Yesterday, I wrote what was essentially a love letter to Gaia, and today I bring you yet more amazing science from my favorite telescope. In a new paper published in Monthly Notices of the Royal Astronomical Society and led by Daniel Mikkola, researchers use data from Gaia to discover that white dwarf stars have two different distributions both in how they move and how bright they shine.

White dwarfs are the leftover cores of stars like our Sun and smaller. They are no longer undergoing nuclear reactions, and their outer atmospheres have been exhaled to form planetary nebulae. These stellar remnants start out hot, hot, hot, but over time, cool off and fade away. If white dwarfs were simple objects, we could readily calculate how they cool in the same way a physics student might calculate the cooling of a rock heated up in a fire. This would also allow us to figure out things like how long the white dwarf has been there, hanging out, cooling. And that in turn, would give us an estimate of the age of the stars they are hanging out with.

But white dwarfs aren’t simple, and how they cool changes not just with how big they are, but also according to composition, and potentially also according to crystallization. In some models, the highly-dense material in these Moon-sized objects will change phases, suddenly turning into massive crystals as they cool. This state change causes the star to temporarily reheat before rapidly cooling. The details can be modeled, sort of, but are highly dependent on a lot of details we don’t always know about the stars we’re looking at.

Here is what we actually know: 120,675 white dwarf stars in the Gaia data set were analyzed by color, brightness, and motion. One of the populations is brighter and has stars with a wider distribution of velocities, and some of the white dwarfs are truly massive for still-minuscule white dwarfs. According to the paper: The bifurcation seen in Gaia clearly has contributions from both atmospheric differences as well as different WD mass cooling tracks. It remains unclear whether the origins of the massive WDs are recent bursts of star formation or a pile-up of WDs with a mixture of ages due to delayed cooling from crystallization, or a mixture of both.

And this is what makes science interesting. Sometimes you can get at the same result by following different paths, and we are always trying to understand is it one or the other or both? Here, it looks like the universe is playing a game of “yes, and” and allowing two different families of stars to emerge.

MORE INFORMATION

Lund University press release

“The velocity distribution of white dwarfs in Gaia EDR3,” Daniel Mikkola, Paul J. McMillan, David Hobbs, and John Wimarsson, to be published in Monthly Notices of the Royal Astronomical Society (preprint on arxiv.org)