Starship Asterisk*

by **neufer** » Fri Mar 30, 2012 2:34 pm

Click to play embedded YouTube video.

by **Chris Peterson** » Sun Mar 25, 2012 1:07 am

dlw wrote:Thanks. That was what I wondered - whether they were actually in orbital motion around a center of mass or whether they weren't or weren't moving at the required radial velocity and thus would that indicate the presence of dark energy which, as I've read, has a repulsive effect. So I thank you for clarifying that for me.

Dark energy is repulsive, but is very, very weak. It's effect is only significant over cosmological distances. Within a cluster, or even an entire galaxy, any repulsion it could provide would be infinitesimally small.

FWIW, globular clusters are small enough that they can be well modeled by dynamical simulations, so it's virtually certain that nothing is going on that can't be explained by ordinary (Newtonian) orbital dynamics. How the globulars formed in the first place remains pretty much a mystery, though.

by **dlw** » Sun Mar 25, 2012 12:16 am

Thanks. That was what I wondered - whether they were actually in orbital motion around a center of mass or whether they weren't or weren't moving at the required radial velocity and thus would that indicate the presence of dark energy which, as I've read, has a repulsive effect. So I thank you for clarifying that for me.

by **Chris Peterson** » Sat Mar 24, 2012 9:50 pm

dlw wrote:Chris, I did allude to that when I mentioned orbiting. Let me answer with another question: can you place a geostationary satellite in orbit over the north pole?

Of course, you can't place a geostationary satellite in a polar orbit. But you can place a geosynchronous satellite in a polar orbit, and dynamically the two are the same.

I can imagine 300,000 objects orbiting in the same plane - we probably have more that that in our solar system. 300,000 objects in randomly inclined orbits around a common 'core' without occasionally colliding or at least interacting gravitationally is harder to imagine. Gravitational interaction would lead to distorted orbits, etc...

300,000 bodies in randomly inclined orbits are much less likely to come close to each other than they would if they were coplanar. Statistically, it is unlikely that any stars in a globular cluster will ever collide. Like multiple bodies in coplanar orbits, bodies in orbit at different inclinations are inherently chaotic. Nevertheless, it takes tens of billions of years for a globular cluster to evaporate due to angular momentum transfer during near misses. Indeed, the orbits are distorted from simple ellipses by gravitational perturbations... but that doesn't result in significant instability.

Globular clusters don't collapse because each star is in its own orbit around the center of mass of the cluster as a whole. Ultimately, the individual stars are ejected- they don't ever end up in the center.

by **dlw** » Sat Mar 24, 2012 6:49 pm

Chris, I did allude to that when I mentioned orbiting. Let me answer with another question: can you place a geostationary satellite in orbit over the north pole?

I can imagine 300,000 objects orbiting in the same plane - we probably have more that that in our solar system. 300,000 objects in randomly inclined orbits around a common 'core' without occasionally colliding or at least interacting gravitationally is harder to imagine. Gravitational interaction would lead to distorted orbits, etc...

by **Chris Peterson** » Sat Mar 24, 2012 5:58 pm

dlw wrote:Pardon a naive question: why doesn't a globular (essentially spherical) cluster collapse? To counteract gravity there needs to be a repulsive force as well - dark energy perhaps? - or the stars need to be orbiting. If they are orbiting, I would expect a lot of collisions or at least gravitational interactions.

Rather than simply answering your question, let me play the teacher's trick, and turn it back into a few questions for you. Why doesn't our Solar System simply collapse into the Sun? What is the "repulsive force" that prevents that from happening? How are a globular cluster and the Solar System similar?

by **dlw** » Sat Mar 24, 2012 5:27 pm

Pardon a naive question: why doesn't a globular (essentially spherical) cluster collapse? To counteract gravity there needs to be a repulsive force as well - dark energy perhaps? - or the stars need to be orbiting. If they are orbiting, I would expect a lot of collisions or at least gravitational interactions.

Thanks.

by **Chris Peterson** » Sat Mar 24, 2012 4:43 am

neufer wrote:Metallicity, however, generally refers to the heavier elements in the photosphere or chromosphere.

Metallicity refers to the heavier elements, period. It is only as applied to stellar measurements that it refers to the outer layers- for two reasons. First, that's all we can observe spectroscopically, and second, that's the interesting part in terms of considering the original composition (since, as you say, the synthesized elements tend to be found in the core).

by **neufer** » Sat Mar 24, 2012 1:34 am

Chris Peterson wrote:
b24eagle wrote:
I thought (wrongly perhaps) that in general, older stars had more heavy elements. If that is correct, what is the thinking about why M9, twice the age of the sun, is lacking in heavy elements?
While any star produces some heavier elements as it undergoes ordinary nucleosynthesis, this effect is small compared with the heavy elements produced and dispersed when a star ends its life, either through the production of a planetary nebula, or by way of a supernova.

Every star produces some heavier elements (potentially up to iron) IN ITS CORE as it undergoes ordinary nucleosynthesis.

Metallicity, however, generally refers to the heavier elements in the photosphere or chromosphere.

by **Chris Peterson** » Fri Mar 23, 2012 11:08 pm

b24eagle wrote:I thought (wrongly perhaps) that in general, older stars had more heavy elements. If that is correct, what is the thinking about why M9, twice the age of the sun, is lacking in heavy elements?

While any star produces some heavier elements as it undergoes ordinary nucleosynthesis, this effect is small compared with the heavy elements produced and dispersed when a star ends its life, either through the production of a planetary nebula, or by way of a supernova. The low metallicity of stars in globular clusters argues for them being Population II stars- just the second generation of stars in the Universe. Such stars formed before many heavy elements had formed, so they consist only of hydrogen and helium. Very old stars are low metallicity because they formed a long time ago, from nothing but hydrogen and helium.

It seems likely that there were once massive stars, which have short lifetimes and are now gone. Any heavy elements they produced have either dissipated, or maybe remain but are largely invisible since there is nothing to ionize them. The environment of a globular cluster is not suitable for new star production, so eventually all that is left are smaller, old stars.

by **Chris Peterson** » Fri Mar 23, 2012 10:53 pm

RaygunRon wrote:Are there Black holes at the center of these clusters?

In general, no. There may be a small number of large globular clusters that contain massive black holes, but that isn't the norm (as it is for spiral galaxies, for instance).

by **Qweenie** » Fri Mar 23, 2012 7:17 pm

This photo reminds me of one of Isaac Asimov's stories: The Stars Like Dust.

by **amazed** » Fri Mar 23, 2012 5:12 pm

how long do blue stars last? I thought they'd all be red if the cluster is 10 billion years old (twice the age of the sun).

by **RaygunRon** » Fri Mar 23, 2012 5:12 pm

Are there Black holes at the center of these clusters?

by **rchristo** » Fri Mar 23, 2012 4:12 pm

It's "renowned," an adjective modifying "18th century astronomer Charles Messier," not "renown," a noun.

by **b24eagle** » Fri Mar 23, 2012 4:03 pm

I thought (wrongly perhaps) that in general, older stars had more heavy elements. If that is correct, what is the thinking about why M9, twice the age of the sun, is lacking in heavy elements?

by **jinger** » Fri Mar 23, 2012 3:03 pm

Astronomical archeology! This cluster, with stars void of heavy elements, is ancient. Akin to digging up a fossil here on terra firma. So now like any good archeologist, you try to reconstruct the story. This cluster, from the earlier generation of stars -- was it absorbed by the milky way or was it a founding father, part of the early collection of stars, the seeds around which this massive galaxy has grown?

Though come to think of it, all astronomy has a strong kinship to archeology, in that most of what we find from our vantage point are ancient snapshots of a universe in various stages of its evolution.

Hey, Beyond and Indigo_Sunrise - we are already rich! Our physical forms are made up of stuff from stars. Though I haven't figured out a way to bank that. But I hear that medical stocks have a good future...

by **Chris Peterson** » Fri Mar 23, 2012 3:00 pm

rstevenson wrote:Volume of a sphere is 4/3pi x r³, so for a radius of 45 ly the volume is about 381,703 ly³. With 300,000 stars you get less than 1 per cubic light year (averaged over the whole cluster.) Still that's a little crowded compared to our neighbourhood, where we have only our own sun in a sphere of about 310 ly³ before we get out to our nearest neighbour, Proxima Centauri.

All the more crowded when you consider that the stars are not uniformly distributed in a globular cluster. The center is several orders of magnitude denser than the outer regions. A small cluster like this probably has a central density on the order of 10 stars per cubic light-year.

by **neufer** » Fri Mar 23, 2012 12:58 pm

starstruck wrote:
If you were able to look up from the surface of a planet going around one of those stars in that crowded cluster, the nightsky would surely look pretty bright! By my rough reckoning, going by what the the GJ catalogue says, about 100 times as many stars as we have in our neighbourhood

I recently pointed out how much the recent Jupiter/Venus conjunction resembled the constellation Canis Minor except for it being ~100 times brighter. So just imagine 88 constellations in the sky as impressive as the Jupiter/Venus conjunction.

by **ritwik** » Fri Mar 23, 2012 12:44 pm

thanks!!i just jumped in to conclusion, i remember that formula 4/3pir^3

by **rstevenson** » Fri Mar 23, 2012 12:18 pm

ritwik wrote:300000 stars within 90ly diameter !!!!!are you kidding me???? ie 300000/90=3333.33 stars in a diameter of 1 light year ???

You only divided the number of stars by the diameter of the cluster. You should have used the volume of the cluster.

Volume of a sphere is 4/3pi x r³, so for a radius of 45 ly the volume is about 381,703 ly³. With 300,000 stars you get less than 1 per cubic light year (averaged over the whole cluster.) Still that's a little crowded compared to our neighbourhood, where we have only our own sun in a sphere of about 310 ly³ before we get out to our nearest neighbour, Proxima Centauri.

Rob (who hopes he did the math right)

by **space vacuum** » Fri Mar 23, 2012 11:48 am

Someone was asking me recently if stars really fly by the way they do in the movies (think specifically Star Trek) and I said not. Even in a globular cluster, if the stars are close enough to have them fly by at ten a second, why don't you see them in the background. I seemed to remember a figure of 1/10th of a light year apart but don't know if that is accurate or not. Anyone know? I can do the math assuming an even distribution but that doesn't account for the higher density at the center.

Whatever that number is I've always though globular clusters were beautiful. I remember seeing my first one in a 16" scope. Viewing conditions weren't that good but it was quite pretty nonetheless.

Just an aside, anyone have any mathematically accurate visualizations of what it would look like in the center?

by **Baffled** » Fri Mar 23, 2012 11:42 am

It also boggles my mind that stars can be "older than our sun" in a cluster that dense. Could anyone please explain how all these stars have stable orbits around each other and avoid collisions?

by **ritwik** » Fri Mar 23, 2012 10:56 am

300000 stars within 90ly diameter !!!!!are you kidding me???? ie 300000/90=3333.33 stars in a diameter of 1 light year ???

i guess it's due to the curvature of space ...like looking inside a well you see the sides coming together as we look deep inside ..something like that idk??? how the heck is that possible???

i'm blocked from physicsforums could someone please explain me a bit more on this 90ly diameter ?

by **Indigo_Sunrise** » Fri Mar 23, 2012 10:39 am

What a spectacular pile of diamonds!

And I'm with you, Beyond: why can't I be rich?
(instead of just good looking?!)

Awesome image!

Starship Asterisk*

APOD: Messier 9 Close Up (2012 Mar 23)

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