Philippa Kaur

A solitary celestial object — more massive than the sun, yet far smaller — is wandering the galaxy a few thousand light-years from Earth. It might be the first isolated stellar-mass black hole to be detected in the Milky Way. Or it might be one of the heaviest neutron stars known.

The interstellar wanderer first revealed itself in 2011, when its gravity briefly magnified the light from a more distant star. But at the time, its true nature eluded researchers. Now, two teams of astronomers have analyzed Hubble Space Telescope images to unmask the traveler’s identity — and have come to somewhat different conclusions.

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The mysterious rogue is a black hole roughly seven times as massive as the sun, one team reports in a study in press in the Astrophysical Journal. Or it’s a bit lighter — a mere two to four times the weight of our nearest star — and therefore either an unusually lightweight black hole or a curiously hefty neutron star, another group reports in a study in press in the Astrophysical Journal Letters.  

Neutron stars and stellar-mass black holes form when massive stars — at least several times the heft of the sun — collapse under their own gravity at the end of their lives. Astronomers believe that about a billion neutron stars and roughly 100 million stellar-mass black holes lurk in our galaxy (SN: 8/18/17). But these objects aren’t easy to spot. Neutron stars are so tiny — about the size of a city — that they don’t produce much light. And black holes emit no light at all.

To detect these kinds of objects, scientists typically observe how they affect their surroundings. “The only way that we can find them is if they influence something else,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore.

To date, scientists have detected nearly two dozen stellar-mass black holes. (These relatively lightweight black holes are puny compared to the supermassive behemoths that sit at the center of most galaxies, including our own (SN: 1/18/21).) To do so, researchers have watched how these objects interact with their nearby celestial neighbors. When a black hole is locked in a gravitational dance with another star, it rips away matter from its partner. As that material falls onto the black hole, it emits X-rays, which telescopes orbiting the Earth can detect.

But finding black holes in binary systems doesn’t paint a whole picture of the black hole kingdom. Because these objects are continually accreting matter, it’s challenging to determine the mass at which they formed. And since birthweight is a key characteristic of a black hole, that’s a significant drawback to looking at binary systems, Sahu says. “If we want to understand the properties of black holes, it’s best to find isolated ones.”

For more than a decade, researchers have been scanning the heavens for solitary black holes. The searches have hinged on Einstein’s theory of general relativity, which states that any massive object, even an unseen one, bends space in its vicinity (SN: 2/3/21). That bending causes light from background stars to be magnified and distorted, a phenomenon known as gravitational microlensing. By measuring changes in the brightness and apparent position of stars, scientists can calculate the mass of the intervening object that’s acting like a lens — a technique that’s rounded up a few extrasolar planets as well (SN: 7/24/17).

In 2011, researchers announced that they had spotted a star that suddenly had gotten more than 200 times brighter. But those initial observations, made using telescopes in Chile and New Zealand, were unable to reveal whether the star’s apparent position was also changing. And that information is key to pinning down the mass of the intervening object. If it’s a heavyweight, its gravity would distort space so much that the star would appear to move. But even a “big” shift in the star’s position would have been extremely small and hard to detect. And unfortunately fine details in astronomical images captured by ground-based telescopes tend to be blurred out because of our planet’s turbulent atmosphere (SN: 7/29/20).

To circumvent this Earthly limitation, two independent teams of astronomers turned to the Hubble Space Telescope. This observatory can capture extremely detailed images since it orbits above most of Earth’s atmosphere.  

Both groups found that the star’s location shifted over the course of several years. One of the teams, led by Sahu, concluded that the star’s apparent dance was caused by an object roughly seven times as hefty as the sun. A star of that mass would have been blazingly bright in the Hubble images, but the researchers saw nothing. Something that heavy and dark must be a black hole, the team reports.

But another group of researchers, led by astronomer Casey Lam at the University of California, Berkeley, found different results. Lam and her colleagues calculated that the mass of the lensing object was lower, only about two to four times the mass of the sun. It could therefore be either a neutron star or a black hole, the group concluded.

Whatever it is, it’s an intriguing object, says astronomer Jessica Lu, a member of Lam’s team also at UC Berkeley. That’s because it’s a bit of an oddball in terms of mass. It’s either one of the most massive neutron stars discovered to date, or it’s one of the least massive black holes known, Lu says. “It falls within this strange region we call the mass gap.”

Despite the disagreement, these are thrilling results, says Will M. Farr, an astrophysicist at Stony Brook University in New York not involved in either study. “To be working at the instrumental limit at the real forefront of what’s measurable is very exciting.” More

1.6 billion stars. 11.4 million galaxies. 158,000 asteroids.

One spacecraft.

The European Space Agency’s Gaia space observatory, which launched in 2013, has long surpassed its goal of charting more than a billion stars in the Milky Way (SN: 10/15/16). On June 13, the mission extended that map into new dimensions, releasing more detailed measurements of hundreds of millions of stars, plus — for the first time — asteroids, galaxies and the dusty medium between stars.

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“Suddenly you have a flood of data,” says Laurent Eyer, an astrophysicist at the University of Geneva who has worked on Gaia for years. For some topics in astronomy, the new results effectively replace all the observations that were taken before, Eyer says. “The data is better. It’s amazing.”

Data in the new survey, which were collected from 2014 to 2017, are already leading to some discoveries — including the presence of surprisingly massive  “starquakes” on the surfaces of thousands of stars (SN: 8/2/19). But more than anything, the release is a new tool for astronomers, one that will aid their efforts to understand how stars, planets and entire galaxies form and evolve.

Here are a few of the long-standing puzzles the data could help solve. 

Asteroid mishmash

The asteroid belt between Mars and Jupiter is a mess of history. After the Earth and other planets formed, the rocky building blocks that were left over smashed into each other, leaving behind jumbled fragments. But if scientists know enough about individual asteroids, they can reconstruct when and where they came from (SN: 4/13/19). And that can provide a peek into the solar system’s earliest days.

Using new Gaia data, astronomers plotted the June 13, 2022, positions of 156,000 asteroids. The trails show their orbits for the last 10 days, and the colors mark different groups of asteroids based on their location (blue, inner solar system; green, the main asteroid belt between Mars and Jupiter; orange, the Trojan asteroids near Jupiter).DPAC/Gaia/ESA, CC BY-SA 3.0 IGO

Gaia’s massive new dataset may help solve this puzzle, says Federica Spoto, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. It includes data on the chemical makeup of over 60,000 asteroids — six times more than researchers had such details on before using other tools. That information can be essential for tracing asteroids back to their shattering origins.

“You can go back in time and try to understand all the formation and evolution of the solar system,” says Spoto, a Gaia collaborator. “That’s something huge that before Gaia we couldn’t even think about.” 

Asteroids aren’t just pieces of the past, though; they’re also dangerous. The new data could reveal asteroids that are next to impossible to spot from Earth because they orbit too close to the sun, says Thomas Burbine, a planetary scientist at Mount Holyoke College in South Hadley, Mass., who is not involved with the mission (SN: 2/15/20). Since these asteroids would have originally come from farther out (say, the asteroid belt), they can tell us about the rocks going past Earth that can potentially hit us. “We’ll know our neighborhood better,” Burbine says.

Dating a star

It is notoriously difficult to measure the age of stars (SN: 7/23/21). “It’s not uncommon to have uncertainty of more than a billion years,” says Alessandro Savino, an astrophysicist at the University of California, Berkeley who is not involved with Gaia. Unlike brightness or location, age is not directly visible. Astronomers have to rely on theories of how stars evolve to predict ages from what they can measure.

If past versions of the Gaia survey were like a photograph of stars, the new release is like shifting the photograph from black and white to color. It provides a deeper look at hundreds of millions of stars by measuring their temperature, gravity and chemistry. “You imagine the star as this point in space, but then they have so many properties,” Spoto says. “That’s what Gaia is giving you.”

Although these kinds of measurements are far from new, they have never been collected in the Milky Way on such a scale before. Those data could provide more insight into how stars evolve. “We can improve the resolution of our clocks,” Savino says. 

Milky Way snacks

Though it may seem unchanging, the Milky Way is actually gorging on a steady diet of smaller galaxies —it’s even in the process of eating one right now. But for decades, predictions of when and how these cosmic mergers happen have been at odds with evidence from our galaxy, says Bertrand Goldman, an astrophysicist at the International Space University in Strasbourg, France, who is not involved in the Gaia data release.  “That has been controversial for a long time,” Goldman says, “but I think that Gaia will certainly shed light.”

The key is to be able to pick apart different structures in the Milky Way and see how old they are (SN: 1/10/20). Gaia’s latest release helps in two ways: By mapping the chemistry of stars and by measuring their motion. Previous versions of the survey described how millions of stars were moving, but mostly in two dimensions. The new catalog quadruples the number of stars with full 3-D trajectories from 7 million to 33 million. 

This has implications beyond our neighborhood. Most of the mass in the universe is contained in galaxies like the Milky Way, so knowing how our own galaxy works goes a long way to understanding space on the largest scales. And the more scientists understand the parts of galaxies they can see, the more they can learn about dark matter, the mysterious substance that exerts gravity but doesn’t interact with light (SN: 6/25/21).

Even as astronomers mine this latest dataset, they are already looking ahead to future treasure hunts. The next round is years off, but it is expected to enable the discovery of thousands of exoplanets, produce rare measurements of black holes and help astronomers clock how fast the universe is expanding. In part, this is because Gaia is designed to track the motion of objects in space, and that gets easier as more time passes. So Gaia’s observations can only get more powerful. “Like good wine, they age very, very well,” Savino says. More