The life cycle of a star is usually described as being born in huge clouds of gas and dust, and then ending up as either a planetary nebula or a supernova explosion. However, over the past 70 years, a number of massive stars seem to simply disappear! According to models of stellar evolution, they should explode as supernovas, but instead they seem to just disappear. A team of researchers has studied the behavior of the star VFTS 243 – a main sequence star with a black hole companion – and now believe that, like the others, it simply collapsed and imploded into a black hole!
During the lifetime of a star, the inward-pulling force of gravity is balanced by the outward-pushing thermonuclear force (the result of fusion in the core). Once the core is rich in iron, as is the case in massive stars about eight times as massive as the Sun, the fusion process stops, as does the thermonuclear force. With the end of the force, the core collapses, the outer layers fall onto the core and bounce back out in a massive explosion called a supernova. The actual mechanism of the explosion and the formation of the compact object left behind by the core is still the subject of much debate.
The supernova process is one of the most powerful explosions in the universe. As the star collapses, a shock wave is created that can trigger fusion in the outer shell of the predecessor star. The reactions can create new elements that are heavier than iron. In a recently published paper by an international team of astronomers led by Alejandro Vigna-Gómez of the Max Planck Institute for Astrophysics in Germany, the team has shed new light on the process. They showed that a star can be so massive that its enormous gravity can be so strong that even a supernova explosion cannot take place.
The Fred Lawrence Whipple Observatory's 48-inch telescope captured this visible-light image of the Pinwheel Galaxy (Messier 101) in June 2023. The location of supernova 2023ixf is circled. The observatory, located on Mount Hopkins in Arizona, is operated by the Center for Astrophysics | Harvard & Smithsonian. Hiramatsu et al. 2023/Sebastian Gomez (STScI)
The team's discovery appears to be related to the concept of vanishing stars. In recent years, it has become clear that some stars simply appear to disappear from view without going through the planetary nebula phase or ending up as a supernova. The discovery of supermassive stars that collapse completely without a supernova now provides a good explanation for the phenomenon.
The team came to this conclusion while studying an object called VFTS 243, a binary star system that contains a star estimated to be 25 times more massive than the Sun and a black hole 10 times more massive than the Sun. Both objects orbit a common center of mass for 10.4 days and are located in the Tarantula Nebula in the Large Magellanic Cloud – 160,000 light-years away. The binary star system is not the first of its kind to be discovered; such systems have been known for decades.
30 Doradus, also known as the Tarantula Nebula, is a region in the Large Magellanic Cloud. Streamlines show the magnetic field morphology from the SOFIA HAWC+ polarization maps, superimposed on a composite image taken by the European Southern Observatory's Very Large Telescope and the Visible and Infrared Survey Telescope for Astronomy. Image credits: Background: ESO, M.-R. Cioni/VISTA Magellanic Cloud Survey. Acknowledgements: Cambridge Astronomical Survey Unit. Streamlines: NASA/SOFIA
Studying the system revealed that the orbit was nearly circular. Since one of the stars had collapsed into a black hole, the nearly circular orbit and the lack of any evidence of an explosion suggest a star that completely collapsed. The complete collapse meant that all of the star's matter collapsed into the black hole, and no material escaped as a supernova. So could the team have finally uncovered the mechanism by which massive stars disappear? It certainly looks like it, but more observations of binary star systems with stars and black holes are needed to confirm this.
Source: Constraints on neutrino natal kicks from the black hole binary system VFTS 243
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