Some interesting information about galaxy and nebula:
A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others.
Primary characteristics used in the classification of nebulae - the absorption or emission of a dispersion of light, that is by this criterion nebula divided into dark and light. First observed by absorption of radiation sources located behind them, the second - by its own radiation or reflection (scattering) of light near stars. The nature of light emission nebulae, energy sources, stimulating their radiation, depending on their origin and may have a diverse nature; often one of the nebula are several mechanisms of radiation.
Most nebulae are of vast size, even hundreds of light years in diameter.Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created in an Earthen environment – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula.
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 , literally "milky". Galaxies range in size from dwarfs with just a few thousand stars to giants with one hundred trillion stars, each orbiting their galaxy's own center of mass. Galaxies are categorized according to their visual morphology, including elliptical,spiral, and irregular.Many galaxies are thought to have black holes at their active centers.
The exact number of galaxies in the observable universe is not known, but apparently, there are about a hundred billion.