The Milky Way is older than astronomers thought, or part of it is. A newly-published study shows that part of the disk is two billion years older than we thought. The region, called the thick disk, started forming only 0.8 billion years after the Big Bang.
A pair of astronomers pieced together the Milky Way’s history in more detail than ever. Their results are based on detailed data from the ESA’s Gaia mission and China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The key to this discovery lies in subgiant stars.
The paper is “A time-resolved picture of our Milky Way’s early formation history,” and it’s online in the journal Nature. The authors are Maosheng Xiang and Hans-Walter Rix, both from the Max-Planck Institute for Astronomy (MPIA.)
“Our results provide exquisite details about that part of the Milky Way, such as its birthday, its star-formation rate and metal enrichment history. Putting together these discoveries using Gaia data is revolutionizing our picture of when and how our galaxy was formed.”
Maosheng Xiang, study co-author, MPIA.
One of the most difficult things to determine about a star is its age. A star’s composition, or metallicity, is key to finding its age. The more accurately astronomers can measure metallicity, the more accurately they can determine its age. The early Universe contained hydrogen and helium almost exclusively. Elements heavier than hydrogen and helium are produced in stars and spread out into the Universe when those stars die and explode. Astronomers call every element heavier than the two primordial elements “metals.”
Stars with lower metallicity are older because they formed when mostly just hydrogen and helium were available. So when astronomers identify a population of stars mainly containing hydrogen and helium, they know those stars are older. When they find a population of stars with higher proportions of metals, they know those stars must be younger.
Precision age measurements are the holy grail in some aspects of astronomy, which is true in this case. Xiang and Rix used more than just metallicity to determine stellar ages. They focused on a specific type of star: subgiants. The subgiant phase in a star’s life is relatively brief, so astronomers can determine a star’s age most accurately when it’s a subgiant. Subgiants are transitioning to red giants and no longer produce energy in their cores. Instead, fusion has moved into a shell around the core.
This figure from the study shows some of the detail for the 247,000 subgiant stars in the sample. (a) shows the subgiant selection by magnitude and temperature. (b) shows the distribution in the relative age precision as a function of age. Image Credit: Xhiang and Rix 2022.
In this study, the pair of scientists used LAMOST data to determine the metallicity of about 250,000 stars in different parts of the Milky Way. They also used Gaia data which gives the precise position and brightness data for about 1.5 billion stars.
The ESA’s Gaia mission is responsible for increased accuracy in this study and many others. Before Gaia, astronomers routinely worked with stellar age uncertainties between 20% to 40%. That meant that ages could be off by one billion years, which is a lot. But Gaia has changed all this. The current data release from the mission is Gaia EDR 3 or Early Data Release 3, and it’s a vast improvement. EDR3 gives precise 3D positions of over 330,000 stars. It also gives high-precision measurements of the stars’ motions through space.
The researchers used all this data from Gaia and LAMOST and compared it to known models of stellar parameters to determine the subgiants’ ages with greater accuracy. “With Gaia’s brightness data, we are able to determine the age of a subgiant star to a few percent,” said Maosheng. The subgiants are spread throughout the different parts of the Milky Way, allowing the researchers to piece together the ages of the other components and build a timeline of the Milky Way’s history.
The study shows two distinct phases in our galaxy’s history. The first phase started 0.8 billion years ago when the thick disk
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