Glossary: A thru E

Active Galactic Nuclei (AGN)
A galaxy which emits a huge amount of energy from a compact central source.

Andromeda Galaxy
The Andromeda galaxy is the largest of the local galaxies, and is visible to the unaided eye. It is about 2 million light years from our own galaxy and has a beautiful spiral structure. The Andromeda galaxy is also known as M31. The Andromeda galaxy and the Milky Way galaxy will eventually collide.

Asteroid
Small, rocky bodies that orbit the Sun, asteroids range in diameter from a few millimeters to over 250 km. Although all asteriods reflect sunlight, only one, Vesta, can be seen with the unaided eye.

Astronomical Unit (AU)
The average distance between the Earth and the Sun, one astronomical unit is defined to be 150 million kilometers, or about 93 million miles.

Astrophysics
The study of the history, structure and dynamics of objects in the universe. Astrophysicists study stars, solar systems and interstellar material.

Autumnal Equinox
The place in the sky where the Sun crosses the celestial equator moving southwards. The autumnal equinox marks the beginning of autumn and falls on, or near, September 21. See celestial sphere for a helpful diagram.

Baryons

Black Holes
An object with a gravitational field so high that even light cannot escape from it. For more about black holes, check out the Black Hole FAQ.

Big Bang Theory
The widely held belief that the Universe began in a fiery explosion, 10-15 billion years ago. The cosmic microwave background, big bang nucleosynthesis, and the observed Hubble expansion of the Universe are considered proof of the big bang theory.
For more on big bang cosmology, check out this primer.

Big Bang Nucleosynthesis
Lighter elements, such as Hydrogen, Helium and Lithium, were formed as a result of the big bang in a process called big bang nucleosynthesis. Scientists can calculate how much of which elements were formed, and the fact that observation agrees with calculation is considered proof of the big bang theory.
For more on big bang cosmology, check out this primer.

Celestial Equator
An imaginary line around the sky directly above the equator. See celestial sphere for a helpful diagram.

Celestial Sphere
A huge sphere centered on the Earth, which provides a convenient way to fix a coordinate system on the sky. Every object in the sky can be found by knowing its declination and right ascension.

Cephied Variables
Cephied variables are a class of star that have very well understood luminosity curves. This means that scientists know how bright they are. Because brightness depends on distance, scientists can figure out how far away a cephied variable is by comparing how bright it appears with how bright it should be. Because of this, cephied variables are used as standard candles.

Comet
Huge dirty snowballs that orbit the Sun.
For more about comets, check out this excellent Comet's tale page.

Cosmic Microwave Background
Observable radiation (in the microwave portion of the electromagnetic spectrum) left over from the big bang, the cosmic microwave background is considered proof of the big bang.

Check out these CfPA cosmic microwave background links:
• Max and Minima
• CMB astrophysics research page
• Big bang cosmology primer

Cosmology
The study of the history, structure and dynamics of the Universe. Cosmologists study how the Universe might have formed, why galaxies are where they are today, and the ultimate fate of the Universe.
For more on cosmology check out this primer.

Dark Matter
Dark matter is any matter in the Universe which does not give off any light of its own, or does not interact with light the way typical matter does.

Check out these CfPA dark matter links:
• The question of dark matter
• The WIMP experiment
• The MACHO experiment
• The CMB Theory Group
• Big bang cosmology primer

Declination
A coordinate used on the celestial sphere just as latitude is used on Earth. An object's declination is measured in degrees from the celestial equator; positive degrees if north of the equator, and negative if south of the equator. Also see right ascension.

Diurnal Path
The apparent daily path of the Sun in the sky. See celestial sphere for a helpful diagram.

Dog Star
Another name for Sirius, the brightest star visible in the night sky.

Dwarf Galaxy
A very faint galaxy. Dwarf galaxies are either small or just not very bright, or sometimes both.

Earth
Orbiting the Sun at a distance of 150 million km, one AU, Earth is a terrestrial planet, and home to us all.
Check out SEDS' Nine Planets, for more about the Earth. Or see StarDate's Solar System Guide.

Ecliptic
The orbits of the planets are all (more or less) in the same plane called the ecliptic and defined by the plane of the Earth's orbit. Put another way, the ecliptic is the apparent path of the Sun around the sky during the year. See celestial sphere for a helpful diagram.

Electron
A sub-atomic particle found orbiting the nuclei of an atom. Unlike the proton and Neutron, the electron has a negative charge.

Elementary Particles
Particles which cannot be broken into smaller pieces. For example, a table can be broken into table legs and the table top, but a table leg cannot be broken into smaller complete pieces. Electrons, quarks, and other elementary particles cannot be broken into smaller pieces because they are the smallest building blocks of nature.