Download Groups of Stars

26.4
 When Giovanni
Riccioli used a
telescope like this
one to observe a
star in the handle
of the Big Dipper,
he discovered two
stars that orbit
each other.
Groups of Stars
 A group of stars that appear
to form a pattern as seen from
Earth is called a
constellation.
 Stars in a constellation are
generally not close to one
another.
 Lie in the same general
direction of the sky as seen
from Earth.
More than half of all stars
are a part of star systems
Star Systems
Most stars occur in groups of two or more.
• A star system is a group of two or more stars that are
held together by gravity.
• A star system with two stars is called a binary star. The
two stars orbit each other.
Star Systems
 Sometimes the smaller star in a binary star is too dim
to be seen easily from Earth but can still be detected
from the motion of the other star.
If one star passes in front of the other, blocking some of
the light from reaching Earth, the star system is called an
eclipsing binary.
The brightness of an eclipsing binary varies over time in a
regular pattern.
Star Clusters
 There are three basic kinds of star clusters: open
clusters, associations, and globular clusters.
Star Clusters
 Studying star clusters is
useful because all the stars
formed together in the
same nebula, so they are
about the same age and the
same distance from Earth.
Astronomers plot the
stars of a cluster on an H-R
diagram to estimate the
cluster’s age.
Star Clusters
A. The Pleiades are an open star cluster that is visible to
the unaided eye.
B. 47 Tucanae is a spectacular globular cluster that is
visible in southern skies.
Star Clusters
 An open cluster has a disorganized or loose
appearance and contains no more than a few thousand
stars that are well spread out.
 Open clusters often contain bright supergiants and gas
and dust clouds.
Associations are temporary groupings of bright,
young stars. In time, gravity from nearby stars breaks
these groups apart.
Associations are typically larger than open clusters.
Star Clusters
A globular cluster is a large group of
older stars.
 Globular clusters usually lack sufficient
amounts of gas and dust to form new
stars.
 They are spherical and have a dense
concentration of stars in the center.
 Can contain more than a million stars.
 Usually do not have short-lived blue
stars because these stars have already died
out.
Astronomers estimate that the oldest
globular clusters are about 12 billion years
old. Thus, the universe must be at least
that old.
Galaxies
 Astronomers classify galaxies into four main types:
spiral, barred-spiral, elliptical, and irregular.
Galaxies
A galaxy is a huge group
of individual stars, star
systems, star clusters,
dust, and gas bound
together by gravity.
• There are billions of
galaxies in the universe.
• The largest galaxies
consist of more than a
trillion stars. Galaxies
vary widely in size and
shape.
Galaxies
Spiral and Barred-Spiral Galaxies
 Spiral galaxies have a bulge of stars at the center,
with arms extending outward like a pinwheel.
• These spiral arms contain gas, dust, and many bright
young stars.
• The Milky Way is a spiral galaxy.
Galaxies
 Some spiral galaxies have a bar through the center with
the arms extending outward from the bar on either
side. These are called barred-spiral galaxies.
Galaxies
Elliptical Galaxies
Elliptical galaxies are
spherical or oval, with no
trace of spiral arms.
• Elliptical galaxies come in a
wide range of sizes.
• Elliptical galaxies have very
little gas or dust between
stars. They contain only
old stars.
Galaxies
Irregular Galaxies
 A small fraction of all galaxies are known as irregular
galaxies.
Irregular galaxies have a disorganized appearance.
They have many young stars and large amounts of gas and
dust.
Irregular galaxies come in many shapes, are typically smaller
than other types of galaxies, and are often located near larger
galaxies.
Galaxies
A.
B.
C.
D.
A spiral galaxy in the constellation Coma Berenices
A barred-spiral galaxy in the Fornax cluster
Elliptical galaxy M87
An irregular galaxy with many areas of star formation
Our Galaxy
The Milky Way Galaxy
The Milky Way galaxy has an
estimated 200 to 400 billion
stars and a diameter of more
than 100,000 light years.
Every individual star that you
can see with the unaided eye is
in our galaxy.
The solar system lies in the
Milky Way’s disk within a
spiral arm, about two thirds of
the way from the center.
Our Galaxy
 In a side view, the Milky Way appears as a flat disk
with a central bulge. An overhead view of the Milky
Way shows its spiral shape.
Location of
solar system
Central bulge
Nucleus
Overhead View of Our Galaxy
Disk of spiral arms
containing mainly
young stars
Halo containing
oldest stars
Central bulge
containing mainly
older stars
Nucleus
Side View of Our Galaxy
Our Galaxy
The Milky Way’s flattened disk
shape is caused by its rotation.
The sun takes about 220
million years to complete one
orbit around the galaxy’s center.
Recent evidence suggests that
there is a massive black hole at
our galaxy’s center.
Stars are forming in the
galaxy's spiral arms
Other Galaxies
Quasars
Quasars are the enormously bright centers of distant,
young galaxies.
Identified by spectra
Quasars produce more light than hundreds of galaxies the
size of the Milky Way.
What makes a quasar so bright?
The most likely explanation involves matter spiraling into a
super-massive black hole with the mass of a billion suns.