Galaxy

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Galaxy Types

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The majority of galaxies fall into three broad categories in terms of their configuration: those exhibiting a "galactic disk" of stars, those with a spheroidal or ellipsoidal distribution of stars, and those with no identifiable pattern to their stellar distribution. The majority of these galaxies are classified as "dwarf" galaxies, the larger types (such as Galaxias and Andromeda) being much rarer. Both disk-type and ellipsoidal galaxies are characterized by a distribution of "redder" stars (types F thru M) throughout their disks or volumes, respectively. Ellipsoidal and Lenticular Disk galaxies, however, are populated almost entirely by such stars, with comparatively few brighter stars of types O thru A. The various "spiral" type disk galaxies, on the other hand, generally exhibit additional young and hot stars of the O thru A types (in addition to the F thru M types) distributed in higher density star-forming regions of gas and dust within their disks that trace out spiral patterns. Note that the observed "spiral" in these galaxies is largely an optical phenomenon, as the younger, hotter and brighter stars located in this region generally outshine the dimmer cooler stars that populate the disk region as a whole.

Most galaxies have a super-massive central black hole massing on the order of several million solar masses around which the stars of the galaxy orbit. Stars toward the central spheroidal "bulge" of a galaxy are typically older and redder stars, and are distributed in a much denser distribution than farther out from the center, sometimes on the order of 10's of thousands of stars per cubic parsec. Within a parsec of the central black hole, densities can reach as high as millions of stars per cubic parsec. In general, stellar densities drop off exponentially as one moves outward from the galactic center.

Most galaxies exhibit small dense concentrations of stars known as Globular Clusters which orbit in wide elliptical orbits about their centers in a roughly spherical distribution generally referred to as a galaxy's "galactic halo". Globular Clusters are composed almost entirely of old red stars at an average density of 0.4 stars per cubic parsec, increasing in density to between 100 and 1000 stars per cubic parsec as one approaches the core of the cluster. The average distance between stars in a globular cluster is generally about 1 lightyear, but decreases to less than 100 AU as one approaches the core of the cluster.

In evolutionary terms, the early Universe seems to have been populated primarily by Spiral-type and Irregular-type galaxies, both of which typically evidence dense regions of interstellar gas and dust that are regions of active star formation. It is believed that as galaxies age, and as the abundance of interstellar gas and dust in a galaxy's spiral arm regions is used up via the star formation process, the older, redder stars become more readily visible as star-formation ceases and the young hot stars die out, leading to the galaxy becoming a "lenticular" disk galaxy in which the prominent, bright spiral arm structures begin to fade into the background stars of the disk. As time increases, the chance that galaxies will gravitationally interact with one another and coalesce also increases, eventually leading to multiple galaxies merging into single composite elliptical galaxies in which the disk or spiral arm structure has been disrupted.

Spiral Disk Galaxies

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Spiral galaxies are generally galaxies of younger to intermediate age, and are characterized by significant amounts of interstellar gas and dust in a flattened disk shaped distribution about a central spheroidal bulge. In general, this gas and dust is not distributed uniformly throughout the disk region, but rather is found in higher densities along radial wave-fronts that curve in a roughly logarithmic spiral formation. These density wave-fronts are formed by the compression of gas and dust due to the slower rotation of the regions of greater density as compared to the stars and gas in the rest of the disk-medium. As these regions become compressed, they cause local gravitational collapse of regions of interstellar molecular clouds, giving rise to regions of star-formation in the direction immediately to trailing of the density wave-front. As these star-forming regions tend to have an abundance of young stars of all types, including many hot bright stars of classes O thru A, the denser spiral regions tend to stand out more brightly to visual observation than the less dense "inter-arm" regions that tend to more exclusively contain older "redder" stars of types F thru M, as the hotter stars have mostly already died out in these regions.

As with most galaxies, the central "bulge" of a spiral galaxy tends to be populated by older redder stars that increase in density exponentially as one approaches the central supermassive black hole at the center of the galaxy. Likewise, a sparsely populated spherical region known as a galactic halo surrounds most spiral galaxies. The halo of a spiral galaxies has very little dust, and is largely inhabited by dense globular clusters of old red Population II stars, along with a sparse scattering of individual old red Population II stars. The number of globular clusters orbiting a typical spiral galaxy's center is generally about 100 to 200 on average. The globular clusters and sparse stars of the halo orbit the galactic center on elliptical paths highly inclined relative to the galactic disk at distances of 10's of thousands of kiloparsecs relative to the galactic center. Additionally, the majority of a spiral galaxy's distribution of dark matter is found in the spheroidal halo region, although the distribution does not perfectly overlap, generally occupying a more ellipsoidal region perpendicular to the galactic plane along its long axis.

Spiral galaxies, along with Irregular galaxies, together make up approximately 60% of observed galaxies.

• Spiral Galaxies (Types Sa/SAa thru Sd/SAd)

Spiral Galaxy NGC 4414

Spiral galaxies are generally classified according to the tightness of the winding of their spiral arms, from most tightly wound to least lightly wound, according to the following nomenclature:

• Sa (or SAa) – Tightly wound arms with a large and bright central bulge
• Sb (or SAb) – Less tightly wound arms than Sa/SAa with a somewhat fainter and smaller central bulge
• Sc (or SAc) – Loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge than Sb/SAb
• Sd (or SAd) – Very loosely wound, fragmentary arms with most of the luminosity in the arms and not the central bulge

Non-barred spiral galaxies are generally believed to be younger galaxies, as 80% of spirals in the early universe were unbarred, as compared to 30% today. However, it is also believed that the bar-like structure of some spiral galaxies may actually appear and disappear over large timescales due to dynamics within the galactic core causing periodic compression of interstellar gas outward from the core region.

• Barred Spiral Galaxies (Types SBa thru SBd)

Barred Spiral Galaxy UGC 12158

Barred-spiral galaxies are also generally classified according to the tightness of the winding of their spiral arms, from most tightly wound to least lightly wound, according to the following nomenclature:

• SBa – Tightly wound arms with a large and bright central bulge
• SBb – Less tightly wound arms than SBa with a somewhat fainter and smaller central bulge
• SBc – Loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge than SBb
• SBd – Very loosely wound, fragmentary arms with most of the luminosity in the arms and not the central bulge

Barred spiral galaxies are generally believed to be more mature spiral galaxies, as only 20% of spirals in the early universe were barred, as compared to 70% today. However, it is also believed that the bar-like structure of some spiral galaxies may actually appear and disappear over large timescales due to dynamics within the galactic core causing periodic compression of interstellar gas outward from the core region.

Galaxias falls into this general category and is currently classified as type SBbc.

• Intermediate SA/SB Galaxies (Types SABa thru SABd)

Intermediate Spiral NGC 6744

Intermediate-spiral galaxies fall somewhere between regular spirals and barred spirals, and are also generally classified according to the tightness of the winding of their spiral arms, from most tightly wound to least lightly wound, according to the following nomenclature:

• SABa – Tightly wound arms with a large and bright central bulge
• SABb – Less tightly wound arms than SABa with a somewhat fainter and smaller central bulge
• SABc – Loosely wound spiral arms, clearly resolved into individual stellar clusters and nebulae; smaller, fainter bulge than SABb
• SABd – very loosely wound, fragmentary arms; most of the luminosity is in the arms and not the bulge

Lenticular Disk Galaxies

Lenticular Galaxy NGC 2787

Lenticular galaxies are also disk galaxies like the spiral types described above. However, unlike spiral-type galaxies they show no evidence of any spiral arm structure, being a flat and featureless disk with a central bulge. The central bulge of Lenticular galaxies can be either spheroidal (type S0 or SA0) or show evidence of a bar-like structure (type SB0). Lenticular galaxies are further graded with a subscript from 1-3 based on the amount of dust-absorption observed in the disk structure. In the case of barred lenticular galaxies, this gradation is sometimes related to length of the central bar.

• Unbarred-types: S0_1-3 or SA0_1-3,
• Barred-types: SB0_1-3

Lenticular galaxies, like ellipticals, have little or no evidence of interstellar gas, though unlike ellipticals they still possess a fair amount of dust. As a result of this dearth of gas, very little star-formation occurs within lenticular galaxies. It is believed that lenticular galaxies are actually aging spiral galaxies that have used up all of their interstellar gas in star-formation, and hence the young bright hot stars have all since evolved off the main-sequence or ended their lives, the spiral structure having slowly faded as a result, leaving behind only a disk filled with older and redder stars. As a result, lenticular galaxies are sometimes thought of as a possible transition-state between spiral and elliptical galaxies.

Lentiuclar galaxies also show a greater abundance of globular clusters in their halos than comparable spiral galaxies.

Ring Galaxies (-r)

Ring Galaxy - Hoag's Object

Formed by collision of a smaller galaxy passing through the center of a larger galaxy

Ring evidences significant star formation

Central region largely devoid of luminous matter

Elliptical Galaxies E(0-7)

Elliptical Galaxy ESO 325-G004

Oldest Galaxies - formed from gravitational mergers of other galaxies

Little Gas & Dust - No star Formation - Old Red Stars

Notation: "En", where n = eccentricity x 10 and e = 1 − (b/a) for an ellipse with semimajor and semiminor axes of lengths a and b respectively.

• Shell Galaxies

Irregular Galaxies (Irr)

Irregular Galaxy - Large Magellanic Cloud NGC1910

"Dark Galaxies"

A Dark Galaxy, or "Ultra-diffuse Galaxy" (such as Dragonfly 44, for example) is a galaxy which has a mass comparable to other galaxies, but which lacks much of the star-forming molecular clouds of gas that typify a more mundane galaxy. As a result, while such galaxies have masses similar to other galaxies, they may have as little as 1% of the number of actual stars in their morphological structure as compared to a more typical galaxy, most of which tend to be old "red" stars, with little current star-forming activity ongoing. It is theorized that much or most of the composition of such galaxies may be mostly dark matter.

Dark Matter

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Galactic Clusters & Superclusters

• Please see main article at Universe

Galaxies are not distributed uniformly throughout the universe, but rather tend to clump together hierarchically in groups. Galaxias belongs to a cluster of galaxies known simply as the "Local Group". The three largest members of the Local Group, in descending order according to size, are the Andromeda Galaxy (M31), Galaxias, and the Triangulum Galaxy (M33), the remainder of the Local Group being composed primarily of dwarf galaxies. In total, the Local Group contains about 51 galaxies.

The Local Group itself is associated with a number of other galactic clusters that together form the Local Supercluster otherwise known as the Virgo Supercluster. In all, the Virgo Supercluster contains on the order of 100,000 galaxies.

The Virgo Supercluster itself is a spur of the larger Laniakea Supercluster, which is but one of many superclusters visible in the observable universe. In all, the observable universe contains approximately 100-200 billion galaxies.