Astronomy Picture of the Day 10-12-04

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.

Sorry APOD has been missing for a few days; my computer was having some problems loading Windows. Hopefully I resolved the issue; we'll see.

Another view of M3:

Cassini eyes the culprit
CASSINI PHOTO RELEASE
Posted: October 11, 2004

Gazing beyond Saturn's magnificent rings, Cassini spotted the cause of the dark gap visible in the foreground of this image: Mimas, which is 398 kilometers (247 miles) wide. The gravitational influence of Mimas is responsible for the 4,800 kilometer- (2,980 mile-) wide Cassini division, which stretches across the lower left portion of this view. The little moon is at a nearly half-full phase in this view.

Credit: NASA/JPL/Space Science Institute
Download larger image version here

A small clump of material is visible in the narrow F ring, beyond the edge of the main rings.

The image was taken in visible light with the Cassini spacecraft narrow angle camera at a distance of 8.9 million kilometers (5.5 million miles) from Mimas and at a Sun-Mimas-spacecraft, or phase, angle of 88 degrees. The image scale is 54 kilometers (34 miles) per pixel. The image was magnified by a factor of four to aid visibility.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras, were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

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