Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2007 April 15
See Explanation. Go online for the highest resolution available.

M3: Inconstant Star Cluster
Credit & Copyright: J. Hartman & (Harvard CfA) & K. Stanek (Ohio State U.)

Explanation: Star clusters appear constant because photographs of them are frozen in time. In reality, though, cluster stars swarm the center and frequently fluctuate in brightness. Although the time it takes for stars to cross a cluster is about 100,000 years, the time it takes for a star to fluctuate noticeably can be less than one night. In fact, the above time lapse movie of bright globular cluster M3 was taken over a single night. Most of the variable stars visible above are RR Lyrae stars, stars that can quickly double their brightness while becoming noticeably bluer. Furthermore, RR Lyrae stars vary their light in a distinctive pattern that allows unique identification. Lastly, since RR Lyrae stars all have the same intrinsic brightness, identifying them and measuring how dim they appear tells how far they are, since faintness means farness. These distances, in turn, help calibrate the scale of the entire universe.

View this page (with external links) online at
http://apod.gsfc.nasa.gov/apod/ap070415.html.


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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Official: Phillip Newman. Specific rights apply.
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