Stars can survive their partner's supernova transformation

A binary star system consisting of two stars: a dense neutron star (lower right) and a normal Sun-like star (upper left). The neutron star was formed in a supernova explosion, while the Sun-like star survived it.

When a massive star dies in a supernova explosion, it’s not good news for nearby planets and stars. Typically, the cataclysmic event destroys nearby worlds and sends companion stars flying into space. So astronomers were surprised to find 21 neutron stars — essentially the stellar cores left behind by supernova explosions — in binary systems with stars like the Sun.

A team led by Karim El-Badry of the California Institute of Technology discovered these cosmic oddities using observations from the European Space Agency's Gaia mission. Its astrometric data revealed “wobbles” in the orbits of the sun-like companions. The team then took spectral observations of the objects. In essence, this work helped to discover a new population of “dark” neutron stars that are still orbiting their sun-like partners. The challenge now, El-Badry says, is to explain why these unusual pairs exist.

“We still don't have a complete model of how these binaries form,” he says. “In principle, the progenitor neutron star would have to be huge and interact with a solar-type star late in its evolution.”

Astronomers have discovered 21 stars like our Sun orbiting neutron stars (formed in supernova explosions). The European Space Agency's Gaia mission discovered the wobble by monitoring the orbits of Sun-like stars (yellow dots) for three years. In this animation, the Sun-like stars are colored green, and the neutron stars (and their orbits) are colored purple.

Survive a supernova?

It seems counterintuitive to think that a star would survive such a close encounter. The process itself begins with the massive parent star expanding as it ages. This process pushes away the smaller star. Just before the supernova occurs, the dying star probably engulfs its companion for a time. Some theories suggest that the engulfment itself could destroy the smaller star. Others say that it affects the star, but does not destroy it entirely.

At some point, the core of the big star collapses when it runs out of fuel. All the other layers collapse onto the core. The resulting heat and pressure compress what's left of the core into a ball of neutrons. The outer layers then bounce off the core and fly out into space. This is the part we see as a supernova explosion. If the companion star still exists, it should be ejected from the system. But in these strange binary systems, that's not what happens. The neutron star and companion star stay together.

Now it's up to El-Badri's team to figure out why. “The discovery of these new systems shows that at least some binary stars survive these cataclysmic events, although models cannot yet fully explain how,” he said. In a paper announcing the discovery, the team also says they have not ruled out the possibility that the neutron stars could be supermassive white dwarfs or white dwarf binaries.

Search for neutron stars and their satellites

The Gaia mission aims to scan the sky and look for “wobbles” in the motions of more than a billion stars. The wobbles are caused by planets orbiting the stars. But they are also caused by the gravitational pull of nearby black holes, neutron stars, or more massive stars.

Neutron stars are massive balls of neutrons, about 20 kilometers (12 miles) across but denser than the Sun. They form when collapsing stellar layers crush the core of a supernova progenitor star. As the neutron star and its companion orbit their common center of mass, the neutron star's gravity tugs on its companion, causing it to wobble back and forth, creating a characteristic “wobble.” Gaia detected these wobbles, and scientists then used follow-up observations from several ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawaii, the La Silla Observatory in Chile, and the Whipple Observatory in Arizona. This gave them more information about the masses and orbits of hidden neutron stars.

The neutron stars orbited other stars like the Sun, and these orbits were quite close and tight. In such cases, the transfer of mass between the two companions makes the neutron star brighter in X-rays or radio waves. This is not the case for the 21 systems studied by El-Badry's team. The bodies in them are much further apart, in wider orbits. This limits the amount of material the neutron star can steal from its companion. As a result, these objects are dark and quiet. “These are the first neutron stars that have been discovered solely through their gravitational effects,” says El-Badry.

Animation of a binary star system containing a supernova-formed neutron star and a Sun-like companion.

Trace the path from a supernova to a binary system

So now astronomers have a population of neutron star/sun-like binaries to explain. The team will now work to figure out why these rare pairs still exist. “We estimate that about one in a million sun-like stars orbits a neutron star in a wide orbit,” El-Badry says.

He’s also interested in similar coincidences between dormant (and mostly invisible) black holes and sun-like stars. He knows of two of them, including one called Gaia BH1, which is just 1,600 light-years away. The fact that these strange pairs also exist opens up a host of questions. “We don’t know for sure how these black hole binaries formed,” says El-Badry. “There are clearly gaps in our models of binary star evolution. Finding more of these dark companions and comparing their population statistics to predictions from different models will help us understand how they form.”