Astronomers find six stars moving through the Milky Way at incredibly high speeds

According to a report by Space.com on June 18, 2023. Astronomers have discovered six runaway stars racing across the Milky Way. Two hypervelocity stars, named J0927 and J1235, are traveling at 5.1 million miles per hour (2,285 kilometers per second) and 3.8 million miles per hour (1,694 kilometers per second), respectively. Its velocity is faster than any previous object of this type. At this speed, an object can circle the Earth 694 times in one hour.

The other four stars are no slouch when it comes to speed, all traveling at more than 2.2 million miles per hour (1,000 kilometers per second). These stars are all traveling faster than the galactic escape velocity (1.2 million miles per hour).

"These stars are extraordinary in that they travel through the galaxy much faster than ordinary stars. Because they travel faster than the galactic escape velocity, they eventually escape the galaxy into intergalactic space," said team leader and Harvard/Smith Kareem El-Badry, a researcher at the Sunny Center for Astrophysics, told Space.com.

The research team believes that the incredible speed of the four stars may be the result of their launch in a special type of cosmic explosion called a Type Ia supernova. This also gave them unusually high surface temperatures, which surprised the team. "They're also much hotter than ordinary stars—likely a result of their unusual formation history, which included a supernova exploding next to them!" the astrophysicist explained.

Type Ia supernovae occur in binary star systems that contain stellar remnants called white dwarfs, which form when stars like the Sun die and feed on material from a companion star.

White dwarfs — also known as "degenerate stars" — result from the collapse of stellar cores and are incredibly dense, about the mass of the sun, squeezed into an Earth-sized sphere, but they're not nearly as massive To cross the so-called Chandrasekhar limit — the mass at which a star reaches a mass that triggers a "normal" supernova and produces a neutron star or even a black hole when it dies.

While any supernova would release enough energy to create runaway stars, the team thinks that more violent and powerful supernovas may be needed to accelerate these stars to hypervelocity. These particular explosions are known as "helium-ignited violent mergers" or "dynamically driven double degenerate double explosions," a moniker shortened to D6 supernovae.

A D6 supernova occurs when a white dwarf strips helium instead of hydrogen from the outer layers of its companion star, which is thought to be another white dwarf (hence double degeneracy). This leads to a second Big Bang (hence the double explosion), with the result that the companion white dwarf is ejected as a hypervelocity runaway star.

"The composition of runaway stars is very unusual," explains El-Badri. "The atmospheres of nearly all the stars in the Milky Way consist primarily of hydrogen and helium, but these objects contain no hydrogen or helium at all and consist primarily of carbon and oxygen."

This suggests that these runaway stars are degenerate white dwarfs, and further supports the idea that they were blasted to extreme speeds by D⁶ supernovae.

El-Badri and colleagues used the standard light output of type Ia standard candles to calculate the emission rate of runaway stars. They found that the rate of hypervelocity star production is consistent with that of Type Ia supernovae, suggesting that many of these events may be D6 explosions.

This led them to conclude that a large number of these hypervelocity escaping stars in the universe have yet to be discovered.

Although they are within the Milky Way, these runaway stars are all traveling at more than 2.2 million miles per hour (1000 kilometers per second) and will one day leave our galaxy because they are traveling faster than the escape velocity of the Milky Way (approx. 1.2 million miles, or 550 kilometers per second). The team doesn't think they will be the first stars launched by the Milky Way.

"If a significant fraction of Type Ia supernovae produced a D6 star, the Milky Way could have launched more than 10 million supernovae into interstellar space," El-Badri and his co-authors wrote.

Schematic illustration of hypervelocity stars escaping from the Large Magellanic Cloud, a satellite galaxy of the Milky Way. (Image source: ESO)

References:
https://www.space.com/fastest-runaway-stars-milky-way