G

Galactic cluster

An open cluster of stars found in the spiral arms of the Milky Way.

 

Galactic equator

The plane of the great circle on the celestial sphere that locates the center line of the Milky Way.

 

Galactic poles

The north and south galactic poles that are 90^o from the galactic equator.

  

Galaxy

A large assemblage of stars and interstellar material held together by gravitation. A galaxy usually contains from millions to hundred of billions of stars.

 

Gamma ray

The most energetic from of electromagnetic radiation yet discovered, and the one possesing the shortest known wavelength.

 

Giant star

A star of large radius and luminosity.

 

Gibbous

A phase of the moon or a planet during which more than half but less than the whole disk appears illuminated.

 

Globular cluster

A large spherical cluster of stars located  in the halo which surrrounds the Milky Way and other galaxies.

 

Globule

a small, dark, relatively dense nebula that may be a protostar.

 

Granules

Small, bright spots in the sun's photosphere that give it a mottled appearance similar to rice grains. They are produced by hot gases rising from below the photosphere.

 

Gravitation

The property of matter  by which one  mass exerts a force of attraction on another.

 

Great circle

The curve formed on the surface of a sphere  where a plane that passes through the sphere intersects its center. It divides the sphere into two equal parts.

 

Greenwich meridian

The meridian that passes through a point at the Royal Greenwhich Observatory in England, the reference or prime meridian for determining longitude.

 

Gregorian calendar

The calendar in common use today, which was introduced by Pope Gregory XIII in 1582.