The Boundary Between Black Holes & Neutron Stars

A new gravitational-wave detection by LIGO and Virgo has opened a fresh window on the universe by revealing an object caught in the mysterious mass gap between neutron stars and black holes. The event, detected in August 2019, involved a 23-solar-mass black hole merging with a 2.6-solar-mass companion, a mass that is too heavy for a typical neutron star yet far lighter than any known black hole. No electromagnetic flash accompanied the merger, leaving astronomers unable to determine whether the smaller body was an unusually massive neutron star or an unexpectedly lightweight black hole. This uncertainty is significant because the maximum possible neutron star mass is thought to be around 2.2 to 2.4 solar masses, while the smallest known black holes are roughly five solar masses. Confirming the object's true nature could force a rewrite of stellar evolution models or reveal the extreme states of matter inside neutron-star cores, making this event one of the most informative and surprising discoveries in gravitational-wave astronomy.

• ▶ 0:00 The episode highlights a new gravitational wave detection that opens a new window on the universe, featuring an object on the boundary between neutron stars and black holes.
• ▶ 0:24 Gravitational-wave astronomy has become routine with LIGO and VIRGO detecting many events, but the latest event is one of the most informative and possibly most surprising so far.
• ▶ 0:59 The event involved a 23 solar-mass black hole merging with a 2.6 solar-mass companion—a mass that is both near the upper limit for a neutron star and lighter than expected for a black hole, leaving the object's true identity an open question.
• ▶ 2:31 On August 14, 2019, a gravitational wave was detected by LIGO and VIRGO, revealing the merging objects' masses as 23.2 and 2.59 solar masses — a "weird" combination that the episode will revisit.
• ▶ 3:14 Telescopes scanned the sky for an electromagnetic counterpart to the merger, but no flash of light was found, unlike the 2017 neutron star collision.
• ▶ 3:58 The absence of light was explained by the event being 6 times farther away than the 2017 merger, or because there was no explosion at all — meaning both objects were black holes, or a neutron star was swallowed whole without a peep.

• ▶ 4:30 The central mystery is what exactly the smaller body detected in the gravitational wave signal was, and why its mass is so exciting to physicists.

• ▶ 5:43 Neutron stars are described as being on the verge of becoming black holes: if you could cram more matter into one, its escape velocity would rise and cross the threshold to become a black hole.

• ▶ 6:36 As a neutron star gains mass, its surface gravity and escape velocity increase, while its actual surface shrinks—when it meets the growing "phantom event horizon," it becomes a black hole, setting a maximum possible neutron star mass.

• ▶ 7:11 The exact maximum mass of a neutron star before it collapses into a black hole is uncertain because the exotic states of quark matter inside its core are extremely difficult to calculate.
• ▶ 8:14 Observations from the 2017 neutron star merger and gamma-ray bursts suggest the maximum neutron star mass is between 2.2 and 2.4 solar masses, while the most massive pulsar measured is about 2.1 solar masses.
• ▶ 9:13 A newly found 2.6 solar-mass object is significant because it sits right at the theoretical mass limit, but it might not be a neutron star at all.
• ▶ 9:27 The 2.6 solar-mass object cannot easily be a black hole because no black hole that small has ever been observed; the smallest known black holes are around 5 solar masses.
• ▶ 9:48 A mass gap between neutron stars and black holes is actually predicted by stellar evolution models, which explain why star death produces a minimum black hole mass of roughly 5 solar masses.
• ▶ 10:36 Confirming the object is not a neutron star would force reworked stellar death models or require an alternative process for making tiny black holes — while if it is a neutron star, it reveals extreme states of matter.
• ▶ 11:22 Matt gives deep thanks to all viewers for their ongoing support, noting subscriptions and weekly tuning help keep the show going.
• ▶ 11:29 He thanks Patreon contributors, highlighting that their support is especially meaningful since content is free, and reminds them about the active Discord for all tiers.
• ▶ 11:43 He gives an extra special shout-out to Ahmad Jodeh at the Big Bang level, joking that their gift is a "brand new 2.6 solar mass black hole" that will arrive in a few million years.
• ▶ 12:35 Dissolving an event horizon by adding charge gets harder, but it never becomes truly impossible if you can supply enough energy—though highly charged black holes are unlikely in nature.
• ▶ 13:03 Crossing the horizon just before Hawking evaporation wouldn't let you pop back out; you'd fall in from your perspective, while outside observers would see you smeared and eventually re-emitted as scrambled Hawking radiation—so you never report what you saw.
• ▶ 13:54 Matter and antimatter couldn't have simply separated into different regions or galaxies; pairs are created close together and smoothly mixed, with no separation mechanism—and any antimatter region would produce an explosive annihilation "great wall" at the boundary.

• ▶ 0:00 Introduction and Journal Club Setup (0:00 - 1:59) - - Opening with gravitational-wave astronomy, routine detections, and the journal-club study of the latest event.
• ▶ 2:00 Detecting the August 2019 Merger (2:00 - 4:30) - - Recapping how LIGO/VIRGO work, localizing the event, and finding no electromagnetic counterpart.
• ▶ 4:30 The Neutron Star Mystery (4:30 - 7:11) - - Exploring whether the small companion was a neutron star and how quantum effects set a maximum mass.
• ▶ 7:11 Neutron Star Mass Limits (7:11 - 9:27) - - Using mergers, gamma-ray bursts, and pulsar measurements to constrain the maximum neutron star mass.
• ▶ 9:27 The Black Hole Mass Gap (9:27 - 11:22) - - Explaining the puzzling lack of black holes between about 2 and 5 solar masses.
• ▶ 11:22 Thanks and Supporter Shout-outs (11:22 - 12:17) - - Acknowledging viewers and supporters.
• ▶ 12:18 Comments: Event Horizons and Antimatter (12:18 - 14:45) - - Answering viewer questions on dissolving event horizons, crossing horizons, and antimatter asymmetry.