Distance to Messier Objects
STARS: In general stars are large balls of mostly hydrogen. Due to crushlng gravita- tlonal forces, the center of the star becomes extremely hot. At this point nuclear reactions occur causing hydrogen nuclei to turn into helium nuclei. This change causes very large amounts of energy to be released in the form of the electro-magnetic spectrum, which includes visible light. The closest star to Earth is our Sun and the next closest star is Proxima (V645 Cen.). It is 4.2 light years away from Earth.
OPEN CLUSTERS: These are loose collections of stars with no well defined shape. Some open clusters are only a few hundred light years from Earth.
NEBULAE: Nebulea are large areas of gas and dust containing mostly hydrogen. Some of these nebula glow due to young, hot stars formed from the gas. These stars generate large amounts of ultraviolet radiation that causes the gas in the nebula to glow. One of the finest examples of a bright or emission nebula is M42, the Orion Nebula. M42 is about 1600 light years from Earth.
PLANETARY NEBULAE: Some stars discard or eject gas and sometimes we can see this gas asa glowing mass that can, at times, look llke a planet through a telescope.
THE MILKY WAY: Our galaxy is a very large system of stars, gas, etc. It has the form of a slowly rotating, disk, with denser areas that look like the curved arms of a spiral. The Milky Way contains about 400 billion stars and is about 100,000 light years in diameter. Our Sun is about 2/3 of the way out from the galactic center in the galactic disk.
GLOBULAR CLUSTERS: Globular clusters are seemingly dense "balls" of stars. They look like a burst from a fireworks shell when viewed through a large telescope. Globular clusters surround the main galaxy in the form of a halo.
GALAXIES: Galaxies are very large systems of stars, gas, etc. that often take a form of a slowly rotating disk, like our Milky Way. Galaxies can also have a rounded look and some are more irregular in shape. The closest major galaxy to our Milky Way galaxy is the Andromeda Galaxy. It is shaped much like our galaxy and is considered one third larger. This large system of stars and gas is 2.2 million light years away. It is one of the most distant "naked eye" objects. There are billions of galaxies.
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