1. Chapter 15 Preview Section 1 Stars
Stars, Galaxies, and the Universe
Preview
Section 1 Stars
Section 2 The Life Cycle of the Stars
Section 3 Galaxies
Section 4 Formation of the Universe
Concept Map

2. Chapter 15 Bellringer List ways that stars differ from one another.
Section 1 Stars
Bellringer
List ways that stars differ from one another.
How is the sun like other stars? How is it different?
Write your answers in your Science Journal.

3. Chapter 15 What You Will Learn
Section 1 Stars
What You Will Learn
Stars differ in size, temperature, composition, brightness, and color.
Distances between stars are very large and are measured in light-years.

4. Chapter 15
Section 1 Stars
Color of Stars
The color of a star indicates the star’s temperature.
Red stars are the coolest, and blue stars are the hottest.
If two stars differ in color, you can conclude that they differ in temperature too.

5. Chapter 15 Composition of Stars
Section 1 Stars
Composition of Stars
Astronomers use an instrument called a spectroscope to separate a star’s light into a spectrum (plural, spectra).
A spectrum is the band of colors produced when white light passes through a prism.

6. Composition of Stars, continued
Chapter 15
Section 1 Stars
Composition of Stars, continued
A continuous spectrum shows all of the colors, while an absorption spectrum shows which wavelengths of light are absorbed.
The spectrum of a star is an absorption spectrum because the atmosphere of the star absorbs certain portions of the light produced by the star.

7. Composition of Stars, continued
Chapter 15
Section 1 Stars
Composition of Stars, continued
The pattern of lines in a star’s absorption spectrum is unique to that star.
A star’s absorption spectrum can be used to determine the elements in that star’s atmosphere.
Stars are made of mostly hydrogen and helium gases.

8. Composition of Stars, continued
Chapter 15
Stars, Galaxies, and the Universe
Composition of Stars, continued

9. Chapter 15 Classifying Stars
Section 1 Stars
Classifying Stars
Stars are now classified by how hot they are.
Temperature differences between stars result in color differences that can be seen. For example, class O stars are blue—the hottest stars.

10. Classifying Stars, continued
Chapter 15
Section 1 Stars
Classifying Stars, continued
Magnitude is used to compare the brightness of one object with the brightness of another object.
To express the brightness of stars, astronomers use a system of magnitudes.

11. Classifying Stars, continued
Chapter 15
Section 1 Stars
Classifying Stars, continued
Positive magnitude numbers represent dim stars. Negative magnitude numbers represent bright stars.
The brightest star in the night sky, Sirius, has a magnitude of -1.4.

12. Chapter 15
Stars, Galaxies, and the Universe
Classifying Stars

13. Chapter 15 How Bright Is That Star?
Section 1 Stars
How Bright Is That Star?
The apparent magnitude is the brightness of a star as seen from Earth. The absolute magnitude is the brightness that a star would have at a distance of 32.6 light-years from Earth.
If all stars were the same distance away, their absolute magnitudes would be the same as their apparent magnitudes.

14. Absolute and Apparent Magnitude
Chapter 15
Stars, Galaxies, and the Universe
Absolute and Apparent Magnitude

15. Chapter 15 Distance to the Stars
Section 1 Stars
Distance to the Stars
Because stars are so far away, astronomers use a unit called a light-year to measure the distance from Earth to the stars.
A light-year is the distance that light travels in one year, about 9.46 trillion kilometers.

16. Chapter 15 Motions of Stars
Section 1 Stars
Motions of Stars
This apparent motion of the stars is due to Earth’s rotation and its revolution around the sun.
Each star is actually moving in space. But because stars are so distant, their actual motion is hard to see.

17. Chapter 15 Bellringer List 3 stages in the life cycle of an insect.
Section 2 The Life Cycle of Stars
Bellringer
List 3 stages in the life cycle of an insect.
List 3 stages in the life cycle of a human.
List any stages you may know of in the life cycle of a star.
Write your answers in your Science Journal.

18. Chapter 15 What You Will Learn
Section 2 The Life Cycle of Stars
What You Will Learn
During star formation, gravity pulls dust and gas into a sphere, and when the sphere gets dense enough, nuclear fusion begins.
Nuclear Fusion is an atomic reaction that fuels stars.

19. Chapter 15
Section 2 The Life Cycle of Stars
Types of Stars
Stars can be classified by mass, size, brightness, color, temperature, composition, and age.
A star is classified differently as its properties change. Its properties often relate to whether the star can generate energy by nuclear fusion.

20. The Life Cycle of Sunlike Stars
Chapter 15
Section 2 The Life Cycle of Stars
The Life Cycle of Sunlike Stars
New stars form from gas and dust, which are pulled into a sphere by gravity.
As the sphere becomes denser, hydrogen combines into helium and energy is released in a process called nuclear fusion.

21. Chapter 15 Bellringer What do you think galaxies are made of?
Section 3 Galaxies
Bellringer
What do you think galaxies are made of?
What is the name of the galaxy in which we live?
Draw a sketch of what you think a spiral galaxy looks like when viewed from outside that galaxy.
Record your answers in your Science Journal.

22. Chapter 15 What You Will Learn
Section 3 Galaxies
What You Will Learn
The three types of galaxies are spiral galaxies, elliptical galaxies, and irregular galaxies.
Galaxies are composed of stars, planetary systems, nebulas, and star clusters.
Looking at distant galaxies reveals what young galaxies looked like.

23. Chapter 15 Types of Galaxies
Section 3 Galaxies
Types of Galaxies
A galaxy is a collection of stars, dust, and gas bound together by gravity.
Astronomers classify a galaxy as spiral, elliptical, or irregular according to its shape and rate of star formation.

24. Types of Galaxies, continued
Chapter 15
Section 3 Galaxies
Types of Galaxies, continued
Spiral galaxies have a bulge at the center and spiral arms.
The spiral arms are made of gas, dust, and new stars that have formed.
The galaxy in which we live is a spiral galaxy called the Milky Way.

25. Types of Galaxies, continued
Chapter 15
Section 3 Galaxies
Types of Galaxies, continued
Most elliptical galaxies are round or oval and seem to have stopped making new stars.
Elliptical galaxies are among the largest galaxies in the universe.
Irregular galaxies have no definite shape and form new stars slowly.

26. Chapter 15 Contents of Galaxies
Section 3 Galaxies
Contents of Galaxies
Some galaxies contain nebulas and star clusters.
A nebula is a large cloud of gas and dust in interstellar space.
Nebulas are a region in space where stars are born.

27. Contents of Galaxies, continued
Chapter 15
Section 3 Galaxies
Contents of Galaxies, continued
A globular cluster is a highly concentrated group of up to 1 million stars that looks like a ball.
Open clusters are groups of 100 to 1,000 stars that are close together relative to other stars.

28. Chapter 15
Stars, Galaxies, and the Universe
Contents of Galaxies

29. Gravity and the Universe
Section 4 Formation of the Universe
Chapter 15
Gravity and the Universe
Gravitational attraction caused the matter distributed throughout the universe to form galaxies.
Because gravity acts over such great distances, gravity controls the size and shape of the universe.
Every object in the universe is part of a larger system and gravity acts within each system.

30. Structure of the Universe
Chapter 15
Stars, Galaxies, and the Universe
Structure of the Universe