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Hubble images some of galaxy's dimmest stars
August 21, 2006Using the Hubble Space Telescope, astronomers have imaged some of the galaxy's oldest and dimmest stars, offering a rare experimental glimpse of two mysterious star types - tiny, slow burners less than one-tenth the size of our sun and once giant stars that still glow more than 10 billion years after their deaths.
The research appears in this week's issue of the journal Science.
"This project pushed the limits of what even Hubble can do," said study co-author Jay Anderson, a research scientist at Rice University. "These stars can't be reliably detected in a single image. You have to combine a large number of images to find them."
In total, the research team trained Hubble's cameras on the same patch of sky for more than 75 hours, gathering 378 overlapping images. The target was a region of space containing about 1 percent of the globular cluster NGC 6397 - a collection of stars that formed early in our galaxy's history.
"When we look at random stars in the sky they have a variety of ages," Anderson said. "Globular clusters offer unique opportunities for astronomers to study a population of stars that are all the same age. All the stars we see in clusters are ancient, because they were created when the galaxy was forming. They're fossils from the galaxy's earliest days."
There are about 150 globular clusters in our galaxy, and most contain between 100,000 and 1 million stars. While most of the galaxy's stars - including our own sun - orbit the galactic center in the plane of the galaxy, globular clusters predate the flattening of the Milky Way, so they're scattered in a more spherical distribution.
NGC 6397 is one of the nearest clusters to Earth, located just 8,500 light years away. But even at this relatively close astronomical distance, the light from NGC 6397's faintest stars is easily lost in the glare from its brightest stars.
To survey the dimmest objects, Anderson and colleagues relied on computers. Anderson, whose specialty is writing programs to sift through astronomical data, spent months writing and refining software that could examine each Hubble image, pixel by pixel, and find the faintest stars.
The two types of object imaged represent the heavy end and the light end of the stellar mass spectrum.
A star's destiny is determined by its mass. There's a minimum mass that a star must have in order to burn hydrogen, and objects below that threshold cool rapidly and fade away. From the NGC 6397 survey, Anderson and his colleagues identified the smallest visible stars yet seen in a globular cluster, stars less than one-tenth the mass of Earth's sun. This is very near the predicted theoretical threshold, and Anderson said data from the survey will be helpful for verifying and refining theories about the structure and evolution of low-mass stars.
On the other end of the stellar mass spectrum are stars that are significantly larger than the sun. Stars about eight times the mass of the sun burn quickly and die in spectacular planetary nebulae, explosions that spew much of the star's material into space. Upon their final collapse, these stars become white dwarfs, extremely dense objects that radiate heat for billions of years as they slowly fade into darkness. Anderson said that while the brightest - and therefore youngest - white dwarfs have been seen in many clusters, the new survey yielded the first images of the faintest and oldest white dwarfs in an ancient cluster. The brightness of the white dwarfs at this end of the scale can help astronomers find out how long the stars have been cooling. From that, they can better determine the age of the cluster, which in turn can be used to narrow estimates of the lower limit of the age of the universe.
Rice University
In total, the research team trained Hubble's cameras on the same patch of sky for more than 75 hours, gathering 378 overlapping images. The target was a region of space containing about 1 percent of the globular cluster NGC 6397 - a collection of stars that formed early in our galaxy's history.
"When we look at random stars in the sky they have a variety of ages," Anderson said. "Globular clusters offer unique opportunities for astronomers to study a population of stars that are all the same age. All the stars we see in clusters are ancient, because they were created when the galaxy was forming. They're fossils from the galaxy's earliest days."
There are about 150 globular clusters in our galaxy, and most contain between 100,000 and 1 million stars. While most of the galaxy's stars - including our own sun - orbit the galactic center in the plane of the galaxy, globular clusters predate the flattening of the Milky Way, so they're scattered in a more spherical distribution.
NGC 6397 is one of the nearest clusters to Earth, located just 8,500 light years away. But even at this relatively close astronomical distance, the light from NGC 6397's faintest stars is easily lost in the glare from its brightest stars.
To survey the dimmest objects, Anderson and colleagues relied on computers. Anderson, whose specialty is writing programs to sift through astronomical data, spent months writing and refining software that could examine each Hubble image, pixel by pixel, and find the faintest stars.
The two types of object imaged represent the heavy end and the light end of the stellar mass spectrum.
A star's destiny is determined by its mass. There's a minimum mass that a star must have in order to burn hydrogen, and objects below that threshold cool rapidly and fade away. From the NGC 6397 survey, Anderson and his colleagues identified the smallest visible stars yet seen in a globular cluster, stars less than one-tenth the mass of Earth's sun. This is very near the predicted theoretical threshold, and Anderson said data from the survey will be helpful for verifying and refining theories about the structure and evolution of low-mass stars.
On the other end of the stellar mass spectrum are stars that are significantly larger than the sun. Stars about eight times the mass of the sun burn quickly and die in spectacular planetary nebulae, explosions that spew much of the star's material into space. Upon their final collapse, these stars become white dwarfs, extremely dense objects that radiate heat for billions of years as they slowly fade into darkness. Anderson said that while the brightest - and therefore youngest - white dwarfs have been seen in many clusters, the new survey yielded the first images of the faintest and oldest white dwarfs in an ancient cluster. The brightness of the white dwarfs at this end of the scale can help astronomers find out how long the stars have been cooling. From that, they can better determine the age of the cluster, which in turn can be used to narrow estimates of the lower limit of the age of the universe.
Rice University
Hubble Space Telescope Current Events and Hubble Space Telescope News RSS'Dropouts' pinpoint earliest galaxies
Astronomers, conducting the broadest survey to date of galaxies from about 800 million years after the Big Bang, have found 22 early galaxies and confirmed the age of one by its characteristic hydrogen signature at 787 million years post Big Bang.
Opening up a colorful cosmic jewel box
Star clusters are among the most visually alluring and astrophysically fascinating objects in the sky. One of the most spectacular nestles deep in the southern skies near the Southern Cross in the constellation of Crux.
Scientists use world's fastest supercomputer to model origins of the unseen universe
Understanding dark energy is the number one issue in explaining the universe, according to Salman Habib, of the Laboratory's Nuclear and Particle Physics, Astrophysics and Cosmology group.
Rebirth of an icon: Hubble's first images since Servicing Mission 4
Astronomers today declared the NASA/ESA Hubble Space Telescope a fully rejuvenated observatory ready for a new decade of exploration, with the release of observations from four of its six operating science instruments.
Astronomers discover stars in early galaxies had a need for speed
A team of astronomers has measured the motions of stars in a very distant galaxy for the first time and discovered they are whizzing around at astonishingly high speeds-about one million miles per hour, or twice the speed at which the Sun circles our own Milky Way galaxy.
Dark Energy From the Ground Up: Make Way for BigBOSS
Several ways have been proposed to examine dark energy, in hopes of finding out just what it is. One of them, "supernovae" for short, certainly works: it's how dark energy was discovered in the first place. Other independent techniques, such as weak gravitational lensing and baryon acoustic oscillation, also promise great power but are as yet unproven.
Hubble captures rare Jupiter collision
The checkout and calibration of the NASA/ESA Hubble Space Telescope has been interrupted to aim the recently refurbished observatory at a new expanding spot on the giant planet Jupiter.
NASA celebrates Chandra X-Ray Observatory's 10th anniversary
Ten years ago, on July 23, 1999, NASA's Chandra X-ray Observatory was launched aboard the space shuttle Columbia and deployed into orbit.
Jupiter pummeled, leaving bruise the size of the Pacific Ocean
Something slammed into Jupiter in the last few days, creating a dark bruise about the size of the Pacific Ocean.
Herschel Space Telescope's SPIRE instrument package makes first-light observations
A scientific instrument package developed in part by the University of Colorado at Boulder for the $2.2 billion orbiting Herschel Space Observatory that was launched in May by the European Space Agency has made its first successful observations, targeting two star-forming galaxies near the Milky Way.
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