Glossary: F thru J

F

Fermions
Particles which obey Fermi-Dirac statistics.

G

Galactic Cannibalism
The (gravitational) swallowing of one galaxy by another. Our own galaxy, the Milky Way, will eventually eat several dwarf galaxies and will probably be eaten by the Andromeda galaxy.

Galaxy
A large grouping of stars, galaxies may have 10 billion stars in them. And there may be as many as ten billion galaxies in the Universe. Our own galaxy is named the Milky Way, and is a spiral galaxy. Although there are many close dwarf galaxies, the nearest full sized galaxy is named the Andromeda galaxy.

Gas Giant Planet
A type of planet characterized by being composed almost entirely of gas. Since gas weighs much less than rock, and since gravity is the reason planets form, gas giant planets are large. Jupiter, Saturn, Uranus, and Neptune are gas giant planets. Gas giant planets are also called Jovian planets. Check out SEDS' Nine Planets, for more about planets. Or see StarDate's Solar System Guide.

Horizon
The plane tangent to the Earth's surface at an observer's position. See celestial sphere for a helpful diagram.

Hubble Expansion
The observed phenomenon that all galaxies in the Universe (outside the local group of galaxies) appear to be moving away from us, implying that the Universe is expanding. The phenomenon was discovered by Edwin Hubble in 1929. Along with cosmic microwave background and big bang nucleosynthesis, Hubble expansion is considered compelling proof of the big bang theory. For more about Hubble expansion, check out this big bang cosmology primer.

Inflation Theory
A theory proposed by Alan Guth in 1980 which suggests that the size of the early Universe greatly increased in a very short period of time. Although the theory is unverifiable, it is widely accepted within the astrophysics community because of its ability to explain certain difficulties.

Jupiter
With an orbit of 778 million km, and a diameter of 143,000 km, Jupiter is the largest planet in our solar system. It is a gas giant planet. Four of its moons (Io, Europa, Ganymede and Callisto) are visible with binoculars. Check out SEDS' Nine Planets, for more about Jupiter. Or see StarDate's Solar System Guide.