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30.3 Star Groups
• Constellations: patterns of stars and the
region of space around them.
• Standard set of 88 constellations set by
group of astronomers in 1930.
– Acts as a road map to locate particular stars.
• Orion
• Big Dipper
• Pleiades- “seven sisters” 7 bright stars
Naming Constellations
• Modern names come from Latin.
• Names come from…
– Real animals e.g. Ursa Major, “The Great
Bear” (The Big Dipper)
– Imaginary animals e.g.Draco, “The Dragon”
– Ancient Gods
– Legendary Heroes
Multiple-Star Systems
• Binary Stars : pairs of stars that revolve
around each other and are held together
by gravity.
• Barycenter: center of mass. i.e. the point
at which the stars revolve around.
• Estimated that more than half of all
observed stars are multiple-star systems.
Star Clusters
• Clusters: groups of hundreds or thousands
of stars
• Globular Clusters: Spherical shape and
can contain up to 100,000 stars.
• Open Clusters: loosely shaped and rarely
contains more than a few hundred stars.
• Galaxy: a large-scale group of stars, gas,
and dust that is bound together by gravity.
• Major building blocks of universe
• Typical galaxy is 100,000 l.y. in diameter
and contains 200 billion stars
• Estimated that universe contains hundreds
of billion of galaxies
Types of Galaxies
• Spiral Galaxies: has a nucleus of bright stars
and flattened arms that spiral around the
• Elliptical Galaxies: vary in shape from nearly
spherical to very elongated. Extremely bright in
the center and have no spiral arms.
• Irregular Galaxies: has no particular shapes.
Have low total masses and are fairly rich in dust
and gas. More rare than other galaxies.
• Elliptical Galaxies-Virgo Clusters
• Irregular
The Milky Way
• Spiral galaxy in which the sun is one of
hundreds of billions of stars.
• All stars orbit around the center of the
Milky Way Galaxy.
• Takes sun 225 million years to complete
one orbit.
• Closest galaxies are 170,000 l.y. away
from Earth.
• Milky Way
• Discovered in 1963
• Quasar: quasi-stellar radio source; a very
luminous object that produces energy at a
high rate.
• Not related to star but related to galaxies.
• Located in the center of galaxies and are
VERY bright
• Believed that black holes are present in
those galaxies
The arrow in this image points out the record-breaking redshift 5.0 quasar
discovered by the Sloan Digital Sky Survey. That faint red dot of light
represents an object that is actually a hundred times as luminous as a
typical galaxy. Sky Survey astronomers identified this object as a possible
high-redshift quasar on the basis of its exceptionally red color compared to
ordinary stars and galaxies. Followup spectroscopy with the ARC 3.5-meter
telescope confirmed that this unassuming speck was indeed the most
distant quasar known to date.