Typically, scientists don’t even hope to spot individual stars in distant galaxies. Meanwhile, the study of stars in the early stages of the development of the Universe is necessary for understanding the evolution of galaxies and the Universe as a whole. And then chance saves the day, the effect of gravitational lensing and the emergence of more advanced telescopes, such as the James Webb. And the stars aligned.
Image source: NASA
Astronomers from the University of Arizona were lucky enough to simultaneously discover dozens of stars in the galaxy at a time when the Universe was half its age – only 6.5 billion years old. Under normal conditions, such a galaxy would appear in astronomical photographs as a faint spot. Thanks to gravitational lensing, it was possible to see 40 individual stars in it and gain a sufficient understanding of them.
The discovery was made thanks to two Webb observations of the supercluster of galaxies Abell 370 at a distance of about 4 billion years from Earth. On the line of sight between the Earth and the cluster, far beyond it is the Dragon Arc galaxy. Studying images of the cluster taken by Webb about a year apart helped identify four dozen stars that turned out to come from a distant galaxy.
Some of the distant stars discovered were brighter in one image, while others were brighter in the second. The analysis showed that the stars increased both by the entire mass of the Abell 370 cluster, the effect of which was called gravitational macrolensing, and by individual stars in the cluster that were not part of any local galaxies (flying freely). It was these stars that produced the effect of gravitational microlensing, changing the magnification (and brightness) of distant stars in a short period of time – weeks and even days. And if the cluster magnified the Arc of the Dragon galaxy and individual stars in it by about 100 times, then individual stars in the cluster increased the light of distant stars by about 10 times more.
The combination of rare conditions and the insight and persistence of scientists produced an astonishing result – 40 observed stars in a galaxy 6.5 billion light years away from Earth. All of them turned out to be red giants at the end of their lives, like the relatively nearby bright star Betelgeuse. It is noteworthy that Webb became the instrument that for the first time was able to see relatively cool stars so far away, because previously the most distant stars detected were bright blue giants. With the help of Webb, astronomy took another step and expanded the observable Universe for terrestrial science.