The ability of a telescope to collect light. Proportional to the area of the telescope's objective lense or mirror.
light pollution
The illumination of the night sky by waste light from cities and outdoor lighting, which prevents the observation of faint objects.
light-year (ly)
A unit of distance. The distance light travels in one year. 1 ly = 9.46*10^15 meter
= 5.9 billion miles.
local group
The small cluster of a few dozend of galaxies that contains our Milky Way Galaxy.
look-back time
The amount by which we look into the past when we look at a distant stars and galaxies.
A time equal to the distance to the time light takes to travel from the objects to us. Examples of look-back times: Moon: 1 second; Sun - 8 minutes; Jupiter 32 to 48 minutes; closest star - 4.2 years; closest galaxy -
150,000 years.
luminosity
The total maount of energy a star radiates in on second.