Galaxy
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.
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| 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.
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| types of morphology |
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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.
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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.
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| NGC 3923 |
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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.
Reference:
Wikipedia
