Faintest Stars Seen in a Globular Cluster

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Hubble Sees Faintest Stars in a Globular Cluster

The press release is reproduced below:

Aug. 17, 2006

Erica Hupp
Headquarters, Washington
202-358-1237

Donna Weaver
Space Telescope Science Institute, Baltimore
410-338-4493

RELEASE: 06-292

Hubble Sees Faintest Stars in a Globular Cluster

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.

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 they have at last unequivocally reached the faintest stars. Richer's team announced their findings Thursday 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," Richer said.

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 the dimmest white dwarfs have such low temperatures 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 in globular clusters as a measure of the universe's age. The universe must be at least as old as the oldest stars. White dwarfs cool down at a predictable rate. The older the dwarf, the cooler it is, making it a perfect "clock" that has been ticking for almost as long as the universe has existed. Richer and his team are using the same age-dating technique to calculate the cluster's age. NGC 6397 is 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.

For images and additional information about NGC 6397, visit:

http://www.nasa.gov/hubble

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Source: NASA Press Release 06-292

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Faintest Stars in Globular Cluster NGC 6397

Looking like glittering jewels, the stars in this Hubble Space Telescope image at left are part of the ancient globular star cluster NGC 6397. Scattered among these brilliant stars are extremely faint stars. Hubble's Advanced Camera for Surveys has taken a census of the cluster stars, uncovering the faintest stars ever seen in a globular cluster. Globular clusters are spherical concentrations of hundreds of thousands of old stars.

The Advanced Camera found the faintest red dwarf stars (26th magnitude), which are cooler and much lower in mass than our Sun, and the dimmest white dwarfs (28th magnitude), the burned-out relics of normal stars. The light from the dimmest white dwarfs is equal to the light produced by a birthday candle on the Moon as seen from Earth.

The image at lower right shows the faintest red dwarf star (the red dot within the red circle) spied by Hubble.

The image at upper right pinpoints one of the dim white dwarfs (the blue dot within the blue circle) seen by Hubble. The white dwarf has been cooling for billions of years. It is so cool that instead of looking red, it has undergone a chemical change in its atmosphere that makes it appear blue.

The images were taken with visual and red filters. NGC 6397, one of the closest globular clusters to Earth, is 8,500 light-years away in the southern constellation Ara.

The data for these images were obtained in March and April 2005.

Credit: NASA, ESA, and H. Richer (University of British Columbia)

Image Type: Astronomical
STScI-PRC2006-37

Source: HubbleSite - Newsdesk Edited August 17, 2006 by Waspie_Dwarf

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Tale of Two Stellar Populations in Globular Cluster NGC 6397

This graph schematically plots two populations of stars Hubble has seen in a neighboring star cluster. The graph, a simplified version of the original data, plots stellar brightness (vertical axis) against stellar color-temperature. The normal main sequence stars in the cluster are plotted on the right curve. The cooler a star is, the redder it appears, and it diminishes in brightness. Hubble has identified the very coolest and faintest normal stars in the cluster.

The curve on the left plots the white dwarf population of burned-out sunlike stars. It follows a normal "cooling slope" until the white dwarf begins to look bluer and hook toward the left. The dwarfs aren't getting hotter. Chemical changes in their atmospheres make them look cooler. This predicted "white dwarf hook" has never been seen before.


Credit: NASA, ESA and A. Feild (STScI)

Image Type: Illustration

Source: HubbleSite - Newsdesk

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Hubble Observes NGC 6397 in Our Milky Way (Artist's Concept)

Globular star cluster NGC 6397 is one of the closest such clusters to Earth. It lies out of the plane of our galaxy and in the direction of the galactic center.

Credit: NASA, ESA and A. Feild (STScI)

Image Type: Illustration

Source: HubbleSite - Newsdesk

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Location of ACS detail in NGC 6397

Globular Cluster NGC 6397 taken in red light by the UK Schmidt Telescope of the Anglo-Australian Telescope. The green rectangle to the left and below the bright central of the cluster shows the location of the detailed image made by Hubble Space Telescope's Advanced Camera for Surveys.

Credit: NASA, ESA, Anglo-Australian Telescope, Digitized Sky Survey

Image Type: Astronomical

Source: HubbleSite - Newsdesk