Chapter 1

Question 1

Our Cosmic Address

Answer 1

• Planet
• Solar system
• Galaxy
• Local group
• Galaxy clusters
• Superculsters
• Universe

Question 2

One Astronomical Unit (AU)

Answer 2

The Earth’s average distance from the sun; about 150 million kilometers. More technically, 1 AU is the length of the semimajor axis of Earth’s orbit.

We commonly describe distances within our solar system in astronomical units.

Question 3

One light-year

Answer 3

The distance that light can travel in one year; 10 trillion kilometers (6 trillion miles).

We generally use light-years to describe the distances of stars and galaxies.

Question 4

Star

Answer 4

A large, glowing ball of gas that generates heat and light through nuclear fusion in its core. Our Sun is a star.

Question 5

Planet

Answer 5

A moderately large object that orbits a star and shines primarily by reflecting light from its star.

According to a definition adopted in 2006, an object can be considered a planet only if it (1) orbits a star, (2) is large enough for its own gravity to make it round, and (3) has cleared most other objects from its orbital path.

An object that meets the first two criteria but has not cleared its orbital path, like Pluto, is designated as a dwarf planet.

Question 6

Moon (or satellite)

Answer 6

An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object.

Question 7

Asteroid

Answer 7

A relatively small and rocky object that orbits a star; asteroids are officially considered part of a category known as “small solar system bodies.”

Question 8

Comet

Answer 8

A relatively small and ice-rich object that orbits a star. Comets are officially considered part of a category known as “small solar system bodies.”

Question 9

Small solar system body

Answer 9

A asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet.

Question 10

Solar System

Answer 10

the Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system (solar means “of the Sun”), it is often applied to other star systems as well.

Question 11

Star System

Answer 11

A star (sometimes more than one star) and any planets and other materials that orbit it.

Question 12

Galaxy

Answer 12

A great island of stars in space, containing from a few hundred million to a trillion or more stars, all held together by gravity and orbiting a common center.

Question 13

Cluster (or group) of galaxies

Answer 13

A collection of galaxies bound together by gravity. Small collections (up to a few dozen galaxies) are generally called groups, while larger collections are called clusters.

Question 14

Supercluster

Answer 14

A gigantic region of space in which many groups of clusters and galaxies are packed more closely together than elsewhere in the universe.

Question 15

Universe (or cosmos)

Answer 15

The sum total of all matter and energy–that is, all galaxies and everything in between them.

Question 16

Observable universe

Answer 16

The portion of the entire universe that can be sen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe.

The extent of the observable universe is 14 billion light years. Though the universe may extend beyond that distance, we are only able to observe those objects no further away in distance of light years than the age of the universe. All other light would take longer to reach our eyes than time has allowed.

Question 17

Rotation (spin)

Answer 17

The spinning of an object around its axis. For example, Earth rotates once each day around its axis, which is an imaginary line connecting the North and South Poles.

Question 18

Orbit (revolution)

Answer 18

The orbital motion of one object around another due to gravity. For example, Earth orbits the Sun once each year. An orbit may be bound (elliptical) or unbound (parabolic or hyperbolic)

Question 19

Expansion (of the universe)

Answer 19

The increase in the average distance between galaxies as time progresses.

Question 20

Milky Way Galaxy

Answer 20

Used both as the name of our galaxy and to refer to the band of light we see in the sky when we look into the plane of the Milky Way Galaxy.

Question 21

Big Bang

Answer 21

The name given to the event thought to mark the birth of the universe.

Question 22

Nuclear Fusion

Answer 22

The process in which two (or more) smaller nuclei slam together and make one larger nucleus.

Question 23

Ecliptic plane

Answer 23

The plane of Earth’s orbit around the Sun

Question 24

Axis tilt

Answer 24

The amount by which a planet’s axis is titled with respect to a line perpendicular to the ecliptic plane.

Question 25

Andromeda Galaxy

Answer 25

M31; the Great Galaxy in Andromeda; the nearest large spiral galaxy to the Milky Way;

Question 26

Cosmos

Answer 26

An alternative name for the universe

Question 27

Dwarf Planet

Answer 27

An object that orbits the Sun and is massive enough for its gravity to have made it nearly round in shape, but that does not qualify as an official planet because it has not cleared its orbital neighborhood. The dwarfs of our solar system include the asteroid Ceres and the Kuiper belt objects Pluto, Eris, Haumea, and Makemake

Question 28

Ecliptic

Answer 28

The Sun’s apparent annual path among the constellations.

Question 29

Group (of galaxies)

Answer 29

A few to a few dozen galaxies bound together by gravity. See also cluster of galaxies.

Question 30

Gravitationally bound system

Answer 30

Any system of objects, such as a star system or a galaxy, that is held together by gravity.

Question 31

Geocentric Universe (ancient belief in)

Answer 31

The idea that Earth is the center of the entire universe.

Question 32

Large Magellanic Cloud

Answer 32

One of two small, irregular galaxies (the other is the Small Magellanic Cloud) located about 150,000 light-years away; it probably orbits the Milky Way Galaxy.

Question 33

Local Supercluster

Answer 33

The supercluster of galaxies to which the Local Group belongs.

Question 34

Local Solar Neighborhood

Answer 34

The portion of the Milky Way Galaxy that is located relatively close (within a few hundred to a couple thousand light-years) to our Sun.

Question 35

Local Group

Answer 35

The group of about 40 galaxies to which the Milky Way Galaxy belongs.

Question 36

Minor planets

Answer 36

An alternative name for asteroids

Question 37

Synchrotron Radiation

Answer 37

A type of radio emission that occurs when electrons moving at nearly the speed of light spiral around magnetic fields.

Question 38

Small Magellanic Cloud

Answer 38

One of two small, irregular galaxies (the other is the Large Magellanic Cloud) located about 150,000 light-years away; it probably orbits the Milky Way Galaxy.