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A galaxy is made of billions of
stars, dust, and gas all held
together by gravity.
Galaxies are scattered throughout
the Universe.
They vary greatly in size and
shape. Not all galaxies look alike.
Types of Galaxies
• Galaxies come in a variety of shapes.
• In the 1920s Edwin Hubble was the first to study the
morphology (shape) of galaxies.
• Using the 100-inch Hooker reflector telescope at Mount
Wilson Observatory in California between 1922-1926,
Hubble photographed numerous galaxies.
• He categorized these shapes or basic schemes as spiral,
barred spiral, elliptical, irregular, and peculiar.
• This system was known as the Hubble morphological
sequence of galaxy types.
Spiral Galaxies
• Some galaxies, like the M31 Andromeda Galaxy, appear as
disks and have arms of stars and dust which appeared in a
spiral pattern.
• These galaxies appear nearly uniform in brightness and in
some the arms are more tightly wound around the galaxy.
• These are called spiral galaxies. Our Galaxy, the Milky Way,
is an example of a spiral galaxy.
Barred Galaxies
• Barred spirals are spiral galaxies that have a straight bar
across their centre from the ends of which the spiral arms
• The arms of spiral galaxies contain gas and dust from
which new stars are still forming.
Elliptical Galaxies
• Elliptical galaxies contain old stars and very little gas.
• They include the most massive galaxies known, containing
a trillion stars.
• At least some elliptical galaxies are thought to be formed
by merges between spiral galaxies.
Irregular Galaxies
• A fourth type of galaxy observed by Hubble was neither
spiral nor elliptical, but was irregular in shape.
• These galaxies were called irregular.
• An example of this is the Magellanic Clouds.
Peculiar Galaxies
• Finally, there are some galaxies that fit none of these
• These are called peculiar galaxies, one example of which is
Centaurus A
Classifying Galaxies
• Astronomers now have decided that the morphology
classification should consist of only two types of
galaxies: the spiral and the elliptical.
• Barred spirals are a subclass of spirals. Irregulars may be
either spiral or barred spiral.
• Peculiars are not fundamentally a different type. They
are simply galaxies in the act of colliding; the collision
distorts their shape and makes them appear “peculiar”
• Our own galaxy, the Milky Way, is about 100 000 light
years in diameter, and contains at least 100 billion stars.
It is a member of a small cluster of the local group.
• The Sun lies between two spiral arms, about 25 000 light
years from the centre.
Classifying Galaxies
• Spiral galaxies are denoted by “S”, and barred spirals by
“SB”. Letters “a”, “b”, “c” denote how tightly the spiral
arms are wound, with “a” being most tightly wound. The
Andromeda Galaxy is an Sb.
• Elliptical galaxies are denoted by “E”, with a number
from 0-7 indicating how circular it appears. An example
of this would be M87, which is an E0 galaxy.
• Irregulars, such as the Small Magellanic Cloud, are
denoted by “Irr”.
• Peculiar galaxies, such as Centaurus A, are denoted by
Hubble’s Classification of Galaxies
Formation of Galaxies
• Galaxies form from
the clumping of
primordial matter
that arise after the
Big Bang and grow
under inflation.
• This clumping of
primordial matter
forms filaments, and
galaxies form in knots
along the filaments.
Spiral vs. Elliptical Galaxies
• The final type of galaxy depends on initial rate of star
• If stars form quickly, then galaxy becomes elliptical.
Stars form within initial distribution of gas, and follow
their initial orbits.
• If stars form later, the gas has time to collapse into a
disk. Most stars from within the disk. The galaxy
becomes a spiral.
Formation via Galaxy Mergers
• In clusters, galaxies can pass close to one another.
• Galaxies can become distorted, and often merge.
• Mergers often lead to giant elliptical galaxies at the
heart of large clusters.