31

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 age. The majority either are never gravitationally bound to begin with or become unbound in the first 10 Myr. The Sun was likely born in a cluster of $10^3-10^...


17

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 the Milky Way. Objects in other galaxies are too far and too dim to be visible with the naked eye. 1The Triangulum galaxy (M33) may also be visible to the ...


13

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 incredibly close to us and so clearly within the Milky Way Galaxy.


12

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 unbound as you suggest. The stars in a cluster have a distribution of velocities and there will always be a tail of high speed stars that will be able to ...


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 ...


7

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 millions of stars in "super" star clusters. This is likely because collapsing clouds of gas are normally much more massive than a star and the collapse process ...


7

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.


6

The Arecibo Message was not broadcast with the aim that it would intercept with a notable astronomical object. Neither do we realistically expect that anyone will ever listen to the Voyager Golden Records. Such symbolic gestures are simply vectors for public relations and education. We send these "time capsules" into space because we can, as a showcase of ...


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

You can't without assuming something about the overall velocity. The radial velocity is one component of a velocity vector; you are missing the other two components, which could in principle be anything. However, you could assume that as open clusters are mostly quite young and members of the Galactic disc population, that they are moving in the disc on ...


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 ...


4

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 smallest known systems are so-called ultrafaint dwarf galaxies (UFDs), which can have as little as $\sim 10^{4}$ solar masses worth of baryons (in the form of ...


3

Finally, after searching quite a deal, I found this paper which would aptly answer the question. The paper from 1999 published by Pietrzynski and Udalski in Acta Astronomica lists the Cepheids in the star clusters of Magellanic Clouds.


3

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 observatory's websites for their data. You will have to look past the pages targeted at the general public and find links for data or science, or user portal or ...


3

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 observational errors, which are contributed to by the instrument, the data reduction, the weather conditions, etc. This error usually gets worse as you get ...


3

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. Wait 2 years. Now go and find the other grains of sand that you dropped. It should be easy, right? They have identical composition as the sample you have. They ...


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

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 stellar clusters, smaller than a few pc, the only chance of measuring age differences occurs in the first 10 million years of life. There is no evidence for age ...


2

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 tidal radius; you need two bodies in order to have a defined tidal radius. The tidal radius of a cluster is directly related to the gravitational potential ...


2

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 bound together, so the stars in a stellar association will gradually separate, forming a moving group of stars. An example is the Scorpius–Centaurus Association


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

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 square degrees (with about a 15% uncertainty), but there is probably a few hundred solar masses beyond this (Jeffries et al. 2001). This agrees with the figures of ...


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)(...


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 \begin{equation}\tag{1} L_0 = \frac{G M_{\odot}}{c} \approx 8.816 \times 10^{41} \, \text{kg} \, \text{m}^2/\text{s}. \end{equation} 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 ...


1

The galactic coordinate system is centred on the Earth (not the galactic centre) with 0 degrees longitude in the direction of the galactic centre. (as measured by observation of the distribution of neutral hydrogen). The physical centre, marked by Sagittarius A* is offset by less than 0.1 degrees. The coordinate transform from equatorial (right ascension, ...


1

The angle you are looking for $$\cos \theta = \sin \delta_1 \sin \delta_2 + \cos \delta_1 \cos \delta_2 \cos (a_1-a_2),$$ where $(a_1, \delta_1)$ are the RA and Dec of the star and $(a_2,\delta_2)$ the RA and Dec of the convergent point.


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