1. 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

2. 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.

3. Star Systems
How are stars distributed in space?
Astronomers have determined that more than half of all stars are members of star systems.

4. Most stars occur in groups of two or more.
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.

5. 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.

6. Star Clusters
The Pleiades are an open star cluster that is visible to the unaided eye.
47 Tucanae is a spectacular globular cluster that is visible in southern skies.

7. 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.

8. Galaxies
What are the types of galaxies?
Astronomers classify galaxies into four main types: spiral, barred-spiral, elliptical, and irregular.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. Galaxies
A spiral galaxy in the constellation Coma Berenices
A barred-spiral galaxy in the Fornax cluster

15. Galaxies
Elliptical galaxy M87
An irregular galaxy with many areas of star formation

16. 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
Nucleus
Halo containing oldest stars
Side View of Our Galaxy
About 100,000 light-years

17. 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.

18. 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 super-massive black hole with the mass of a billion suns.

19. Assessment Questions A constellation is
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.

20. Assessment Questions A constellation is
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

21. Assessment Questions
A large group of older stars without sufficient gas and dust to form new stars is a(n)
open cluster.
galaxy.
association.
globular cluster.

22. Assessment Questions
A large group of older stars without sufficient gas and dust to form new stars is a(n)
open cluster.
galaxy.
association.
globular cluster. ANS: D

23. What type of galaxy is the Milky Way?
Assessment Questions
What type of galaxy is the Milky Way?
spiral galaxy
barred-spiral galaxy
elliptical galaxy
irregular galaxy

24. What type of galaxy is the Milky Way?
Assessment Questions
What type of galaxy is the Milky Way?
spiral galaxy
barred-spiral galaxy
elliptical galaxy
irregular galaxy ANS: A