Images taken by the Hubble Space Telescope have been
analyzed by a team of astronomers, uncovering a population of very
old white dwarf stars. It's the first time the ancient stars have
been observed, and their study provides clues regarding the early
construction phase of our home galaxy.
The new observations are the most detailed of their kind, looking closely at the foundational bulge at the very heart of the Milky Way. Looking specifically at the white dwarf star remnants gives us valuable information about the galaxy's distant past.
Carefully studying the data, the team was able to pick out the white dwarfs, selecting 70 of the hottest examples for in-depth analysis. The stellar remnants are only about the size of the Earth, but are are a staggering 200,000 times denser. As they're so small, they're also pretty dim, making them challenging to observe.
To find the remnants, the team looked at different Hubble images of the same spot of sky, which contains around 240,000 stars, taken nine years apart. They then carefully analyzed the movement of the stars, picking out those in the bulge thanks to their slower speed. The region itself resides some 26,000 light years away, and can be seen particularly clearly thanks to the unusually dust-free space between the it and the Earth.
Having picked out the bulge stars, of which there were some 70,000, the team then set to work identifying the white dwarfs. They did so by studying the colors of the stars and comparing them to theoretical models that suggested that hot white dwarfs should appear with a blue tinge relative to younger stars. The team picked out 70 prime examples – those still burning hot enough to be detectable in the Hubble imagery.
The data from the sample supports theories that the bulge of the Milky Way formed early on, and that the stars within it ignited within the first two billion years of its existence. The vast disk of stars that surround the bulge formed much more slowly.
More broadly, there were also found to be more low-mass stars in the bulge than in the surrounding disk, suggesting that the environment in the bulge, early on the lifespan of the Milky Way, was a little different experienced by later stars, located farther from the central core.
"These 70 white dwarfs represent the peak of the iceberg," says study leader Kailash Sahu. "We estimate that the total number of white dwarfs is about 100,000 in this tiny Hubble view of the bulge."
To get a better look at the full population of white dwarfs, we'll have to wait until next-generation installations like NASA's James Webb Space Telescope become operational. Such telescopes should be able to view practically all of the stars in the bulge, right down to the very faintest bodies.
In the meantime, the team plans to expand its sample of white dwarfs by analyzing other Hubble data, an endeavour that it hopes will lead to a more precise estimation of the age of the Milky Way's galactic bulge, as well as giving us a clearer picture of potential differences between star formation in the core and arms of the galaxy.
The researchers published the results of their findings in The Astrophysical Journal.