|Hess, Frances. Earth Science. New York: Glencoe Mc Graw-Hill, 2002: 348.||"The Sun's orbit around the galaxy is about 220 km/s and thus its orbital period is about 240 million years."||240 million years|
|Morris, Mark. "The Milky Way." The World Book Encyclopedia, 2002, Vol. 13: 551.||"The Sun's completes an almost circular orbit of the center (of the galaxy) about every 250 million years."||250 million years|
|Croswell, Ken. The Alchemy of the Heavens Searching for the meaning of the Milky Way. New York: Doubleday, 1995: 2.||"The Galaxy is so huge that the Sun requires 230 million years to complete one orbit around the Milky Way's center."||230 million years|
|Moore, Patrick. The International Encyclopedia of Astronomy. New York: Mitchell Beazly Publishers, 1981: 45.||"Cosmic Year: the time taken for one complete revolution of the Sun around the entire center of the galaxy; about 225 million years."||225 million years|
|Kerrod, Robin. Encyclopedia of Science Heavens 2. New York: MacMillian Reference USA, 1997: 35.||"The Sun takes 225 million Earth years to make one rotation. This period of time is called a cosmic year."||225 million years|
The sun is one of hundreds of billion of stars in our galaxy, the Milky Way. The galaxy is composed of gaseous interstellar medium, neutral or ionized, sometimes concentrated into dense gas clouds made up of atoms molecules, and dust. All of the matter -- gas, dust, and stars -- rotate around a central axis perpendicular to the galactic plane. The centrifugal force caused by the rotation balances out the gravitational force, which draw all the matter toward the center.
The mass is located within the circle of the Sun's orbit through the galaxy is about 100 billion times the mass of the Sun. Because the Sun is about average in mass, astronomers have concluded that the galaxy contains about 100 billion stars within its disk.
All stars in the galaxy rotate around a galactic center but not with the same period. Stars at the center have a shorter period than those farther out. The Sun is located in the outer part of the galaxy. The speed of the solar system due to the galactic rotation is about 220 km/s. The disk of stars in the Milky Way is about 100,000 light years across and the sun is located about 30,000 light years from the galaxy's center. Based on a distance of 30,000 light years and a speed of 220 km/s, the Sun's orbit around the center of the Milky Way once every 225 million years. The period of time is called a cosmic year. The Sun has orbited the galaxy, more than 20 times during its 5 billion year lifetime. The motions of the period are studied by measuring the positions of lines in the galaxy spectra.
Stacy Leong -- 2002