1. What makes up the universe and how does gravity affect the universe?
The universe is made up of stars, gas, and dust, as well as invisible dark matter. Material in the universe is pulled by gravity into galaxies, including our own Milky Way galaxy.

2. The View from Earth How do astronomers divide the night sky?
What can astronomers learn about stars from their light?
How do scientists measure the distance and the brightness of objects in the sky?

3. Looking at the Night Sky
The star Polaris is almost directly above the North Pole.
Earth’s rotation causes other stars to appear to revolve around Polaris.
CORBIS

4. Present-day astronomers use many ancient constellations to divide the sky into 88 regions.

5. Dividing the sky helps scientists communicate to others what area of sky they are studying.

6. Looking at the Night Sky (cont.)
Telescopes can collect more light than the human eye can.
Steve Allen/Brand X Pictures
Michael Matisse/Getty Images

7. Looking at the Night Sky (cont.)
The electromagnetic spectrum is a continuous range of wavelengths.

8. Different parts of the electromagnetic spectrum have different wavelengths and different energies. You can see only a small part of the energy in these wavelengths.

9. Looking at the Night Sky (cont.)
The set of wavelengths that a star emits is the star’s spectrum.
A spectroscope spreads light into different wavelengths.
Using spectroscopes, astronomers can study stars’ characteristics, including temperatures, compositions, and energies.

10. Measuring Distances
Parallax is the apparent change in an object’s position caused by looking at it from two different points.
Astronomers use parallax to calculate how far an object in space is from Earth.
The object is viewed from two extreme points in Earth’s orbit.

12. An astronomical unit is the average distance between Earth and the Sun, about 150 million km.

13. Measuring Distances (cont.)
A light-year is the distance light travels in 1 year.
One light-year is about 10 trillion km.

14. Measuring Brightness
The apparent magnitude of an object is a measure of how bright it appears from Earth.

15. Measuring Brightness (cont.)
Luminosity is the true brightness of an object.
The luminosity of a star, measured on an absolute magnitude scale, depends on the star’s temperature and size, not its distance from Earth.

16. Galaxies and the Universe
What are the major types of galaxies?
What is the Milky Way, and how is it related to the solar system?
What is the Big Bang theory?

17. Galaxies Galaxies are huge collections of stars.
Gravity holds stars and galaxies together.
When astronomers examine how galaxies rotate and gravitationally interact, they find that most of the matter in galaxies is invisible.
Matter that emits no light at any wavelength is dark matter.

18. Types of Galaxies: Spiral Galaxies
Spiral arms of stars, gas, and dust extend from central disk.
Spiral galaxies have a central bulge.
A spherical halo surrounds the disk.
NASA/JPL-Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics)

19. Types of Galaxies: Elliptical Galaxies
Elliptical galaxies have a spherical or elliptical shape and no internal structure.
They contain more older stars than spiral galaxies, and may have formed as spiral galaxies merged.
JPL/NASA

20. Types of Galaxies: Irregular Galaxies
Irregular galaxies are oddly shaped and contain many young stars.
Local Group Galaxies Survey Team, NOAO, AURA, NSF

21. The Milky Way
The solar system is in the Milky Way, a spiral galaxy that contains gas, dust, and almost 200 billion stars.
The Milky Way is a member of the Local Group, a cluster of about 30 galaxies.

23. The Big Bang Theory
According to the Big Bang theory, the universe began from one point billions of years ago and has been expanding ever since.
Most scientists agree that the universe is billion years old.
Scientists observe how space stretches by measuring the speed at which galaxies move away from the Earth.

24. The Big Bang Theory (cont.)
When light travels away from you, its wavelength stretches out, shifting to the red end of the electromagnetic spectrum.
The shift to a different wavelength is called the Doppler shift.

25. Doppler Shift