37 votes
Accepted

Is our sun in a star cluster?

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 ...
James K's user avatar
  • 121k
24 votes
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Why aren't globular clusters disk shaped

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 ...
ProfRob's user avatar
  • 152k
15 votes

Is our sun in a star cluster?

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 ...
ProfRob's user avatar
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11 votes
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Is it possible for a star cluster to be composed of main sequence stars?

The opposite is true. You should check your sources. Globular clusters do primarily consist of small main-sequence stars. Globular clusters are believed to be formed in the halo of the Galaxy at the ...
planetmaker's user avatar
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10 votes
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What cluster of stars is this with a "dark donut" to one side?

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 ...
Peter Erwin's user avatar
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10 votes

Why aren't globular clusters disk shaped

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 ...
Nbody sim's user avatar
  • 101
8 votes
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What would our night sky look like if the solar system was 100 light years from the centre of the Milky Way

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'...
Fattie's user avatar
  • 1,126
8 votes

Is it possible for a star cluster to be composed of main sequence stars?

Most stars in a cluster form at the same time, hence the most massive ones, which have the shortest lifetimes, will have evolved onto the giant branches, while the majority of the stars sits happily ...
AtmosphericPrisonEscape's user avatar
7 votes
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Are there really confined Globular Clusters?

You are asking for a star cluster that sits at the bottom of an infinite potential well. Such clusters do not exist because they are largely the source of the potential that they reside in. There are ...
ProfRob's user avatar
  • 152k
6 votes
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How far apart are stars in Terzan 5 cluster?

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 $10^6\ M_\odot.\mathrm{pc}^{-3}$ ...
cphyc's user avatar
  • 603
6 votes
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Why don't globular clusters flatten with a galactic disc?

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 ...
ProfRob's user avatar
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6 votes
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How many stars are there in a Globular Cluster of 10^5 solar masses?

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 ...
J. O'Brien Antognini's user avatar
6 votes
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Jacobi vs tidal radius for star cluster

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, $...
charles's user avatar
  • 86
5 votes
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Difference in HR diagram clusters

The main difference is due to the age of the cluster. In our Galaxy the globular clusters are old ($>10^{10}$ years) and as a result the main sequence turn-off is down at something like $0.9 M_{\...
ProfRob's user avatar
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5 votes
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How to locate M13, Ring Nebula?

The ring nebula (also called M57) is quite beautiful and relatively easy to find! It is located in the constellation Lyra---which will be easy to find since it has one of the brightest stars in the ...
Keyan Gootkin's user avatar
5 votes
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Core radius of King's model

The first thing you should know (as Peter Erwin and eshaya have implied) is that the King Model is notorious for being a mishmash of observational and theoretical definitions. From what I understand, ...
Tom Donlon's user avatar
4 votes

Is it possible for a star cluster to be composed of main sequence stars?

So, unfortunately, there are some imprecisions in the answers to this question. Indeed, it is true that globular clusters are mostly composed of main-sequence (dwarf) stars just like @planetmaker said....
guilimberg's user avatar
3 votes
Accepted

Average velocity of stars in globular cluster

$\sigma^2$ is the variance and the (general) definition of variance (for any set of data) is $$\sigma^2=\frac{\sum x^2}{N} - \bar x^2$$ That is the mean of the squares minus the square of the mean. ...
James K's user avatar
  • 121k
3 votes

Are there really confined Globular Clusters?

Disregarding the example of globular clusters (which are all undergoing evaporation, as ProfRob points out), there are a couple other candidates for "isolated" astrophysical systems. The ...
Tom Donlon's user avatar
2 votes

Natural units of star clusters angular momentum and energy

Using unit analysis, I've found that a natural unit of angular momentum should be \begin{equation}\tag{1} L_0 = \frac{G M_{\odot}}{c} \approx 8.816 \times 10^{41} \, \text{kg} \, \text{m}^2/\text{s}. \...
Cham's user avatar
  • 273
2 votes

Dataset containing list of known globular clusters

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 ...
MCG's user avatar
  • 795
2 votes
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Do the heliospheres of stars at the center of globular clusters interact?

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 ...
userLTK's user avatar
  • 24k
2 votes

Jacobi vs tidal radius for star cluster

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 ...
George's user avatar
  • 21
2 votes

Difference in HR diagram clusters

So, @ProfRob's answer is absolutely amazing and covers the bulk of star-cluster properties that I could think of. Nevertheless, since the question specifically asked for HR diagrams of open vs. ...
guilimberg's user avatar
1 vote

Are there really confined Globular Clusters?

Well, things like small asteroids are self-gravitating and confined to a pretty well defined volume. We believe they are approximately in thermal equilibrium. But it isn't gravity that brings them to ...
John Doty's user avatar
  • 1,876
1 vote
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Help me identify a cluster

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 ...
Mike G's user avatar
  • 18.7k
1 vote

Jacobi vs tidal radius for star cluster

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.....
charles's user avatar
  • 86
1 vote

Is it true that open clusters have $E > 0$ and globular clusters have $E < 0$?

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. ...
Stijn D'hondt's user avatar

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