# Tag Info

Accepted

### How hard is it to find the Sun's "sisters?"

Here are the problems/issues: Most stars are born in clusters/associations but a cursory investigation of cluster demographics with age reveals that the vast majority of clusters do not survive to old ...
• 129k
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With the exception of the Andromeda galaxy and the Magellanic clouds1, every star, star cluster and nebula that is visible to the naked eye is part of the Milky Way. The Pleiades is a star cluster in ...
• 101k

To give you a perspective the Milky Way Galaxy is between 150,000 and 200,000 light years in across. The Pleiades is less than 450 light years from Earth. In a galactic perspective the Pleiades is ...
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### Why do stars born in a cluster finally disperse?

The boundedness or otherwise of clusters remains to be established in most cases. The vast majority of clusters become unbound and disperse at a much younger age than the Pleiades. Or they may be born ...
• 129k
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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 ...
• 15.1k
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### Is every star formed in an open star cluster?

The question is still an open matter of current research. It seems to be true that the vast majority of star formation takes place in groups and aggregates of various sizes - from a few stars to ...
• 129k

This screen grab from the program Where is M13? shows the location of the Sun and Pleiades in our galaxy. The Sun is the orange dot, the Pleiades the yellow dot.
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• 129k
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### What are the smallest star clusters affected by Galaxy Rotation Curve?

Yes, dwarf galaxy rotation curves are affected -- in fact, they tend to require relatively more dark matter than is required to explain the rotation curves of giant galaxies like the Milky Way. The ...
• 15.1k
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### integration of velocity distribution f(v)

You need to divide 60,000 by the integral of this function from zero to infinity. That will then give you the fraction of stars with a velocity (I think it is probably speed, not velocity?) between 0 ...
• 129k
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### Which A. M. Fridman is the author of this paper on gravitational stability?

It's Alexey Maksimovich Fridman. I think that's pretty clear from the respective research interests of the two scientists, but we can go deeper... Let's start off with the author of your 1971 paper. A ...
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### How hard is it to find the Sun's "sisters?"

Stand on dune in a desert. Take a handfull of sand, all crushed from the same rock. Now close your eyes, Hold your hand up to the wind, let the wind blow all but one of the grains of sand, somewhere. ...
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### Understanding Scatter in Data collected for HR Plot

So, scatter in observational measurements of clusters in the HR diagram is mainly, in my experience, caused by 3 things: observational errors, binaries and contamination. As you mentioned, there are ...
• 161
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### Astronomical databases for machine learning?

The European Southern Observatory has catalogues with image data available from http://www.eso.org/qi/, you will have to register before you are able to access them. I'd suggest you look at other ...
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### 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. ...
• 61
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### What exactly is a stellar association?

A stellar association is a loose cluster of stars, that formed at the same time from the same molecular cloud, and so have the same proper motion. Unlike open clusters, they are not gravitationally ...
• 101k

### Natural units of star clusters angular momentum and energy

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}. \...
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### 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 ...
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### 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 ...
• 21

### Fitting isochrone on color magnitude diagram obtained from growth india telescope for M67

To put an isochrone on your colour-magnitude diagram, then you need those isochrones to be predictions in terms of colour and magnitude. There are models out there which do predict isochrones in terms ...
• 129k

### Is there a relationship between stars' absolute magnitude, spectral class and relative location in a cluster?

Stars born together in clusters have more-or-less the same age. As a rule of thumb, any spread in age, measured in millions of years, is smaller than the extent of the cluster in parsecs. For most ...
• 129k
Accepted

### how to calculate the half-mass radius and tidal radius of a (simulated) globular cluster

Your first question is about computing, not astronomy. You find a cumulative histogram of distance from the centre of mass and where that reaches 50% of the stars. As to your second point, there is no ...
• 129k
1 vote
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### Mass estimate for cluster NGC2516?

NGC 2516 is somewhat richer and more massive than the Pleiades. A careful look at the mass function suggests there is about $1000 M_{\odot}$ in stars $\geq 0.3 M_{\odot}$, within the central 0.9 ...
• 129k
1 vote
Accepted

### 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 ...
• 17k
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.....
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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. ...
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