments!

Thanks!

AFAIR the count of stars in our own Galaxy is an estimated tenth of that trillion of yours, maybe two tenths (star population counts enjoy inflation, just like money) - out of these, I have no idea how many are "small red" stars, I bet rather a small proportion - Chris will know (wild guess : 1%? would be in the billions, not trillions!).

In his book Planet Quest, Ken Croswell claims that our own Sun is unusual as stars go, because it is more massive than about 90% of the stars in our galaxy. (Read about it in his chapter "A Shaky Start", page 79 in the book I have got.) In fact, according to Croswell, the Sun may beat 95% of the stars in our galaxy when it comes to mass. So contrary to belief, our Sun is not an average star.

On the other hand, according to Croswell, the red M-type stars are indeed average! Croswell claims that 80% of the stars in the Milky Way are red dwarfs of spectral class M.

Last year, a group of researchers found evidence that the M dwarfs were even more plentiful than expected. Pieter van Dokkum, an astronomer at Yale University, claimed that his group had found so many M-type dwarfs in the large elliptical galaxies that the total number of stars in the universe may actually triple when you take all the elliptical galaxy red dwarf denizens into account:

Elliptical galaxies are some of the largest galaxies in the universe. The largest of these galaxies were thought to hold more than 1 trillion stars (compared with the 400 billion stars in our Milky Way).
The new finding suggests there may be five to 10 times as many stars inside elliptical galaxies than previously thought, which would triple the total number of known stars in the universe, researchers said.

The researchers' computer models based on these findings suggest that red dwarfs are far more common than expected, with these galaxies each possessing roughly 20 times more red dwarfs on average than the Milky Way, said researcher Charlie Conroy, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

And that's why I think that there may actually be trillions of red dwarfs in a large elliptical galaxy.

Ann

by **Donnageddon** » Wed Dec 21, 2011 6:42 pm

Absolutely fantastic APOD and discussion! Thanks you Chris and Ann for your helpful explanations.

But this one still confuses me

Chris Peterson wrote: When mass distorts spacetime, it makes it appear (from outside the distorted region) that photons are following a curved path, rather than a straight line (they aren't... but that's another matter). This is very similar to what happens in a regular glass lens, where the direction of a photon is altered. That's really all lensing is- in a glass lens or in distorted spacetime- a change in direction of photons, where the magnitude of that change varies with the radial distance from the center.

I understand that the photons "curved" with gravitational lensing are not actually curved, but just following a straight line in a "warped" space time fabric. But is this also the case with a glass lens? Are the photons passing through a glass prism still following a straight line, is the glass prism altering the space-time fabric?

Thanks again!

by **Chris Peterson** » Wed Dec 21, 2011 6:13 pm

Al Denelsbeck wrote:I've been looking everywhere for this information and can't find it. Can anyone shed some light on how distant the LRG (the 'lens' itself) is?

The lensing galaxy, LRG 3-757, has a redshift of z=0.44 (comoving distance 5.5 bly, light travel time 4.6 billion years). The lensed galaxy has a redshift of z=2.38 (comoving distance 18.5 bly, light travel time 10.9 billion years).

by **Al Denelsbeck** » Wed Dec 21, 2011 5:37 pm

I've been looking everywhere for this information and can't find it. Can anyone shed some light on how distant the LRG (the 'lens' itself) is?

by **Chris Peterson** » Wed Dec 21, 2011 5:12 pm

Sskfan wrote:I look at images like today's and can't help thinking that they, and today's especially, look like anamorphic art. If you put a reflective cone in the middle of the front galaxy and look straight down at the tip of the cone, you might see what the background galaxy looks like - except that the tip of the cone would have to be slightly off-center, since the image isn't distributed perfectly around the circle. Some geek with far more math and computer skills than I might be able to determine what deformity the cone should have in order to get a coherent image...

It is sort of like that. But you can't reproduce a coherent image, because you don't know what the lens is actually shaped like. But since the background galaxy is so distant, it subtends only a tiny angle (on the order of an arcsecond), and it can therefore be treated as a point source. By assuming it is a point source, the characteristics of the gravitational lens can be worked out, and therefore the mass distribution of the foreground galaxy determined (most of which is invisible dark matter).

A gravitational lens differs from a more common optical lens in a couple of important ways. First, an ordinary lens has no effect on a ray passing through its center, and an increasingly greater effect on rays as they get farther from the center. That is, the power of an ordinary lens increases with radius. A gravitational lens is just the opposite: it has the greatest power at its optical center, and the effect on light decreases with increasing radius. Optically, a gravitational lens can be simulated with a lens like this one:

: Click to view full size image

However, this simulation will only be approximate, for another reason: with a typical glass lens, the light is only bent at the surfaces- entering the lens and exiting it. It travels in a straight line inside the lens. With a gravitational lens, there is no surface, and the light ray is curved as it passes through the distorted region of spacetime. In fact, there are lenses that do this, called gradient index lenses, but these are quite specialized, and not the sort of lenses found in most common applications.

Note the resemblance of the simulator lens to the base of a wine glass. As you sit at the Solstice or Christmas table this year, with your wine in front of you and some candles burning, you can divert yourself from the boring conversation of your relatives by playing around with focusing candlelight through the base of your glass, and develop a sense of the sort of images produced by gravitational lensing. (Just remember to say "uh-huh" every now and then, to keep everybody happy.)

by **Sskfan** » Wed Dec 21, 2011 4:01 pm

Czerno-1 wrote:Oldfart asked earlier and I am bumping his question, as I too would be very interested in an informed answer by one of our local, friendly experts :

(1) Do we, or do our computers, have the capability to produce an accurate image of the galaxy that was lensed?
I suspect the requirement of nan accurate[ image is a bit too much, but from image of mirages such as today's APOD can we reconstruct something of the original, making a few hypotheses & applying appropriate mathematical transforms, (de)convolution and what-have-you... ?

I look at images like today's and can't help thinking that they, and today's especially, look like anamorphic art. If you put a reflective cone in the middle of the front galaxy and look straight down at the tip of the cone, you might see what the background galaxy looks like - except that the tip of the cone would have to be slightly off-center, since the image isn't distributed perfectly around the circle. Some geek with far more math and computer skills than I might be able to determine what deformity the cone should have in order to get a coherent image...

Starship Asterisk*

APOD: A Horseshoe Einstein Ring from Hubble (2011 Dec 21)

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