Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/~jkalirai/PressN6397FaintStars.pdf Дата изменения: Fri Sep 19 22:26:07 2008 Дата индексирования: Tue Oct 2 01:25:57 2012 Кодировка: Поисковые слова: ngc 6520

HubbleSite - NewsCenter - Hubble Sees Faintest Stars in a Globular Cluster (08/17/2006) - The Full Story

4/2/08 12:36 PM

News Release Archive: Entire Collection
News Release 81 of 678 BACK TO ENTIRE COLLECTION PREVIOUS RELEASE NEXT RELEASE

News Release Number: STScI-2006-37

August 17, 2006 02:00 PM (EDT)

Hubble Sees Faintest Stars in a Globular Cluster
Introduction The Full Story Release Images Fast Facts Related Links

The full news release story:

NASA's Hubble Space Telescope has uncovered what astronomers are reporting as the dimmest stars ever seen in any globular star cluster. Globular clusters are spherical concentrations of hundreds of thousands of stars.
View this image

These clusters formed early in the 13.7 - billion - year - old universe. The cluster NGC 6397 is one of the closest globular star clusters to Earth. Seeing the whole range of stars in this area will yield insights into the age, origin, and evolution of the cluster.

Although astronomers have conducted similar observations since Hubble was launched, a team led by Harvey Richer of the University of British Columbia, Vancouver, is reporting that it has at last unequivocally reached the faintest stars. Richer's team announced its findings today at the 2006 International Astronomical Union General Assembly in Prague, Czech Republic, and in the August 18 edition of Science. "We have run out of hydrogen - burning stars in this cluster. There are no fainter such stars waiting to be discovered. We have discovered the lowest- mass stars capable of supporting stable nuclear reactions in this cluster. Any less massive ones faded early in the cluster's history and by now are too faint to be observed," said Richer. Hubble's Advanced Camera for Surveys completed a census of two distinct stellar populations in NGC 6397. Hubble surveyed the faintest red dwarf stars, which fuse hydrogen in their cores like our sun, and the dimmest white dwarfs, which are the burned - out relics of normal stars. The light from these faint stars is as dim as the light produced by a birthday candle on the Moon seen from Earth. NGC 6397 is 8,500 light- years away from Earth. Analyzing the burned - out remnants of stars that died long ago, Hubble showed that the dimmest white dwarfs have such low temperatures that they are undergoing a chemical change in their atmospheres that makes them appear bluer rather than redder as they cool. This phenomenon had been predicted, but never observed. These white dwarfs are the relics of stars up to eight times as massive as the sun, which have exhausted the fuel capable of supporting nuclear reactions in their cores. Stars that were initially even more massive died as supernovae very early in the cluster's life, leaving behind neutron stars, black holes or no debris at all. Astronomers have used white dwarfs be at least as old as the oldest stars. cooler it is, making it a perfect "clock" and his team are using the same age in globular clusters as a measure of the universe's age. The universe must White dwarfs cool down at a predictable rate -- the older the dwarf, the that has been ticking for almost as long as the universe has existed. Richer - dating technique to calculate the cluster's age. NGC 6397 is currently
Page 1 of 2

http://hubblesite.org/newscenter/archive/releases/2006/37/full/


HubbleSite - NewsCenter - Hubble Sees Faintest Stars in a Globular Cluster (08/17/2006) - The Full Story

4/2/08 12:36 PM

and his team are using the same age- dating technique to calculate the cluster's age. NGC 6397 is currently estimated to be nearly 12 billion years old. A globular cluster's dimmest stars have eluded astronomers because their light is too feeble. Richer's team used Hubble's Advanced Camera to probe deep within the cluster for nearly five days to capture the faint stars. The camera's resolution is so sharp that it is capable of isolating cluster stars in this crowded cluster field, enabling cluster members to be distinguished from foreground and background stars. The cluster stars move together as the cluster orbits the Milky Way Galaxy, and Hubble was able to pinpoint which stars were moving with the cluster. The Hubble team used this technique together with archival Hubble images taken as much as a decade earlier to make sure they had a pure sample of cluster stars.
CONTACT

Dwayne Brown/Erica Hupp Headquarters, Washington Phone: 202- 358- 1726/1237 Donna Weaver Space Telescope Science Institute, Baltimore, Md. (Phone: 410- 338- 4493; E- mail: dweaver@stsci.edu) Lars Lindberg Christensen ESA/Hubble, Garching, Germany (Phone: 011- 49 - 89 - 3200- 6306; E- mail: lars@eso.org ) Harvey Richer University of British Columbia, Vancouver, BC, Canada Phone: 604- 822- 4134 [office], 011- 420- 7761- 79656 [Cell phone], 011- 420- 261- 177- 075 [IAU Press Office]; E- mail: richer@astro.ubc.ca )

BACK TO ENTIRE COLLECTION

PREVIOUS RELEASE

NEXT RELEASE

http://hubblesite.org/newscenter/archive/releases/2006/37/full/

Page 2 of 2