Glenn Elert
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The Physics Factbook™
Edited by Glenn Elert -- Written by his students
An educational, Fair Use website
topic index | author index | special index
| Bibliographic Entry | Result (w/surrounding text) | Standardized Result |
|---|---|---|
| Chaisson, Eric, & McMillan, Steve. Astronomy Today.New Jersey: Prentice-Hall, 1993: 533. | "Measurements of gas velocities in the solar neighborhood show that the sun, and everything in its vicinity, orbits the galactic center at a speed of about 220 km/s …." | 220 km/s |
| "Milky Way Galaxy. " The New Encyclopedia Britannica.15th ed. Chicago: Encyclopaedia Britannica, 1998: 131. | "The Sun, which is located relatively far from the nucleus, moves at an estimated speed of about 225 km per second (140 miles per second) in a nearly circular orbit." | 225 km/s |
| Goldsmith, Donald. The Astronomers.New York: St. Martin's Press, 1991: 39. | "If the solar system … were not moving in orbit around the center, we would fall straight in toward it, arriving a hundred million years from now. But because we do move (at about 150 miles per second) along a nearly circular path …." | 240 km/s |
| Norton, Arthur P. Norton's Star Atlas.New York: Longman Scientific & Technical, 1978: 92. | "… the sun's neighborhood, including the Sun itself, are moving around the centre of our Galaxy in approximately circular orbits with velocities of the order of 250 km/s." | 250 km/s |
| Recer, Paul (Associated Press). Radio Astronomers Measure Sun's Orbit Around Milky Way. Houston Chronicle. 1 June 1990. | "Using a radio telescope system that measures celestial distances 500 times more accurately than the Hubble Space Telescope, astronomers plotted the motion of the Milky Way and found that the sun and its family of planets were orbiting the galaxy at about 135 miles per second." "The sun circles the Milky Way at a speed of about 486,000 miles per hour." | 217 km/s |
The Sun is one of some 300 billion stars that travel aroundthe Milky Way in a near circular orbit. These stars are closerto the galactic center than the Sun. The distance from the centerof our Galaxy to the Sun is about 26,000 light years (a lightyear is about 6 trillion miles), which is approximately halfwayout on one of the Milky Way's curving arms. The Sun and its planetstake a period of 225 million years to revolve around the galacticcenter. The time it takes for each orbit is sometimes referredto as a cosmic year or a galactic year. The Sun has completedabout 20 orbits since the solar system was formed. For each orbit,the Sun traveled 150,000 light years of distance.
The orbit of the Sun around the Milky Way is influenced bythe galaxy's matter, which does not solely occupy the galacticcenter. Instead, it is distributed all over space. Some of thegalaxy's mass is inside the sun's orbit and some of it is outside.The Sun's orbital period is determined by the galaxy's mass withinthe orbit of the Sun.
Newton's explanation of the speed of stars in the Milky Wayis as follows. He showed that stars closer to the galactic center,including the Sun, experience a gravitational pull equal to thepull created by the mass that is equal to that of all the starscloser to the galactic center. Hence, the mass of the galacticcenter is equal to the total mass of all the stars closer to thecenter.
He also showed that stars farther from the center have a combinedgravitational force of zero. Those stars pull in all differentand opposite directions, canceling out one another. Therefore,the stars closer to the center experience a gravitational pulltowards the center and they move at greater speeds, since thereis more force acting upon them. Conversely, more distant starshave less force acting upon them and in turn, they travel at lowerspeeds. In addition, stars beyond this distance have speeds thatstop decreasing and eventually remain constant.
Angela Chan -- 2001
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