Most of the stars in the Milky Way orbit silently and methodically around the galactic center, but not all of them: Every now and then, a maverick breaks ranks, hurtling along at such speeds that he or she is eventually pushed out into intergalactic space.
These “Lightning fast“Stars are extremely rare, but we’ve found a particularly special example. The star, called CWISE J124909+362116.0 (J1249+36 for short), not only exceeds the galactic escape velocity of about 600 kilometers (373 miles) per second, but it is also a very rare type of small, ancient main-sequence star, which is extremely rare. L SubdwarfIt is also one of the oldest galaxies in the Milky Way.
J1249+36 was first discovered by a citizen scientist scouring telescope data for signs of the mysterious Planet Nine. It’s one of the few hypervelocity stars confirmed in the Milky Way. While it’s far from the fastest ever observed, it poses something of a conundrum for astronomers: how does it get to such breathtaking speeds?
The findings were presented at the 244th meeting of the American Astronomical Society and a paper was recently published in Astrophysical Journal Letters.
There are several possible explanations for the star’s speed, and the researchers investigated three of them.
The first is from binary star systems that contain white dwarfs – the cores left behind when a sun-like star runs out of hydrogen, expels most of its outer material, dies, and enters the afterlife. Super-dense white dwarfs glow hot from residual heat rather than nuclear fusion, and can be a bit unstable if they have a binary companion.
When two stars orbit close together, a white dwarf can steal material from its companion. The problem is that there is an upper limit to how much mass a white dwarf can have: it only needs to gain a small amount of mass before it can sustain its existence by repeatedly exploding. Rising StarHowever, if the white dwarf becomes too massive, it will explode as a Type Ia supernova and be completely destroyed.
“In this type of supernova, the white dwarf is completely destroyed, and the companion star is freed to fly away at its original orbital velocity, plus there’s a small jolt from the supernova explosion.” Astrophysicist Adam Burgasser says: of the University of California, San Diego.
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“Our calculations show that this scenario holds true. However, because the white dwarf no longer exists and the remnants of the explosion that probably occurred millions of years ago have already dissipated, we have no conclusive evidence that this is its origin.”
The second possibility is that a many-body interaction becomes unstable and causes one of the objects on the other side of the galaxy to collide. Within our Milky Way galaxy, there is an environment that is favorable for such interactions to occur: globular clusters are dense masses containing millions of stars. They are thought to have a swarm of black holes at their centers, and contain a higher-than-usual density of binary black-hole pairs.
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“When a star encounters a black hole binary, the complex dynamics of this three-body interaction can cause the star to be ejected from the globular cluster.” “It’s a very powerful force,” said Kyle Kremer, an astrophysicist at the California Institute of Technology.who will soon be attending the University of California, San Diego.
This seems plausible, but by tracing the star’s orbit backwards, the researchers have yet to pinpoint the specific globular cluster from which it began.
A third possibility is that J1249+36 isn’t part of the Milky Way, but one of many satellite dwarf galaxies orbiting it. A 2017 study into the origins of hypervelocity stars found that an extragalactic origin was likely, and the researchers’ calculations suggest that this is also possible for J1249+36.
All three options are under consideration. The best way to answer this is to look more closely at the star’s chemical composition. If J1249+36 is a companion to a white dwarf, trace elements left behind by the supernova could be contaminating the L-type subdwarf’s atmosphere. Conversely, it may be possible to link the star to its parent star group in a globular cluster, as these clusters all contain stars with similar compositional properties.
And if neither of those work, we may need to turn to the Milky Way’s moons to determine if this tiny, faint star is a stranger from outside our galaxy that’s simply winking hello as it passes by.
This discovery 224th Meeting of the American Astronomical Society.