Astronomers have discovered 21 “one-in-a-million” binary systems in which Sun-like stars orbit a hidden “dead star,” an extremely dense and compact object called a neutron star.
One of the neutron stars discovered in this set of binaries is one of the most massive dead stars of its type ever observed.
Although we are used to stars being singletons like our Sun, at least half of all stars of the Sun’s mass exist in binary systems. This proportion increases to 75% for more massive stars. Therefore, stellar remnants such as neutron stars and black holes (which are born from the death of giant stars) also exist in binary systems, both with other dead stars and with massive stars.
There is, however, one extremely rare binary configuration: a neutron star orbited by a Sun-like star. Using the European Space Agency’s (ESA) Gaia probe, astronomers have discovered a new population of these elusive binary systems.
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Neutron stars are often observed orbiting ordinary stars like the Sun, but at very close distances that cause these dead stars to feed on their companions like cosmic vampires. What makes this group of neutron and normal star binaries so special is that they are much further apart, which could challenge our understanding of how such systems are born.
“Binary evolution models predict that neutron stars and normal binary stars should be born primarily in tight orbits, with the neutron star and its companion nearly touching,” Kareem El-Badry, discovery team leader and a research scientist at the Harvard-Smithsonian Center for Astrophysics (CfA), told Space.com. “These binaries are much wider than that, with separations of about 300 to 1,000 times the size of the stars.”
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“This means that alternative models of neutron star formation and normal stars may be needed.”
gif diagram showing the orbit of neutron stars around 21 Sun-like stars
A GIF shows the orbit of neutron stars around 21 Sun-like stars discovered by Europe’s Gaia mission. (Photo credit: Caltech/Kareem El-Badry)
These neutron stars resist the urge to feed on their stellar companion
Neutron stars are born when stars at least eight times the mass of the Sun exhaust their fuel for nuclear fusion, ending the outward pressure that supports them against the inward pressure of their own gravity.
As the cores of these stars collapse, their outer layers are blown away by massive supernova explosions, resulting in a stellar core with a mass between one and two times that of the Sun and a width of about 20 kilometers.
One of the newly discovered neutron stars falls just outside this theoretical mass range, at 1.9 times the mass of the Sun, making it one of the most massive neutron stars ever observed.
What is really significant about these binaries is that they raise the question of how they survived the transformation from a star to a neutron star.
In the process, the massive star should have violently attacked its smaller stellar companion, probably even engulfing it, even temporarily. When the future neutron star exploded as a supernova, current models suggest that the smaller star should have been “driven out,” ending the existence of the binary.
“The discovery of these new systems shows that at least some binaries survive these cataclysmic processes, even if models cannot yet fully explain how,” El-Badry said.
A glowing white and blue sphere surrounded by an orange smoke ring next to a much larger orange sphere
An illustration shows a “vampire” neutron star feeding on a nearby companion star. (Image credit: Robert Lea (created with Canva))
When neutron stars have been found orbiting “living” main-sequence stars, they tend to rip matter away from those stars like cosmic vampires. These binaries are remarkable because the neutron star and the main-sequence star are close enough together to facilitate the transfer of matter.
This material has angular momentum, so it cannot fall directly into the neutron star, but forms a flattened, swirling cloud called an “accretion disk.” Although the material is gradually funneled from the accretion disk toward the neutron star, the result when it reaches the surface is anything but gentle. This cannibalistic feeding process can release as much energy in one second as it takes the Sun to radiate in a million years.
This emission of energy, much of it pouring out in powerful, high-speed jets, makes these neutron-donor binaries very visible to astronomers, particularly in X-ray and radio wavelengths. In the absence of such emission across the entire electromagnetic spectrum, more distant binaries composed of quiet, dim neutron stars are harder to identify.
“Most neutron binary stars are discovered using X-ray or radio data from current or past accretion,” El-Badry explained. “These neutron stars have not accreted anything and do not produce X-ray or radio data at a detectable level, so they are harder to find.”
Related: ‘Star vampires’ could feed on stars hidden in their systems
A top hat shaped spaceship next to a glowing orange sphere and a bright blue sphere
An illustration shows the Gaia space telescope observing a binary star system. (Photo credit: SALT/ESA)
Gaia is able to make discoveries like this because it can precisely measure the positions and motions of billions of stars in the background sky. Precisely tracking stellar motion reveals the gravitational influences exerted on these stars, which are the visible components of these binaries, by their dark neutron star companions, even though the two objects in the duo are widely separated.
This influence is manifested by a slight oscillation in the star’s motion caused by the attraction exerted by the accompanying neutron star. This is the first time that neutron stars have been detected solely by their gravitational influence.
“Gaia can detect the very small shifts of solar-like stars on the plane of the sky, measuring their position with a precision comparable to the width of a human hair observed from 3,000 miles away. [5,000 kilometers] “This is the only astronomical facility that can do this currently,” El-Badry said.
In fact, Gaia is more sensitive to the wider orbits and longer orbital periods of these systems. El-Badry explained that if these neutron stars were closer to their companion stars, the wobble they cause would have been too weak for ESA’s spacecraft to detect.
Gaia was also helped by the fact that these binary stars are close to Earth, at a distance of just about 3,000 light-years. That may seem like a huge distance, but compared to the Milky Way’s 100,000 light-year width, it’s relatively small.
While the detection of 21 such binary systems is a major event, El-Badry said it is unlikely to change thinking about how rare large binaries of neutron stars and normal stars are in the cosmos.
“We estimate that about one in a million solar-type stars orbit a neutron star in a wide orbit,” he explained. “But we weren’t surprised to find them! We were hoping that a population like this might be detectable with Gaia, and characterizing these objects has been one of the main projects of my research group over the past two years.”
El-Badry explained that the next step in this research is for the team to create models to explore the formation and future evolution of neutron star-normal star binaries.
The CfA researcher also plans to use Gaia to search for wide binaries with a normal star and a non-feeder black hole.
“We also don’t know for sure how these black hole binaries formed,” El-Badry concludes. “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.”
Gaia has already demonstrated its ability to detect silent, dark black holes.
In April 2023, Gaia detected two black holes, called Gaia BH1 and Gaia BH2, located 1,560 and 3,800 light-years from Earth, respectively. This year, the space telescope discovered Gaia BH3, just 2,000 light-years from Earth. These are the three closest black holes to our planet ever detected.
The team’s research is published July 15 in the Open Journal of Astrophysics.
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