These observations are very important because we learn a lot about the physics of stars and galaxies.
NASA, ESA, CSA, A. Pagan (STScI)
The study of globular clusters
Globular clusters like M92 are very important for our understanding of stellar evolution. For decades they have been a primary reference point for understanding how stars work and how they evolve. M92 is a classic globular cluster, close to us and well understood enough to be one of our references in the studies of evolution and stellar systems.
Another reason why M92 matters is that it is one of the oldest globular clusters in the Milky Way, if not the oldest. It is believed that M92 is between 12 and 13 billion years old, contains some of the oldest stars we can find, or at least that we can observe and characterize well as traces of the ancient universe.
M92 was also chosen for its density: inside there are many stars close together, the center of the cluster is thousands of times denser than the region around the Sun. Observing M92 allows you to test how the James Webb behaves in this particular situation where we have to make measurements of stars very close to each other.
One of the interesting features of this cluster is that most stars in M92 would have formed at roughly the same time and with the same mix of elements, but with a wide range of masses. Thanks to this you can get an excellent analysis of this particular population of stars.
Also, since the stars all belong to the same object, the same globular cluster M92, we know that they are all at the same distance from us. This is very helpful because we know that the differences in luminosity between the different stars must be intrinsic rather than simply related to their distance. This makes comparison with existing models much easier.
A detailed analysis of the stars
Credit: NASA, ESA, CSA, A. Pagan (STScI)
The image was taken using Webb’s near infrared camera (NIRCam) and features a black stripe in the center that is the result of the separation between the two detectors. The cluster’s center is extremely crowded and extremely bright, not ideal for James Webb’s observations, and would have limited the usefulness of that region’s data. Images of the central region are already available thanks to the Hubble telescope and overlap perfectly with the new data collected by Webb.
The James Webb was able to capture the lowest mass stars, as low as 0.1 times the mass of the Sun. Such a dimension is very close to the border where the stars stop being stars. Below this limit, only brown dwarfs have such a small mass that they are not able to ignite hydrogen in their nuclei. And these are the most numerous stars in the universe.
From a theoretical standpoint, they are very interesting as they have always been very difficult to observe and characterize. Especially for stars less than half the Sun, where our current understanding of stellar patterns is a little more uncertain.
Looking at the light emitted by these low-mass stars can also help us better define the age of the globular cluster. This helps us to better understand when different parts of the Milky Way have formed just like the halo, where M92 is located. And this may have implications for our understanding of cosmic history.
References: NASA
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