\n<\/p>\n
When a massive star dies in a supernova explosion, it\u2019s not great news for any planets or stars that happen to be nearby. Generally, the catastrophic event crisps nearby worlds and sends companion stars careening through space. So, astronomers were pretty surprised to find 21 neutron stars\u2014the crushed stellar cores left over after supernova explosions\u2014orbiting in binary systems with Sun-like stars.<\/p>\n
<\/p>\n A team led by Caltech\u2019s Kareem El-Badry detected these cosmic oddities using observations made by the European Space Agency\u2019s Gaia mission. Its astrometrical data revealed \u201cwobbles\u201d in the orbits of the Sun-like companions. The team then followed up with spectral observations of the objects. Essentially, this work helped uncover a new population of what the team terms \u201cdark\u201d neutron stars still in orbital dances with their sunlike partners. Now the trick is to explain why these unusual pairs exist, according to El-Badry.<\/p>\n \u201cWe still do not have a complete model for how these binaries form,\u201d he said. \u201cIn principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution.\u201d<\/p>\n It seems counterintuitive to think of the nearby star surviving the nearby catastrophe. The process itself begins as the massive progenitor star ages and expands. That pushes the smaller star around. Just before the supernova occurred, the dying star probably engulfed the companion for a time. Some theories suggest that the engulfment itself could destroy the smaller star. Others say that it affects the star but doesn\u2019t completely obliterate it.<\/p>\n At some point, the larger star\u2019s core collapses when it runs out of fuel. All the other layers come crashing down on the core. The temperatures and pressures in the event compress what\u2019s left of the core into a ball of neutrons. Then, the outer layers rebound off the core and blast out into space. That\u2019s the part we see as the supernova explosion. The outburst should eject it from the system if there\u2019s still a companion star. However, for these strange binaries, that didn\u2019t happen. The neutron star and a companion remain. <\/p>\n Now it\u2019s El-Badry\u2019s team task to figure out why. \u201cThe discovery of these new systems shows that at least some binaries survive these cataclysmic processes even though models cannot yet fully explain how,\u201d he said. In a paper about the finding, the team also suggests that they cannot rule out that the neutron stars may be ultramassive white dwarfs or white dwarf binaries.<\/p>\n The Gaia mission aims to scan the sky and look for \u201cwobbles\u201d in the motions of more than a billion stars. The orbits of planets around the stars cause wobbles. However, the gravitational tug of nearby black holes, neutron stars, or more massive stars also induces them.<\/p>\n Neutron stars are massive balls of neutrons about 20 km across but denser than the Sun. They\u2019re created as the collapsing stellar layers crush the core of the supernova progenitor star. As the neutron star and its companion orbit around a common center of mass, the neutron star tugs on its companion and that makes it shift back and forth\u2014creating the telltale \u201cwobble\u201d. Gaia detected those wobbles, and then scientists used data from follow-up observations at several ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawai\u2018i; La Silla Observatory in Chile; and the Whipple Observatory in Arizona. That gave them more information about the masses and orbits of the hidden neutron stars.<\/p>\n Now, there have been neutron stars in orbit with other Sun-like stars, those orbits have been pretty tight and close-in. In those cases, the mass transfer between the two companions makes the neutron star brighter in X-ray or radio wavelengths. That\u2019s not true for the 21 systems El-Badry\u2019s team studied. They are much farther apart in wider orbits. This limits how much material the neutron star can steal from its companion. As a result, those objects are dark and quiet. \u201cThese are the first neutron stars discovered purely due to their gravitational effects,\u201d El-Badry said.<\/p>\n \nSurviving a Supernova?<\/h3>\n
The Search for Neutron Stars and their Companions<\/h3>\n