The globular cluster M92 in Hercules is a familiar target for amateur astronomers, a compact swarm of stars near M13, one of the brightest globulars visible from the northern hemisphere. M92 is also one of the oldest known globulars in the Milky Way, made up of stars between 12 billion and 13 billion years old.
Astronomers using the James Webb Space Telescope imaged M92 last summer as part of an ongoing study of stellar evolution. The ancient cluster was also targeted to help calibrate Webb’s science instruments using a field of relatively close-packed, resolved stars.
“We … chose M92 because it is very dense,” said Rutgers University researcher Roger Cohen. “There are a lot of stars packed together very closely. Looking at M92 allows us to test how Webb performs in this particular regime, where we need to make measurements of stars that are very close together.”
While M92 has been extensively studied with the Hubble Space Telescope and other ground- and space-based instruments, Webb operates at infrared wavelengths and is ideally suited for studying cool, low-mass stars.
“We were actually able to reach down to the lowest mass stars, stars less than 0.1 times the mass of the sun,” said Cohen. “This is very close to the boundary where stars stop being stars. … These observations weren’t actually designed to push very hard on the limits of the telescope. So it’s very encouraging to see that we were still able to detect such small, faint stars without trying really, really hard.”