Stars that vanish from the sky may be collapsing directly into black holes without going supernova first, according to a new study of a bizarre binary star system.
**Stellar Evolution and Supernovae**
To understand the importance of this finding, let’s review stellar evolution. Stars generate energy through nuclear fusion in their cores, converting hydrogen into helium. When stars with at least eight times the mass of our sun exhaust their hydrogen supply, they start fusing heavier elements like helium, carbon, and oxygen. Eventually, they form an inert iron core that requires more energy to fuse than it can produce, leading to the cessation of fusion reactions. Without fusion, the star’s energy production stops, and gravity causes the core to collapse while the outer layers rebound and explode outward in a supernova, sometimes shining brighter than an entire galaxy.
The collapsing core forms a compact object, often a spinning neutron star (pulsar) or, under certain conditions, a stellar-mass black hole. This is the standard scenario. However, new evidence suggests some stars might form black holes without a supernova explosion.
**Evidence of Direct Collapse**
Researchers have observed occurrences of failed supernovas — stars that brighten as if about to explode but then fade away. The Vanishing and Appearing Objects During a Century of Observations (VASCO) project, led by Beatriz Villarroel, found stars on old photographic plates that have vanished without a trace. These failed supernovas and vanishing stars may indicate stars collapsing into black holes before exploding.
Alejandro Vigna-Gómez of the Max Planck Institute for Astrophysics noted that astronomers have observed the sudden disappearance of bright stars, suggesting they might collapse directly into black holes.
**The Case of VFTS 243**
Vigna-Gómez and his team studied a strange binary system, VFTS 243, in the Tarantula Nebula in the Large Magellanic Cloud. This system contains a 25-solar-mass star and a 10-solar-mass black hole. Despite the black hole’s formation, no explosion traces are found, as the orbits of the star and the black hole remain nearly circular. Supernova explosions are typically asymmetric, giving the compact object a “natal kick” that accelerates it, causing its orbit to become elongated. The black hole in VFTS 243 shows minimal kick, suggesting it formed without a supernova.
Natal kicks result from the ejection of debris, neutrino bursts, and gravitational waves during a supernova. Without a supernova, there is no debris, only neutrinos and gravitational waves, providing a smaller kick, as observed in VFTS 243.
**Implications and Consequences**
If some massive stars collapse directly into black holes, bypassing supernova explosions, this has significant implications. Supernovas are element factories, blasting elements like oxygen, carbon, and nitrogen into space and forming heavier elements in the explosion’s heat and energy. If some stars skip this process, they cannot contribute to element creation and recycling. This concept, if true, will need to be factored into models of element formation and distribution, helping us understand the chemical evolution of galaxies and the accumulation of elements necessary for planets like Earth.
