An invisible cosmic behemoth might be tearing apart the closest star cluster to the Sun, leaving one side of the cluster eerily dark and devoid of stars, according to a new study.
The culprit may be a dark matter substructure, a relic that contains the mass of 10 million Suns and is made of a mysterious non-luminous substance. The possible presence of this “Galactic lump” was detected in a new map that charts out the enormous extent of the Hyades star cluster, located only 153 light years from Earth, which was published on Wednesday in the journal Astronomy & Astrophysics.
Scientists led by Tereza Jerabkova, a research fellow at the European Space Agency (ESA), came across the unnerving lump while examining the Hyades cluster using data collected by ESA’s Gaia satellite.
“This is the amazing thing about the data from the Gaia satellite—we have the chance, for the first time in history, to search for stellar structures that are hiding in the huge amount of field stars in the galaxy,” Jerabkova said in an email.
“It is a unique time for an astronomer: when looking into the data and comparing the finding with theoretical models, it definitely raises one's heart beat with huge excitement!” she continued. “I mean, we look up at the stars that seem to be ‘just points’ on the dark sky and it is incredible how much we can learn about the universe and from them.”
While Gaia has been able to resolve features of the Hyades cluster in unprecedented detail, the bright central region of this stellar group, which spans about 20 light years, is visible even to the naked eye. You can look for some of its most radiant stars in the V-shape at the head of the constellation Taurus. The cluster dates back some 700 million years and has changed significantly, as stars become unbound due to both interior cluster dynamics as well as gravitational forces from the larger Milky Way galaxy.
These outside forces that tug at the cluster have, over the eons, sculpted two structures known as “tidal tails” that sweep out in front and behind the central hub of stars. These tails have long been observed in large stellar populations, but Hyades is the first “open” star cluster—a much smaller and younger version of these groups—that scientists have been able to pinpoint tails on. That breakthrough was published by a different team in 2019, and also relies on Gaia’s advanced surveying power.
Now, according to the new study, it looks like something is ripping apart one of those tails. Something we can’t see. Something big.
Jerabkova and her colleagues noticed this presence using the most recent Gaia data-dump in December 2020, which enabled the team to identify far-flung stars that originated within the Hyades.
The researchers first produced a simulation of the cluster that predicted the current positions and velocities of stars that might have drifted out of it over time. Because Gaia’s goal is to catalog the movement and distance of every observable star in the Milky Way, the team was then able to compare the simulation to the real data and spot the stars with trajectories and motions that matched a Hyades origin. This approach extended the known range of the two tails to an astonishing breadth of several thousand light years each.
But while the simulated map of the tails predicted that they would be relatively symmetrical, the real observations showed that the trailing tail was comparatively unpopulated with stars, an asymmetry that had also been noted by the 2019 study of the cluster.
The team is the first to suggest that “a close encounter with a massive Galactic lump can explain the observed asymmetry in the tidal tails of the Hyades,” according to the study. Based on the observations, this lump would have to be incredibly massive and elusively hidden, because there is no sign of a visible gas cloud or star cluster that might be tugging stars off the trailing tail.
“We see that stars that belong to the nearest star cluster are moving in a way they should not be moving if we apply our known and widely used models,” Jerabkova said. “Either these models are wrong and this would have big implications for physics, or the motions are changed due to a dark matter lump, and this would also be an important discovery.”
To that point, the team proposes that the lurking lump may be a dark matter substructure, also known as a sub-halo. These clumps emerge in the early years of galactic formation and drift across galaxies thereafter. As the name suggests, they are made of dark matter, a non-luminous material that is far more abundant in the universe than the regular matter that makes up stars and planets. Scientists only know about dark matter because of its gravitational effects on luminous objects—potentially including, in this case, the Hyades cluster.
The missing stars aren’t being gobbled up, as they might be by a black hole, Jerabkova said. Rather “the orbits of the stars in the Galaxy are being affected/changed by the encounter” which may cause them to disappear from view because of the “disruption of the cluster and the tails,” she added.
These sub-haloes are like smaller versions of galactic dark matter haloes, which are gargantuan structures that account for about 84 percent of the total mass of galaxies. The Milky Way’s galactic halo, for instance, is estimated to be more than a trillion times more massive than the Sun, far bigger than the 10-million-Sun mass of the entity that might be causing the asymmetry in the Hyades’ trailing tail.
If you’re wondering whether our own solar system might end up wandering too close to one of these haloes, you shouldn’t sweat it.
“Such an encounter is basically impossible—simply because the solar system is very small in comparison to the Hyades and its tails and thus the chances of a close meet-up with any massive lump (molecular cloud or dark matter) are extremely small,” Jerabkova said.
Stars that are bundled into star clusters might swap planets, or disrupt the orbits of planets in neighboring systems, she added, but we also shouldn’t worry about that outcome because the Sun is a lonely star that left its natal cluster long ago.
It’s thrilling to imagine that scientists may have stumbled across such a huge and poorly understood dark matter monster, let alone one that is casually plucking stars off of the Hyades cluster. But to get a better idea of what’s really going on in this cluster tail, we’ll have to wait for future Gaia data releases, among other observational advances. This data will not only help to resolve the mystery of the missing stars in the Hyades, but could yield insights into other stellar and galactic enigmas as well.
“Tidal tails are actually great connective links between small scales (individual star clusters) and the galaxy as a whole,” Jerabkova said. “The tidal tails are life books of each star cluster as every star from the tail was born in the cluster and left the cluster at different times.”
“The stars thus carry lots of information about the cluster’s birth conditions and its evolution,” she concluded. “On top of this, the shape and structure of the tail depends on the mass distribution of the galaxy and thus we can even constrain galactic potential by studying tidal tails!”