Stellar Runaways From Another Galaxy

Stellar Runaways From Another Galaxy
By Judith E Braffman-Miller

Hypervelocity stars are fascinating stellar sprinters that travel so fast that they can reach escape velocity–and in this way break free from the powerful gravitational grip of our Milky Way Galaxy. Stars with only moderate speeds, like our own Sun, are tightly fastened to our Milky Way by the gravitational ties that bind–and because they can reach speeds of only tens to a few hundreds of kilometers per second, they peacefully orbit the heart of their Galactic host throughout their entire stellar existence. However, there are a small number of hypervelocity stars that are known to travel so quickly that they become unbound. Some astronomers have proposed that an unfortunately close brush past our Galaxy’s resident supermassive black hole, haunting the center of our Milky Way, is the most plausible mechanism that causes these free-spirited stars to swiftly escape to freedom, flying into the wilds of intergalactic space. In July 2017, a team of astronomers from the University of Cambridge in the UK, using data from the Sloan Digital Sky Survey (SDSS) and supercomputer simulations, demonstrated that these stellar speed demons did not originate in our Milky Way, but were really born in the Large Magellanic Cloud (LMC), a dwarf galaxy that orbits our own giant spiral.

These speedy children of another galaxy were able to break free from their original birthplace when one of the stars in a binary system went supernova. This catastrophic explosion caused its unfortunate companion star to go screaming away at such a high speed that it was able to escape the powerful gravity of the LMC–thus becoming an adopted stellar child of our Milky Way. The results of this study appear in the journal Monthly Notices of the Royal Astronomical Society (UK), and were also presented on July 5, 2017 at the National Astronomy Meeting (NAM) in Hull (UK.

Many astronomers originally thought that hypervelocity stars, which are extremely hot giant blue stars, may have been rudely evicted from the heart of our Galaxy by the voracious supermassive black hole that lurks there. A second scenario, that also involves dwarf galaxies, suggests that these small galaxies are falling apart. A third proposal puts the blame on messy, chaotic star clusters as the true culprit behind these swift stellar travelers. However, all three of these explanations fail to account for why hypervelocity stars are only found in a certain region of the sky.

A close brush with our Galaxy’s resident supermassive black hole, dubbed Sagittarius A, or Sgr A, for short (pronounced saj-a-star) is the most widely accepted scenario explaining these stellar speed demons. Many astronomers presume this to be the most plausible mechanism for hurling these unfortunate stars out of our Milky Way.

Speedy Stars

From Earth, the Galactic core is located in the direction of the constellations Ophiuchus, Scorpius, and Sagittarius. Because of the thick shroud of impenetrable interstellar dust blanketing the line of sight, the Galactic center cannot be seen by astronomers using visible, ultraviolet, or soft X-ray wavelengths. For this reason, all of the information that astronomers have obtained over the years, in respect to the hidden heart of our Milky Way, has been obtained from observations at gamma ray, hard X-ray, infrared, and sub-millimeter and radio wavelengths.

Although Sgr A is certainly a powerful gravitational beast, that exerts a mighty pull on anything that wanders too close to where it waits in sinister secret, it is relatively puny–at least by supermassive black hole standards. Sgr A sports a mass of “only” four million–as opposed to billions–of suns. There are many supermassive black holes that possess billions of solar-masses, and these strange gravitational beasts are thought to inhabit the mysterious hearts of perhaps every large galaxy in the Universe–and possibly many smaller galaxies, as well.

Sgr A is a complex radio source, and it is situated almost precisely at our Milky Way’s center. Sgr A harbors a very powerful compact radio source, which suggests that it is, indeed, a supermassive black hole. Accretion of gas onto Sgr A*–which is also surrounded by a disk–should hurl out immense quantities of energy to power the radio source, which is considerably larger than the black hole.

The classic scenario for the tragic eviction of hypervelocity stars, involves two sibling stars bonded together gravitationally in a binary system. The duo of unfortunate stars wander too close to the snatching gravitational claws of Sgr A*, and as one member of the doomed pair spirals inward toward the hungry dark-hearted beast, its companion star is thrown outward at a tremendous speed, becoming a hypervelocity star.

Currently, approximately 20 hypervelocity stars have been observed. Most of these stellar speed demons reside in the northern hemisphere, although it is possible that there are many more that can only be observed in the southern hemisphere.

“Earlier explanations for the origin of hypervelocity stars did not satisfy me. The hypervelocity stars are mostly found in the Leo and Sextans constellations–we wondered why that is the case,” commented Douglas Boubert, a doctoral student at the University of Cambridge’s Institute of Astronomy and the paper’s lead author.

An alternative scenario explaining the origins of hypervelocity stars–that they are the runaways from an ill-fated binary system–proposes that the closer the duo of stars are to one another, the faster they will orbit each other. As a result, if one of this stellar duo goes supernova, blasting itself to smithereens, it can break up the binary. The surviving star, that is still in one piece, becomes the runaway–and goes screeching for its life into space. However, stellar runaways in our Milky Way Galaxy are not fast enough to be hypervelocity. This is because blue stars cannot orbit each other too closely–without the two stars crashing into one another and merging. But a fast-moving galaxy could produce these stellar speed demons.

The Runaway Stellar Children Of Another Galaxy

The LMC is the largest and fastest of the numerous dwarf galaxies that are in orbit around our Milky Way. It sports only about 10% of the mass of our Galaxy. This means that the small galaxy’s swiftest stellar sprinters can readily attain escape velocity and run for their lives. The LMC circles our Milky Way at the breath taking pace of 400 kilometers per second and, like a traveling bullet shot from a moving train, the speed of these stellar runaways is the velocity they were ejected at plus the velocity of the LMC. This is fast enough for them to become hypervelocity stars.

“These stars have just jumped from an express train–no wonder they’re fast. This also explains their position in the sky, because the fastest runaways are ejected along the orbit of the LMC towards the constellations of Leo and Sextans,” study co-author Dr. Rob Izzard explained. Dr. Izzard is a Rutherford fellow at the Institute of Astronomy.

At almost 200,000 light-years from our planet, the LMC shines its way through space, in a lazily long waltz, around our Milky Way. Enormous clouds of gas within it ultimately collapse to give birth to sparkling, new baby stars. In turn, these stellar newborns light up the clouds of gas, creating a beautiful, varicolored cradle for the newborn stars. Indeed, the LMC is on fabulous fire with star-birthing regions. The LMC’s Tarantula Nebula is the most brilliant stellar nursery in our cosmic neighborhood, and the LMC itself is scattered with glistening, glowing nebulae–which are the most noticeable sign that brilliant baby stars are being born.

The team of astronomers used a combination of data derived from the SDSS, as well as supercomputer simulations, to model how hypervelocity stars might flee from the LMC, and ultimately wind up as the adopted stellar children of our Milky Way. The scientists simulated the birth and death of stars inhabiting the LMC over the past two billion years, and studied every stellar runaway. The orbit of the hypervelocity stars after they escaped from the LMC was then followed in a second supercomputer simulation that included the gravity of the LMC and our Galaxy. These simulations enabled the scientists to predict where on the sky astronomers would expect to observe runaway stars originating from the LMC.

“We are the first to simulate the ejection of runaway stars from the LMC–we predict that there are 10,000 runaways spread across the sky,” Boubert noted. About 50 percent of the simulated stars, which flee from the LMC, grow speedy enough to escape the gravitational clutches of our Milky Way, making them hypervelocity stars. If the previously known hypervelocity stars are runaway stars it would also explain their position in the sky.

Massive blue stars reach the unhappy end of that long stellar road by collapsing to become either a neutron star or a black hole after hundreds of millions of years–and runaway stars are no different. The majority of runaway stars in the simulation perished in mid-flight after escaping the LMC. Their stellar corpses, the neutron stars and black holes, that are left behind simply continue on their way and so, along with the 10,000 stellar runaways, the astronomers also predict about a million runaway neutron stars and black holes soaring through our Milky Way.

“We’ll know soon enough whether we’re right. The European Space Agency’s Gaia satellite will report data on billions of stars next year, and there should be a trail of hypervelocity stars across the sky between the Leo and Sextans constellations in the North and the LMC in the South,” Boubert added.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.

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