University researchers develop new techniques to explore chemistry of the galaxy
Marcelle Couto | Monday, February 21, 2022
This month, the “Astrophysical Journal” published an article highlighting the advances for the exploration of the Milky Way Galaxy. Namely, it brings individuals closer to unlocking the history behind the formation of the elements in the universe.
Physics professor and one of the leading contributors behind this research Timothy C. Beers described the work he does within the University.
“What I and my colleagues do are observations of individual stars,” he added. “Ideally, they are stars that are extremely old, because the oldest ones provide the cleanest signatures of where and how the different elements came to be.”
Beers also discussed expanding the physics department to include astronomy.
“Although it is a physics department, we are in the process of making it a physics and astronomy department,” Beers said. “Within this context, what we do is broadly referred to as nuclear astrophysics. What we’re trying to understand is the origin of the elements, or how the periodic table emerged.”
Beers exemplified the challenges of probing into the mysteries of origins by giving the example of the sun.
“Even though the sun is 4.5 billion years old now, at the time it was born, it inherited the previous 7.5 billion years of previous chemical evolution,” he said. “So, the gas with which the sun was made had already been ‘polluted,’ or ‘enriched,’ by previous generations of stars. What that means is, if you only have the sun to look at in terms of details regarding the elements, they are hard to interpret because you have many different contributors.”
Beers continued by explaining the process of dating stars.
“If you go back in time, to the point where individual stars are being born of more and more pristine gas, they have not had much previous contribution from other generations,” Beers said. “Then you stand a chance to say ‘Oh, this signature came from this kind of object’ and so forth.”
The difficulties in observing the night sky lie in unveiling what Beers appropriately deemed as ‘fossils.’
“These long-lived stars were formed just after the time the previous massive, short-lived stars died, so they really are just like fossils,” Beers illustrated. “The challenge is finding them and interpreting them.”
Beers noted his research team is trying to determine the composition of objects, like stars that have exploded.
“What we are trying to figure out for the most part is the nature of objects which are no longer there,” Beers said. “These are stars that live typically short lives [a couple million years], explode, their gas gets distributed and then the low-mass stars that do live billions of years are formed from this gas.”
Discerning the chemical makeup of stars principally lies within the field of spectroscopy, a technique Beers has dedicated decades perfecting for large-scale surveys. Spectroscopy is the study of light, which is dispersed into constituent colors and absorption lines. By examining these probes, it is possible to determine any number of properties of the studied objects, as the spectra are influenced by the composition of their gas. Spectroscopy may be used to determine the elemental compounds within the field of chemistry or gain insights into the composition and velocity of astronomical bodies.
Until recently, Beers stated scientists did not believe high-resolution spectroscopy could be used in studies.
“In the early days, low-resolution spectroscopy was thought to be the end product of our studies,” he said.
Now, researchers find they are instead, “the calibration objects, the ones that tell you what to pick out for a more detailed study,” Beers said.
Beers also noted most stars have similar material to the sun.
“The vast majority of stars in the night sky are spectroscopically similar to the sun,” he said. “You have to find the rare ones that are deficient in their heavy metals. Metal–poor stars in our part of the galaxy only occur about one in a thousand, and the most metal poor only occur about one in 10,000. I have worked, along with my colleagues, to develop new ways to find these kinds of objects, and we did.”
Beers detailed the logarithmic nature of the scale used to determine the metallicity of stars. It is helpful to recall the Richter scale, where each level on the scale represents a drastic change in the strength of an earthquake.
In essence, the metallicity of stars is compared to that of the sun, which has a ratio of iron to hydrogen, represented by [Fe/H], set to zero. [Fe/H] = -1 represents a star whose metallicity is 10 times below that of the Sun, and –2 equates to 100 times lower, and so on.
“When I got started there were only a few stars known below [Fe/H] = –2,” Beers explained. “Now, we have a handful below [Fe/H] = –5 —100,000 times below the sun — and at least two approaching [Fe/H] = –8, or 100 million times lower than the sun.”
Despite the progress over the years of research dedicated to these star searches, the problem was to acquire larger samples for study, according to Beers.
“How do we make that transition from the relatively small numbers from the spectroscopic samples?” Beers posed, “What we’ve done along the way is to train a new technique, that is, precision filter photometry. All you have to do is take a picture through a given filter, and it relates to the metal signatures of the object. Now, we have known about this for a long time, but the problem was that the accuracy was always low.”
Along with his colleagues, Beers has worked to refine this process, and employ a multiplexed approach to target stars of the greatest interest with high accuracy.
Beers affirmed that he is “working hard so the next generation of astronomers does not have to work quite as hard.”
He discussed how this next generation will have better technology at their disposal.
“The next generation will still be doing spectroscopy, but at much higher resolution; without the tools and my colleagues have been building, they would have to target many stars at random. These higher resolution surveys will already have a long list of ‘targets’ — not one star at a time, but 500, 1,000 or 5,000 stars in one go, fed into multiple spectrographs.”
“By the 1980s , we had only found about 20 stars below [Fe/H] = -2 on the scale,” he said. “We have confidence that we have now identified 500,000 stars below thus value.”
On a good night, Beers used to spectroscopically examine fifteen to twenty stars. Now, a good night can yield information regarding 15 to 20,000 stars.
“You want to make sure those are the 15 to 20,000 you care about,” he said. “You can ‘fairly’ study a sizable sample, or you can ‘unfairly’ study the ones you care most about.”
Beers remarked that he and his team are truly interested in the origins of the universe.
“Ultimately, we are interested in origins,” he said. “We want to know something deeper about not only where we came from, but a detailed picture that emerges from work like this — one that fills in the gaps that we don’t usually consider.”