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Frequently Asked Questions
How are galaxies classified?
Astronomer Edwin Hubble classified galaxies into four major types: spiral,
barred spiral, elliptical and irregular. Most of the nearby, bright
galaxies are spirals, barred spirals or ellipticals.

/Spiral galaxies/ have a bulge at the center and a flattened disk
containing spiral arms. Spiral galaxies have a variety of shapes and are
classified according to the size of the bulge and the tightness and
appearance of the arms. The spiral arms, which wrap around the bulge,
contain numerous young blue stars and lots of gas and dust. Stars in the
bulge tend to be older and redder. Yellow stars like our Sun are found
throughout the disk of a spiral galaxy. The disks of spiral galaxies rotate
somewhat like a hurricane or a whirlpool.

/Barred spiral galaxies/ are spiral galaxies that have a bar-shaped
collection of stars running across the center of the galaxy.

/Elliptical galaxies/ do not have a disk or arms. Instead, they are
characterized by a smooth, oval-shaped appearance. Ellipticals contain old
stars, and possess little gas or dust. They are classified by the shape of
the ball, which can range from round to oval (baseball-shaped to
football-shaped). In contrast to


Source: hubblesite.org/reference_desk/faq/answer.php.id=40&cat=galaxies


how are galaxies classified


Galaxy morphological classification - Wikipedia, the free encyclopedia

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** Galaxy morphological classification **

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Artist's concept illustrating bulge & no bulge spiral galaxies.

*Galaxy morphological classification* is a system used by astronomers to
divide galaxies into groups based on their visual appearance. There are
several schemes in use by which galaxies can be classified according to
their morphologies, the most famous being the Hubble sequence, devised by
Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan
Sandage.

*Contents*

· 1 Hubble sequence
· 2 De Vaucouleurs system

· 2.1 Numerical Hubble stage

· 3 Yerkes (or Morgan) scheme
· 4 See also
· 5 References
· 6 External links

*Hubble sequence[edit]*

Main article: Hubble sequence

The Hubble sequence is a morphological classification scheme for galaxies
invented by Edwin Hubble in 1926.^[1]^[2] It is often known colloquially as
the “Hubble tuning-fork” because of the shape in which it is
traditionally represented. Hubble’s scheme divides galaxies into 3
broad classes based on their visual appearance (originally on photographic
plates):

Tuning-fork style diagram of the Hubble sequence

· *Elliptical galaxies* have smooth, featureless light distributions and
appear as ellipses in images. They are denoted by the letter E, followed by
an integer n representing their degree of ellipticity on the sky.
· *Spiral galaxies* consist of a flattened disk, with stars forming a
(usually two-armed) spiral structure, and a central concentration of stars
known as the bulge, which is similar in appearance to an elliptical galaxy.
They are given the symbol S. Roughly half of all spirals are also observed
to have a bar-like structure, extending from the central bulge. These
barred spirals are given the symbol SB.
· *Lenticular galaxies* (designated S0) also consist of a bright central
bulge


Source: en.wikipedia.org/wiki/Galaxy_morphological_classification

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