At the heart of our galaxy is Sagittarius A*, a supermassive black hole nearly four
million times the mass of our sun.
Its gravitational effects are quite intense due to its size, and the effects can be detected
by looking at the stars in its immediate vicinity.
Orbiting Sagittarius A* is a handful of stars, as well as some mysterious objects, locked
in a cosmic two-step with the invisible monster, moving at mind-boggling speeds.
And scientists have just discovered the fastest of the bunch, clocking in at 8 percent the
speed of light near the supermassive black hole.
A study, published in August 2020, examined the area surrounding Sagittarius A*, looking
for the tell-tale signs of stars.
Previous research has detected dozens of stars moving around the supermassive black hole
on exceedingly peculiar orbits.
This group of stars is known as the S-stars, and some of them orbit the black hole extremely
close, making them difficult to identify.
However, the research team scoured through images collected between 2004 and 2016 using
instruments deployed at the European Southern Observatory's Very Large Telescope in Chile,
adding five new stars, S4711-S4715, to the population and tracing their movements around
Sagittarius A*.
Their findings add to the growing body of evidence indicating a distinct population
of stars orbit Sagittarius A* at distances comparable to our solar system's size.
They are also privy to some extreme physics due to their proximity to the petrifying,
bottomless hole at the center of the Milky Way.
A team of astronomers at the University of Cologne in Germany have been studying the
region of space close to the black hole so intensely.
In January 2020, they reported observations of the star S62.
According to their observations, S62 orbited the black hole once every 9.9 years, giving
it the shortest orbital period and making it the fastest star to travel around the Milky
Way's black hole.
But S62 lost both of its records due to the new information that came up.
New information stated that one of the newly-discovered stars, S4711, orbits the Milky Way's black
hole once every 7.6 years, setting the record for the shortest orbital period.
How the stars developed a stable orbit under "extreme conditions" remains a mystery.
A possible explanation could be the Hills mechanism, in which a binary system is disrupted
by the presence of a supermassive black hole (SMBH).
According to this theory, the Milky Way's supermassive black hole disrupts a pair of
stars, ejecting one into space and encircling the other in a tight orbit, as seen in S4711.
In search to find even more stars close to the black hole, a team known as the GRAVITY
collaboration devised a new analysis technique that allowed them to collect the deepest and
sharpest photos of the galactic center yet.
The VLTI (Very Large Telescope Interferometer) provides this outstanding spatial resolution,
and the new images allow scientists to go further into space than ever before.
They are astounded by the attention to detail and the action and number of stars they disclose
near the black hole.
Surprisingly, they discovered a star called S300 that had never been spotted before, demonstrating
how effective this method is for detecting faint objects near Sagittarius A*.
With their most recent observations, which took place between March and July 2021, the
scientists focused on taking exact measurements of stars as they approached the black hole.
This includes the record-holder star S29, which made its nearest approach to the black
hole in late May 2021.
It passed it at a distance of 13 billion kilometers, roughly 90 times the sun-Earth distance, at
an astounding speed of 8740 kilometers per second.
No other star has ever been seen passing so close to the black hole or traveling so fast
around it.
The team's measurements and images were made possible thanks to GRAVITY, a one-of-a-kind
instrument that the collaboration developed for ESO's VLTI, located in Chile.
Following stars on close orbits around Sagittarius A* allows scientists to investigate the gravitational
field around the nearest enormous black hole to Earth in great detail, test General Relativity,
and identify the black hole's properties.
When paired with the team's previous findings, the new findings confirm that the stars follow
paths exactly as predicted by general relativity for objects moving around a black hole of
mass 4.30 million times that of the sun.
This is the most precise estimate of the mass of the central black hole in the Milky Way
to date.
The scientists were also able to fine-tune the distance to Sagittarius A*, finding it
to be 27,000 light-years away.
GRAVITY will be replaced by GRAVITY+ later this decade, which will be installed on ESO's
VLTI and will increase sensitivity to reveal fainter stars even closer to the black hole.
The team eventually hopes to identify stars that are close enough to feel the gravitational
effects of the black hole's rotation on their orbits.
ESO's future Extremely Large Telescope (ELT), which is currently under construction in Chile's
Atacama Desert, will further allow the team to measure the velocity of these stars with
even greater precision.
Scientists will be able to determine how fast the black hole spins by combining the GRAVITY+
and ELT's power.