Astronomers uncover tons of of high-speed stars, many on their means out of the Milky Approach

In our galaxy there are thousands of stars orbiting the center of the Milky Way at high speeds. Occasionally some of them pick up so much speed that they break free from our galaxy and become intergalactic objects. Because of the extreme dynamic and astrophysical processes, astronomers are most interested in studying these stars – especially those that are able to reach escape speeds and leave our galaxy.

However, an international team of astronomers led by the National Astronomical Observatories of China (NAOC) recently announced the discovery of 591 high-speed stars. Based on data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and ESA’s Gaia Observatory, they showed that 43 of these stars are fast enough to one day escape the Milky Way.

The study was published in the Astrophysical Journal Supplement Series on December 17. The study was conducted by Dr. LI Yinbi, a NAOC astronomer, directed and included researchers from the Chinese Academy of Sciences (CAS), the Max Planck Institute for Astronomy (MPIA), the Institute for Advanced Study and the European Southern Observatory (ESO)), the ExtantFuture Technology Co., the Institute of Statistical Mathematics in Tokyo and several universities.

An artist’s imagination of a hypervelocity star that escaped the Milky Way. Photo credit: NASA

In terms of astrophysical studies, high-speed stars are a relatively recent discovery. The first was observed in 2005, and over the next 15 years over 550 were discovered by multiple observatories. From these, astronomers were able to derive four subclasses of high-speed stars, including: hypervelocity stars, runaway stars, hyper-runaway stars, and fast halo stars.

This latest discovery is particularly significant because it effectively doubles the number of known high-speed stars, which are quite rare in our galaxy. “The 591 high-speed stars discovered this time doubled the total number previously discovered, bringing the current total to over 1,000,” said Dr. Li.

Hypervelocity stars (HVS), the fastest of the group, are particularly interesting because they have reached relativistic speeds (a fraction of the speed of light). In fact, astronomers have estimated that with the right kind of gravitational acceleration, hypervelocity stars can reach 1/10 to 1/3 the speed of light – roughly 30,000 to 100,000 km / s (18,640 to 62,130 mi / s).

It is these stars that have the escape speed required to leave the Milky Way. Prof. LU Youjun from NAOC, co-author of this paper, said:

“Although rare in the Milky Way, high-speed stars with unique kinematics can provide deep insights into a broad spectrum of galactic science, from the central supermassive black hole to the distant galactic halo.”

The positions and reconstructed orbits of 20 high-speed stars, depicted on an artistic view of the Milky Way. Photo credit: ESA / Marchetti et al. 2018 / NASA / ESA / Hubble

“Rare” is certainly a fitting description. According to earlier estimates by astrophysicists, there are probably only 1000 HVS in our galaxy (that’s 0.0000005% of the galactic population). Given their speed and the great distances they travel, tracking these stars and creating a database of their motions could tell us a lot about a number of cosmic mysteries.

For their study, the international team relied in part on data from LAMOST. LAMOST is not only the largest optical telescope in China, but also has the highest spectral acquisition rate of any telescope in the world and can observe around 4,000 celestial objects with a single exposure. Since starting surveys in 2012, the company has established the world’s largest database of spectra.

In addition, the team relied on astrometric measurements from the Gaia Observatory, launched in 2013 by the European Space Agency (ESA). Since then, it has accumulated information on the location, proper movement, and speed of over 1.3 billion celestial objects, making it the largest astrometric database in the world. Both observatories and their extensive databases have been invaluable in detecting and studying HVS.

Based on the movement and composition of the observed objects, the research team identified 591 HVS that came from the inner atrium of the Milky Way. “Their low levels of metallicity suggest that most of the star halos formed as a result of the accretion and tidal disturbance of dwarf galaxies,” said co-author Prof. Zhao Gang of the CAS School of Astronomy and Space Science.

One of the biggest takeaways from this study is how combining several large surveys can lead to the discovery of rare objects. In the future, astronomers will have even larger databases containing survey data provided by next-generation instruments. These data will be particularly useful in studying dark matter, the mysterious mass that makes up 27% of the universe’s mass-energy density.

By tracking the movement of HVS, astrophysicists can better constrain the shape of the dark matter halo of the Milky Way. In addition, they could tell us a lot about the formation and evolution of the Milky Way itself, as HVS is believed to be the result of galactic fusions and other extreme gravitational forces (i.e., supermassive holes). Studying more, therefore, could help astronomers compile a history of past galactic mergers.

It has also ventured that HVS could allow astrophysicists to precisely constrain the mass of our galaxy that remains unsolved. Additionally, previous research has shown that HVS can carry their planetary systems with them, which could be one of the ways life spreads throughout the cosmos (intergalactic panspermia).

Further reading: Chinese Academy of Sciences, The Astrophysical Journal

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