Astrophysical Directions by Michael Erlewine

 

 

In Astrophysical Directions
Introduction Coordinate Systems The Solar System The Solar Neighborhood The Galaxy Galactic Objects The Fixed Stars Star Clusters & Nebulae Non-Visual Astronomy External Galaxies Finder-Lists, etc.
Star Clusters & Nebulae

 

  Galactic Star Clusters

  Open Clusters

  Globular Star Clusters

  Moving Clusters

  Stellar Associations

  O-Associations

  T-Associations

  Galactic Nebulae

  Emission Nebulae

  Dark Nebulae

  Planetary Nebulae

 

Galactic Star Clusters

The first and only astrology that I am aware of who has as concerned himself with star clusters is Charles A. Jayne. Jayne points out that clusters of dozens and thousands of stars exist at various distances and directions from our Sun. These clusters cohere for hundreds of millions of years in most cases. Jayne goes on to point out that these star clusters are at least as deserving of our attention as the more familiar constellations, composed of stars (in most cases) at different distances and having no physical relationship with one another.

The reader should understand that single stars are not the rule, but very much the exception. Clusters of stars are the rule, and in fact clusters of almost all astrophysical objects are the rule. ALL OBJECTS IN SPACE SHOW A DECIDED PREFERENCE TOWARD GROUPS, CLUMPS AND CLUSTERS.

This tendency of stellar objects toward clustering helps to define the various planes of cosmic structure and the larger or more vast the distances we consider, the greater is this tendency of objects to clumps. In other words, galaxies show an even greater preference for clustering than do the stars!

 

Gallectic Model

 

The most obvious and gigantic cluster of stars is our galaxy itself. It is estimated that our galaxy contains some hundred billion stars. We will examine these vast clusters (galaxies) in a later section. Here we want to look at the various kinds of star clustering within our galaxy. There are two major cluster types: the Open or Galactic Cluster and the Globular Star Cluster. Each of these types shows a preference for different parts of the galaxy. The open clusters are groups of dozens or hundreds of stars that occur in the equatorial plane of the galaxy and seem to form the very backbone of our Milky Way. The globular clusters are systems of hundreds of thousands of stars packed tight in a globe or sphere and these great globes circle the galactic nucleus in highly inclined orbits. The globular clusters act as beacon lights to indicate the overall dimensions of our galaxy in all directions. Figure A illustrates both of these basic cluster, types.

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Open Clusters

The Open Star Clusters (OC) are ragged and irregular groups of dozens or hundreds of stars that have a closer gravitational relation than that of the surrounding star field. These open clusters define the galactic equatorial plane and form a flattened disk-like system 1,000 parsecs thick and with a diameter of 10,000 parsecs. There is a distinct anti-center emphasis among open clusters with amaximum concentration of these objects toward a point 350 parsecs from our Sun, in the vicinity of galactic longitude 280°. The spatial distribution of the open clusters defines the spiral arm structure of our galaxy much as the globular clusters define the nucleus and spherical shape of the galactic system. The open clusters are much smaller than the globulars, with a maximum linear diameter for the largest of not over 15 parsecs, the smallest around 1.5 parsecs, and a range of 2-6 parsecs for those clusters of average size.

They are also much closer to us than the globulars, or at least the ones visible from our vantage point in the galaxy. The open clusters occur in the disk or plane of the galaxy along with the great concentration of gas and dust clouds. For this reason, we cannot see the more distant members of this group. Keep in mind that not only are the globular clusters intrinsically brighter, but their relatively higher galactic latitudes put them out of the dusty galactic plane and into view. All open clusters show a concentration to the plane of the galaxy with the exception of those clusters, which are situated so near to us that they appear projected in high galactic latitudes. The Coma Berenices cluster is one of these. While some 1000 open clusters are known, it is estimated that there are about 18,000 of these objects distributed throughout the galactic plane.

We have noted that the dense globular clusters are able to defy the disruptive tidal forces within our galaxy and survive forever in terms of the life of the galaxy, not so for the open clusters. The great disk of our galaxy revolves like some great wheel through space and time. While to us, this spinning disk appears stationary (we are like a flashbulb picture), it has a powerful motion in terms of the life span of stellar objects. The open clusters have much shorter life spans than do the globular clusters. The fact that so many open clusters are found tells us that these objects are continuously being born or formed. Otherwise they would have vanished from the galaxy long ago.

In fact, the open clusters are the Johnny Appleseeds of our galaxy. In endless formation, they arise together in clusters from the great dust and gas clouds and move together through space, away from their birthplace. They cohere or hang together as long as possible in defiance of the galactic tidal waves, yet suffer loss of star after star until all are dispersed in an endless trail across the galactic plane. The speed of "evaporation" of the cluster stars depends greatly on their density in the cluster. A dense, compact cluster will be much more stable than a cluster of low density. The Hyades cluster is perhaps safe for about a thousand million years, whereas the Pleiades and Praesepe may endure for ten times that period.

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The Globular Star Clusters

The globular clusters form a, more or less, spherical system of satellites around the galactic nucleus. If we could speed up our time process, we would find that these objects circle the nucleus of our galaxy, in their very inclined orbits, at high speeds, very much like diagrams of the atomic nucleus surrounded by orbiting electrpns.The anticenter of the galaxy is almost devoid of globulars and most occu between our Sun and the center of the galaxy (GC).

With the exception of the very much more diffuse stellar associations (to be mentioned later), the globular clusters are the most massive and vast star clusters known (the Associations are vast but not massive). A globular cluster may contain hundreds of thousands of stars packed into a spherical region of space some 100 parsecs in diameter. Globulars, therefore, are very bright and can be observed at relatively greater distances than any other form of star cluster. The largest are visible to the naked eye as fuzzy patches and the two brightest are Omega Centauri and 47 Tucanae. Messier Object 13 (M.13) in Hercules is very well known.

Globular clusters are also very old and are essentially permanent members of the galactic system. They are the eternal guardians of the galactic nucleus. The large number of stars in these clusters and their density or compactness create a self-gravitating system that is very effective against the disruptive tidal forces in the galaxy. There is a distinct absence of blue giants and supergiant stars (superluminous stars of short lifetimes) in the globular clusters. This along with the absence of dust and gas clouds points to their great age. The globular clusters seem to move, more or less, at random about the galactic center, very much like individual stars in a cluster seem to have mainly incoherent motions about the center of the cluster. Some astronomers claim that the globulars make radial oscillations (in and out) rather than spherical orbits.

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Moving Clusters

Certain open clusters, for which the individual stars have a pronounced motion toward a convergent point, are known as Moving Clusters. The Hyades is one of the earliest known and the nearest of these clusters. Except for small peculiar motions, all stars that are members of a moving cluster move in space along more or less parallel paths, much like meteors in a meteor stream. The individual proper motions of these stars seem to converge toward or diverge from a common point in the sky in the same way that meteors in showers appear to diverge from their radiants (see Meteors). The point of divergence marks the direction in space toward which the Sun is moving with respect to the cluster. The point of convergence of the proper motions (opposite the point of divergence on the celestial sphere) marks the direction toward which the cluster is moving with respect to the Sun.

The Ursa Major cluster is of particular interest to us since it occupies the same volume of space as our Sun. In fact, it is moving through our space, although the Sun is not a member of this cluster. The Ursa Major cluster is composed of two subgroups which consist of a moderately compact cluster of 14 stars with the same proper motion and an extended stream of stars which has approximately the same motion. The nucleus of this cluster is located about 23 parsecs from the Sun and occupies (roughly) ellipsoidal region 4x6x10 parsecs in diameter. The shortest diameter is perpendicular to the galactic plane, while the longest is in the direction of the motion of the cluster. The motion of the local centroid is 29 km/sec.

Another moving cluster of great interest is the Scorpio-Centaurus or Southern Stream cluster. The Sco-Cen moving cluster is a relatively ancient one as indicated by its rather elongated shape. It is also part of the local system of stars (which see) of which our Sun is a member. The total annual proper motions for the Sco-Cen cluster range from O.02" to O.05." The group formed about 70 million years ago in a region 2200 parsecs distant, in the direction of galactic longitude 59 degrees.

The seven most conspicuous open-type clusters are the Pleiades, Hyades, Praesepe, Coma Bereneces, IC 2602, NGC 3532, and Messier Objects 6 & 7. The Pleiades, perhaps the most photographed object in this universe, has been recorded in ancient times by the Chinese, Hindus, Chaldeans, and the Greeks and is mentioned in the Bible. Called the seven sisters or pigeons, only six stars can be seen today with the unaided eye. It is thought that one of the original stars may have faded since ancient times. There is somewhere between 300-500 stars in the Pleiades cluster with the center 6' west of Alcyone, the lucida, at R.A. 56° 01'01" and Declination of +23°57126." The brightest Pleiades are late B-type, an indication of the youth of this particular cluster. The Pleiades has always received much attention from astrologers and it is more than interesting to note that the position of the cluster in the Zodiac coincides with the intersection of the Galactic and Supergalactic equators, as projected onto the ecliptic.

Notes & Legend for Moving Clusters (Listed with O-Associations)

The listed position for the Moving Clusters is that of the CONVERGENT POINT relative to the Sun. This is the point toward which the cluster seems to be headed. The physical positions for the various clusters, that is, their celestial positions, are listed in the various star cluster categories.

S = The velocity of the cluster relative to the Sun
C = The velocity of the cluster corrected for the Sun's velocity

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The Stellar Associations

Very loose galactic clusters are called Stellar Associations and were discovered in the late 1940s. These associations are almost spherical in shape and yet have very low spatial density. The differential revolution about the galactic center should have elongated these groups of stars in a few hundred million years. This is due to the fact that the portion of the galactic disc nearer the center of the galaxy revolves more rapidly than the outer edge of the disk, in a pseudo-Keplerian motion. The lack of elongation noted in associations indicates that they must have been recently formed and are expanding at velocities of the order of 5 to 10 km/sec. If so, their ages cannot exceed a few millions or tens of millions of years. Young associations are of spherical shape (circular in outline) and older associations suffer distinct elongation as would be expected from the effect of galactic differential rotation, the spinning galactic disk.

The official definition of stellar associations in the words of their founder V.A. Ambartsumian: "O-Associations are stellar systems where the partial density of '0' and 'B2' stars [spectral class] is larger than the average field density of these stars in such a way that this difference cannot be explained by chance fluctuations; moreover, '0' or 'BO' stars are present."

The radii of these associations range up to 200 parsecs, far exceeding a typical open cluster or even that of the mighty globulars. It has been found that many associations outline the three major spiral arms of our galaxy. An association may contain 100 stars and it is estimated that ten million stellar associations have passed through their evolutionary cycle during the lifetime of our galaxy, each ending in total disintegration and the scattering of its members through the galaxy.

Perhaps the most famous association is that in Orion, where the great nebula and its central cluster form the nucleus of a large expanding association. Many associations include well-known galactic clusters and emission nebulae. Some are so young that only their most massive stars have had time to condense out of the interstellar medium and reach the hydrogen burning stage on the main sequence. The less massive young stars (still in the contracting stage) are usually embedded in bright and dark nebulosity and many of these stars are variable. It has even been reported that a star (FU Orion) has become visible in the Orion association where a few years prior none appeared in the photographs.

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O-Associations

Astronomer V. A. Ambartsumian describes O-Associations as:
"... stellar systems where the partial density of 'O-B2' stars is larger than the average field density of these stars in such a way that this difference cannot be explained by chance fluctuations; moreover, 'O' or 'BO' stars are present."

The properties of O-associations may be described as follows:

  • The linear diameters range between 30 and 200 parsecs.
  • The associations contain an open star cluster of type 'O' as nucleus.
  • They include, besides O-B2 stars, also stars of types later than B2, sometimes even Wolf-Rayet stars (though it is difficult to ascertain the number of faint stars).
  • Sometimes multiple star systems of Trapezium type and star chains may be part of the nuclei,
  • hot stars occur also outside the nuclei.
  • There are reasons for presuming the O-associations to be unstable systems.

The Associations have also been called aggregates and groups, but the lack of basic data on individual members of the associations or groups often resulted in a specific system receiving a completely different and independent description, which produced confusion among the identifications.

Extensive lists of early spectral-class stars with known luminosities enable us to form a notion of their distribution in space. Systematic research resulted in establishing space condensations of these stars. W. W. Morgan called them aggregates and K. H. Schmidt groups. Both of these authors introduced designations, which differed from Ambartsumian's and Markarian's original nomenclature (the founders). The lack of basic data for individual members of the associations or groups often resulted in a specific system receiving a completely independent description, which produced confusion among the identifications.

In Markarian's designation, the association is characterized by a Roman number following the order of detection of the object (e.g. Cassiopeia V), while Morgan and (later) Schmidt arranged the associations according to increasing galactic longitude with the Roman number preceding the name of the Constellations (e.g. I Cassiopeia). The basic designations used here are those of The Catalogue of Clusters and Associations by Alter, Ruprecht, and Vanysek. The alternate designations are listed with the initials of the author: (Mo)= Morgan, (Ru)= Ruprecht, (Sch)= Schmidt, (Ma)= Markarian, and (Ko)= Kopylov.

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T-Associations

T-Associations are groups or clusters of very young stars that are very near to our Sun. Along with the O-Associations, these are the nearest groups of celestial objects. T-Associations are loose groups of T Tauri and RW Aurigae stars at distances from about 100 to 1000 parsecs from the Sun. There are about forty of these associations known, most with few then thirty members, although there may be as many as four hundred. T Associations have received much attention in recent years from astronomers due to the fact that T Tauri stars are closely associated with strong clouds of interstellar dust and are often observed near or within these dark nebulae. Armenian astronomer V. A. Ambartsumian sees in these compact associations of variable stars a special class of stars, possible in the condensing stage -- young or very young stars. These T Tauri stars help to bridge the gap between protostars (newborn stars) and the younger stars. (T Tauri stars = age less than 107 The spherical shape of the T-Associations or clusters is an indication that these young stars will disband in a relatively short period of time. They are unstable.

The T Tauri stars exhibit erratic variations that may be, in part, extrinsic. They have luminosity comparable to our Sun, although their spectra are peculiar, with bright lines of hydrogen and the metals and continua of unusual energy distribution. These stars are strong emitters of infrared radiation.

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Galactic Nebulae

Not all of the material in our universe is contained in stars. A large amount of interstellar matter exists in the form of cloud-like objects called nebulae. A nebula is made up of very tenuous gas and dust. Gaseous nebulae are of such great importance because it seems that it is here that stars are born. It is here that fresh stars are condensing out of the nebular material. The three main types of nebulae are illustrated above. In most of space, these interstellar dust clouds are cold and dark. Dark clouds block our view to the very center of our Galaxy. Were it not for these obscuring dark nebulae, the nuclear region of the galaxy would be a blaze of light filling the night sky in that direction.

There are two types of nebulae that 'shine': emission nebulae and reflection or continuum nebulae.

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Emission Nebulae

A nebula containing a very hot star can be excited to self luminosity, resulting in what is termed an emission nebula. A nebulous region which is excited to luminosity in this way is also called an H-II region since hydrogen (H) is the most abundant element. Emission nebulae are huge masses of gas that absorb ultraviolet radiation from nearby hot stars and reradiate it as bright-line emission. The most famous example of an emission nebula is Messier Object 42 (M.42), the great nebula in Orion. Another is the Eta Carinae Nebula in the southern sky. The larger emission nebulae are most often associated with the very hot 'O' and 'BO 'stars and may contain dense groups of these most luminous stars. The hot central stars in the emission nebulae often appear to have cleared away the dust from their immediate surrounding, creating a hole or dust-free bubble inside an otherwise dusty region of space. (See the section on Solar Wind)

Nebulae can also become luminous when a nearby bright star causes them to shine by reflected light. The Pleiades was the first reflection nebula to be observed. The reflection nebulae reflect the light of stars embedded within them. It is not known whether reflection nebulae are only dark clouds that happen to be near a bright star or whether some actual physical relationship may exist between the reflection nebulae and the stars that illuminate them. It has been noted that stars of 'B1' or later (see section on spectral Type) produce reflection nebulae, while stars of 'BO' or earlier produce emission nebulae.

 

Emission Nebulae

 

In some nebulae, the star producing the illumination is not hot enough to make the nebulosity shine by its own light and the result is a reflection nebula. Perhaps the most famous reflection nebula is the one in the Pleiades star cluster.

 

Reflection Nebulae

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Dark Nebulae

If there are no stars in or near the nebulosity, the nebula will obscure or block all light beyond or behind itself. The result are dark patches or 'holes' in the sky. The most celebrated dark nebua is the Coal Sack in the Southern Cross.

 

Dark Nebulae

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dark Clouds and Interstellar Dust

Until the 20th century, astronomers assumed that the immense distance between stars was empty, in effect a perfect vacuum. Numerous dark patches were thought to be some sort of 'holes in space" where there were no stars. A few of these dark areas are visible to the naked eye, in particular the "Coal Sack" near the Southern Cross and the "Great Rift" in the Milky Way. The Great Rift splits the luminous background from Cygnus to Sagittarius through a succession of large overlapping dark clouds in the equatorial plane of the galaxy. It has been discovered that these "holes in the stars" are in fact obscuring clouds of small grains of matter, dust. Like cigarette smoke, this dust diffuses the light coming from behind them. There is no essential difference between a bright nebula and a dark one; it all depends on whether there are any suitable stars to provide illumination.

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Planetary Nebulae

A Planetary Nebula, in essence, appears to be a roughly spheroidal or ellipsoidal shell of gas with a nuclear star in or close to its center. These nebulae (planetaries) received their name not due to any possible generic relationship to planets, but because early observers, while searching for planets with primitive telescopes, sometimes came across these disc-like objects and they at first glance looked like planets. The central star of a planetary is usually quite dim. It is not often brighter than eleventh magnitude and it is the exception that can be seen at all. The body or expanded shell-like ring of gas of the nebula is also faint and tends to blend into the sky background. The hot central stars in these nebulae seem to be evolving to the white-dwarf stage and radiate by far the greatest portion of their energy in the far ultraviolet portion of the electromagnetic spectrum. Since these objects are expanding, they are probably short-tem and will disappear as a result of expansion in something like 30,000 years. The Ring Nebula in Lyra is perhaps the best known of the planetaries. The Crab Nebula, while often considered as an example of this category, is not a true planetary, but rather the remains of a massive supernova, the ghost of a cosmic fireworks.

 

 

© Copyright © 1997 Michael Erlewine

 

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