Hubble images some of galaxy's dimmest stars

Using 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.