Starship Asterisk*

M72: A Globular Cluster of Stars

Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Pictured above by the Hubble Space Telescope are about 100,000 of M72's stars. M72, which spans about 50 light years and lies about 50,000 light years away, can be seen with a small telescope toward the constellation of the Water Bearer (Aquarius).

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I should think that the viewer looking through the 'small' telescope, would be looking at a 'certain' constellation, instead of a globular cluster.

Beyond wrote:I should think that the viewer looking through the 'small' telescope, would be looking at a 'certain' constellation, instead of a globular cluster.

Well....Constellations are hard to fit into the view of a small telescope...you really need a regular camera, that can take good pictures at night. As for "small" it is a relative term. My old Jason 60mm refractor could have done the job of seeing it, but not resolve it....but my 10" Meade, definitely can do the job...and it is a "small" telescope. My Jason would not be wide enough to see the whole constellation though. But my wide angel eye piece on my Meade gets a good swath of sky....

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Hubble sure take some nice pictures...I wish they would send it into a higher orbit and save it....

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Beyond wrote:I should think that the viewer looking through the 'small' telescope, would be looking at a 'certain' constellation, instead of a globular cluster.

Any telescope can be used at any magnification. As a telescope gets larger (which usually refers to aperture, not magnification) it can be used at higher magnification without resulting in a dimmer image.

About the smallest and lowest power telescope that anybody uses to look at stars is a pair of binoculars, and even at the lowest power the field of view is not wide enough to see entire constellations. Constellations are naked eye objects. At 7 or 8 power, typical of binoculars, M72 is very apparent. I think the reference to a "small telescope" was simply intended to make clear that nothing more than the most basic optical equipment is required to see this object.

(And FWIW, you would point your telescope "toward" Aquarius to see M72, but the cluster itself is "in" Aquarius.)

Chris Peterson wrote:
About the smallest and lowest power telescope that anybody uses to look at stars is a pair of binoculars, and even at the lowest power the field of view is not wide enough to see entire constellations.

Some birder binoculars have a field of view exceeding 8º which is sufficient to view Crux.

neufer wrote:

Chris Peterson wrote:
About the smallest and lowest power telescope that anybody uses to look at stars is a pair of binoculars, and even at the lowest power the field of view is not wide enough to see entire constellations.

Some birder binoculars have a field of view exceeding 8º which is sufficient to view Crux.

Yes. Delphinus is another constellation that fits in a binocular field. (Technically, Delphinus is too large for a binocular field, but the asterism we associate with the constellation is not... something that is true for a handful of cases.) But I think my point stands, that constellations are not telescopic objects.

Nice to see your globular cluster pictures here, Boomer12k!

Ann

Ann wrote:Nice to see your globular cluster pictures here, Boomer12k!

Ann

Thanks, Ann....Do you see any Blue Stragglers in today's APOD????

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The thing that I've always wondered about globular clusters is how they relate to and interact with the spiral arms of the galaxy. It seems that the mechanism which creates the arms should also perturb the clusters, smearing them out over time. Perhaps it's just my lack of appreciation for the intrinsic low density of a galaxy that makes it seem odd that they continue to survive rotation after rotation.

It also seems obvious, but aren't all identified clusters in our own galaxy? There's no way we can identify them in any other galaxies, right?

jonpugh wrote:The thing that I've always wondered about globular clusters is how they relate to and interact with the spiral arms of the galaxy. It seems that the mechanism which creates the arms should also perturb the clusters, smearing them out over time. Perhaps it's just my lack of appreciation for the intrinsic low density of a galaxy that makes it seem odd that they continue to survive rotation after rotation.

They are far enough outside the disc, and small enough relative to their distance from the galaxy, that tidal forces are small compared with the internal gravitation holding them together. So they are relatively stable, with their evaporation resulting from internal transfers of angular momentum, not external.

It also seems obvious, but aren't all identified clusters in our own galaxy? There's no way we can identify them in any other galaxies, right?

There are probably a few around our own galaxy that haven't been identified; we can't see very well through the dense, dusty core. And the vast majority of globular clusters we know of- tens of thousands or more- are around other galaxies (nearly 500 around Andromeda alone).

Boomer12k wrote:

Ann wrote:Nice to see your globular cluster pictures here, Boomer12k!

Ann

Thanks, Ann....Do you see any Blue Stragglers in today's APOD????

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You know about blue stragglers! That's good!

Can't say that I spotted any. It's hard to extract that kind of color information from a color picture made from exposures through only two filters, one clear and one infrared one.

One star is a standout, however. It is found around 6 o'clock (or 5.45-ish). This star is extremely red. It looks stellar to me, so I assume it's a member of M72. Does anyone have any idea why this star would be so incredibly infrared? Chris?

Ann

The mystery to me is what sustains them. According to n-body simulations I've read about, they should either collapse into, perhaps, a black hole or simply evaporate with individual stars flying off every which way. So, why do we see some that are apparently extremely old? Are these stable structures with stars in complex stable orbits or is something else keeping them alive?

zbvhs wrote:The mystery to me is what sustains them. According to n-body simulations I've read about, they should either collapse into, perhaps, a black hole or simply evaporate with individual stars flying off every which way. So, why do we see some that are apparently extremely old? Are these stable structures with stars in complex stable orbits or is something else keeping them alive?

N-body simulations show them surviving just fine. Why would they collapse? There's certainly no mechanism that could cause that. The stars are in orbit around the center of mass of the cluster. Because of the density of the cluster, those orbits are somewhat perturbed, but it is rare for enough angular momentum to be transferred that an individual star ends up in a hyperbolic orbit (that is, reaches escape velocity with respect to the cluster). But it does happen, and over a long time- tens of billions of years- globular clusters do evaporate away. Like all naturally occurring orbital systems, they are only semi-stable.

Ann wrote:One star is a standout, however. It is found around 6 o'clock (or 5.45-ish). This star is extremely red. It looks stellar to me, so I assume it's a member of M72. Does anyone have any idea why this star would be so incredibly infrared? Chris?

No idea, but there are lots of stars that are bright in the IR- maybe because they are very cool, because they are surrounded with a dusty shell, or because of stimulated IR emission (OH/IR stars).

Boomer12k wrote:

Beyond wrote:I should think that the viewer looking through the 'small' telescope, would be looking at a 'certain' constellation, instead of a globular cluster.

Well....Constellations are hard to fit into the view of a small telescope...you really need a regular camera, that can take good pictures at night. As for "small" it is a relative term. My old Jason 60mm refractor could have done the job of seeing it, but not resolve it....but my 10" Meade, definitely can do the job...and it is a "small" telescope. My Jason would not be wide enough to see the whole constellation though. But my wide angel eye piece on my Meade gets a good swath of sky....

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It's not what is being looked at that i was referring to. It's who/what that's doing the looking. Did you actually click on the small telescope link

Beyond wrote:
It's not what is being looked at that i was referring to. It's who/what that's doing the looking.

Did you actually click on the small telescope link

That dog's been well trained

Missing / Wanted: Supernovae and Remnants in Star Clusters.
There are a lot of old Red Giants. Will they never die?

I have to get Sassy to use a telescope! She's kind of snoopy anyhow!

Chris Peterson wrote:

neufer wrote:

Chris Peterson wrote:
About the smallest and lowest power telescope that anybody uses to look at stars is a pair of binoculars, and even at the lowest power the field of view is not wide enough to see entire constellations.

Some birder binoculars have a field of view exceeding 8º which is sufficient to view Crux.

Yes. Delphinus is another constellation that fits in a binocular field. (Technically, Delphinus is too large for a binocular field, but the asterism we associate with the constellation is not... something that is true for a handful of cases.) But I think my point stands, that constellations are not telescopic objects.

well, if a very simple binocular is used reversed, holding it on the eyes with the wide part to the night sky, a whole constellation could be theoretical seen, but it would make it
hard to a human eye to see a constellation...physically correct? But totally useless

orin stepanek wrote:I have to get Sassy to use a telescope! She's kind of snoopy anyhow!

Click to view full size image

Do you see where the sun's shadow is? Is the telescope positioned toward the sun?

Moonlady wrote:Do you see where the sun's shadow is? Is the telescope positioned toward the sun?

Looks like it's pretty close to noon... so the dog is safe enough.

ThePiper wrote:Missing / Wanted: Supernovae and Remnants in Star Clusters.
There are a lot of old Red Giants. Will they never die?

Supernova remnants are not missing in globular clusters.

http://en.wikipedia.org/wiki/47_Tucanae wrote:

47 Tuc's dense core contains a number of exotic stars of scientific interest.

...

47 Tuc has 23 known millisecond pulsars, the second largest population of pulsars in any globular cluster.

A pulsar is almost certainly a supernova remnant. If there are 23 millisecond pulsars in globular cluster 47 Tuc, then this globular may well have seen at least 23 supernovae in the past.

However, the red giants that exist today in the globular clusters of the Milky Way are not going to explode as supernovae. The reason is that they are not even remotely massive enough to explode. Most of the brightest red giants in today's Milky Way clusters are probably not that much more massive than the Sun. The Sun, too, will become a bright red giant when it runs out of hydrogen in its core, and it will be even brighter when it runs out of helium in its core. But the Sun will never go supernova.

Of course there were massive stars in globular clusters in the past. The massive stars were there long long ago, when the globular clusters were young, and the universe was young. But the massive stars exploded as supernovae long ago, leaving, for example, 23 millisecond pulsars behind in 47 Tucanae.

The bright red giants in the globular clusters of today are actually dying and disappearing "all the time", although the process still takes too long for us to have "caught one in the act". But as a matter of fact, the brightest red giants in globular clusters are on their way out. Very soon, in cosmic terms, they will shed their outer envelopes and become white dwarfs. And as white dwarfs they may actually still go supernova, provided they have just the right sort of companion to interact with. But since the last white dwarf supernova actually witnessed in the Milky Way was seen in the sixteenth (or possibly seventeenth) century, don't hold your breath waiting for one to explode in a globular cluster.

As red giants die in globular clusters, others take their place. Not only are red giants dying all the time in globular clusters, but low-mass stars are leaving the main sequence to become red giants and other stars are leaving the blue horizontal branch to become (red) asymtotic giant branch stars.

Ann

Say Ann, if the sun runs out of hydrogen and becomes a red giant, then also runs out of helium and gets brighter, just how the heck can it keep getting brighter when it keeps running out of things to burn?? What's left after the hydrogen and helium goes, that it could burn at all, never mind get brighter??

This is how I understand it (but I may very well be wrong, mind you).

Even though the star runs out of hydrogen in its core, and helium in its core, it still contains hydrogen and helium in shells around the core. So the stars fuse hydrogen or helium (not both at the same time, I think) in those shells. The fusion process takes place closer to the surface of the star, making it swell. It could be, too, although I'm not sure, that more of the mass of the star is being used for fusion when the fusion process "has moved outwards" in the star than when it was confined to the core.

Stars also swell and get very bright just when their cores "go dead", or stop fusing. Then the cores shrink, a lot of energy is released, and the stars swell and get brighter.

Ann