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Science File / An exploration of issues and trends
affecting science, medicine and the environment | I
Didn't Know That

January 08, 1998

Q: A recent story noted that there are 300 million stars in the Andromeda galaxy. How did astronomers determine that?

A: In much the same way that police estimate the number of people present at a rally at City Hall or at a Garth Brooks concert in Central Park: They count the number of people (or stars) in a small area and use that to estimate the total number, said Sallie Baliunas, deputy director of the Mt. Wilson Observatory.

The Andromeda galaxy, like the Earth's Milky Way galaxy, is a middle-sized specimen, she said. Elliptical galaxies, like M-87 in the constellation Virgo, can have as many as 100 trillion stars. Dwarf galaxies, in contrast, can have as few as 1 million stars.

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