Stellar black hole

• 1. Introduction

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  In September 2015, gravitational waves carried news of a collision across more than a billion light years of space. Two black holes had merged into one, a rotating object of 62 solar masses, and the event announced itself by warping spacetime itself. This is the realm of the stellar black hole, an object born from the gravitational collapse of a star. These black holes carry masses ranging from about 5 to several tens of solar masses. Some are the remnants of supernova explosions, but other formation mechanisms may also operate. How does a dying star cross the threshold into a black hole, and why are some masses apparently forbidden? How do astronomers spot something that emits no light, and what happens to the matter that falls in? The answers reach from the cores of collapsing giants to binary systems scattered across the Milky Way and far beyond it.

• 2. The Threshold Of Collapse

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  At the end of a massive star's life, every internal energy source runs out, and gravitational collapse becomes inevitable. What the star becomes depends entirely on the mass of the collapsing part. Below the Chandrasekhar limit, the result is a white dwarf. Above that but below the Tolman-Oppenheimer-Volkoff limit for neutron-degenerate matter, the star ends as a neutron star. Cross the TOV limit, and the crush continues without pause until a black hole forms. These boundaries are approximate, shifting with a star's chemical composition and spin. The exact maximum mass a neutron star can hold before collapsing further remains poorly understood. In 1939 it was estimated at 0.7 solar masses, the origin of the TOV limit itself. A later estimate in 1996 placed the upper bound somewhere between 1.5 and 3 solar masses. The most massive neutron star yet observed is PSR J0740+6620, discovered in September 2019, which sits near the edge of what neutron matter can sustain.

• 3. Three Numbers Define Them

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  By the no-hair theorem, a black hole can be described by only three fundamental properties: mass, electric charge, and angular momentum. Nothing else survives the collapse. The angular momentum of a stellar black hole comes directly from the conservation of angular momentum of the star or objects that produced it. The spinning of a dying star becomes the spinning of its remnant. In general relativity, a black hole could in principle exist at any mass at all. The lower the mass, the higher the density of matter required to form one. Yet there are no known stellar processes that can produce black holes lighter than a few times the mass of the Sun. If such tiny black holes exist anywhere, they are most likely primordial, dating to the early universe rather than to any star. Until 2016, the largest known stellar black hole measured 15.65 solar masses, and the smallest known sits in the binary system 2MASS J05215658+4359220, a mere 3.3 solar masses across a diameter of only 19.5 kilometers.

• 4. Seen Only In X-Rays

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  Several hundred million degrees: that is how hot matter becomes as it spirals from a companion star toward a black hole in a close binary system. The energy released in that fall is so enormous that the infalling material glows in X-rays. The black hole reveals itself through that radiation, while its companion star can be watched through optical telescopes. This is how astronomers identify them, by combining X-ray and optical data from compact sources. There is a complication. Black holes and neutron stars release energy of the same order of magnitude, which makes the two often difficult to tell apart. The dividing line comes from mass. Every identified neutron star carries a mass below 3.0 solar masses, and none of the compact systems above that threshold show neutron star properties. This makes it increasingly likely that compact stars heavier than 3.0 solar masses are genuinely black holes. The proof, though, is not purely observational. It leans on theory, because astronomers can imagine no other object that fits these massive compact binaries. A truly direct proof would require watching a particle or a cloud of gas actually fall into one.

• 5. Natal Kicks

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  Some black hole binaries sit at startling distances above the galactic plane, far from where stars are born. These heights are the result of black hole natal kicks, violent shoves delivered at the moment of formation. The velocity distribution of these kicks looks similar to that of neutron star kicks. That similarity is itself a puzzle. One might expect the momenta to match instead, which would give black holes lower velocities than neutron stars because of their greater mass. That does not seem to be the case. A possible explanation lies in the fall-back of asymmetrically expelled matter, which could add momentum to the resulting black hole and even out the velocities. The kicks help explain why some of these systems wander so far from home.

• 6. The Forbidden Masses

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  Two ranges of mass appear to be off limits to black holes formed directly by a single collapsing star. Some models of stellar evolution call them the lower and upper mass gaps, covering roughly 2 to 5 and 50 to 150 solar masses. The lower gap is suspected from a scarcity of observed candidates just above the maximum neutron star mass, though its existence and theoretical basis remain uncertain. Any black holes found there may have come from merging binary neutron stars rather than from stellar collapse. The LIGO/Virgo collaboration reported three candidate events in its O3 observing run with component masses falling in this lower gap. A bright, rapidly rotating giant star was also seen orbiting an unseen companion of 3.3 solar masses that emits no light at all, not even X-rays. The upper gap traces to pair-instability supernovae, where pair production of electrons and positrons in collisions between atomic nuclei and energetic gamma rays drops the pressure supporting the core. The star undergoes a runaway thermonuclear explosion and is blown completely apart, leaving no remnant behind. Such supernovae occur only in stars of around 130 to 250 solar masses with low to moderate metallicity, common in Population III stars. Pulsational mass loss can extend this gap down to about 45 solar masses. The LB-1 system was first read as a 70 solar mass black hole that the upper gap should forbid, but further investigation has weakened that claim.

• 7. A Catalogue Of The Unseen

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  The Milky Way holds several stellar-mass black hole candidates closer to us than the supermassive black hole at the galactic center. Most belong to X-ray binary systems, where the compact object draws matter from its partner through an accretion disk. The probable black holes in these pairs range from three to more than a dozen solar masses. Cygnus X-1, at 21.2 solar masses, ranks among the heaviest, lying some 6,000 to 8,000 light years away. GRS 1915+105 carries about 14 solar masses, and V404 Cygni roughly 12. A0620-00 sits at only 0.33 light years in its listed orbital separation, with a mass of 11. The disputed LB-1 system appears at 7 solar masses. Beyond the Milky Way, candidates come almost entirely from gravitational wave detections. GW150914 united black holes of 36 and 29 solar masses into a 62 solar mass remnant, about 1.3 billion light years away. GW190521 reached an estimated 155 solar masses. X-ray binaries in other galaxies add more, including M33 X-7 in the Triangulum Galaxy at 15.65 solar masses, 2.7 million light years off. Then there is N6946-BH1, a star in NGC 6946 that simply disappeared following a failed supernova, an event that may have produced a black hole without any explosion at all.