Astronomers have detected an extremely compact, massive object unlike any seen before near the core of the Milky Way galaxy. This mysterious object, located in the globular cluster NGC 1851 about 27,000 light years from Earth, is either the lightest black hole or heaviest neutron star ever found, presenting a cosmic puzzle for researchers.
Strange Object Defies Known Models for Neutron Stars and Black Holes
The unusual object was spotted by South Africa’s new MeerKAT radio telescope array using its recently commissioned MeerTRAP survey. MeerTRAP is performing a census of exotic objects emitting radio pulses, including exotic binary star systems containing neutron stars or black holes. Most neutron stars and stellar-mass black holes fall within predictable mass ranges based on models of dying stars. This strange new find, however, falls right between the expected maximum mass for neutron stars and minimum mass for black holes.
“The mass of the object is right at the upper boundary of what models predict for neutron stars and the lower boundary for black holes,” explained Dr. Mallory Smith, an astrophysicist at Manchester University involved in the discovery. “Depending on what it turns out to be, it will significantly change our understanding of the evolutionary process for neutron stars and black holes.”
|Typical Mass Range
|1.1 – 2.5 solar masses
|Stellar-Mass Black Hole
|At least 5 solar masses
At around 3 solar masses, the peculiar object’s mass falls in what researchers call the “mass gap” between theoretical limits for neutron stars and black holes. Determining whether it is an exotic neutron star or the lightest known black hole holds important clues about the life cycles of massive stars.
“This ‘Goldilocks’ object is either the heaviest known neutron star or the lightest known black hole,” said Dr. Smith. “In either case, it breaks records and challenges our assumptions about the end stages of stellar evolution.”
Is It a Super Dense Neutron Star or Tiny Black Hole?
Neutron stars represent one possible end state for large stars after supernova explosions. Their cores collapse from an initial mass around 10-29 solar masses down to a city-sized ball just 12-25 km wide, composed purely of densely packed neutrons. Any heavier and the neutron degeneracy pressure that holds up a neutron star supposedly can’t prevent further collapse into a black hole.
If the odd cosmic body turns out to be an incredibly dense neutron star, astronomers may have to revise models used to describe matter at extraordinarily high densities.
“This might be the first discovered neutron star that’s over the theoretical upper mass limit,” commented Prof. Michael Brown of Monash University, an expert on neutron stars. “That would shake the foundations of nuclear physics established from decades of theory and experiments.”
Alternatively, researchers speculate the anomalous object could be the smallest stellar-mass black hole yet found if too heavy for even neutron degeneracy pressure to resist gravitational collapse.
Stellar-mass black holes normally start around 5 solar masses. Any smaller and models indicate a dying star’s core shouldn’t be heavy enough to overcome neutron degeneracy, instead forming a neutron star. This oddball might therefore be the first observed “intermediate-mass” black hole smaller than the usual lower limit.
“Regardless of the final classification, this discovery has the potential to transform our understanding of the death throes of massive stars,” said Manchester University’s Dr. Mallory Smith.
Unusual Binary System Source of Mystery
The puzzling object was initially detected because of the radio pulses emanating from it as part of a bizarre binary system. It has a companion neutron star spinning at 11,000 rpm that beams out radio waves creating a lighthouse-like sweep of the surrounding space picked up by MeerKAT. Researchers realized the energy powering these bright pulses implied the neutron star has an extraordinarily massive nearby companion.
Binary systems with a neutron star closely orbiting a stellar-mass black hole are theoretically predicted but have never been conclusively identified before. The extreme gravitational forces and stellar material swirling close to the black hole usually obscures detection in most wavelengths. If the newly uncovered system contains such a pairing, it would finally confirm models predicting the existence of neutron star-black hole binaries. But the object could also end up being a novel type of neutron star binary never seen before.
“Whether the companion is a black hole or an entirely new species of neutron star, this system is forcing us to reconsider models of binary evolution,” explained Dr. Smith.
Follow up observations across different wavelengths are underway to definitively characterize the true nature of the object. Neutron stars and black holes have very distinct x-ray and infrared signatures that may enable unambiguous categorization.
“Further study of this system offers a rare opportunity to directly compare observations with our predictions for how stellar binaries containing neutron stars or black holes evolve,” said Prof. Brown. “In astronomy, these kinds of controlled experiments are very hard to come by.”
What Happens Next With the Perplexing Discovery?
Astronomers are working to gain additional data from new telescopic observations and astrophysical simulations that could reveal whether the unusual object lurking at the heart of NGC 1851 is an impossibly massive neutron star or improbably tiny black hole.
Over the next year, researchers will leverage capabilities like the VLT Survey Telescope’s near-infrared imaging in Chile and the XMM Newton orbiting x-ray observatory to analyze the mysterious compact object. Computer models incorporating complex general relativity and nuclear physics will also explore potential evolutionary pathways.
The strange cosmic body’s final classification as either the heaviest known neutron star or lightest known black hole holds profound implications for a wide range fundamental physics questions:
- How do massive stars end their life cycles?
- What are the true upper mass limits for neutron stars?
- Can neutron degeneracy pressure prevent collapse at such extreme densities?
- Do intermediate-mass black holes created from stellar death actually exist?
“While we likely won’t have definitive answers for a while, the tantalizing observations so far are already fueling innovative new theoretical and computational research,” said Dr. Smith.
Unraveling the puzzles surrounding this peculiar object orbiting perilously close to utter gravitational oblivion in a distant globular cluster promises to teach physicists fundamental new lessons about the nature of matter, the death of stars, and the properties of spacetime itself. The world eagerly awaits definitive word on whether an impossibly massive neutron star or impossibly tiny black hole has been found drifting through the Milky Way’s orbiting swarm of ancient suns.
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