Tuesday, 5 July 2016



A galaxy is gravitationally bound structure of stellar remnants, interstellar gas, stars, dark matter, and dust. On the basis of quantity of stars, It’s size ranging from dwarf to huge. Dwarf galaxies are contains few thousands of stars whereas huge galaxies are containing trillions of stars.
pink galaxy

Stars are orbiting around galaxies center of mass. Galaxies are also categories on the basis of their visual appearance or morphology. They are normally three types, elliptical, spiral and irregular. Many galaxies have black holes at their active center. Our milky way’s galaxies also have a black hole at their center. The milky way’s galaxy black hole is known as Sagittarius A*.
Sagittarius A* is four million times greater than sun in mass. On the basis of March 2016, 32 billion light year away from earth a most distant and oldest galaxy observed known as GN-z11. There are approx 170-200 billions of galaxies exist in our observable universe. They are 1000 to 100,000 parsecs in diameter, separated by distance of millions of parsecs. The space between them is filled with tenuous gas of density one atom/cubic meter. Galaxies are associates together because of galaxies in galaxy groups, cluster and super clusters. Overview they are arranged as sheets and filament surrounded by immense voids.
Types of morphology:
Galaxies are normally three main types spirals, ellipticals, irregulars. More types are given by Hubble sequence on the basis of their visual appearances.
types of morphology

Ø  elliptical galaxy
on the basis of their Ellipticity Hubble classification system ranging elliptical galaxies. They are ranging from E0 to E7, where it became nearly spherical. They are classified according to their ellipsoid profile regardless of their viewing angle.
Elliptical galaxies are believed as they form due to interaction of galaxies. Due to their interaction they collide and merge. Giant elliptical are larger galaxies, they found near the larger galaxies clusters. Grow to enormous size, larger than spiral.
Ø  Shell galaxy
The shell galaxies are also a type of elliptical galaxy. Stars in this galaxies halo are arranged in concentric shells. One tenth of elliptical galaxies are shell like structure. They are not observed in spiral galaxies. They thought to form when larger galaxies are absorbs smaller one. As their center approach and stars vibrate at their center. due to this gravitational ripples are form which arrange stars like shells. NGC 3923 galaxy have 20 shells.
NGC 3923

Ø  Spiral galaxy
Spiral galaxies resemble huge spiraling pinwheels. The Mass of galaxies are exist in roughly spherical halo of dark matter. Which extends beyond the visual appearance and the constituent are arrange in nearly flat disc.
Spiral galaxies have a rotating disc which contains stars and interstellar material. they have central bulge contains normally older stars. The arms are extending outward from centre bulge and they are relatively brighter. On the basis of Hubble classification scheme, “spiral galaxies” are listed as type “S” followed by letter (a, b or c). which indicates size of central bulge and degree of tightness of spiral arms.




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