Hubble Uncovers Fading Cinders of Some of Our Galaxy's Earliest Homesteaders

 

White Dwarf Stars in the Milky Way Bulge

Credit for Hubble Images: NASA, ESA, A. Calamida and K. Sahu (STScI), 
and the SWEEPS Science Team. Credit for Ground-based Image: A. Fujii
Object Name: SWEEPS Field

About this image: NASA's Hubble Space Telescope has detected for the first time a population of white dwarfs embedded in the hub of our Milky Way galaxy. The Hubble images are the deepest, most detailed study of the galaxy's central bulge of stars. The smoldering remnants of once-vibrant stars can yield clues to our galaxy's early construction stages that happened long before Earth and our sun formed.

[Left] — This is a ground-based view of the Milky Way’s central bulge, seen in the direction of the constellation Sagittarius. Giant dust clouds block most of the starlight coming from the galactic center. Hubble, however, peered through a region (marked by the arrow) called the Sagittarius Window, which offers a keyhole view into the galaxy's hub.

[Upper right] — This is a small section of Hubble's view of the dense collection of stars crammed together in the galactic bulge. The region surveyed is part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field and is located 26,000 light-years away.

[Lower right] — Hubble uncovered extremely faint and hot white dwarfs. This is a sample of 4 out of the 70 brightest white dwarfs spied by Hubble in the Milky Way's bulge. Astronomers picked them out based on their faintness, blue-white color, and motion relative to our sun. The numbers in the inset images correspond to the white dwarfs' location in the larger Hubble view.

Hubble's Advanced Camera for Surveys made the observations in 2004 and 2011-2013.

Using NASA's Hubble Space Telescope to conduct a "cosmic archaeological dig" at the very heart of our Milky Way galaxy, astronomers have uncovered the blueprints of our galaxy's early construction phase.

Peering deep into the Milky Way's crowded central hub of stars, Hubble researchers have uncovered for the first time a population of ancient white dwarfs — smoldering remnants of once-vibrant stars that inhabited the core. Finding these relics at last can yield clues to how our galaxy was built, long before Earth and our sun formed.

The observations are the deepest, most detailed study of the galaxy's foundational city structure — its vast central bulge that lies in the middle of a pancake-shaped disk of stars, where our solar system dwells.

As with any archaeological relic, the white dwarfs contain the history of a bygone era. They contain information about the stars that existed about 12 billion years ago that burned out to form the white dwarfs. As these dying embers of once-radiant stars cool, they serve as multi-billion-year-old time pieces that tell astronomers about the Milky Way's groundbreaking years.

An analysis of the Hubble data supports the idea that the Milky Way's bulge formed first and that its stellar inhabitants were born very quickly — in less than roughly 2 billion years. The rest of the galaxy's sprawling disk of second- and third-generation stars grew more slowly in the suburbs, encircling the central bulge like the brim of a giant sombrero.

"It is important to observe the Milky Way's bulge because it is the only bulge we can study in detail," explained Annalisa Calamida of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, the science paper's lead author. "You can see bulges in distant galaxies, but you cannot resolve the very faint stars, such as the white dwarfs. The Milky Way's bulge includes almost a quarter of the galaxy's stellar mass. Characterizing the properties of the bulge stars can then provide important information to understanding the formation of the entire Milky Way galaxy and that of similar, more distant galaxies."

The Hubble survey also found slightly more low-mass stars in the bulge, compared to those in the galaxy's disk population. "This result suggests that the environment in the bulge may have been different than the one in the disk, resulting in a different star-formation mechanism," Calamida said.

The observations were so sensitive that the astronomers also used the data to pick out the feeble glow of white dwarfs. The team based its results on an analysis of 70 of the hottest white dwarfs detectable by Hubble in a small region of the bulge among tens of thousands of stars.

These stellar relics are small and extremely dense. They are about the size of Earth but 200,000 times denser. A teaspoon of white dwarf material would weigh about 15 tons. Their tiny stature makes them so dim that it would be as challenging as looking for the glow of a pocket flashlight located on the moon. Astronomers used the sharp Hubble images to separate the bulge stars from the myriad stars in the foreground of our galaxy's disk by tracking their movements over time. The team accomplished this task by analyzing Hubble images of the same field of 240,000 stars, taken 10 years apart. The long timespan allowed the astronomers to make very precise measurements of the stars' motion and pick out 70,000 bulge stars. The bulge's stellar inhabitants move at a different rate than stars in the disk, allowing the astronomers to identify them.

The region surveyed is part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field and is located 26,000 light-years away. The unusually dust-free location on the sky offers a keyhole view into the "downtown" bulge. Hubble's Advanced Camera for Surveys made the observations in 2004 and 2011-2013.

"Comparing the positions of the stars from now and 10 years ago we were able to measure accurate motions of the stars," said Kailash Sahu of STScI, the study's leader. "The motions allowed us to tell if they were disk stars, bulge stars, or halo stars."

The astronomers identified the white dwarfs by analyzing the colors of the bulge stars and comparing them with theoretical models. The extremely hot white dwarfs appear bluer relative to sun-like stars. As white dwarfs age, they become cooler and fainter, becoming difficult even for sharp-eyed Hubble to detect.

"These 70 white dwarfs represent the peak of the iceberg," Sahu said. "We estimate that the total number of white dwarfs is about 100,000 in this tiny Hubble view of the bulge. Future telescopes such as NASA's James Webb Space Telescope will allow us to count almost all of the stars in the bulge down to the faintest ones, which today's telescopes, even Hubble, cannot see."

The team next plans to increase their sample of white dwarfs by analyzing other portions of the SWEEPS field. This should ultimately lead to a more precise estimate of the age of the galactic bulge. They might also determine if star formation processes in the bulge billions of years ago were different from what's seen in the younger disk of our galaxy.

The team's results appeared in the Sept. 1, 2015, issue of The Astrophysical Journal. A companion paper appeared in The Astrophysical Journal in 2014.

CONTACT 

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Annalisa Calamida / Kailash Sahu
Space Telescope Science Institute, Baltimore, Maryland
410-338-4341 / 410-338-4930
calamida@stsci.edu / ksahu@stsci.edu

Source: Hubble Site

NASA's Hubble Finds Rare Blue Straggler Stars in the Milky Way's Hub

Blue Straggler Stars in the Galactic Bulge

Image Type: Astronomical/Illustration

Credit: NASA, ESA, W. Clarkson (Indiana University and UCLA),
and K. Sahu (STScI)

View this image

Artist's View of a Blue Straggler Star

Artwork Credit: NASA, ESA, and G. Bacon (STScI)

Science Credit: NASA, ESA,
W. Clark (Indiana University and UCLA),
and K. Sahu (STScI)

This is an artist's concept of a close binary pair of stars that are merging to form a blue-straggler-class star. Blue stragglers are so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed. The merger stirs up hydrogen fuel and causes the resulting more massive star to undergo nuclear fusion at a faster rate, causing it to burn hotter and bluer. Probing the star-filled, ancient hub of our Milky Way, the Hubble Space Telescope has found blue stragglers for the first time within our galaxy's bulge.

Probing the star-filled, ancient hub of our Milky Way, NASA's Hubble Space Telescope has found a rare class of oddball stars called blue stragglers, the first time such objects have been detected within our galaxy's bulge.

The size and nature of the blue straggler population detected in the bulge will allow astronomers to better understand if the bulge is exclusively old stars, or a mixture of both young and old stars. In addition, the discovery provides a new test case for formation models of the blue stragglers themselves.

Blue stragglers — so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed — were first found inside ancient globular star clusters half a century ago. They have been detected in many globular and open clusters, as well as among the stars in the solar neighborhood. But they have never been seen inside the core of our galaxy until Hubble was trained on the region.

Hubble astronomers found blue straggler stars in an extensive set of Hubble exposures of the Milky Way's crowded hub. Blue stragglers are much hotter — and hence bluer — than they should be for the aging neighborhood in which they live. Now that blue stragglers have at last been found within the bulge, the size and characteristics of this population will allow astronomers to better understand the still-controversial processes of star formation within the bulge.

The results, to be published in The Astrophysical Journal, are being reported by lead author Will Clarkson of Indiana University and the University of California, Los Angeles, at the American Astronomical Society meeting in Boston, Mass.

These results support the idea that the Milky Way's central bulge stopped making stars billions of years ago. It is now home to aging Sun-like stars and cooler red dwarfs. Giant blue stars that once lived there exploded as supernovae billions of years ago. If our galaxy were the size of a dinner plate, the central bulge would be roughly the size of a grapefruit placed in the middle of the plate.

This discovery is a spin-off from a seven-day-long survey conducted in 2006 called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). Hubble peered at and obtained variability information for 180,000 stars in the crowded central bulge of our galaxy, 26,000 light-years away. The survey was intended to find hot Jupiter-class planets that orbit very close to their stars. But the SWEEPS team also uncovered 42 oddball blue stars among the bulge population with brightness and temperatures typical for stars much younger than ordinary bulge stars.

Blue stragglers have long been suspected to be living in the bulge. Until now, it has never been proven because younger stars in the disk of our galaxy lie along the line-of-sight to the core, confusing and contaminating the view.

But Hubble's view is so sharp that astronomers could distinguish the motion of the core population from foreground stars in the Milky Way. Bulge stars orbit the galactic nucleus at a different speed than foreground stars. Plotting their motion required returning to the SWEEPS target region with Hubble two years after the first-epoch observations were made.

Hence, the blue stragglers were identified as moving along with the other stars in the bulge. It's like looking into a deep, clear pond where the fish at the bottom of the pond are swimming at a faster rate than the fish closer to the surface.

"The size of the field of view on the sky is roughly that of the thickness of a human fingernail held at arm's length, and within this region, Hubble sees about a quarter million stars towards the bulge," Clarkson says. "Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field."

From the 42 candidate blue stragglers, the investigators estimate 18 to 37 of them are likely to be genuine blue stragglers, with the remainder consisting of a mixture of foreground objects and at most a small population of genuinely young bulge stars.

It's not clear how blue stragglers form, or if there is more than one mechanism at work. A common idea is that blue stragglers emerge from binary pairs. As the more massive star evolves and expands, the less massive star accretes material from the companion. This stirs up hydrogen fuel and causes the accreting star to undergo nuclear fusion at a faster rate. It burns hotter and bluer.

The seven-day observation allowed the fraction of blue straggler candidates presently in close binaries to be estimated by virtue of their changing light-curve. This is caused by the change of shape induced in one star due to the tidal gravitational pull of its companion. "The SWEEPS program was designed to detect transiting planets through small light variations. Therefore the program could easily detect the variability of binary pairs, which was crucial in confirming these are indeed blue stragglers," says Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md., the principal investigator of the SWEEPS program.

The observations clearly indicate that if there is a young star population in the bulge, it is very small, and it was not detected in the SWEEPS program. "Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood," Clarkson says. "While the consensus is that the bulge largely stopped forming stars long ago, many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge."

CONTACT

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Will Clarkson
Indiana University, Bloomington, Ind., and
University of California, Los Angeles, Calif.
812-855-6911
wiclarks@indiana.edu

Kailash Sahu
Space Telescope Science Institute, Baltimore, Md.
410-338-4930
ksahu@stsci.edu