Astronomer Judy Cheng of the University of California, Santa Cruz was part of a science team that used a giant survey of stars to reveal something new about how our Milky Way galaxy formed. The team found evidence that the inner disk of the Milky Way grew organically from the inside out, like the rings of a tree. The surrounding outer disk, according to Cheng, likely formed all at once.
Cheng’s study looked at data from the Sloan Extension for Galactic Understanding and Exploration 2 (SEGUE-2), which is part of the Sloan Digital Sky Survey-III project, operating from the Apache Point Observatory in New Mexico. The star survey collected starlight of more than 118,000 stars to measure their motions and inner chemistry.
The first generation of stars in the Milky Way are thought to have consisted entirely of the elements hydrogen and helium. Over time, those early stars turned some of their hydrogen and helium into heavier elements, like calcium or iron. When those stars died, the heavier elements they produced became part of the next generation of stars. As new stars were born and the Milky Way disk grew, each generation had more calcium, iron, and other heavy elements. Thus, scientists can learn which parts of our galaxy have seen several generations of stars come and go, simply by looking at the metal content of stars in that part of the galaxy. Judy Cheng told EarthSky:
What we did was, we looked at the elemental abundances of stars at different positions in the disk of the Milky Way. The disk is what you normally see when you go out on a clear night and you see a bright band of stars across the sky. That’s the Milky Way disk. What we looked at was how the amount of metals in the stars varied as you looked at different positions in the galaxy.
Cheng’s team looked at different parts of the entire galactic disk. That disk can be divided into two components. There’s a thin disk, which is the milky band of stars what one can actually see in the sky on a clear night. Then there’s a surrounding thick disk, which is not visible from the Earth but is made up of much older stars, according to Cheng. Those stars can go very far from the plane of the galaxy, which is why it’s called a thick disk. Cheng told EarthSky what she found:
What we are finding is that when we look at how the metals vary as a function of distance from the center of the galaxy, when we looked at the thin disk, we see that there are more metals in the inner part of the galaxy than in the outer parts. And that tells us that the thin disk of the galaxy formed from the inside out. But when we look at the thick disk of the galaxy, we don’t see any change in the metals as you look farther from the center of the disk. So that tells us either that the thick disk all formed at once, and so that all of the metals in the disk accumulated the metal content at the same rate. Or it could also be that the thick disk formed inside out, like the thin disk, but things have gotten mixed around so well over time that we don’t see any change in the metal content farther from the center of the galaxy.
Cheng explaineed that one goal of the SEGUE surveys is to understand how the Milky Way got assembled. In a sense, how all the pieces of the puzzle of our galaxy came together. Cheng told EarthSky:
The idea behind SEGUE is to use the stars that we observe and study their chemical compositions, look at their motions and also their positions in the galaxy to understand how the stars got there, when they were born, and how they fit into our picture of the galaxy as having this thin disk, and a thick disk. And there’s also different components of the galaxy as well. There’s a halo that goes out farther and is made up of even older stars. And by looking at all of these types of stars in SEGUE, we can try to put together a picture of how these stars got to where we see them today.
Bottom line: Astronomer Judy Cheng of UC Santa Cruz led a survey of stars called SEGUE-2 that found the thin disk of our Milky Way galaxy grew differently than its thick disk.