# Tag Info

38

No the sun is not part of a cluster. There are several types of clusters that we see in the sky. The most familiar is the "open cluster", like the Pleiades. These are a group of stars that formed together and have remained close. As the stars drift apart they can become part of a "moving group", a collection of stars that don't appear to be a cluster, but ...

23

In order to result in a disk-like stellar system there are two conditions that need to be satisfied. (a) The initial gas from which the stars form must have a significant ratio of rotational to gravitational energy. (b) The star formation would have to occur slowly enough, that the gas collapses to a disk before star formation is complete. The formation of ...

15

It is very likely that most stars are born in clusters, ranging in size from 100 stars to a million or more (Lada & Lada 2003). It has been suggested (from indirect evidence) that our Sun was born in a cluster of somewhere between 1000 and 10,000 siblings (Adams 2010). Unfortunately, most clusters do not survive beyond about 10 million years from their ...

14

Globular clusters occupy an interesting place in the spectrum of composite stellar systems. As you point out, they are highly concentrated populations of stars, and seem to lack any dark matter component, unlike more massive dwarf galaxies. Binary interactions become very important in simulating globular clusters, and interestingly enough (maybe ...

10

While some galaxies are disk-shaped, others (elliptical galaxies) are more spherical. Disk-shaped systems are those in which conservation of angular momentum of the star-forming gas played a central role; for more spherical systems, this was not the case. Globular clusters are thought to form in the densest portions of gas in galaxies with the highest ...

9

OK, having (finally) actually looked at the video, it's clear that Szymanek is looking at the center of M33. There is in fact a nuclear star cluster in the center of that galaxy; not knowing the field of view or the resolution, I can't tell how much of the central condensation is simply the unresolved, smeared-out nuclear star cluster (which is small -- you ...

8

I have tried to collate some info from various other answers. Density of stars where we are: About 0.15 per cubic parsec Density of stars "at the middle of the galaxy". Unfortunately I don't know how "100 ly" affects this. It's probably about this figure. About 50-100 stars per cubic parsec So in this QA it's "about 500x as dense&...

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In fact, some people had looked at it more seriously recently and conducted a computer simulation to visualize the night sky as seen from within a globular cluster. The article has appeared recently in Astronomy journal. This is just one example of a typical image inside a globular cluster: Some more discussion can be found here: http://io9.com/what-the-...

6

In a typical position in a globular cluster (maybe halfway between center and edge), there'd be many more bright stars in the sky due to the star density. These would be distributed unevenly in the sky, with more light coming from the center of the globular cluster. Depending on the globular cluster's orbit, we might be able to see the Milky Way face-on. ...

6

You have to assume a characteristic mass for the stars in the cluster. Usually, to get an approximate I go with stars of mass $1 M_\odot$. Then, given the density of $10^6\ M_\odot.\mathrm{pc}^{-3}$ you have a density of stars of $\rho_\star = 10^6\ \mathrm{pc}^{-3}$. In a box of $1\ \mathrm{pc}^{3}$, you have $10^6$ stars, so the characteristic distance ...

6

Globular clusters formed whilst the gas of the proto Milky Way was still approximately spherically distributed. The gas forms a dissipative system that loses energy and collapses (within the first billion years) to a disk whilst conserving angular momentum. Formed stars and clusters are essentially collisionless so the halo stars continue to have a ...

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The mean mass of the initial mass function is ~0.3 solar masses (Maschberger 2013). A GC with 10^5 solar masses will therefore have ~3 x 10^5 stars in it. Since the most massive stars will already have died in a GC, the mean mass of the present-day mass distribution of the GC will actually be slightly lower, implying a slightly larger number of stars, but ...

5

There are a couple of important distinctions between the two types of objects. Galaxies are objects which range in mass from about $10^{9}-10^{12} M_{\odot}$, and contain 'halos' of dark matter which represent the majority of the mass of the object. Now, there are things called dwarf galaxies (which are less massive than regular galaxies; I'd imagine that ...

5

In the book Galactic Dynamics by Binney and Tremaine (second edition) there is a whole section explaining the difference between the Jacobi radius and the tidal radius (page 677-chapter 8). Here, $r_J$ is defined as: $$r_J= R_0\left(\frac{m}{3M}\right)^{1/3}$$ The Jacobi radius $r_J$ (also, Roche or Hill radius) of an orbiting stellar system is ...

5

Particles in a gas approximate to point-like objects that interact roughly elastically through short-range forces when they collide, but are otherwise non-interacting. Stars interact gravitationally over long ranges, occasionally with each other, but always with the overall gravitational potential of the system. Sometimes people do talk thermodynamically ...

4

Let us assume the data for a globular cluster to be equivalent to that of M13. Given 300,000 stars and a radius of 1 ly, let us assume uniform density. Another assumption is to consider all stars to be Sun-like. The number density can be calculated as $$\rho = \frac{300000*3}{4\pi(1\ ly)^3} \approx 9 \times10^{-44}\ m^{-3}$$ Now, using the formula for ...

4

Check out the MASSCLEAN package, it can be used to generate artificial/synthetic clusters of arbitrary metallicities, ages, mass, radius (based on a King profile distribution) and even includes field stars contamination. Here's an example of a cluster generated with the code (right) imitating a known cluster (NGC 3603, left): See the original article for a ...

4

The usual thing is a King model. There are indeed free parameters. These are the central density, the "core radius" and a tidal truncation radius. The background and rationale for these models is given in the link. They provide a pretty good representation of the surface density of globular clusters (or indeed open clusters). They require a numerical ...

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There are many differences starting from size to the dark matter content... Please check the following link it has given much information : http://www.answers.com/topic/dwarf-spheroidal-galaxy

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Similar to what @Barrycenter said in his comment, There's a wide range of distances between 100 or 1,000 times closer on average and as close as binary stars. Alpha Centauri A and B get as close as 11.2 AU. The Sun and Alpha Centauri are roughly 270,000 AU apart so at 1,000 times closer, the close stars in a globular closer would still be (ballpark), 250-...

2

In the paper "A million binaries from Gaia eDR3: sample selection and validation of Gaia parallax uncertainties" El-Badry et al (2021) the Jacobi radius, in the context of orbiting binary White Dwarf's, is defined as the separation between two orbiting binaries beyond which the Galactic tidal field dominates a binary’s internal acceleration. The ...

2

So far we have about 150ish known globular clusters in the milky way. A list of all the known ones can be found HERE. This includes their locations (may be slightly shifted by now as they were correct as of 2011, the time of writing) and where applicable, the Messier/NGC/IC numbers are included for ease.

1

Since this appears to be a screenshot of Eyes on Exoplanets, I searched for "exoplanet 485" and found Kepler-485 b. With the colors exaggerated, the label area could be "Kepler-485" in yellow overwritten with "Sun" in orange. If this is correct, it's not a cluster, just a star with a planet, plus some artifacts from the instrument which took the image.

1

thanx Gabriel for adding the detailed definition to my reply.. an additional definition of the tidal radius hereafter in this definition it describes molecular clouds rather than stellar systems..again notice the difference (2 is in the numerator).. there is no reference on where he got this formalism from..this is from Tan 2000 eq (9) check also eq (8)(...

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An overly simplistic distinction might be that globular clusters do not have any cloudy/gaseous regions, with no current star forming activity occurring. Globulars tend to have very similar stars, which together with their lack of gas regions, indicates that they formed early on in a single star forming period - though actually some globulars show evidence ...

1

Stars in an open star cluster are - according to wikipedia - "loosly bound by mutual gravity". Whereas a globular cluster is more dense and will be "less loosly" bound - to use the same terminology. Since the stars in the open star cluster still are gravitationally bound, the total energy $E$ must be less than zero. This criterium that $E<0$ is always ...

1

Using unit analysis, I've found that a natural unit of angular momentum should be $$\tag{1} L_0 = \frac{G M_{\odot}}{c} \approx 8.816 \times 10^{41} \, \text{kg} \, \text{m}^2/\text{s}.$$ In the case of a bounded system of two equal masses, circularly moving around their center of mass, the total angular momentum (relative to the ...

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