APOD: Globular Star Cluster 47 Tuc (2015 May 19)

[APOD Robot]

Globular Star Cluster 47 Tuc

Explanation: Globular star cluster 47 Tucanae is a jewel box of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with over 150 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, 47 Tuc lies about 17,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Recent observations have shown that 47 Tuc's white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass. Other colorful low mass stars including yellowish red giant stars are easy to pick out on the outskirts of the cluster in this recently released sharp telescopic portrait by the Hubble Space Telescope.

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[Ann]

I thought this picture was the most interesting. It shows how the white dwarfs (inside green circles) are often found in the outer parts of 47 Tuc, while the more massive stars crowd at the center.

I note that a few blue stars that are not white dwarfs (because they are too bright) can be seen in 47 Tuc. But there is nothing resembling a large population of medium bright blue horzontal stars. The blue stars inside 47 Tuc are other kinds of creatures in the cosmic zoo. Could they possibly be freshly hatched, brilliantly bright new white dwarfs? Or could they be blue stragglers? They seem too bright and blue for that, I think.

Ann

[bystander]

http://asterisk.apod.com/viewtopic.php?t=34774

[ChrisW]

I wonder, what the brichtness in the center of 47 Tucanae is.

[JeffKLass]

The expression in the note "white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass" seems somewhat awkward and is probably the reverse of what is actually happening (most likely in ALL globular clusters), namely, that the heavier stars are attracting each other and slowly 'shuffling' themselves toward the 'attractor' at the center, thus leaving the lighter white dwarfs as a 'residue' at the outer edge. Question: are all stars in a globular cluster eventually going to 'shuffle' toward the center to form one gigantic black hole, and if so, how long will that take?

[Chris Peterson]

The expression in the note "white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass" seems somewhat awkward and is probably the reverse of what is actually happening (most likely in ALL globular clusters), namely, that the heavier stars are attracting each other and slowly 'shuffling' themselves toward the 'attractor' at the center, thus leaving the lighter white dwarfs as a 'residue' at the outer edge. Question: are all stars in a globular cluster eventually going to 'shuffle' toward the center to form one gigantic black hole, and if so, how long will that take?

Globular clusters are not stable. Stars are occasionally ejected, and since there are only a finite number of stars, they eventually evaporate away. Stellar collisions are vanishingly rare, so there's no mechanism to create a black hole (although some globulars already have one, probably for the same reason most galaxies do... although that reason is not well understood).

[BMAONE23]

I wonder, what the brichtness in the center of 47 Tucanae is.

The brightness at the center of 47 Tuc is the light of stars, stars so closely packed that their distances apart could be measured in AU rather than light years

Click to view full size image

http://www.universetoday.com/14380/clos ... 7-tucanae/

[neufer]

Globular clusters are not stable. Stars are occasionally ejected, and since there are only a finite number of stars, they eventually evaporate away. Stellar collisions are vanishingly rare, so there's no mechanism to create a black hole (although some globulars already have one, probably for the same reason most galaxies do... although that reason is not well understood).

<<The results of N-body simulations have shown that [globular cluster] stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 10¹⁰ years.>>
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[b][color=#0000FF][size=125]Messier 53 has surprised astronomers[/size] with its unusual number of blue stragglers.[/color][/b]

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<<Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars formed at the beginning of the cluster.

Astronomers have searched for black holes within globular clusters since the 1970s. The resolution requirements for this task, however, are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000 M☉ intermediate-mass black hole has been suggested to exist based on HST observations in the globular cluster M15 and a 20,000 M☉ black hole in the Mayall II cluster in the Andromeda Galaxy. Both x-ray and radio emissions from Mayall II appear to be consistent with an intermediate-mass black hole. These are of particular interest because they are the first black holes discovered that were intermediate in mass between the conventional stellar-mass black hole and the supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies. Claims of intermediate mass black holes have been met with some skepticism. The heaviest objects in globular clusters are expected to migrate to the cluster center due to mass segregation. As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15 and Mayall II.>>

The expression in the note "white dwarf stars are in the process of being gravitationally expelled to the outer parts of the cluster due to their relatively low mass" seems somewhat awkward and is probably the reverse of what is actually happening (most likely in ALL globular clusters), namely, that the heavier stars are attracting each other and slowly 'shuffling' themselves toward the 'attractor' at the center, thus leaving the lighter white dwarfs as a 'residue' at the outer edge.

[MarkBour]

I don't think we have yet thought of any way that, by simple astronomical observation, we could expect to see signs of intelligent life from immense distances. In our own case, humanity has not yet done anything on or around Earth that humanity could see from a distance of over 1000 light years, agreed?

Nevertheless, when I look at images of globular clusters, one reaction I have is: "If we could see the signs, this is the kind of place I would most want to look." I guess it just seems like the chances for life are greatest in places such as these. I don't know if anyone agrees with that hunch. Are there any facts about globular clusters that might work against this? For example, are they places that are somehow over-radiated for beings such as ourselves? Are the interactions destructive of planets? Perhaps in a place such as this the formation of proto-planetary disks is uncommon... It'd be nice to be able to find out if these stars are as planet-rich as the stars found by Kepler.

[neufer]

when I look at images of globular clusters, one reaction I have is: "If we could see the signs, this is the kind of place I would most want to look." I guess it just seems like the chances for life are greatest in places such as these. I don't know if anyone agrees with that hunch. Are there any facts about globular clusters that might work against this? For example, are they places that are somehow over-radiated for beings such as ourselves? Are the interactions destructive of planets? Perhaps in a place such as this the formation of proto-planetary disks is uncommon... It'd be nice to be able to find out if these stars are as planet-rich as the stars found by Kepler.

<<In 2000, the results of a search for giant planets in the globular cluster 47 Tucanae were announced. The lack of any successful discoveries suggests that the abundance of elements (other than hydrogen or helium) necessary to build these planets may need to be at least 40% of the abundance in the Sun. Terrestrial planets are built from heavier elements such as silicon, iron and magnesium. The very low abundance of these elements in globular clusters means that the member stars have a far lower likelihood of hosting Earth-mass planets, when compared to stars in the neighborhood of the Sun. Hence the halo region of the Milky Way galaxy, including globular cluster members, are unlikely to host habitable terrestrial planets.

In spite of the lower likelihood of giant planet formation, just such an object has been found in the globular cluster Messier 4. This planet was detected orbiting a pulsar in the binary star system PSR B1620-26. The eccentric and highly inclined orbit of the planet suggests it may have been formed around another star in the cluster, then was later "exchanged" into its current arrangement. The likelihood of close encounters between stars in a globular cluster can disrupt planetary systems, some of which break loose to become free floating planets. Even close orbiting planets can become disrupted, potentially leading to orbital decay and an increase in orbital eccentricity and tidal effects.>>

[Craine]

Nevertheless, when I look at images of globular clusters, one reaction I have is: "If we could see the signs, this is the kind of place I would most want to look." I guess it just seems like the chances for life are greatest in places such as these. I don't know if anyone agrees with that hunch. Are there any facts about globular clusters that might work against this? For example, are they places that are somehow over-radiated for beings such as ourselves? Are the interactions destructive of planets? Perhaps in a place such as this the formation of proto-planetary disks is uncommon... It'd be nice to be able to find out if these stars are as planet-rich as the stars found by Kepler.

I was wondering about possible planetary systems as well. I imagine that Kuiper Belts and Oort Clouds would likely be constantly perturbed to the point of extinction. Planetary orbits themselves would be affected by passing stars as well. But, especially close in, perhaps they might survive? No idea about the radiation levels, but a decent magnetosphere might protect against that.

Ultimately I think life would find it very difficult in such clusters. But who knows...

[BMAONE23]

Most of the central mass of stars are separated by less than 80AU, our own solar system wouldn't fit between any 2 given stars so most planetary systems would have little chance of being unaffected / distorted / distroyed by the gravitational tug of neighboring stars. The central region of 47 Tuc would likely be to disruptive to allow for societal development if planets could form.
The less dense outer regions might form and retain planets though, provided the stars initial formation was in an area offering fewer competition for the heavier elements needed to create them

[Craine]

Most of the central mass of stars are separated by less than 80AU.

You know, I never did the math on that until I saw your numbers. That's just crazy in there.

[Chris Peterson]

Most of the central mass of stars are separated by less than 80AU.

You know, I never did the math on that until I saw your numbers. That's just crazy in there.

And yet, it's still almost entirely composed of empty space. You could zip back and forth through there in your ultrafast starship, thousands of times, and never come close to hitting a star.

[Craine]

And yet, it's still almost entirely composed of empty space. You could zip back and forth through there in your ultrafast starship, thousands of times, and never come close to hitting a star.

I could, and I would, except for that FTL starship thingie. Minor hiccup there. I'll get right on that.

[BMAONE23]

Be a good place for Pluto Pinball though

[Craine]

Can you imagine what the sky would be like on a (yeah I know, non-existent) planet in there?
Hundreds of those stars must be bright enough to be seen even during a nice sunny day.

[geckzilla]

Can you imagine what the sky would be like on a (yeah I know, non-existent) planet in there?
Hundreds of those stars must be bright enough to be seen even during a nice sunny day.

At some point in some other thread (this comes up every time we have a glob on APOD) someone mentioned that the brighter ones would be about as bright as Jupiter or maybe Venus. I know you can observe Venus sometimes during the day, but it might not be all that easy to see something like that even in a globular cluster.

[Nitpicker]

I imagine that any intelligent life in a globular cluster would be rather stressed out. The likelihood of having a model good enough to predict all the perturbations, is probably low. One could never be sure that one's planet would not suddenly be hurtled too close to, or too far from, a life giving star (or stars), for significant periods of time. It seems more likely that intelligent life would not have time to evolve in such an environment.

[Ann]

I imagine that any intelligent life in a globular cluster would be rather stressed out. The likelihood of having a model good enough to predict all the perturbations, is probably low. One could never be sure that one's planet would not suddenly be hurtled too close to, or too far from, a life giving star (or stars), for significant periods of time. It seems more likely that intelligent life would not have time to evolve in such an environment.

My thoughts exactly. The problem with globular clusters when it comes to habitable planets is probably not finding enough Earth-sized planets which could be coaxed into being habitable. The problem is the crazy pin-ball game going on in there.

Ann

[Nitpicker]

Of course, we can never be sure, either, that we won't collide with some asteroid, or comet, or some such dreaded thingy (which could sneak up on us at any moment, especially from behind the Sun). Still, I'd take the relatively low risk Solar System, over a globular cluster, every time.

[alter-ego]

Can you imagine what the sky would be like on a (yeah I know, non-existent) planet in there?
Hundreds of those stars must be bright enough to be seen even during a nice sunny day.

At some point in some other thread (this comes up every time we have a glob on APOD) someone mentioned that the brighter ones would be about as bright as Jupiter or maybe Venus. I know you can observe Venus sometimes during the day, but it might not be all that easy to see something like that even in a globular cluster.

In detail, I attempted to answer this question last year for Omega Centauri. I included the best known population density profile(s), total population estimates out to 120 light years and assumed all members had the same absolute magnitude. The results were interesting and impressive. So the number of stars visible (full sky, 2 hemispheres) for such an observer is: ~25 brighter than -4.4 mag (Venus), and ~200 brighter than Jupiter at opposition (-2.4 mag). There's on average, ~1 star in the entire sky at about -8 mag.

Defining a conservative limiting visual magnitude of between +4 and +5 due to sky brightness (avoid the brightest stars!), the full-sky star-count visibility is between and 400,000 and 1.1 million stars ( somewhere between 10 and 25 stars per square degree)! This is roughly 40x to 100x the number of stars we can see from Earth. The integrated sky brightness is ~40% to ~50% of a full moon distributed over both sky hemispheres (unlike here). Imagine looking up and seeing visible stellar densities exceeding the Pleiades everywhere!

Also, one result surprised me is that I could realistically simulate nighttime views by magnitude scaling a view of our own Milky Way star population, thus bringing a more realistic brightness distribution into the mix.

[Ann]

That's amazing, alter-ego. Thanks for sharing this stunning information with us.

Ann

[Craine]

According to http://en.wikipedia.org/wiki/Apparent_m ... al_objects on Sedna at aphelion our Sun has an Apparent Magnitude of -11.2, far brighter then Venus at ~-4. And that is at 936 AU. So, yeah...lotsa stars in the day sky.

[BMAONE23]

Speaking of distant objects and close stars, the stars of 47 Tuc orbit so close that no star near the central mass could develop planetary systems to the size of ours or share our design with a similar Kuiper belt or Oort cloud at least to the size of ours. KBO's would be within the area of influence of other stars and Oort objects would be regularly passed through
https://solarsystem.nasa.gov/planets/pr ... bject=KBOs
Now Kepler has detected planetary systems where most of the detected planets orbit within the orbital area of Venus. These could be stable within a large mass of stars, it would just expend on how orbital dynamics were tweaked by other passing stars.