Everything about Globular Clusters totally explained
A
globular cluster is a
spherical collection of
stars that orbits a
galactic core as a
satellite. Globular clusters are very tightly bound by
gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of
star cluster is derived from the
Latin globulus—a small sphere. A globular cluster is sometimes known more simply as a
globular.
Globular clusters, which are found in the
halo of a galaxy, contain considerably more stars and are much older than the less dense
galactic, or open clusters, which are found in the disk. Globular clusters are fairly common; there are about 158 currently known globular clusters in the
Milky Way, with perhaps 10–20 more undiscovered. Large galaxies can have more:
Andromeda, for instance, may have as many as 500. Some giant
elliptical galaxies, such as
M87, may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40
kiloparsecs (approximately 131 thousand
light-years) or more.
Every galaxy of sufficient mass in the
Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters. The
Sagittarius Dwarf and
Canis Major Dwarf galaxies appear to be in the process of donating their associated globular clusters (such as
Palomar 12) to the Milky Way. This demonstrates how many of this galaxy's globular clusters were acquired in the past.
Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their
origins and their role in galactic evolution are still unclear. It does appear clear that globular clusters are significantly different from
dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy rather than as a separate galaxy. However, recent conjectures by astronomers suggest that globular clusters and
dwarf spheroidals may not be clearly separate and distinct types of objects.
Observation history
The first globular cluster discovered was
M22 in 1665 by Abraham Ihle, a German amateur astronomer. However, due to the small
aperture of early
telescopes, individual stars within a globular cluster were not
resolved until
Charles Messier observed
M4. The first eight globular clusters discovered are shown in the table. Subsequently,
Abbé Lacaille would list
NGC 104,
NGC 4833,
M55,
M69, and
NGC 6397 in his 1751–52 catalogue. The
M before a number refers to the catalogue of Charles Messier, while
NGC is from the
New General Catalogue by
John Dreyer.
William Herschel began a survey program in 1782 using larger telescopes and was able to resolve the stars in all 33 of the known globular clusters. In addition he found 37 additional clusters. In Herschel's 1789 catalog of deep sky objects, his second such, he became the first to use the name
globular cluster as their description.
The number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. A total of 151 globular clusters have now been discovered in the
Milky Way galaxy, out of an estimated total of 180 ± 20. While his distance estimate was significantly in error, it did demonstrate that the dimensions of the galaxy were much greater than had been previously thought. His error was due to the fact that dust in the Milky Way diminished the amount of light from a globular cluster that reached the earth, thus making it appear farther away. Shapley's estimate was, however, within the same
order of magnitude of the currently accepted value.
Shapley's measurements also indicated that the Sun was relatively far from the center of the galaxy, contrary to what had previously been inferred from the apparently nearly even distribution of ordinary stars. In reality, ordinary stars lie within the galaxy's disk and are thus often obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.
Shapley was subsequently assisted in his studies of clusters by
Henrietta Swope and
Helen Battles Sawyer (later Hogg). In 1927–29, Harlow Shapley and Helen Sawyer began categorizing clusters according to the degree of concentration the system has toward the core. The most concentrated clusters were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the
Shapley–Sawyer Concentration Class. (It is sometimes given with numbers [Class1–12] rather than
Roman numerals.)
Composition
Globular clusters are generally composed of hundreds of thousands of
low-metal, old stars. The type of stars found in a globular cluster are similar to those in the
bulge of a
spiral galaxy but confined to a volume of only a few cubic
parsecs. They are free of gas and dust and it's presumed that all of the gas and dust was long ago turned into stars.
While globular clusters can contain a high density of stars (on average about 0.4 stars per cubic
parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster), they're not thought to be favorable locations for the survival of planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters due to the perturbations of passing stars. A planet orbiting at 1
astronomical unit around a star that's within the core of a dense cluster such as
47 Tucanae would only survive on the order of 10
8 years. However, there has been at least one planetary system found orbiting a
pulsar (
PSR B1620−26) that belongs to the globular cluster
M4.
With a few notable exceptions, each globular cluster appears to have a definite age. That is, most of the stars in a cluster are at approximately the same stage in
stellar evolution, suggesting that they formed at about the same time. All known globular clusters appear to have no active star formation, which is consistent with the view that globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form. Very large regions of star formation known as
super star clusters, such as
Westerlund 1 in the
Milky Way, may be the precursors of globular clusters.
Some globular clusters, like
Omega Centauri in our
Milky Way and
G1 in
M31, are extraordinarily massive (several million
solar masses) and contain multiple stellar populations. Both can be regarded as evidence that supermassive globular clusters are in fact the cores of dwarf galaxies that are consumed by the larger galaxies. Several globular clusters (like
M15) have extremely massive cores which may harbor
black holes, although simulations suggest that a less massive black hole or central concentration of neutron stars or massive white dwarfs explain observations equally well.
Metallic content
Globular clusters normally consist of
Population II
stars, which have a low metallic content compared to
Population I stars such as the
Sun. (To astronomers,
metals includes all elements heavier than
helium, such as
lithium and
carbon.)
The
Dutch astronomer
Pieter Oosterhoff noticed that there appear to be two populations of globular clusters, which became known as
Oosterhoff groups. The second group has a slightly longer period of
RR Lyrae variable stars. Both groups have weak
lines of metallic elements. But the lines in the stars of Oosterhoff type I (OoI) cluster are not quite as weak as those in type II (OoII).
In the Milky Way it has been discovered that the large majority of the low metallicity clusters are aligned along a plane in the outer part of the galaxy's halo. This result argues in favor of the view that type II clusters in the galaxy were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought. The difference between the two cluster types would then be explained by a time delay between when the two galaxies formed their cluster systems.
Exotic components
Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as
blue stragglers,
millisecond pulsars and
low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the
main sequence stars formed at the beginning of the cluster.
Astronomers have searched for
black holes within globular clusters since the 1970s. The resolution requirements for this task, however, are exacting, and it's only with the
Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000
solar mass intermediate-mass black hole has been suggested to exist based on HST observations in the globular cluster
M15 and a 20,000 solar mass black hole in the
Mayall II cluster in the
Andromeda Galaxy. Both
x-ray and
radio emissions from Mayall II appear to be consistent with an intermediate-mass black hole.
These are of particular interest because they're the first black holes discovered that were intermediate in mass between the conventional
stellar-mass black hole and the
supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies.
Claims of intermediate mass black holes have been met with some skepticism. The densest objects in globular clusters are expected to migrate to the cluster center due to
mass segregation. These will be white dwarfs and neutron stars in an old stellar population like a globular cluster. As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15 and Mayall II.
Color-magnitude diagram
The
Hertzsprung-Russell diagram (HR-diagram) is a graph of a large sample of stars that plots their visual
absolute magnitude against their
color index. The
color index, B−V, is the difference between the magnitude of the star in blue light, or B, and the magnitude in visual light (green-yellow), or V. Large positive values indicate a red star with a cool surface
temperature, while negative values imply a blue star with a hotter surface.
When the stars near the
Sun are plotted on an HR diagram, it displays a distribution of stars of various masses, ages, and compositions. Many of the stars lie relatively close to a sloping curve with increasing absolute magnitude as the stars are hotter, known as
main sequence stars. However the diagram also typically includes stars that are in later stages of their evolution and have wandered away from this main sequence curve.
As all the stars of a globular cluster are at approximately the same distance from us, their absolute magnitudes differ from their
visual magnitude by about the same amount. The main sequence stars in the globular cluster will fall along a line that's believed to be comparable to similar stars in the solar neighborhood. (The accuracy of this assumption is confirmed by comparable results obtained by comparing the magnitudes of nearby short-period variables, such as
RR Lyrae stars and
cepheid variables, with those in the cluster.)
By matching up these curves on the HR diagram the absolute magnitude of main sequence stars in the cluster can also be determined. This in turn provides a distance estimate to the cluster, based on the visual magnitude of the stars. The difference between the relative and absolute magnitude, the
distance modulus, yields this estimate of the distance.
When the stars of a particular globular cluster are plotted on an HR diagram, nearly all of the stars fall upon a relatively well-defined curve. This differs from the HR diagram of stars near the Sun, which lumps together stars of differing ages and origins. The shape of the curve for a globular cluster is characteristic of a grouping of stars that were formed at approximately the same time and from the same materials, differing only in their initial mass. As the position of each star in the HR diagram varies with age, the shape of the curve for a globular cluster can be used to measure the overall age of the collected stars.
The most massive main sequence stars in a globular cluster will also have the highest absolute magnitude, and these will be the first to evolve into the
giant star stage. As the cluster ages, stars of successively lower masses will also enter the
giant star stage. Thus the age of a cluster can be measured by looking for the stars that are just beginning to enter the giant star stage. This forms a "knee" in the HR diagram, bending to the upper right from the main sequence line. The absolute magnitude at this bend is directly a function of the age of globular cluster, so an age scale can be plotted on an axis parallel to the magnitude.
In addition, globular clusters can be dated by looking at the temperatures of the coolest white dwarfs. Typical results for globular clusters are that they may be as old as 12.7
billion years. This is in contrast to open clusters which are only tens of millions of years old.
The ages of globular clusters place a bound on the age limit of the entire universe. This lower limit has been a significant constraint in
cosmology. During the early 1990s, astronomers were faced with age estimates of globular clusters that appeared older than cosmological models would allow. However, better measurements of cosmological parameters through deep sky surveys and satellites such as
COBE have resolved this issue as have computer models of stellar evolution that have different models of mixing.
Evolutionary studies of globular clusters can also be used to determine changes due to the starting composition of the gas and dust that formed the cluster. That is, the change in the evolutionary tracks due to the abundance of heavy elements. (Heavy elements in astronomy are considered to be all elements more massive than
helium.) The data obtained from studies of globular clusters are then used to study the evolution of the Milky Way as a whole.
In globular clusters a few stars known as
blue stragglers are observed, apparently continuing the main sequence in the direction of brighter, bluer stars. The origins of these stars is still unclear, but most models suggest that these stars are the result of mass transfer in multiple star systems.
Morphology
In contrast to open clusters, most globular clusters remain gravitationally-bound for time periods comparable to the life spans of the majority of their stars. (A possible exception is when strong tidal interactions with other large masses result in the dispersal of the stars.)
At present the formation of globular clusters remains a poorly understood phenomenon. It remains uncertain whether the stars in a globular cluster form in a single generation, or are spawned across multiple generations over a period of several hundred million years. This star-forming period is relatively brief, however, compared to the age of many globular clusters. Observations of globular clusters show that these stellar formations arise primarily in regions of efficient star formation, and where the interstellar medium is at a higher density than in normal star-forming regions. Globular cluster formation is prevalent in
starburst regions and in
interacting galaxies.
After they're formed, the stars in the globular cluster begin to gravitationally interact with each other. As a result the velocity vectors of the stars are steadily modified, and the stars lose any history of their original velocity. The characteristic interval for this to occur is the
relaxation time. This is related to the characteristic length of time a star needs to cross the cluster as well as the number of stellar masses in the system. The value of the relaxation time varies by cluster, but the mean value is on the order of 10
9 years.
Ellipticity of Globulars>
| Galaxy |
Ellipticity |
| Milky Way |
0.07±0.04 |
| LMC |
0.16±0.05 |
| SMC |
0.19±0.06 |
| M31 |
0.09±0.04 |
Although globular clusters generally appear spherical in form, ellipticities can occur due to tidal interactions. Clusters within the Milky Way and the Andromeda Galaxy are typically
oblate spheroids in shape, while those in the
Large Magellanic Cloud are more elliptical.
Radii
Astronomers characterize the morphology of a globular cluster by means of standard radii. These are the core radius (
rc), the half-light radius (
rh) and the tidal radius (
rt). The overall luminosity of the cluster steadily decreases with distance from the core, and the core radius is the distance at which the apparent surface luminosity has dropped by half. A comparable quantity is the half-light radius, or the distance from the core within which half the total luminosity from the cluster is received. This is typically larger than the core radius.
Note that the half-light radius includes stars in the outer part of the cluster that happen to lie along the line of sight, so theorists will also use the half-mass radius (
rm)—the radius from the core that contains half the total mass of the cluster. When the half-mass radius of a cluster is small relative to the overall size, it has a dense core. An example of this is
Messier 3 (M3), which has an overall visible dimension of about 18
arc minutes, but a half-mass radius of only 1.12 arc minutes.
Almost all globular clusters have a half-light radius of less than 10
pc. Although there are well-established globular clusters with very large radii (for example
NGC 2419 (R
h = 18 pc) and
Palomar 14 (R
h = 25 pc). An example of a core-collapsed globular is
M15.
Core-collapse is thought to occur when the more massive stars in a globular encounter their less massive companions. As a result of the encounters the larger stars tend to lose
kinetic energy and start to settle toward the core. Over a lengthy period of time this leads to a concentration of massive stars near the core, a phenomenon called
mass segregation.
The Hubble Space Telescope has been used to provide convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at the cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery. The globular star cluster
47 Tucanae, which is made up of about 1 million stars, is one of the densest globular clusters in the Southern Hemisphere. This cluster was subjected to an intensive photographic survey, which allowed astronomers to track the motion of its stars. Precise velocities were obtained for nearly 15,000 stars in this cluster.
The different stages of core-collapse may be divided into three phases. During a globular cluster's adolescence, the process of core-collapse begins with stars near the core. However, the interactions between
binary star systems prevents further collapse as the cluster approaches middle age. Finally, the central binaries are either disrupted or ejected, resulting in a tighter concentration at the core.
A 2008 study by Dr. John Fregeau of 13 globular clusters in the Milky Way shows that three of them have unusually large number of X-ray sources, or X-ray binaries, suggesting the clusters are middle-aged. Previously, these globular clusters had been classified as being in old age because they'd very tight concentrations of stars in their centers, another litmus test of age used by astronomers. The implication is that most globular clusters, including the other ten studied by Fregeau, are not in middle age, as previously thought, but are actually in adolescence.
"It's remarkable that these objects, which are thought to be some of the oldest in the Universe, may really be very immature," said Fregeau whose paper appears in
The Astrophysical Journal. "This would represent a major change in thinking about the current evolutionary status of globular clusters."
The overall luminosities of the globular clusters within the Milky Way and
M31 can be modeled by means of a
gaussian curve. This gaussian can be represented by means of an average magnitude M
v and a variance σ
2. This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). (For the Milky Way, M
v = −7.20±0.13, σ=1.1±0.1 magnitudes.) The GCLF has also been used as a "
standard candle" for measuring the distance to other galaxies, under the assumption that the globular clusters in remote galaxies follow the same principles as they do in the Milky Way.
N-body simulations
Computing the interactions between the stars within a globular cluster requires solving what is termed the
N-body problem. That is, each of the stars within the cluster continually interacts with the other
N−1 stars, where
N is the total number of stars in the cluster. The naive
CPU computational "cost" for a dynamic simulation increases in proportion to
N3, so the potential computing requirements to accurately simulate such a cluster can be enormous. An efficient method of mathematically simulating the N-body dynamics of a globular cluster is done by sub-dividing into small volumes and velocity ranges, and using probabilities to describe the locations of the stars. The motions are then described by means of a formula called the
Fokker-Planck equation. This can be solved by a simplified form of the equation, or by running
Monte Carlo simulations and using random values. However the simulation becomes more difficult when the effects of binaries and the interaction with external gravitation forces (such as from the Milky Way galaxy) must also be included.
The results of N-body simulations have shown that the stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster. Over long periods of time this will result in a dissipation of the cluster, a process termed evaporation. The typical time scale for the evaporation of a globular cluster is 10
10 years.
In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy, which is, in several ways, very similar to globular clusters. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. The clusters also share other characteristics with globular clusters,
for example the stellar populations and metallicity. What distinguishes them from the globular clusters is that they're much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters. Parametrically, these clusters lie somewhere between a (low dark-matter) globular cluster and a (dark matter-dominated)
dwarf spheroidal galaxy.
How these clusters are formed isn't yet known, but their formation might well be related to that of globular clusters. Why M31 has such clusters, while the Milky Way does not, isn't yet known. It is also unknown if any other galaxy contains these types of clusters, but it would be very unlikely that M31 is the sole galaxy with extended clusters. These tails typically both precede and follow the cluster along its orbit. The tails can accumulate significant portions of the original mass of the cluster, and can form clump-like features.
The globular cluster
Palomar 5, for example, is near the
perigalactic point of its orbit after passing through the Milky Way. Streams of stars extend outward toward the front and rear of the orbital path of this cluster, stretching out to distances of 13,000 light-years. Tidal interactions have stripped away much of the mass from Palomar 5, and further interactions as it passes through the galactic core are expected to transform it into a long stream of stars orbiting the Milky Way halo.
Tidal interactions add kinetic energy into a globular cluster, dramatically increasing the evaporation rate and shrinking the size of the cluster.
Not only does tidal shock strip off the outer stars from a globular cluster, but the increased evaporation accelerates the process of core collapse. The same physical mechanism may be at work in
Dwarf spheroidal galaxies such as the Sagittarius Dwarf, which appears to be undergoing tidal disruption due to its proximity to the Milky Way.
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