26.4 Groups of Stars
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.
Red Sea
26.4 Groups of Stars
A group of stars that appear to form a pattern
as seen from Earth is called a constellation.
The stars in a constellation are generally not
close to one another. They just happen to lie
in the same general direction of the sky as
seen from Earth.
26.4 Groups of Stars
Star Systems
How are stars distributed in space?
Astronomers have determined that more
than half of all stars are members of star
systems.
26.4 Groups of Stars
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.
26.4 Groups of Stars
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.
26.4 Groups of Stars
Star Clusters
What are the characteristics of each type of
star cluster?
There are three basic kinds of star clusters:
open clusters, associations, and globular
clusters.
26.4 Groups of Stars
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.
26.4 Groups of Stars
Star Clusters
A. The Pleiades are an open star cluster that
is visible to the unaided eye.
26.4 Groups of Stars
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.
26.4 Groups of Stars
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.
26.4 Groups of Stars
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.
26.4 Groups of Stars
Star Clusters
Globular clusters can contain more than a
million stars. Globular clusters 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.
26.4 Groups of Stars
Galaxies
What are the types of galaxies?
Astronomers classify galaxies into four main
types: spiral, barred-spiral, elliptical, and
irregular.
26.4 Groups of Stars
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.
26.4 Groups of Stars
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.
26.4 Groups of Stars
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.
26.4 Groups of Stars
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.
26.4 Groups of 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.
26.4 Groups of Stars
Galaxies
A. A spiral galaxy in the constellation Coma
Berenices
26.4 Groups of Stars
Galaxies
A. A spiral galaxy in the constellation Coma
Berenices
B. A barred-spiral galaxy in the Fornax
cluster
26.4 Groups of Stars
Galaxies
C. Elliptical galaxy M87
26.4 Groups of Stars
Galaxies
C. Elliptical galaxy M87
D. An irregular galaxy with many areas of
star formation
26.4 Groups of Stars
Galaxies
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.
26.4 Groups of Stars
Galaxies
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
Central bulge
containing mainly
older stars
Halo containing
Nucleus
oldest stars
About 100,000 light-years
Side View of Our Galaxy
26.4 Groups of Stars
Galaxies
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.
26.4 Groups of Stars
Galaxies
Quasars
By studying their spectra, astronomers have
determined that quasars are the enormously
bright centers of distant, young galaxies.
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 supermassive black hole with the mass of a billion suns.
26.4 Groups of Stars
Assessment Questions
1. A constellation is
a.
b.
c.
d.
two stars that orbit each other.
a star system with more than two stars.
an open cluster of stars that are close to one another.
a group of stars that appear to form a pattern.
26.4 Groups of Stars
Assessment Questions
1. A constellation is
a.
b.
c.
d.
two stars that orbit each other.
a star system with more than two stars.
an open cluster of stars that are close to one another.
a group of stars that appear to form a pattern.
ANS: D
26.4 Groups of Stars
Assessment Questions
2. A large group of older stars without sufficient gas
and dust to form new stars is a(n)
a.
b.
c.
d.
open cluster.
galaxy.
association.
globular cluster.
26.4 Groups of Stars
Assessment Questions
2. A large group of older stars without sufficient gas
and dust to form new stars is a(n)
a.
b.
c.
d.
open cluster.
galaxy.
association.
globular cluster.
ANS: D
26.4 Groups of Stars
Assessment Questions
3. What type of galaxy is the Milky Way?
a.
b.
c.
d.
spiral galaxy
barred-spiral galaxy
elliptical galaxy
irregular galaxy
26.4 Groups of Stars
Assessment Questions
3. What type of galaxy is the Milky Way?
a.
b.
c.
d.
spiral galaxy
barred-spiral galaxy
elliptical galaxy
irregular galaxy
ANS: A