YSM Issue 96.2

FEATURE
Materials Engineering
Astrophysics
FEATURE
THE
IMPOSSIBLE
ART BY YUROU LIU
STAR
A NEWBORN STAR LIVING ON
THE CUSP OF A BLACK HOLE
BY ELIZABETH WATSON
Most of the stars we see in the night sky are billions of
years old, their light only just now reaching us from
light-years away. But what lies beyond what we can see?
The reach of the universe extends far beyond the stars that we’re
able to observe with the naked eye.
A team led by Florian Peißker, a postdoctoral researcher at the
University of Cologne’s Institute of Astrophysics in Germany,
recently discovered a newborn star named X3 whose existence
defies all odds. Dubbed “the impossible star,” X3 is located over
twenty-five thousand light-years away and is currently undergoing
early stages of stellar formation in the vicinity of Sagittarius A*, the
supermassive black hole at the heart of our galaxy. Star formation
so close to a black hole was thought to be theoretically impossible,
but X3 persists all the same.
“I enjoy thinking about the opportunity to witness processes
nobody else has seen before,” Peißker said. The paper, published in
The Astrophysical Journal, is the product of two and a half years of
work and explores how X3 was able to form in spite of Sagittarius A*.
Star formation typically requires two conditions: relatively low
temperatures and high gas density, neither of which holds true for
the environments created by black holes. The area that Sagittarius A*
occupies, known as the Galactic Center, is extremely hot and volatile.
“This source should not exist in the first place because of the
harsh environment of the supermassive black hole Sagittarius
A*,” Peißker said. “The fact that we observe such a young object
so close to Sgr A* implies that this is not the only [such object].
It furthermore shows that star formation can occur, although the
classical criteria are not fulfilled.”
Previous research in the field identified clumps of silicon
monoxide (SiO) gas near Sagittarius A* that may have been dense
enough to permit high-mass star formation. A study in 2014
suggested that these clumps originated from the Circumnuclear
Disk (CND), a ring of molecular gas that surrounds Sagittarius A*.
It was proposed that some SiO clumps found within the CND either
had high enough velocity gradients or had experienced a sufficient
decrease in angular momentum to spiral closer to Sagittarius A*
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than would be otherwise possible. This process, called molecular
cloud inspiraling, was thought to be part of what could foster
stellar formation so close to a black hole.
The team behind Peißker’s study sought to build upon this work.
They compiled data on X3 spanning three decades from four
different telescopes, including the Very Large Telescope in Chile, to
better map out the X3 system and its surroundings. The team divided
the X3 system into three components—designating the young stellar
object as X3a and two neighboring thermal blobs as X3b and X3c—
and collected data about nearby stars and gas clusters. The analysis
helped the team confirm the star’s proximity to Sagittarius A* and
better understand its origins by examining its characteristics.
In addition to the accretion of the SiO gas clumps discussed in
previous research, the team believes that rotating regions of dust
and gas in the Galactic Center, called stellar disks, may also have
been key to X3’s formation. The thickness of these disks would have
been sufficient to lower the temperature within the region for star
formation to be feasible, while simultaneously protecting the area
from the black hole’s radiation.
As they rotate, these stellar disks become dense enough to create
massive gas clusters conducive to high-mass stellar formation. The
team believes that one of these clusters, IRS 13, was instrumental
to the formation of the X3 system. Based on the team’s data points,
the timeline for this formation theory aligns with our current
understanding of this region’s stellar history.
Peißker was excited upon confirming X3’s proximity to Sagittarius
A*, but joked that the process of discovery was far more prolonged
than an ordinary surprise reaction. “For six months, I ran day-in and
day-out simulations to fit the data points,” Peißker said. “Back then,
my daughter demanded milk almost every three hours each night.
So I woke up, gave baby milk to my daughter, and started simulations
with the new parameters. I did this around the clock.”
Peißker hopes to build upon this work in the future to learn
more about how the mechanisms of stellar formation respond to
unconventional situations, which could lay the groundwork for a
richer understanding of star evolution and our universe at large. ■
May 2023 Yale Scientific Magazine 27