1. 28.1 Astronomical distances
Astronomers have developed units other than kilometers or meters to measure the vast distances in space.
One light year is equal to the distance that light travels through space in one year.

2. 28.1 Determining distances to closer objects in the universe
Astronomers use a method called parallax to determine the distance of stars that are closer than 1,000 light years to Earth.
As Earth revolves around the Sun, the nearby stars appear to change positions in the sky over the course of one year.

3. 28.1 Determining distances to closer objects in the universe
To use parallax, astronomers determine the position of a closer star (moving) in relation to faraway stars (not moving).
Next, they look at the same star six months later, and measure its change in position relative to the distant stars.

4. 28.1 Studying the universe
Light from other galaxies and stars takes time to reach Earth.
The farther away the object they are viewing, the further back in time astronomers are looking.

5. 28.1 Telescopes
Telescopes work by collecting the light from a distant object with a lens or mirror and bringing that light into a concentrated point, called the focal point.

6. 28.1 Telescopes
A refracting telescope uses lenses to bend, or refract, light, making objects look bigger.
Refracting telescopes are made from a long tube, a glass objective lens that you point toward the sky, and an eyepiece lens.

7. 28.1 Telescopes
A reflecting telescope uses mirrors instead of lenses to gather and focus light.
A concave mirror (called the primary mirror) is placed at the back of a tube.
The secondary mirror deflects the light to an eyepiece lens.

8. 28.1 Telescopes and electromagnetic waves
Astronomers use different types of telescopes to view the different types of waves emitted by objects in space.
A radio telescope works like an extremely powerful receiver that picks up radio waves from space.
Infrared telescopes are often placed on satellites that orbit above Earth.
X-ray telescopes are designed to detect high-energy radiation (X-rays) from space.

9. 28.1 Telescopes and electromagnetic waves
These images of the Crab Nebula were taken with different telescopes.
Each new view gave astronomers more information.

10. 28.1 Satellites and other space craft
The Hubble Space Telescope is a satellite that orbits Earth out of reach of “light pollution.”
It sends images from deep space to computers back on Earth.

11. 28.1 Spacecraft
Space probes are unmanned spacecraft that carry scientific instruments on board.
Launched in 1977, the NASA Voyager 1 and 2 probes are still sending information back to Earth via radio waves.

12. 28.1 Spacecraft
Alan Sheppard of the U.S. followed on May 5.

13. 28.1 Spacecraft
This led to the NASA Manned Lunar Program known as Apollo, which lasted from to 1972.

14. Two generations of Mars Rovers: Sojourner and Spirit
28.1 Spacecraft
NASA’s Mars Exploration Rover (MER) Mission began in 2003.
Two unmanned rovers, Spirit and Opportunity, were sent to explore the surface features and geology of Mars.
Two generations of Mars Rovers: Sojourner and Spirit

15. 28.1 Spacecraft NASA’s Phoenix lander launched in August 2007.
The lander contains a robotic arm that digs through the Martian soil and brings samples onboard for scientific analysis.

16. 28.2 Galaxies
A galaxy is a huge group of stars, dust, gas, and other objects bound together by gravitational forces.
The sun, along with an estimated 200 billion other stars, belongs to the Milky Way galaxy.

17. 28.2 Types of galaxies
The Milky Way is a flattened, rotating system that contains young to middle-aged stars, along with gas and dust.
Astronomers identify it as a spiral galaxy.

18. 28.2 Types of Galaxies
Astronomers classify galaxies according to their shape.
Spiral galaxies consist of a central, dense area surrounded by spiraling arms.
Barred spiral galaxies have a bar-shaped structure in the center.
Elliptical galaxies look like the central portion of a spiral galaxy without the arms.
Lenticular galaxies are lens-shaped.

19. 28.2 The central black hole theory
Recent studies have suggested that a black hole, with a mass of more than a million Suns, exists at the very center of our galaxy.
The evidence for a huge black hole comes from measurements of the orbital velocities of stars and gas at the center.
One of the strangest predictions of Einstein’s theory of relativity is the existence of black holes.

20. 28.2 The central black hole theory
The minimum speed an unpowered projectile must have to escape the planet’s gravity is called the escape velocity.
A black hole is an object with such strong gravity that its escape velocity equals or exceeds the speed of light.

21. 28.2 Distances between galaxies
The distances between stars are 10,000 times greater than the distances between planets.
The distances between galaxies are a million times greater than the distances between stars.

22. 28.2 Distances between galaxies
Figuring out the distance between galaxies is one of the more difficult tasks in astronomy.
A faint object in the night sky could be a dim object that is relatively nearby or a bright object that is far, far away.

23. 28.2 Distances between galaxies
The most reliable method for estimating the distance to a galaxy is to find a star whose luminosity is known.
If the luminosity is known, the inverse square law can be used to find the distance from the observed brightness.

24. 28.2 Distances between galaxies
The inverse square law shows how the brightness of an object decreases as you move away from it.

25. 28.2 Distances between galaxies
The inverse square law is important to astronomers because if they know the brightness and luminosity of an object, they can determine its distance by rearranging equation variables.

26. 28.2 Distances between galaxies
A second type of standard candle is called a Cepheid star.
Cepheid stars “pulsate” in regular periods ranging from a few days to a few weeks.
By measuring the period of a Cepheid star, astronomers determine its luminosity and then calculate its distance.

27. 28.3 Doppler Shift
Doppler shift also occurs with electromagnetic waves, such as visible light, X-rays, and microwaves.
This phenomenon is an important tool used by astronomers to study the motion of objects in space.

28. 28.3 The expanding universe
The faster the source of light is moving away from the observer, the greater the redshift.
The opposite (blueshift) happens when an object is moving toward the observer.

29. 28.3 The expanding universe
Edwin Hubble discovered that the farther away a galaxy was, the faster it was moving away from Earth.
This concept came to be known as the expanding universe.

30. 28.3 The Big Bang theory
The theory that the universe was expanding implies the universe must have been smaller in the past than it is today.
It implies that the universe must have had a beginning.
Astronomers today believe the universe exploded outward from a single point.
This idea is known as the Big Bang theory.

31. 28.3 The Big Bang theory
The Big Bang theory says the universe began as a huge explosion between 10 billion and 20 billion years ago.
According to this theory, all matter and energy started in a space smaller than the nucleus of an atom.

32. 28.3 Evidence for the Big Bang theory
In the 1960s, Arno Penzias and Robert Wilson were trying to measure electromagnetic waves given off by the Milky Way.
The “noise” these scientists found was the cosmic microwave background radiation predicted by the Big Bang theory.

33. 28.3 Evidence for the Big Bang theory
The proportion of hydrogen to helium is consistent with the physics of the Big Bang.
If the universe were significantly older, there would be more heavy elements present compared with hydrogen and helium.

34. 28.3 Planetary systems
A star with orbiting planets is called a planetary system.
Scientists now believe that planets are a natural by-product of the formation of stars.

35. 28.3 How the solar system formed
Scientists think that the solar system was formed out of the same nebula that created the Sun.

36. 28.3 Binary stars
A binary star is a system with two stars that are gravitationally tied and orbit each other.
About half of the 60 nearest stars are in binary (or multiple) star systems.