Explain why globular clusters spend

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Ren Jie T. Answer Globular clusters, such as the one shown below, are spherical groupings of stars that form a ring around the Milky Way galaxy. Holt Physics Chapter 15 Interference and Diffraction.

Discussion You must be signed in to discuss. Video Transcript now this question exterior asking ourselves whether, given the following conditions, the image, the process off imaging, the gobbler caster will be easier or harder and why right in this case, so first condition they were given is weapons.

Upgrade today to get a personal Numerade Expert Educator answer! Ask unlimited questions. Test yourself. Join Study Groups. Create your own study plan. Join live cram sessions. As stars evolve, they become redder. The bright orange star in NGC is the member of the cluster that has evolved most rapidly. Figure 5 shows the H—R diagram of the open cluster M41, which is roughly million years old; by this time, a significant number of stars have moved off to the right and become red giants.

Note the gap that appears in this H—R diagram between the stars near the main sequence and the red giants. A gap does not necessarily imply that stars avoid a region of certain temperatures and luminosities. In this case, it simply represents a domain of temperature and luminosity through which stars evolve very quickly. We see a gap for M41 because at this particular moment, we have not caught a star in the process of scurrying across this part of the diagram.

Figure 5. Some of its more massive stars are no longer close to the zero-age main sequence red line. Note that it contains several orange-color stars. These are stars that have exhausted hydrogen in their centers, and have swelled up to become red giants. After 4 billion years have passed, many more stars, including stars that are only a few times more massive than the Sun, have left the main sequence Figure 6.

This means that no stars are left near the top of the main sequence; only the low-mass stars near the bottom remain. The older the cluster, the lower the point on the main sequence and the lower the mass of the stars where stars begin to move toward the red giant region. The location in the H—R diagram where the stars have begun to leave the main sequence is called the main-sequence turnoff. Figure 6. Note that most of the stars on the upper part of the main sequence have turned off toward the red-giant region.

And the most massive stars in the cluster have already died and are no longer on the diagram. The oldest clusters of all are the globular clusters. Figure 7 shows the H—R diagram of globular cluster 47 Tucanae. These parameters are closely related to a number of other cluster properties such as, for example, Galactocentric radius and age. Such a division of globular clusters can therefore provide us with a great deal of information about the evolutionary history of the Galaxy. By comparing the different Galactic subgroups with the cluster systems in nearby dwarf galaxies, i.

In other words, such clusters are most likely to have formed within much smaller galaxies perhaps similar to the Local Group dSphs observed at the present epoch which later merged with the Milky Way system. These authors used cluster core radius R c as their size diagnostic; however, unlike the half-light radius R h , this parameter is quite sensitive to the dynamically evolving state of a globular cluster.

In this paper, we examine the different Galactic globular cluster subsystems in terms of cluster structures, luminosities and surface brightnesses.

In Section 3 we use these data to investigate the collective properties of each cluster subsystem, while in Section 4 we discuss the possible links between half-light radius and luminosity both for Galactic globular clusters and for clusters belonging to some other nearby Local Group dwarf galaxies. Finally, we discuss what these parameters might be able to tell us about the origins and evolution of different types of clusters.

In Table 1 we list all known Galactic globular clusters, including the handful of clusters that are thought to be associated with the Sagittarius dwarf galaxy. The structures of most of these objects have been measured. For each such cluster, we list the half-light radius R h and tidal radius R t. We have adopted the majority of these values from the on-line data base of Harris update. Because this compilation lists R h and R t as angular diameters, we have converted them to parsecs using the cluster distances that are also listed by Harris.

For a few clusters, the data base does not have entries for R h and R t. Using the half-light radius and integrated luminosity for each cluster, we have calculated I h , the half-light intensity. It is essentially a measure of the mean surface brightness of a cluster within R h , but in terms of absolute units rather than observational units.

Because of the evaporation of stars from dynamically evolving clusters, I h is more sensitive to evolutionary effects than is R h.

Nevertheless, this parameter turns out to be useful when examining cluster classifications. As noted in Section 1 , this classification scheme follows closely those of Zinn and van den Bergh — see also references therein.

To briefly review this scheme, the clusters are grouped by metallicity and HB type. They found that the majority of globular clusters associated with nearby dwarf galaxies are, in terms of HB morphology and core-radius distribution, essentially indistinguishable from those in the Galactic YH class. Furthermore, the YH objects are observed to be characterized by large, energetic orbits around the Galaxy.

These orbits are generally of high eccentricity and intermediate inclination, and cover a very large range in orbital angular momentum, including highly retrograde orbits. A small fraction of OH clusters have similar orbits, but most have much smaller energies and eccentricities. Dinescu et al. These authors determined relative ages for 55 globular clusters using several different CMD-based techniques. However, several OH clusters are also seen to be somewhat younger, while a number of YH clusters appear to be just as old as the oldest measured OH clusters.

This shows that YH-type clusters apparently underwent much slower evolution to higher metallicities than did the OH and BD objects. The remainder of the OH group, together with the BD system, appear to have been formed in the proto Galaxy. In the following subsections, we consider some additional properties of the three cluster subsystems, focusing on structural parameters such as R h and ellipticity, as well as on cluster luminosities and surface brightnesses.

This is clearly seen in Fig. Each of the three globular cluster subsystems are quite distinct in terms of their distribution of Galactocentric radii. We can quantify the significance of this statement using simple Kolmogorov—Smirnov K—S tests on each pair of distributions. These show that there is only a 2 per cent chance that the BD and OH samples were drawn from the same parent population. The probability that the YH clusters share the same parent distribution as either the OH or BD clusters is even smaller: 0.

Cumulative distributions in Galactocentric radii R gc for the three Galactic globular cluster subsystems. In this respect, it strongly resembles the Sagittarius clusters Ter. Furthermore, examining Table 1 , Pal. This adds additional weight to the suspicion that these two objects might not be native BD clusters. However, they are not nearly as extended as the YH clusters, which dominate the outer halo. These objects are therefore not exclusively a remote-halo population. In Fig. Furthermore, these two systems contain very few low-luminosity clusters.

This contrasts with the situation for the YH system, which contains a significant population of low-luminosity objects. Again, we can evaluate the significance of the differences between the three distributions using K—S tests. These show that there is only a 5 per cent chance that the OH and YH clusters share the same parent luminosity distribution. However, the K—S tests are unable to discriminate between the OH and BD distributions, and the YH and BD distributions; the results do not confirm statistically significant differences between these parent populations, but they also do not confirm statistically significant similarities.

This result is at least partly due to the relatively small BD and YH sample sizes. As discussed above, the primary difference between the distributions is the presence or not of a small percentage of low-luminosity clusters. Hence, it would be necessary for the sample sizes to be larger to confirm or deny this difference at a significant level.

This is, of course, not possible, because we included all known Galactic globular clusters in Table 1. However, we can clearly state that the luminosity distribution of YH clusters is significantly broader than that for the OH clusters. Combined with our observations from Section 3.

The present result is however distinct from this, as we have not used R gc as a discriminator; indeed, Fig. Our observations imply that LF broadness is intrinsic to cluster subpopulation rather than simply location within the Galaxy. Because YH clusters typically lie at larger Galactocentric radii than do OH and BD clusters, they should be much less strongly affected by destructive external forces.

In addition, if the accretion hypothesis is correct, many of the YH clusters have spent some fraction of their life outside the Galaxy in a more benign tidal environment.

Consider, for example, the clusters associated with the Sagittarius dwarf. In each of these scenarios, the Sagittarius dwarf, and its remaining globular clusters, spend a significant fraction of their lives away from the inner regions of the Galaxy.

The observation that four of the Sagittarius clusters are still associated with the main body of this galaxy provides further evidence that these clusters have not suffered prolonged tidal stresses due to the Galaxy.

Tidal forces strong enough to disrupt a globular cluster are likely to be so strong that they would also pull that cluster out of its parent dwarf galaxy. Together the factors described above may help to explain why the YH system has a greater fraction of low-luminosity clusters than do the OH and BD systems. If destructive effects are indeed responsible, then the YH luminosity distribution may well indicate that globular clusters are formed with a broad LF.

This distribution is plotted in Fig. A K—S test gives a 92 per cent chance that the YH and external clusters have the same parent luminosity distribution, but only a 3 per cent chance that the OH and external clusters share the same parent luminosity distribution. It therefore offers further evidence in favour of the hypothesis that much, or all, of the ensemble of YH clusters is of external origin.

Note how similar this distribution is to that for the YH clusters Fig. Figs 4 and 5 show the distributions of half-light radii R h and tidal radii R t , respectively, for the three Galactic cluster subsystems.

Conversely, the large majority of OH and BD objects are seen to be compact. Once again, we apply K—S tests to determine the significance of the differences between the three distributions. These calculations show that there is a less than 1 per cent chance that the OH and YH distributions in R h were drawn from the same parent population, and a less than 0.

Exactly similar results are obtained for the R t distributions. Distributions of half-light radii R h for the Galactic globular cluster subsystems. Distributions of tidal radii R t for the Galactic globular cluster subsystems.

Given what is known about the distributions in Galactocentric radius for the three subsystems Section 3. The tidal radii of clusters are determined by how deeply their orbits dive into the the Galactic potential in which they reside see, for example, King In fact, it has been shown that R t is most strongly correlated with the distance of a cluster at the pericentre of its orbit van den Bergh Hence, because the majority of clusters which reside at large R gc belong to the YH ensemble, it is to be expected that this group also contains the largest fraction of clusters with large R t.

By the same token, the BD clusters are the most centrally concentrated in R gc , and consequently all the clusters in this subsample have comparatively small tidal radii.

The distributions in R h are somewhat more intriguing. It has long been known that the half-light radii of Galactic globular clusters correlate with their Galactocentric distances, in the sense that the most remote clusters are also the largest e.

Hence, with only this fact in mind, one might expect the YH system to possess most of the clusters with large R h. This is indeed what is observed. However, the fact that the YH clusters are strongly suspected of being accreted in a number of minor merger events presents a challenge for this explanation.

This is so because, for clusters which have been more or less randomly accreted, there is no reason to expect half-light radii to follow the same trend with R gc as that which is observed for clusters which are native to the Galaxy.

We address this puzzle in more detail in Section 4. Irrespective of the above result, the distributions in R h are interesting because, as described in Section 1 , a cluster's half-light radius is stable over many relaxation times.

Hence, the observed distribution of half-light radii for the YH clusters should, to a large extent, reflect the distribution of radii with which these clusters were formed. These most strongly affect diffuse clusters i. For example, in the formalism of Dinescu et al. Furthermore, clusters with short orbital periods and small pericentric radii i.

In their study, Dinescu et al. Clearly, the native population is most strongly affected. In addition, several of the nine have large R h e. NGC , Pal. This is expected in a cluster if it has been subjected to strong external tidal forces. Assuming that some fraction of the native Galactic clusters has been destroyed as discussed above, it is possible to place constraints on the initial population. Examining Fig. It is important to point out that the above argument explicitly depends on the assumption that the OH and BD systems originally had R h distributions similar to that which is presently observed for YH clusters.

Alternatively, we again note the possibility that formation conditions did not allow the production of diffuse clusters in the inner Galaxy. However, not enough information about ancient cluster formation conditions is available at present to allow this possibility to be confirmed or rejected. Because this quantity is closely related to both R h and M V , it is not surprising to see that the vast majority of low surface brightness clusters belong to the YH subsystem.

The distribution of I h for this ensemble is quite broad with no prominent maximum. In this respect it differs significantly from the much sharper singly peaked distributions that are observed for the BD and OH clusters. A K—S test shows that there is only a 1 per cent chance that the YH and BD samples were drawn from the same parent distribution, and a less than 0.

Distributions of half-light intensity I h for the three Galactic cluster subsystems. As was already pointed out in Sections 3. An additional complication is that observational selection effects militate against the discovery of low surface brightness clusters against the rich foreground of stars in the direction of the Galactic bulge. As previously noted, the YH population has apparently not been as strongly affected by tidal forces as have the OH and BD populations.

This subsystem may therefore provide interesting constraints on the initial distribution of cluster surface brightnesses although, as noted previously, I h is more sensitive to cluster evolution than is R h , because the evaporation of stars, along with stellar evolution, causes variations in M V. He was correct on this point, although Edwin Hubble helped prove Shapley's position about nebulae and globular clusters occurring within our galaxy wrong when he demonstrated that the Cepheid variables in the Andromeda galaxy were much further away than Shapley's proposed extent of the Milky Way and that Andromeda was indeed its own "island universe.

Under his leadership, it became one of the world's most important centers for astronomy training and research. Shapley established the graduate school of astronomy as part of the educational structure of Harvard University.

He was also responsible for mandating that public education be made a part of the Observatory's mission; a requirement for students in the Harvard program was lectures and presentations for school children. Shapley remained as the Observatory's director until , and then continued research as the Paine Professor of Practical Astronomy until his retirement in After that, he continued his work as both a lecturer and an author.

A member of numerous academies and the recipient of many prestigious prizes, Harlow Shapley did much to help popularize the field of astronomy. He was active in the professional as well as political interests of science. Shapley was a political liberal and became a victim of McCarthyism. Joseph McCarthy claimed that Shapley was a communist in the State Department, even though Shapley had no real connection to it. Shapley responded to the press saying, "the Senator succeeded in telling six lies in four sentences, which is probably the indoor record for mendacity.

Wallace and attended the Progressive Party convention in He supported Wallace on a personal level, but was against the pro-Soviet positions of the Progressive Party. Next to astronomy, Shapley's greatest interest was myrmecology, the study of ants.

He spent much time studying them in the daylight hours at Mount Wilson, and even published a few papers on ant behavior.

Harlow Shapley died on October 20, —just shy of his 87th birthday—in Boulder, Colorado, during a visit to his son. Harlow Shapley was the first to realize that the Milky Way Galaxy was much larger than previously believed. Using the inch telescope at Mount Wilson, he took photographs and observed globular clusters, which were compact spheres composed of many thousands of stars.

Shapley used Henrietta Swan Leavitt's method for Cepheid variable stars to determine distances to globular clusters. In , American astronomer Henrietta Leavitt used the degree of brightness of Cepheid stars, whose brightness vary at regular intervals, in the Magellanic Clouds to measure their distance from the Earth.

The longer the time a Cepheid star takes to undergo a complete cycle, the higher the star's average brightness or average "absolute magnitude".