If you were attacked by a ravenous vampire star or were in danger of falling into two dueling black holes, you’d probably run too!
One of these terrifying scenarios is likely responsible for sending a low-mass star hurtling through the Milky Way at a staggering speed of a million miles per hour (1.6 million kilometers per hour). That’s about 1,500 times faster than the speed of sound.
The star is designated CWISE J124909+362116.0 (J1249+36) and was first detected by citizen science volunteers from the Backyard Worlds: Planet 9 project, who are exploring the vast amount of data collected by the Wide-field Infrared Survey NASA Explorer (WISE). mission over the course of nearly a decade and a half. J1249+36 immediately stood out for its immense speed of 1.3 million mph (2.1 million km/h), almost three times the speed of the sun in its orbit around the heart of the Milky Way. The speed of this “hypervelocity” star is so great, in fact, that it is likely to escape our galaxy entirely.
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To unlock the secrets of this hypervelocity star, Adam Burgasser, professor of astronomy and astrophysics at the University of California, San Diego, turned to the WM Keck Observatory in Maunakea, Hawaii, with the goal to observe its infrared spectrum.
This investigation revealed that the star belongs to a class of the oldest stars in the Milky Way: L subdwarfs. These stars are very rare and remarkable because of their very low mass and relatively cool temperatures.
The team’s spectral data was combined with a new set of atmospheric models created specifically to study L subdwarfs. This revealed the position and speed of J1249+36 across the Milky Way. “This is where the source became very interesting,” Burgasser said in a statement. “Its speed and trajectory showed it was moving fast enough to potentially escape the Milky Way.”
The question is: what started this sub-dwarf star on its rapid escape trajectory? Well, that brings us to our two suspects.
Is this star fleeing a white dwarf vampire?
In the first scenario used to explain the hypervelocity nature of J1249+36, Burgasser and his colleagues hypothesized that the low-mass star was once the stellar companion of a type of “dead” star called a white dwarf.
White dwarfs are born when smaller stars like the Sun exhaust the supply of hydrogen in their cores. When this happens, a star’s nuclear fusion ceases. This cuts off the outward flow of energy that supports the star against the internal pressure of its own gravity. Although this ends the life of lonely, isolated stars like the Sun, white dwarfs in binary systems can nevertheless return from the grave by feeding cannibalistically on stellar matter extracted from a nearby “donor” star.
This material accumulates on the white dwarf until the mass of this stellar remnant exceeds the Chandrasekhar limit of about 1.4 times the mass of the sun, above which a star can go supernova. The result is a type of cosmic explosion called a “Type Ia supernova” that completely obliterates the white dwarf.
“In this type of supernova, the white dwarf is completely destroyed, so its companion is released and flies away at the orbital speed it was originally traveling at, with also a little kick from the explosion of the supernova,” Burgasser explained. “Our calculations show that this scenario works. However, the white dwarf is no longer there and the remains of the explosion, which probably occurred several million years ago, have already dissipated, we do not We therefore have no definitive proof that it is indeed this dwarf origin.
Could twin black holes have something to do with it?
The second scenario considered by the team sees this hypervelocity star begin its life in a globular cluster, a dense and compact conglomeration of stars bound together by gravity. These spherical clusters can contain tens of thousands to several million stars.
Stars are concentrated toward the centers of globular clusters, where scientists believe black holes of varying masses also lurk. These black holes can come together and form binary systems capable of propelling out of their original system any stars that venture too close to them.
“When a star encounters a binary black hole, the complex dynamics of this three-body interaction can throw that star out of the globular cluster,” said Kyle Kremer, a new assistant professor in the Department of Astronomy and Astrophysics at UC San Diego.
Simulations generated by Kremer revealed that, on rare occasions, these types of interactions can expel a low-mass subdwarf from a globular cluster and place it on trajectories similar to those observed with J1249+36.
The team also traced the path of this hypervelocity star to an extremely populated region of space, which may indeed be the location of a currently unknown globular cluster – or, maybe. more than one.
The team will now examine the elemental composition of J1249+36 to try to determine which of these ejection scenarios is correct. The composition could be a possible indication of the origin, because when white dwarfs “go nova,” they pollute the stars they chase. Additionally, stars born in globular clusters have distinct chemical compositions.
Whatever the origins of this star, its discovery offers scientists the unique opportunity to study hypervelocity stars as a whole. And it’s all very cool.
Burgasser presented the team’s results at a press conference Monday, June 10, at the 244th national meeting of the American Astronomical Society (AAS) in Madison, Wisconsin.