Thursday, December 9, 2010

What are the types of galaxies

Galaxies are certainly among the most popular telescope targets for amateur astronomers. They show an incredibly wide range of size, shape, and internal structure has undoubtedly lead to their fascination among both amateurs and professional astronomers alike. they are vast groups of stars, dust, and gas ranging from a few thousand to nearly a million light-years in diameter. 
Their respective masses show a similarly broad range from less than a million to well over trillion solar masses. This variety of shape and form is far greater than in any other class of deep sky objects.

Elliptical Galaxies

An elliptical galaxy is a galaxy that is elliptical in "observed shape". they are generally composed of only old stars, with little dense gas available for additional star formation. Dynamical measurements show that many are tri-axial, geometric figures with different radii along each axis, and thus their shapes reflect the distribution of stellar orbits rather than being produced by a net rotation. In the "Hubble Classification", ellipticals are described according to apparent shape, from E0 for circular images to E7 for the most flattened ellipticals.Elliptical galaxies span a vast range in size and luminosity, from 100 million stars in a dwarf elliptical to10 million million in the largest giant ellipticals.
Famous Examples:

M32 (NGC 221) about 2.65 million light-years away in the constellation Andromeda.
M49 (NGC 4472) about 49 million light-years away in the constellation Virgo.
M59 (NGC 4621) about 60 million light-years away in the constellation Virgo.
M60 (NGC 4649) approximately 55 million light-years away in the constellation Virgo.
M87 (Virgo A or NGC 4486) a super giant elliptical galaxy located 55 million light-years away in the constellation Virgo.
M89 (NGC 4552) located about 50 million light-years away in the constellation Virgo.
M105 (NGC 3379) located about 32 million light-years away in the constellation Leo.
Maffei 1(the closest giant elliptical galaxy) the closest giant elliptical galaxy to the Milky Way about 9.8 light-years away in the constellation Cassiopeia.

Spiral Galaxies

They are galaxies that has a thin disk of stars, gas and dust, in which a more or less continuous spiral pattern appears. Most spiral galaxies also contain a central spherical bulge of old stars. The spiral pattern may be maintained as a "DENSITY WAVE" moving through the disk, or by differential rotation shearing star-forming regions into locally tilted segments.

Thus spirals range from grand design patterns, with between two and four arms traceable through complete turns around the galaxy, to flocculent galaxies, in which only small,discontinuous pieces of the spiral pattern exist. Most spiral galaxies have at least a weak bar, an elongation of the nuclear bulge, in the plane of the disk. Spirals are classified into various subtypes in the "Hubble Classification". Most maintain active star formation and have a significant reservoir of interstellar gas to fuel additional generations of stars.
Famous Examples:

Triangulum Galaxy (M33 or NGC 598) about 3 million light years away in the constellation Triangulum.
Andromeda Galaxy (M31 or NGC 224) about 2.5 million light-years away in the constellation Andromeda.
Sunflower Galaxy (M63 or NGC 5055) about 37 light years away in the constellation Canes Venatici.
Whirlpool Galaxy (M51a or NGC 5194) an interacting spiral galaxy located at a distance of approximately 23 million light years in the constellation Canes Venatici.
Pinwheel Galaxy (M101 or NGC 5457) a face-on spiral galaxy distanced 25 million light years away in the constellation Ursa Major.

This image shows the evolution of spiral galaxies, from fully formed structures to disheveled collections of stars just beginning to form. These galaxies were captured in the Great Observatories Origins Deep Survey and are presented in the IMAX short film "Hubble: Galaxies Across Space and Time." It takes billions of years for the light of a distant galaxy to reach Earth. Consequently, we see such galaxies as they were in the past, and can thus assemble a rough pictorial history of galaxy evolution. Credit: NASA, ESA, F. Summers and Z. Levay (STScI).

Hubble Space Telescope and NASA's Spitzer Space Telescope joined forces to create this striking composite image of one of the most popular sights in the universe. Messier 104 is commonly known as the Sombrero galaxy.

Barred spiral galaxies

It is a spiral galaxy in which the distribution of stars near the nucleus is elongated in the disk plane. Strong bars are reflected in the Hubble Classification. Weaker bars are common among spirals, with thede Vaucouleurs types indicating that about 60% of spiral shave distinct bars.

Infrared observations, less sensitive to dust absorption and the confusing influence of young stars, may indicate an even greater fraction of spirals with small or weak bars, among them our Galaxy. Stellar dynamics in a disk can produce a bar because of instabilities,and a bar in turn may dissolve into a ring structure over cosmic time. Indeed, many bars are accompanied by stellar rings. Bars can alter the chemical content of a galaxy’s gas, since flow along the bar mixes gas originally located at a wide range of radii, which thus started with different chemical compositions.
Famous Examples:

NGC 4314, a barred spiral galaxy approximately 40 million light years away in the constellation Coma Berenices.
NGC 4921, a barred spiral galaxy in the Coma Cluster, located in the constellation Coma Berenices about 320 million light years away.
M95 (NGC 3351), a barred spiral galaxy about 33 million light years away in the constellation Leo.
NGC 3953, a barred spiral galaxy located in the constellation Ursa Major at a distance of 46 million light years away.
NGC 2903, a barred spiral galaxy about 30 million light-years away in the constellation Leo.
M108 (NGC 3556), a barred spiral galaxy in the constellation Ursa Major.
M58 (NGC 4579), a barred spiral galaxy in the constellation Virgo, approximately 68 million light-years away.

The barred spiral galaxy, NGC 3953

The barred spiral galaxy, NGC 2903

Other morphologies
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies, like:

Ring galaxies
A galaxy, clearly distinct from a spiral or elliptical, in which a ring of stars surrounds the nucleus like the rim of a wheel. Ring galaxies are thought to arise as a result of a "density wave" produced when a small galaxy passes through a larger one. Ring galaxies are also distinct from ringed galaxies, in which normal stellar dynamics (often in the presence of a bar) have channelled stars into a prominent ring about the galaxy’s centre. Collisional rings often have an off-centre nucleus, or none at all.

The most famous example here, is the Hoag's Object (Right Photo), a non-typical galaxy of the type known as a ring galaxy.
The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy is named after Arthur Allen Hoag who discovered it in 1950and identified it as either a planetary nebula or a peculiar galaxy with 8 billion stars.

And another example (Below Image), the Cartwheel Galaxy (ESO 350-40), a ring-lenticular galaxy located 500 million light years away in the constellation Sculptor.

Cartwheel Galaxy (ESO 350-40)

Lenticular Galaxies
A galaxy that appears lens-shaped when seen edge-on. Lenticular galaxies are of a type intermediate in form between the much more common elliptical and spiral types.
They are classified S0, which indicates that they have the flattened form of spirals but no spiral arms. Lenticular galaxies contain both central bulges and surrounding disks of stars, and the disks include significant amounts of dust in some cases.
Famous Examples:

IC 1101, a super giant lenticular galaxy at the center of the Abell 2029 galaxy cluster. It is 1.07 billion light years away in the constellation of Serpens, that has a diameter of approximately 6 million light years, which makes it currently (as of 2010) the largest known galaxy in terms of breadth.
NGC 2787(Right Photo), a beautiful barred lenticular galaxy approximately 24 million light-years away in the constellation Ursa Major.

NGC 5866 (also called the Spindle Galaxy) is a relatively bright lenticular in the constellation Draco.

Irregular Galaxies
A galaxy that shows no symmetry. Some irregular galaxies are smaller than spiral and elliptical galaxies, contain much gas, and are undergoing star formation. Some are classified as irregular for the only reason that they do not fit into other categories of galaxy. Many irregular galaxies have overall rotation and are latively thin, gas-rich disk, sometimes including a bar(as in the Large Magellanic Cloud), forming a continuation of the "Hubble Classification" beyond the spirals of types Sc and SBc.
Famous Examples:

NGC 1427A(Below Photo), an irregular galaxy, located 51.9 million light-years away in the constellation Eridanulip.
The Small Magellanic Cloud (SMC), a dwarf galaxy which has a diameter of about 7,000 light-years, and located only 197,000 light years away, it is also a close companion dwarf to the milky way.
The Large Magellanic Cloud (LMC), a nearby irregular galaxy, and is a satellite of the Milky Way, at a distance of slightly less than 160,000 light years, the LMC is the third closest galaxy to the Milky Way.

Dwarf Galaxies
A galaxy that is much smaller and intrinsically fainter than the familiar spiral and giant elliptical systems. A common definition has absolute visual magnitude fainter than 18; by this criterion the "MAGELLANIC CLOUDS" are not dwarfs but all the other non-spiral members of the "LOCAL GROUP" are.
Dwarf galaxies are usually spheroidal or irregular in structure, and are often found clustered around brighter galaxies. Current galaxy creation theory states that most galaxies,including dwarf galaxies, form in association with dark matter or out of gas containing metals.However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases lacking metals. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo.

Disc Galaxies
Galaxies which have discs, a flattened circular volume of stars. These galaxies may, or may not include a central non-disc-like region (central bulge). Disc galaxy types include spiral galaxies(barless spiral galaxies type S & SA, barred spiral galaxies type SB, intermediate barred spiral galaxies type SAB), lenticular galaxies (type E8, S0, SA0, SB0, SAB0).

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. It is often known colloquially as the "Hubble tuning-fork" because of the shape in which it is traditionally represented.

"X" Structure at Core of Whirlpool Galaxy (M51), forming a cross figure. The M51 core contains a black hole which might be forming these dark lines from our angle of view.

A Few Detailed High-Resolution Images of Galaxies:

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