Free Astronomy Magazine January-February 2016

of the Space Telescope Science Insti- tute (STScI) in Baltimore, Maryland, lead author of the study. "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 impor- tant information to understanding the formation of the entire Milky Way galaxy and that of similar, more distant galaxies." The Hubble survey also found slight- ly more low-mass stars in the bulge, compared to those in the galaxy's disk population. "This result suggests that the envi- ronment in the bulge may have been different than the one in the disk, resulting in a different star-for- mation 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 re- sults on an analysis of 70 of the hot- test 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 dens- er. A teaspoon of white dwarf mate- rial 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 fore- ground of our galaxy's disk by track- ing their movements over time. The team accomplished this task by ana- lyzing Hubble images of the same field of 240,000 stars, taken 10 years apart. The long timespan allowed the astronomers to make very pre- cise measurements of the stars' mo- tion and pick out 70,000 bulge stars. billion-year-old time pieces that tell astronomers about the Milky Way's groundbreaking years. An analysis of the Hubble data sup- ports 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 sprawl- ing disk of second- and third-gen- eration stars grew more slowly in the suburbs, encircling the central bulge like the brim of a giant som- brero. "It is important to observe the Milky Way's bulge because it is the only bulge we can study in de- tail," explained Annalisa Calamida SPACE CHRONICLES 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 Extra- solar Planet Search (SWEEPS) field and is located 26,000 light-years away. The unusually dust-free loca- tion on the sky offers a keyhole view into the "downtown" bulge. Hub- ble'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 mo- tions 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 com- paring them with theoretical mod- els. The extremely hot white dwarfs appear bluer relative to sun-like stars. As white dwarfs age, they be- come cooler and fainter, becoming difficult even for sharp-eyed Hub- ble 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 faint- est 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 ga- lactic bulge. They might also deter- mine if star formation processes in the bulge billions of years ago were different from what's seen in the younger disk of our galaxy. n located 26,000 light-years away. [Lower right] — est white dwarfs spied by Hubble in the Milky Way's to our Sun. The numbers in the inset images corre- and K. Sahu (STScI), and the SWEEPS Science Team]