Two new "shooting stars" have been discovered

Astronomers estimate that only about a thousand very fast stars are in the Milky Way. That is only 0.0000005% of the 200 billion stars in our galaxy. These stars originate in the center of the galaxy and are believed to have been blown from the center of the galaxy due to their proximity to the supermassive black hole at the center of our galaxy known as Sagittarius A*. They managed to get far, and some even escaped outside the galaxy.

Most of the stars in our galaxy behave predictably. They orbit the center of the Milky Way at a speed of 100 kilometers per second, but several stars have managed to reach significantly higher speeds, so much so that they are even able to escape the galaxy's gravity. These are known as "runaway stars" or hypervelocity stars (HVS), a rare type of star that is estimated to have been blown away due to their proximity to the supermassive black hole at the center of the galaxy.

The existence of fast stars was theorized in the late 20s and only twenty of them have been identified so far. Now thanks to a new study by a team of Chinese astronomers, two new stars have been added to the list. These stars, which are named after the sky survey in which LAMOST-HVS2 and LAMOST-HVS3 were discovered, fly at a speed of a thousand kilometers per second, and they estimate that they originate in the center of our galaxy.

The researchers used the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) to identify the two new stars.
The stars observed in the LAMOST sky survey over three years. Photo: LAMOST

Astronomers estimate that only about a thousand very fast stars are in the Milky Way. That is only 0.0000005% of the 200 billion stars in our galaxy. These stars originate in the center of the galaxy and are believed to have been blown from the center of the galaxy due to their proximity to the supermassive black hole at the center of our galaxy known as Sagittarius A*. They managed to get far, and some even escaped outside the galaxy.

This is why astronomers are so interested in the HVS. Given their speed, and the vast distances they can cover - tracking them and creating a database of their movements could provide explanations for the shape of the dark matter halo surrounding our galaxy. Therefore, Dr. Yang Huang of the Southwest Institute for Astronomical Research at Yunnan University in Kuching, China, and his colleagues began sifting through the LAMOST data to find evidence of new HVSs.

The LAMOST observatory located in Hebei Province in northwest China is operated by the Chinese Academy of Sciences. For five years, this observatory conducted a spectroscopic survey of 10 million stars in the Milky Way, as well as millions of galaxies. In June 2017, LAMOST released its third data release (DR3), which included spectra obtained during the pilot survey and the first three years of regular surveys.

The database contains high-quality spectra of 4.66 million stars and data for 3.17 million additional stars. DR3 is currently the largest public spectrum and stellar data catalog in the world. LAMOST data have already been used to identify a B1IV / V type star (the main star is a blue giant and the second is a dwarf) that has 11 solar masses and is 13,500 times brighter than the Sun. Its temperature reached 26 thousand degrees Kelvin.

This star has been named LAMOST-HSV1, after the observatory, so both stars will have the following serial numbers. While HSV2 has a mass of 7.3 solar masses, it is 2399 times brighter than our Sun, and has a temperature of 20,600 degrees Kelvin. The mass of HSV3 is 3.9 solar masses and is 309 times brighter than it. and a temperature of 14,000 degrees Kelvin. The researchers also considered the possible origins of all three HVS based on their spatial location and the time when they began to move outward.
In addition to originating in the center of the Milky Way, they consider alternative possibilities. As they say in their study: “The three HVS are spatially associated with known stellar structures near the galactic center, which supports a galactic center origin. for them. However, two of them, LAMOST-HVS1 and 2, have lifetimes shorter than their flight durations, meaning they may not have had enough time to move from the galactic center to their current location, unless their blue color is due to some other reason (as in the case of HVS HE 0437 -5439). The third planet (LAMOST-HVS3) has a longer lifespan than its flight time, so it has no problem.
In other words, the origins of these stars are still a mystery. Beyond the idea that they were blown away by an encounter with the black hole at the center of our galaxy, the team also considered other possibilities that have been raised over the years. Among other things, it may be the debris of a dwarf galaxy torn apart by the Milky Way, stars that survive from Type Ia supernova explosions, the result of a dynamic encounter between multiple stars, as well as refugees from the Large Magellanic Cloud.

In the future, Huang and his colleagues note that their research will benefit from additional information provided by the European Space Agency's GAIA spacecraft, which they say will shed more light on how fast stars behave and where they come from. As they state in their conclusions: "The correct and accurate motion measurements of the Gaia spacecraft should provide direct data on their origins. Finally, we expect more fast stars to be discovered by ongoing telescopic surveys such as the LAMOST surveys, thus providing additional constraints on the nature and emission mechanisms of these fast stars.

for scientific research