X-ray: NASA/CXC/Penn State Univ./S. Dichiara; IR: NASA/ESA/STScI;
Illustration: ERC BHianca 2026 / Fortuna and Dichiara, CC BY-NC-SA 4.0; Image
Processing: NASA/CXC/SAO/P. Edmonds
A fleet of NASA missions has likely
uncovered a collision between two ultradense stars in a tiny galaxy buried in a
huge stream of gas. Astronomers have never seen this type of explosive event in
an environment like this before — and it may help solve two outstanding cosmic
mysteries. A paper describing these results was published today in The Astrophysical Journal Letters.
Neutron stars are the cores left behind
after a star much heavier than the Sun runs out of fuel, collapses on itself,
and then explodes. They are small (only a dozen or so miles across) but
slightly more massive than the Sun, making them amazingly dense. Astronomers
consider them to be some of the most extreme objects in the universe.
In recent years, astronomers have
collected data on collisions, or mergers, of two neutron stars inside of
moderately sized or large galaxies. This latest discovery, however, shows that
a neutron star collision may take place inside a tiny galaxy.
“Finding a neutron star collision where
we did is game changing,” said Simone Dichiara of Penn State University, who
led the study. “It may be the key to unlocking not one, but two important
questions in astrophysics.”
The first puzzle this unprecedented
location for a neutron star collision may explain may explain is the fact that
gamma-ray bursts (GRBs), which can be produced by the collapse of two neutron
stars, sometimes do not appear within the core of a galaxy, or any galaxy at
all.The other question this result could address is how elements like gold and
platinum have been found in stars located at large distances from the centers
of galaxies.
This neutron star collision is
unexpectedly located in a tiny galaxy, about 4.7 billion light-years away,
embedded within a stream of gas that stretches some 600,000 light-years long.
(For context, our Milky Way galaxy is about 100,000 light-years across.) This
stream was likely created when a group of galaxies collided hundreds of
millions of years ago, stripping gas and dust from the galaxies and leaving it
in intergalactic space.
“We found a collision within a
collision,” said co-author Eleonora Troja of the University of Rome in Italy.
“The galaxy collision triggered a wave of star formation that, over hundreds of
millions of years, led to the birth and eventual collision of these neutron
stars.”
To discover the event dubbed GRB
230906A, which occurred on 2023 September 6th, astronomers needed several NASA
telescopes including the Chandra X-ray Observatory, Fermi Gamma-ray Space
Telescope, Neil Gehrels Swift Observatory, and Hubble Space Telescope.
Fermi discovered the neutron star
collision by picking up the distinctive signal of a gamma-ray burst, or GRB,
explosion. After using the InterPlanetary Network to derive a preliminary
location for the Fermi source, astronomers then needed the sharp vision of
Chandra, Swift, and Hubble to more precisely pinpoint the location of the
object. NASA’s missions are part of a growing, worldwide network that watches
for these changes, to solve mysteries of how the universe works.
“Chandra’s pinpoint X-ray localization
made this study possible,” said co-author Brendan O’Connor, a McWilliams
Postdoctoral Fellow at Carnegie Mellon University. “Without it, we couldn’t
have tied the burst to any specific source. And once Chandra told us exactly
where to look, Hubble’s extraordinary sensitivity revealed the tiny, extremely
faint galaxy at that position. We were only able to make this discovery after
we put all the pieces together.”
This finding may explain why some GRBs
do not appear to have host galaxies. This result implies that some host
galaxies are too small and faint to be seen in most optical light images from
ground-based observatories.
The unusual location of GRB 230906A may
also help explain how astronomers have spotted elements like gold and platinum
in stars at relatively large distances from galaxies. Such stars are generally
expected to be older and to have formed from gas that had less time to be
enriched in heavy elements from supernova explosions.
Through a chain of nuclear reactions, a
collision between two neutron stars can produce heavy elements like gold and
platinum, which astronomers witnessed in a well-documented collision seen in
2017 . Events like GRB 230906A could generate elements like these and spread
them throughout the outskirts of galaxies, eventually appearing in future
generations of stars.
An alternative explanation for the
explosion is that it is located in a much more distant galaxy that is behind
the galaxy group. The team considers this to be a less likely explanation than
the tiny galaxy idea.
NASA’s Marshall Space Flight Center in
Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical
Observatory’s Chandra X-ray Center controls science operations from Cambridge,
Massachusetts, and flight operations from Burlington, Massachusetts.
Visual Description
This release features two artist’s
concepts and a composite image depicting two cosmic collisions that began
hundreds of millions of years ago.
At the center of the large artist’s
concept is a brilliant glowing ball with a nearly white core, and golden orange
outer layers. This brilliant ball represents the brightest galaxy in a
collision between two groups of galaxies, which began hundreds of millions of
years ago. Gas and dust from that collision were tossed into intergalactic
space in long tidal streams. In the illustration, the tidal streams resemble
swooping blue streaks shooting off the brilliant ball. Near the end of each
swooping tidal stream is a glowing orange streak, or ellipse. These glowing
shapes are smaller individual galaxies, some of which are revealed to have
spiraling arms when examined closely.
One of the tidal streams shoots
toward our upper left, then begins to hook back down, passing two glowing
orange galaxies along its path. Near the end of this tidal stream is a tiny
galaxy and an X-ray source presented in the middle of a close-up insert. In the
center of the composite insert, Hubble observations in orange reveal the tiny,
faint galaxy buried in the tidal stream. A pool of neon blue haze shows X-rays
detected by Chandra from the collision of two ultra-dense neutron stars.
Astronomers believe that the tiny
galaxy was born from gas and dust along the 600,000 light-year-long tidal
stream, created by the initial collision of the galaxy groups. Over hundreds of
millions of years, that material contributed to the birth of many stars within
the tiny galaxy. Two of those stars collapsed into neutron stars, and
ultimately collided, producing important elements like gold and platinum, and
gravitational waves that rippled across space.
The artist’s concept in the other insert shows a close-up view from the side of what the aftermath of a neutron star collision might look like. A burst of gamma rays was originally detected by viewing it down the barrel of the jet, which triggered follow-up X-ray observations with Chandra and other X-ray telescopes.
To learn more about Chandra, visit: https://science.nasa.gov/chandra
Source: NASA Discovers Crash of Extreme Stars in Unexpected Site - NASA
