Constellations
Is an area of the sky with defined boundaries, all stars and any other objects within that boundary are considered part of the constellation.There are 88 official constellations which are recognized by the International Astronomical Union.Some constellations are only visible in the northern hemisphere, while others are only visible in the southern hemisphere.Constellations that are visible in both hemispheres may appear upside down in the southern hemisphere.
Galaxies
A galaxy is a gravitationally bound
system of stars, stellar remnants, interstellar gas, dust, and dark matter. The
word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way.
Galaxies range in size from dwarfs with
just a few billion (109) stars to giants with one hundred trillion (1014) stars, each orbiting its
galaxy's center of mass. Galaxies are categorized according to their visual
morphology as elliptical, spiral and irregular. Many
galaxies are thought to have black holes at
their active centers.
