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Period of the Sun's Orbit around the Galaxy (Cosmic Year)

The Physics Factbook

Edited by Glenn Elert -- Written by his students

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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 orbitof 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.

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Stacy Leong -- 2002