The Venerable Hubble Space Telescope has completely established its place in Space history. Some refer to it as the most effective medical experiment in history. Even though the James Webb Space Telescope may challenge the name, the Hubble is able to do things which the more powerful JWST can’t.
Exhibit A: This’s a spectacular picture of NGC 6355.
NGC 6355 is a globular cluster (GC) located in the Ophiuchus constellation approximately 50,000 light years from Earth. GCs are tightly packed spherical groups of stars which may hold countless stars. Nobody is sure the number of stars NGC 6355 possesses.
Original astronomers were unable to tell GCs and believed they were individual stars. Once the telescopes arrived, GCs appeared to be blobs, and some early astronomers believed they were comets. The Hubble Space Telescope’s launch, though, transformed the investigation of globular clusters. Hubble might get clear views of these things from over the Earth’s atmosphere.
Ground telescopes find it tough to resolve particular stars in GCs. The environment on Earth didn’t impede Hubble’s observations. The ground-based astronomers need to cope with the light distortion consequences of the environment, but Hubble is above all.
In nearly all galaxies, GCs happen to be positioned. They’re mainly present within spiral galaxies similar to ours in the galactic halo. GCs tend to be more mature compared to the other cluster type, the open clusters (OCs). GC stars are a few of the earliest objects in the Universe and possess lower metallicities as compared to stars in OCs.
Globular Clusters stay mystified objects. But Hubble examined them, and more than eliminated a bit of the unknown. One pressing concern is whether GCs hold black holes, similar to galaxies do.
Black holes are available in several sizes. Gravitational collapsing of a star creates stellar mass black holes. They vary from approximately 5 to tens of solar masses. Supermassive Black Holes (SMBHs) are enormous and are situated in the center of galaxies. They might have vast amounts of times more mass as compared to the Sun. Then there are Intermediate Mass black holes (IMBHs).
IMBHs, however, are hard to come by.
As soon as Hubble arrived, its capability to identify specific stars in GC made it easier for astronomers to identify IMBHs. Within a GC center, the stars tend to be more tightly packed compared to towards the edges. Within that central region, astronomers believed that IMBHs may be hiding.
The NGC 6397 happens to be a globular cluster within the Ara constellation. It’s around 7,800 light years from the Earth and one of the 2 nearest GCs. There’re approximately 400 000 stars in it. Astronomers utilized Hubble images as well as information from other facilities to look at NGC 6397 in a study in 2021 for proof of an intermediate mass black hole.
“We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not ‘point-like’ (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster,” said Eduardo Vitral, one of the paper’s authors.
Finding a clump of stellar-mass black holes was surprising and went against expectations. Astronomers thought that the cores of GCs were a likely place to find the elusive IMBHs.
The researchers haven’t wholly concluded that the mass in the center of the GC is all black holes. They call it a CUO: a cluster of unresolved objects. The CUO has from 1000 to 2000 solar masses; whatever they are, they’re dense.
The paper’s authors say that stellar evolution explains what’s likely there. They think the 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 to the GCs center 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,” said Mamon. Two other recent studies also proposed that stellar remnants, particularly stellar-mass black holes, could populate the inner regions of globular clusters. “Ours is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the center of a core-collapsed globular cluster,” said Vitral.
None of this would’ve been discovered if it weren’t for the Hubble. It had some help, especially from the ESA’s Gaia spacecraft. But the Hubble’s ability to resolve individual stars was critical.
The authors explain in their paper exactly how Hubble’s observations were crucial to their results. On the flip side, Gaia data has far too few stars with sufficiently precise PMs at low projected radii to examine the CUO or IMBH mass and even less the CUO size. ” This stresses the need for merging Gaia and HST for appropriate mass orbit modelling of the PM of stars in GCs.
Astronomers ‘ability to grasp black holes in the heart of GCs will depend on their ability to measure the correct motion of the stars. Hubble made a great contribution, but as Gaia publishes more data, its PMs will increase in accuracy. The James Webb Space Telescope is going to also contribute, according to the authors.
The authors say that continued pointings of GCs using HST and soon the James Webb Space Telescope will lead to longer baselines and more precise PMs. “The third data release of the Gaia mission will double PM precision, thus making it possible for more exact mass orbit modelling of nearby GCs like NGC 6397, but in addition of distant ones in conjunction with HST data,” he said.
The James Webb Space Telescope is adored by most of us. It’s amazing!
It’s a strange feeling for those of us who grew up with Hubble, watching JWST supplant Hubble and eventually relegate it to a lesser role.