Billions of years ago, what would eventually be the Milky Way galaxy began to form. Young, hot, Sun-like stars within the galactic center were some of the first stars and as such, laid the foundation for what would be our galaxy. The majority of these first residents of our galaxy have long since burned out, leaving behind smoldering stellar cinders known as white dwarfs. Locked away within the dying embers of these “dead stars” is a fossilized account of our galaxy’s earliest history.
However, locating these stellar fossils has proven to be a daunting task. The heart of the Milky Way is packed full of bright stars, outshining their dim, ancient counterparts. Now, astronomers have used images captured by Hubble to carry out a cosmic archeological dig of sorts, searching for any evidence of the Milky Way’s galactic blueprints. The researchers were able to locate an ancient population of white dwarf stars hidden within the crowded galactic center.
These new observations are part of the deepest, most detailed study of the Milky Way’s central bulge ever conducted. Finding these stellar remnants will help astronomers better understand galaxy formation. The ancient relics harbor a vast amount of information on the stellar population that formed over 12 billion years ago, eventually burning out to form the white dwarfs we see today.
The multi-billion-year-old timepieces tell astronomers that the galaxy’s bulge formed first, with its first stellar inhabitants forming less than two billion years later. The rest of the galaxy’s second and third-generation stars grew more slowly, encircling the central bulge like a giant halo. Hubble also detected a higher concentration of low-mass stars within the central bulge, indicating there may be two different star-forming mechanisms at work.
“It is important to observe the Milky Way’s bulge because it is the only bulge we can study in detail,” explained Annalisa Calamida, lead author of the study published in the Astrophysical Journal, in a statement. “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.”
To look for the stellar remnants’ feeble glow, the team analyzed the colors of the stars and compared the data to theoretical models. Even though white dwarf stars appear bluer than their Sun-like counterparts, this was a daunting task. Finding an ancient white dwarf among the bulge’s stellar population is like looking for the glow of a flashlight on the Moon.
There are tens of thousands of white dwarf stars within the galactic bulge, and the team selected 70 of the hottest white dwarfs detected to study as part of this survey. White dwarfs rival the Earth in size, but are 200,000 times more dense. To put that into perspective, a teaspoon full of white dwarf material weighs approximately 15 tons. Astronomers used the incredibly sharp Hubble images to separate bulge stars from the disk stars by tracking their movements with extreme precision over a 10-year time period. Stars within the bulge move at different rates than those in the disk, making it easy to distinguish between the two populations.
“Comparing the positions of the stars from now and 10 years ago we were able to measure accurate motions of the stars,” said study leader Kailash Sahu of STScI. “The motions allowed us to tell if they were disk stars, bulge stars, or halo stars."
Sahu added: "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 region surveyed is located 26,000 light-years from Earth and is part of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field. The observations were made in 2004 and then again in 2011-2013 by Hubble’s Advanced Camera for Surveys. The team hopes to increase their sample size to ultimately estimate the age of the galactic bulge with greater precision.
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