   #copyright

Globular cluster

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The Messier 80 in the constellation Scorpius is located about 28,000
   light years from the Sun and contains hundreds of thousands of stars.
   Enlarge
   The Messier 80 in the constellation Scorpius is located about 28,000
   light years from the Sun and contains hundreds of thousands of stars.

   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. 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.

   A globular cluster is sometimes known more simply as a globular; the
   word is derived from the Latin globulus (a small sphere).

   Globular clusters are fairly common; there are about 150 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.

Observation history

                               CAPTION: Early Globular Cluster Discoveries

                               Cluster name             Discovered by Year
                                        M22              Abraham Ihle 1665
                                      ω Cen             Edmond Halley 1677
                                         M5           Gottfried Kirch 1702
                                        M13             Edmond Halley 1714
                                        M71 Philippe Loys de Chéseaux 1745
                                         M4 Philippe Loys de Chéseaux 1746
                                        M15    Jean-Dominique Maraldi 1746
                                         M2    Jean-Dominique Maraldi 1746

   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. These additional, undiscovered globular
   clusters are believed to be hidden behind the gas and dust of the Milky
   Way.

   Beginning in 1914, Harlow Shapley began a series of studies of globular
   clusters, published in about 40 scientific papers. He examined the
   cepheid variables in the clusters and would use their period–luminosity
   relationship for distance estimates.
   M75 is a highly-concentrated, Class I globular cluster.
   Enlarge
   M75 is a highly-concentrated, Class I globular cluster.

   Of the globular clusters within our Milky Way, the majority are found
   in the vicinity of the galactic core, and the large majority lie on the
   side of the celestial sky centered on the core. In 1918 this strongly
   asymmetrical distribution was used by Harlow Shapley to make a
   determination of the overall dimensions of the galaxy. By assuming a
   roughly spherical distribution of globular clusters around the galaxy's
   center, he used the positions of the clusters to estimate the position
   of the sun relative to the galactic centre. 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 centre 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 [Class 1–12] 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 is presumed that all of the gas and dust was long ago turned into
   stars.

   While globular clusters can contain a high density of stars, they are
   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 is 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.

   Some globular clusters, like Omega Centauri in our Milky Way and G1 in
   M31, are extraordinarily massive (several million solar masses)
   containing 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
   harbour 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). Hence type I are referred to as
   "metal-rich" while type II are "metal-poor".

   These two populations have been observed in many galaxies (especially
   massive elliptical galaxies). Both groups are of similar ages (nearly
   as old as the universe itself) but differ in their metal abundances.
   Many scenarios have been suggested to explain these subpopulations,
   including violent gas-rich galaxy mergers, the accretion of dwarf
   galaxies, and multiple phases of star formation in a single galaxy. In
   our Milky Way, the metal-poor clusters are associated with the halo and
   the metal-rich clusters with the Bulge.

   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 favour 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.
   Globular cluster M15 has a 4,000-solar mass black hole at its core.
   NASA image.
   Enlarge
   Globular cluster M15 has a 4,000- solar mass black hole at its core.
   NASA image.

   Astronomers have searched for black holes within globular clusters
   since the 1970s. The resolution requirements for this task, however,
   are exacting, and it is 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
   (simulations have suggested alternative possibilities) and a 20,000
   solar mass black hole in the Mayall II cluster in the Andromeda Galaxy.

   These are of particular interest because they are 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
   centre of the cluster, even without a black hole, in both M15 and
   Mayall II.

Colour-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 colour index. The colour 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 is believed to be
   comparable to similar stars in the solar neighbourhood. (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.
   Color-magnitude diagram for the globular cluster M3. Note the
   characteristic "knee" in the curve at magnitude 19 where stars begin
   entering the giant stage of their evolutionary path.
   Enlarge
   Colour-magnitude diagram for the globular cluster M3. Note the
   characteristic "knee" in the curve at magnitude 19 where stars begin
   entering the giant stage of their evolutionary path.

   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
   globular cluster, and the age range 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 age of globular clusters, place a bounds 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.
   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. However, 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 are 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.

                                         CAPTION: 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 (r[c]), the half-light
   radius (r[h]) and the tidal radius (r[t]). 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 (r[m])—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 the Messier 3, which has an overall visible
   dimension of about 18 arc seconds, but a half-mass radius of only 1.12
   arc seconds.

   Finally the tidal radius is the distance from the centre of the
   globular cluster at which the external gravitation of the galaxy has
   more influence over the stars in the cluster than does the cluster
   itself. This is the distance at which the individual stars belonging to
   a cluster can be separated away by the galaxy. The tidal radius of M3
   is about 38″.

Mass segregation and luminosity

   In measuring the luminosity curve of a given globular cluster as a
   function of distance from the core, most clusters in the Milky Way
   steadily increase in luminosity as this distance decreases, up to a
   certain distance from the core, then the luminosity levels off.
   Typically this distance is about 1–2 parsecs from the core. However
   about 20% of the globular clusters have undergone a process termed
   "core collapse". In this type of cluster, the luminosity continues to
   steadily increase all the way to the core region. An example of a
   core-collapsed globular is M15.
   47 Tucanae - the second most luminous globular cluster in the Milky
   Way, after Omega Centauri.
   Enlarge
   47 Tucanae - the second most luminous globular cluster in the Milky
   Way, after Omega Centauri.

   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.

   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 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 N^3, 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.

   Binary stars form a significant portion of the total population of
   stellar systems, with up to half of all stars occurring in binary
   systems. Numerical simulations of globular clusters have demonstrated
   that binaries can hinder and even reverse the process of core collapse
   in globular clusters. When a star in a cluster has a gravitational
   encounter with a binary system, a possible result is that the binary
   becomes more tightly bound and kinetic energy is added to the solitary
   star. When the massive stars in the cluster are sped up by this
   process, it reduces the contraction at the core and limits core
   collapse.

Tidal encounters

   When a globular cluster has a close encounter with a large mass, such
   as the core region of a galaxy, it undergoes a tidal interaction. The
   difference in the pull of gravity between the part of the cluster
   nearest the mass and the pull on the furthest part of the cluster
   results in a tidal force. A "tidal shock" occurs whenever the orbit of
   a cluster takes it through the plane of a galaxy.

   As a result of a tidal shock, streams of stars can be pulled away from
   the cluster halo, leaving only the core part of the cluster. These
   tidal interaction effects create tails of stars that can extend up to
   several degrees of arc away from the cluster. 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.

   Retrieved from " http://en.wikipedia.org/wiki/Globular_cluster"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
