APOD: The Reddening of M71 (2014 Dec 10)

[APOD Robot]

The Reddening of M71

Explanation: Now known to be a globular star cluster at the tender age of 10 billion years, M71 is a mere 13,000 light-years away within the narrow boundaries of the faint constellation Sagitta. Close to the plane of the Milky Way galaxy in planet Earth's sky, its 10,000 or so member stars are gathered into a region about 27 light-years across near the center of this color composite view. In fact, the line-of-sight to M71 passes along the galactic plane through much intervening diffuse interstellar dust. The dust dims starlight and scatters blue light more efficiently, masking the brightness of M71's stars and shifting true star colors toward the red. How much are the star colors shifted? Slide your cursor over the image (or follow this link) to use an estimate of the dust reddening or galactic extinction to correct the star colors in M71. Corrections to the brightness and colors of M71 member stars are needed to measure the cluster's distance and age using a Color-Magnitude diagram.

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[Ann]

Interesting demonstration. Thanks.

I note that there appear to be no blue horizontal stars in M71. The evolved blue stars might really be missing, as is the case in 47 Tuc, due to the fact that 47 Tuc is moderately metal-rich as globular clusters go. The same might be true for M71. But it could also be that the blue horizontal stars are really there in M71, and they are just too reddened by intervening dust for us detect them - at least in an optical image like this one.

Ann

[somebodyshort]

A diameter of 27 light years yields a volume of just over 10,000 cubic light years, With 10,000 members that's about a cubic lightyear per star. A sphere of a cubic light year has a diameter of 1.25 light years. I assume from that that stars would be about 1.25 light years apart, much less at a denser center. At those distances it would make observation of neighbouring stars much easier. Apart from still being a huge undertaking, comumication and travel between stars would be much easier.

I assume all of the nebular matter from which the stars have formed has either been conumed in star formation or been blown away. I'm curious why none of the stars has gone supernova and created a new nebula. I'm also curious because the material density would have been high to start with that there does not appear to be any super massive stars in the cluster

[Boomer12k]

A diameter of 27 light years yields a volume of just over 10,000 cubic light years, With 10,000 members that's about a cubic lightyear per star. A sphere of a cubic light year has a diameter of 1.25 light years. I assume from that that stars would be about 1.25 light years apart, much less at a denser center. At those distances it would make observation of neighbouring stars much easier. Apart from still being a huge undertaking, comumication and travel between stars would be much easier.

I assume all of the nebular matter from which the stars have formed has either been conumed in star formation or been blown away. I'm curious why none of the stars has gone supernova and created a new nebula. I'm also curious because the material density would have been high to start with that there does not appear to be any super massive stars in the cluster

Have you taken a look at Westerlund 1? a seemingly newly evolving cluster. Many super massive stars...and even possibly a "star merger"...a very interesting read.

:---[===] *

[loudube]

Is that the same red shift the universe to be expanding? Couldn't dust be the explanation there as well!
Maybe that's the way dark matter shows itself.

[Ann]

somebodyshort wrote:
I'm curious why none of the stars has gone supernova and created a new nebula. I'm also curious because the material density would have been high to start with that there does not appear to be any super massive stars in the cluster

It seems certain that there have been several supernovas in M71, but they must have happened a very long time ago.

M71 is a globular cluster, extremely rich in stars. It seems unavoidable that such a massive and rich cluster would also produce some very massive individual stars that are destined to go supernova eventually.

But M71, like all other Milky Way globulars, is extremely old. I haven't tried to find out the age of M71 specifically, but an age of about 12 billion years seems very reasonable. That means that the very massive stars of M71 formed about 12 billion years ago. Stars that are massive enough to go supernova because they have built up iron in their cores explode after only a few dozen million years at most. In other words, the core-collapse supernovas of M71 exploded about 11 billion years ago or more.

There are also type Ia supernovas, where the progenitor stars are not so massive and can therefore exist for a relatively long time before they blow. But no supernova type Ia has ever been definitely observed in a globular cluster either in the Milky Way or in any of our closest galactic neighbours. It could well be that most of the supernovas type Ia in the globular clusters have also already exploded, although we can't rule out the possibility that a supernova type Ia may still go pop in one of the Milky Way globulars.

Ann

[Bultineer Bosco]

I haven't tried to find out the age of M71 specifically, but an age of about 12 billion years seems very reasonable.

Now known to be a globular star cluster at the tender age of 10 billion years...

Yes?

Also, question: Do you suppose M71 was a satellite galaxy that the Milky Way sucked in?

[MarkBour]

Is that the same red shift the universe to be expanding? Couldn't dust be the explanation there as well!
Maybe that's the way dark matter shows itself.

I like this hypothesis. I'm not an astronomer or physicist -- I'm a total amateur. No doubt one of the more experienced people on this forum will argue with this. But I've been wondering this same thing since I first learned of the redshift years ago.

What I see in Wikipedia leads me to believe that Hubble himself held to the view that his redshift law was an observational truth, but open to various interpretations. That it is perhaps caused by something else, not the recessional velocity of the galaxies nor the expansion of the Universe.

If you look at this article http://en.wikipedia.org/wiki/Hubble%27s_law, the terse mention of it there is that Einstein visited Hubble and thanked him: "In 1931, Einstein made a trip to Mount Wilson to thank Hubble for providing the observational basis for modern cosmology.[32]". However, if you look at this biographical article http://en.wikipedia.org/wiki/Edwin_Hubble, it appears that while Hubble was supportive of certain other interpretations of the redshift, he remained undecided himself on the ultimate interpretation. That article has:

To the very end of his writings he maintained this position, favouring (or at the very least keeping open) the model where no true expansion exists, and therefore that the redshift "represents a hitherto unrecognized principle of nature."[28]

and

In fact, Einstein apparently once visited Hubble and tried to convince him that the universe was expanding.[29]

Which is a very different accounting of the matter.

We have plenty of examples of dust reddening. Our own sun shows it in the sky every morning and evening, and I don't think it is racing away from the Earth at those times. That this cluster's light reaching us is reddened by dust seems pretty "solid".

So, what is the reddening of light as it travels across intergalactic space for billions of years? I have not studied General Relativity. Evidently it predicts it as space itself expands. But for now, I (though I am just one unlearned amateur voice among the noise) believe it is most likely simply a loss of energy from the photons to the medium through which they travel. I would even hazard the guess that the microwave background is made up of the very tired photons that have traveled perhaps even trillions of light years and have lost so much energy that they are now microwaves.

But, I admit that I am sharing much ignorance with you in this response. Perhaps someone with more training in physics and cosmology will come along and either offer more support, or offer contrary arguments, theories, or evidence.

[Visual_Astronomer]

There is a difference between flitering and red-shifting.

The dust in space or in our atmosphere works like a red filter; passing red light and blocking blue.

The "red-shift" of cosmic expansion refers to an actual frenquency shift toward longer wavelengths of known spectral emission lines. Filtering does not shift frenquency, despite how the visual result appears.

[Chris Peterson]

So, what is the reddening of light as it travels across intergalactic space for billions of years? I have not studied General Relativity. Evidently it predicts it as space itself expands. But for now, I (though I am just one unlearned amateur voice among the noise) believe it is most likely simply a loss of energy from the photons to the medium through which they travel.

See Visual_Astronomer's comment about the difference between reddening and redshift. They are very different things, both theoretically and observationally.

What you are suggesting is called the tired light hypothesis, and is not considered a reasonable explanation for cosmological redshift due to the fact that any known or proposed mechanism for a loss of energy has been ruled out by experiment or observation. Since we have no evidence that supports tired light, a good deal that invalidates it, and very strong evidence supporting the actual cause of cosmological redshift as a result of expanding spacetime during the transit of the photon, the hypothesis is not taken seriously by cosmologists.

Believing in it these days would constitute pseudoscience.

[water-logged]

Images of elliptical galaxies such as m87 also suffer from yellowing and reddening for somewhat similar reasons. The dust surrounding elliptical galaxies must be the cause of yellowing and red shifting; to assert that the yellow color is due to old population stars may be a premature conclusion of astronomical survey. Likewise. the images of very distant cosmic objects may be red shifted due to the build up of dust across billions of light years and not necessarily be the result of cosmic expansion. How much of the red shifting is due to dust or cosmic expansion will probably be an issue for future academic debates. Elliptical galaxy images must be corrected to remedy the yellowing due to dust. It is highly unlikely that the yellow color of elliptical galaxies is due to old population stars. The extreme conditions of gravity and magnetic compression associated with elliptical galaxy requires a more in depth observation of its core region. A hint of an elliptical galaxy's brilliant and violent core are the subatomic jets blasting out of the galaxy core region. The dust enveloping an elliptical galaxy must be hiding more than a relativistic galactic core.

[Chris Peterson]

Images of elliptical galaxies such as m87 also suffer from yellowing and reddening for somewhat similar reasons. The dust surrounding elliptical galaxies must be the cause of yellowing and red shifting; to assert that the yellow color is due to old population stars may be a premature conclusion of astronomical survey. Likewise. the images of very distant cosmic objects may be red shifted due to the build up of dust across billions of light years and not necessarily be the result of cosmic expansion. How much of the red shifting is due to dust or cosmic expansion will probably be an issue for future academic debates.

There is no debate at all, nor will there be. You are confusing reddening with red shifting. No amount of dust will cause redshift. The wavelength of any measured emission line will be unaltered by transmission through dust. All the dust does is scatter away shorter wavelengths, making a continuum source appear redder.

Cosmological redshift is caused by the expansion of space. That is supported by so many independent lines of evidence that virtually nobody seriously questions it anymore. But even the handful of cosmologists exploring other options recognize that intervening dust plays no role.

[geckzilla]

Is there a way to take spectroscopic examples of redshift and reddening to illustrate how different they are? I think the verbal explanation is probably the better one, but maybe there is a way to drive the point further with a picture.

[Ann]

I haven't tried to find out the age of M71 specifically, but an age of about 12 billion years seems very reasonable.

Now known to be a globular star cluster at the tender age of 10 billion years...

Yes?

Also, question: Do you suppose M71 was a satellite galaxy that the Milky Way sucked in?

Ooops, I hadn't even read the caption. Thanks for telling me. So M71 is only 10 billion years old. That makes it relatively young for a globular cluster. It might also make it metal-rich as globular clusters go, and if so, it makes sense that this cluster would contain no blue horizontal branch stars.

Was M71 a satellite galaxy that the Milky Way sucked in? To my knowledge, the only Milky Way globular cluster that was originally the core of an independent galaxy is Omega Centauri. And as far as I know, there is only one Milky Way globular that was originally a globular cluster in another galaxy, and still technically belongs to another galaxy, and that is M54. It actually belongs to the Sagittarius Dwarf elliptical galaxy, although it is certainly in the process of being fully integrated into the Milky Way.

But isn't it possible that M71 also belonged to another galaxy a long time ago? Perhaps it is, but if so, astronomers have found no trace of any remnants of this other galaxy. And therefore it seems more likely that M71 has always been a part of the Milky Way.

Ann

[MarkBour]

Thanks, Chris and Visual_Astronomer, for explaining the difference between red-shifting and reddening. I had wondered if you could tell a difference, and evidently it's clearly different. So, one starts with a spectrum of light, and dust-reddening reduces the amount coming through, preferentially reducing the higher frequencies, whereas redshifting of distant galaxies moves the whole spectrum over? I will go read about the "tired light hypothesis" and why it has not held up under scrutiny. I agree with geckzilla, that a picture could be helpful here. I think I get what you said, but I will look for one that portrays it. (Perhaps it would be labelled a spectral chart? Maybe there is one in Dr. Nemiroff's course materials...)

[Chris Peterson]

So, one starts with a spectrum of light, and dust-reddening reduces the amount coming through, preferentially reducing the higher frequencies, whereas redshifting of distant galaxies moves the whole spectrum over?

Exactly. Indeed, redshift can actually make a source appear more blue, depending on its original spectral content (for instance, if invisible UV shifts into visible blue).

[Ron-Astro Pharmacist]

Just as some astrophotographers enhance a nebula's color is it possible to enhance the intrinsic color of stars? Say to get a Christmas tree-like light effect.

Christmas tree.jpg (3.58 KiB) Viewed 21099 times

[Ron-Astro Pharmacist]

Or even to assigned the stars with a color coded distance estimate. I suppose, when Gaia data become available, this might be done to be able to look at objects to get a visual clue for those stars of close proximity.

http://www.spaceflight101.com/gaia-mission-updates.html

[DavidLeodis]

I like the image and also that Bob Franke has given a lot of information about it through the "this color composite view" link. I find it adds greatly to the interest of an APOD when at least the date and location an image was acquired is available (at least through a link) and here Bob also gives information on such as the exposures.

[geckzilla]

I agree with geckzilla, that a picture could be helpful here. I think I get what you said, but I will look for one that portrays it. (Perhaps it would be labelled a spectral chart? Maybe there is one in Dr. Nemiroff's course materials...)

I have been thinking of a way to illustrate the concept. I understand it well enough that I could create a number of images but the problem is containing what is a dynamic concept into a single, easy to understand image. I would hate to make it even more confusing when the fact is, as Chris said, that there really is no confusing dust reddening with redshifting. Dust reddening is something we can easily observe up close and in nearby galaxies but at the distances where redshifting becomes very apparent and there is no mistaking it. The wavelengths are NOT attenuated or scattered. They are clearly shifted because we still receive their signal. In fact they could be both reddened by dust and redshifted at the same time and we are able to see distant dust in distant galaxies. That is why infrared light is so important in astronomy because all of our favorite wavelengths get pushed into this red zone. If the Universe was really full of a fine dust, we'd never even see blue light (as shifted into redder light). Plus, the dust would have to be in an impossibly smooth distribution. What would make the dust do that instead of being thicker in some places and thinner in others? The idea is easily broken down even by an amateur such as myself.

[geckzilla]

Attempt at a drawing.

[Ann]

Or even to assigned the stars with a color coded distance estimate. I suppose, when Gaia data become available, this might be done to be able to look at objects to get a visual clue for those stars of close proximity.

http://www.spaceflight101.com/gaia-mission-updates.html

Gaia will produce tremendously interesting results, if things work out as they are supposed to, but unfortunately Gaia will not tell us much about the color and temperature of huge numbers of stars. Gaia will photograph the stars through two filters, a red and a blue one, but that means that all stars that are dust-reddened will be seen by Gaia as red and cool. This means that Gaia will not be able to see the true nature of for example NGC 3603, a compact young cluster full of hot blue stars. This splendid picture has been processed by stephen 63 using Hubble images taken through no fewer than six different filers, to produce an image where the stars look properly blue. To Gaia the stars will look red, because they are reddened by dust to an apparent color redder than the Sun.

Ann

[Chris Peterson]

I agree with geckzilla, that a picture could be helpful here. I think I get what you said, but I will look for one that portrays it. (Perhaps it would be labelled a spectral chart?

I think it would be difficult to show in an astronomical image. But a spectral chart is doable. Here is the spectrum of M66, with some simulations applied. The top chart shows the actual spectrum, and an example of what we might see through dust. Note the prominent H-alpha spike at 656 nm. The bottom chart shows the same thing, but now we've moved the galaxy 500 million ly away (from its actual 36 million ly distance). It now has a redshift of z=0.037, which pushes the H-alpha line up to an observed 680 nm.

In either case, the dust creates reddening by attenuating the blue end of the spectrum more than the red end. But the dust never changes the position of any of the emission lines.

[Ann]

Attempt at a drawing.

reddning_vs_redshift.png

Great image, Geck, but I think that the two last spectra (those from galaxies) have been blueshifted instead of redshifted.

Otherwise the illustration is indeed extremely instructive and illuminating.

Ann

[geckzilla]

I think that the two last spectra (those from galaxies) have been blueshifted instead of redshifted.

Hm, I don't think so.