What is a light year? A light year is not a unit of time (like an hour or year). Instead, it is a measure of distance: It is the distance that light will travel in one year. Light is a form of electromagnetic radiation, and travels at 186,282 miles per second. So, light will travel 5,865,696,000,000 miles in a year. This is very, very far! One can also come up with a unit known as a "light minute:" The sun is about 93 million miles from Earth, and light from it takes about 8 minutes to travel to Earth. This means that the sun is 8 "light minutes" away. That is, if the Sun "went out," we would not know about it for 8 minutes!

Why is the galaxy called M31? What does the "M" stand for? Many celestial objects are named with an "M" followed by a number. The "M" stands for Messier and comes from the name of a French astronomer, Charles Messier, who put together catalog of celestial objects in the 18th Century. Messier successfully discovered about 20 comets and put together the catalog to record celestial objects easily mistaken for actual comets. Other examples of "Messier Objects" include: M1,"The Crab Nebula," found in the constellation of Taurus. It is also the site of a supernova remnant. M13,"The Great Cluster," is a globular star cluster found in the constellation of Hercules. M42, "The Great Nebula," is also known as "The Orion Nebula" and is one of the brightest celestial objects in the northern hemisphere. It can be seen in the "sword" of the bright constellation of Orion, the hunter.

What is a galaxy? A galaxy is a system of about 100 billion stars, all held together by mutual gravitational attraction. It is truly an "island" in space, and the stars in a particular galaxy are "isolated" from stars in the next galaxy by extreme distance. The Andromeda galaxy is "close" to us by galactic standards, and is in our "local group" of galaxies.

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