The venerable Hubble Space Telescope has cemented its place in history. Some call it the most successful science experiment ever. And while the James Webb Space Telescope may vie for that title, Hubble does things that even the powerful JWST can’t.
Figure A: This stunning image of NGC 6355.
NGC 6355 is a globular cluster (GC) about 50,000 light-years from Earth in the constellation of Ophiuchus. GCs are tightly packed globular clusters of stars that can contain millions of stars. Astronomers aren’t sure how many stars NGC 6355 contains.
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Early astronomers could not distinguish astrocytes and thought they were individual stars. When telescopes appeared, the star planets looked like blobs, and some early astronomers thought they were comets. But the launch of the Hubble Space Telescope revolutionized the study of globular clusters. And above Earth’s atmosphere, Hubble can get clear views of these objects.
Ground-based telescopes struggle to resolve individual stars in GCs. But the Earth’s atmosphere did not hinder the Hubble observations. Ground-based telescopes must deal with the light-distorting effects of the atmosphere, but Hubble is above all else.
CSCs are found in almost all galaxies. In spiral galaxies like our own, it is found primarily in the galactic halo. GCs are older than the other type of cluster, open clusters (OCs.) GC stars have a lower metallicity than stars in OCs and are some of the oldest objects in the universe.
Globular clusters are still mysterious objects. But Hubble studied it and removed at least some of the mystery. One burning question is whether CSCs host black holes, as galaxies do.
There are different sizes of black holes. The gravitational collapse of a star leads to the formation of stellar-mass black holes. They range from about five to tens of solar masses. Supermassive black holes (SMBHs) are giant and are found at the centers of galaxies. It can contain a mass billions of times greater than that of the Sun. Then there are intermediate-mass black holes (IMBHs.)
But IMBHs are elusive.
When Hubble came along, its ability to discern individual stars in the GC helped astronomers discover IMBHs. The stars at the center of the GCs are much tighter than the stars towards the edges. That central region is where astronomers thought IMBHs might be hiding out.
NGC 6397 is a globular cluster in the constellation Ara. It lies about 7,800 light-years from Earth and is one of two complexes that close together. It holds about 400,000 stars. In a 2021 study, astronomers looked at Hubble images and data from other facilities to examine NGC 6397 for evidence of an intermediate-mass black hole.
Black holes cannot be seen, and in order to infer their existence, astronomers study the motion of stars in the cores of CSCs. The black hole’s massive gravitational force affects the trajectories and speeds of those stars, and by measuring it, astronomers can estimate the black hole’s mass. The 2021 paper found something unusual: There wasn’t a single black hole at the center of NGC 6397. There was a bunch of them.
“We found very strong evidence of invisible mass in the dense core of the globular cluster, but we were surprised that this extra mass is not ‘point-like’ (which would be expected in a solitary supermassive black hole) but is extended to a few percent of the mass’s size,” said Eduardo. Vitral, one of the paper’s authors.
Finding a cluster of stellar-mass black holes was surprising and defying expectations. Astronomers thought the nuclei of CSCs were a potential place to find elusive IMBHs.
The researchers haven’t quite concluded that the cluster at the center of the GC is entirely black holes. They call it CUO: Collection of Unresolved Objects. CuO has from 1,000 to 2,000 solar masses; Whatever it is, it is dense.
The paper’s authors say that stellar evolution explains what is likely out there. They believe that CUO includes the remnants of massive stars: neutron stars, white dwarfs, and black holes. Through a process called dynamical friction, more massive objects gradually sink into the center of the GCs while less massive stars migrate to the periphery.
“We used the theory of stellar evolution to conclude that most of the extra mass we found was in the form of black holes,” Mammon said. Two other studies have also suggested that stellar remnants, particularly stellar-mass black holes, could populate the interiors of globular clusters. “Our study is the first to report both the mass and extent of what appears to be a cluster of mostly black holes at the center of mass of a collapsed globular at the core,” Vitrall said.
None of this would have been discovered without Hubble. I got some help, especially from the European Space Agency’s Gaia spacecraft. But Hubble’s ability to resolve individual stars was crucial.
In their paper, the authors explain how important the Hubble observations are to their findings. “on the other side, Jaya The data itself contains very few stars with sufficiently accurate PMs <حركات مناسبة> At a low drop radius to explore the IMBH or CUO mass, and even less than the CUO size. This highlights the requirement to combine HST and Gaia for proper mass-orbit modeling of star-based PMs in GCs. “
Understanding the black holes at the heart of GCs depends greatly on how accurately astronomers measure the proper motion of stars. Hubble made an enormous contribution, but as Gaia releases more data, the resolution of its PMs will increase. According to the paper’s authors, the James Webb Space Telescope will also start contributing.
“Continued signaling of GCs using the HST and soon the James Webb Space Telescope will lead to longer baselines and more accurate PMs,” the authors wrote. “The third data release for the Gaia mission will double the fine particle resolution, enabling more accurate mass orbit modeling, not only of nearby GCs like NGC 6397 but also of more distant ones, in conjunction with the HST data.”
We all love the James Webb Space Telescope. Something amazing!
But for those of us who have come of age alongside Hubble, it’s kind of a strange feeling to watch JWST replace Hubble and maybe eventually relegate it to a lower role.
But as these images and research show, Hubble is still in the game.