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Galaxies

A huge gravitationally bound system of starsis called galaxy. It is a interstellar gas and dust, plasma, and unseen dark matter. Typical galaxies contain ten million to one trillion stars, all orbiting a common center of gravity.

In addition to single stars and a tenuous interstellar medium, most galaxies contains multiple star systems,star clusters and various types of nebulae.

Most galaxies are usually separated from one another by distances on the order of millions of light years.

There is some evidence that supermassive black holes may exist at the center of many galaxies.

The space between galaxies called Intergalactic space, is filled with a tenuous plasma with an average density less than one atom per cubic meter.

There are probably more than a hundred billion (1011) galaxies in our universe.

Etymology

The word galaxy derives from the Greek term for our own galaxy, galaxies meaning “milky circle” for the system’s appearance in the sky.

When astronomers speculated that certain objects which were classified as spiral nebulae were actually vast congeries of stars, this was called the “island universe theory”; but this was an obvious misnomer, since universe means everything there is.

Consequently, this term fell into disuse, replaced by applying the term galaxy generically to all such bodies.

Types of galaxies

Galaxies come in three main types:

elliptical,

spirals,

irregulars.

A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence.

Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate.

Our own galaxy, the Milky Way is a large disk-shaped barred spiral galaxy about hundred light millennia in diameter and three light millennia in thickness.

It contains about three hundred billion stars and has a total mass of about six hundred billion times the mass of Sol.

The spiral arms have the shape of logarithmic spirals in spiral galaxies, it is a pattern that results from a disturbance in a uniformly rotating mass of stars.

The spiral arms like the stars, also rotate around the center with constant angular velocity. That means that stars pass in and out of spiral arms. As stars move into an arm, they slow down, thus creating a higher density. The arms are visible because the high density facilitates star formation and they therefore harbor many bright and young stars.

Most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundred times smaller than the Milky Way.

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites. Dwarf galaxies may also be classified as elliptical, spiral or irregular.

Since small dwarf ellipticals resembles to large ellipticals, they are often called dwarf spheroidal galaxies.

Galaxy formation and evolution

Formation

The method of galactic formation may be divided into two categories:

1. top-down : In top-down theories such as the Eggen–Lynden-Bell–Sandage (ELS) model, protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.

2. bottom-up: In bottom-up theories such as the Searle-Zinn (SZ) model, globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.

Modern theories must be modified to account for the probable presence of large dark matter halos. Baryonic matter begins to condense around cold dark matter halos,shortly after recombination. Zero-metal high-velocity halo stars are the first to develop around a protogalaxy as it starts to contract.These huge stars releases heavy elements into the interstellar medium. Within the next billion years, globular clusters, the central supermassive black hole and galactic bulge of metal-poor Population II stars form.

The galaxy will absorb infalling material from high velocity clouds and dwarf galaxies throughout its life; the cycle of stellar birth and death will increase the abundance of heavy elements forming planets. Within two billion years, the remaining material settles into a galactic disk.

The oldest galaxy yet found by humans was discovered in 2004 by scientists at Caltech using the Hubble Space Telescope and the Keck telescopes. The great mass of this galactic cluster bends and focuses the light passing through it, acting as a lens in space. This galaxy was displaced by galaxy Abell 1835 IR1916 as the most distant galaxy ever seen by humans.

The existence of old protogalaxies suggests that they must have grown in the so-called “Dark Ages” from anisotropic irregularities present during the era of recombination, three hundred thousand years after the Big Bang. Such irregularities of the right scale were observed using the Wilkinson Microwave Anisotropy probe.

More evidence for this model of galactic formation comes from detection of ancient Population III stars. The giant star, HE0107-5240, discovered in 2002 by researchers is believed to be the oldest yet discovered star in the Milky Way and it is virtually metal-free.

Evolution

The Milky Way Galaxy is moving towards Andromeda Galaxy at about 130 km/s, and the two may collide in about five to six billion years. In dense galaxy clusters such galaxy collisions are fairly common.

Although the Milky Way has never collided with Andromeda galaxy before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.

Spiral galaxies, like the Milky Way produces new generations of stars till they continue to have dense molecular clouds of interstellar hydrogen in their spiral arms.

Elliptical galaxies are devoid of this gas and so forms no new stars. As stars convert hydrogen into heavier elements the supply of star-forming material is finite thus fewer stars will form.

After the end of stellar formation the “stellar age” will come to an end as the smallest longest-lived stars in our astrosphere, tiny red dwarfs begin to fade. At the end of the stellar age galaxies will comprise compact objects: brown dwarfs, black dwarfs, cooling white dwarfs, neutron stars, and black holes. As a result of gravitational relaxation, all stars will either fall into the central supermassive black hole of the galaxies, or be flung into the depths of intergalactic space as a result of collisions.