AstronomyThe study of objects and matter outside the earth's atmosphere and of their physical and chemical properties

Light Year• A unit of

distance in astronomy; distance that light travels in one year or about 5.88 trillion miles (9.46 trillion kilometers)

Universe

• The entire celestial cosmos

Star A light-radiating gaseous celestial body of great mass which produces energy by means of nuclear fusion reactions

Galaxy• Any of the

very large groups of stars and matter that are found throughout the universe

System• a group of

related natural objects or forces

Milky Way GalaxyThe galaxy of which the Sun and the solar system are a part and which contains the many stars that make up the galaxy

Big Bang TheoryTheory that states that the universe originated billions of years ago in an explosion from a single point of nearly infinite energy density

Cluster

A collection of stars or galaxies that appear close together in the sky and are gravitationally associated

Telescope• An optical instrument for viewing distant objects.

Eyepiece Lens

• The lens of a telescope that makes the image larger

Objective Lens

• The lens of a telescope that collects and focuses light

Spiral Galaxy

Galaxy that looks like a flat disk with a bulge in its center and spiral arms

Constellation

Groups of stars that seem to form patterns in the sky.

Polaris

The star that is above Earth’s north pole

SpectroscopeAn instrument for forming and examining spectra.

Galaxy

A huge, gravitationally bound system of stars, interstellar gas and dust

SunThe star in our Solar System, which all the planets orbit

Apparent Magnitude

A star’s brightness as it appears from Earth

Absolute Magnitude

A star’s actual size and temperature

Nebula

A cloud of gas and dust in outer space

Astronomical UnitUsed to measure distances within our solar system; the average distance between the Sun and the Earth; about 150 million kilometers; AU

Period of Revolution The motion

of the planet in its orbit around the Sun; the Earth’s revolution is 1 year

Period of RotationThe time it takes for one complete spin of a planet on its axis; the Earth’s rotation is 1 day

Inner PlanetMercury, Venus, Earth, Mars; much smaller and less massive than the outer planets

Outer PlanetJupiter, Saturn, Uranus, Neptune; more gaseous, larger and less dense

CometA small body orbiting the Sun and occasionally exhibits an atmosphere and a tail

AsteroidChunk of rock and metal and found between the orbits of Mars and Jupiter in the asteroid belt.

Meteoroid

A chunk of rock that orbits the Sun

Meteorite

A meteoroid that has entered the Earth’s atmosphere.

Red ShiftA movement of the spectrum of a celestial body toward longer wavelengths that is a result of the Doppler effect

Focal Point

The point at which light rays meet to form an image.

Elliptical GalaxyMore rounded and are often longer in one direction than in the other, like a football

Irregular Galaxy

Appear to have no definite shape.

Spectral Analysis

Studying the “fingerprints” or spectral lines of elements given off by stars

Red GiantA star that has low surface temperature and a diameter that is large relative to the Sun

Super Giant

A star of very great natural luminosity and enormous size

Main SequenceContains most of the stars visible to the naked eye; stars where there is a direct relationship between brightness and size, includes our Sun

White DwarfA small whitish star of low natural brightness usually with a mass approximately equal to that of the sun but with a density many times larger

Neutron StarHypothetical dense celestial object that consists mostly of closely packed neutrons and that results from the collapse of a much larger stellar body

Black HoleA hypothetical celestial object with a gravitational field so strong that light cannot escape from it which is believed to be created in the collapse of a very massive star

SupernovaThe explosion of a very large star in which the star may reach a maximum luminosity one billion times that of the sun

H-R DiagramA chart that plots the data of stars according to their surface temperature and brightness.

Solar SystemThe Sun together with the group of celestial bodies that are held by its gravitational attraction and revolve around it

Local Group

The cluster that contains our galaxy, the Milky Way

Planet

Any of the large bodies that revolve around a star in a star system

Satellite

A celestial body orbiting another of larger size

Axis

A straight line about which a body or a geometric figure spins

SeasonsDivision of the year, based on yearly periodic changes in the weather; results from the tilt of Earth’s axis

HemisphereNorthern or southern half of the earth divided by the equator or the eastern or western half divided by a meridian

EquinoxOccurs during March and September, where all parts of Earth received equal amounts of sunlight, and the day and the night are the same length