Gaia Finds Six Stars Zipping out of the Milky Way

In 2013, the European Space Agency launched the Gaia spacecraft. As the successor to the Hipparcos mission, this space observatory has spent the past three and a half years gathering data on the cosmos. Before it retires sometime next year (though the mission could be extended), this information will be used to construct the largest and most precise 3D astronomical map ever created.

In the course of surveying the cosmos, Gaia has also revealed some very interesting things along the way. For example, after examining the Gaia catalog with a specially-designed artificial neural network, a team of European researchers recently detected six new hypervelocity stars in the Milky Way. And one of these stars is moving so fast that it may eventually leave our galaxy.

Their study – titled “An Artificial Neural Network to Discover Hypervelocity Stars: Candidates in Gaia DR1/TGAS” – was recently published in the Monthly Notices of the Royal Astronomical Society. It was presented late last month at the European Week of Astronomy and Space Science, which was being held from June 26th to June 30th in Prague, Czech Republic.

Artist’s conception of the Gaia telescope backdropped by a photograph of the Milky Way taken at the European Southern Observatory. Credit: ESA/ATG medialab; background: ESO/S. Brunier

Hypervelocity stars are a rare and fascinating thing. Whereas all stars in the Milky Way are in constant motion, orbiting around the center of our galaxy, some are accelerated to speeds of up to hundreds of kilometers per second. In the past, astronomers have deduced that these fast-moving stars are the result of a close stellar encounter or a supernova explosion of a stellar companion.

And a little over a decade ago, astronomers became aware of a new class of high-speed stars that are believed to have been accelerated from past interactions with the supermassive black hole (Sagittarius A*) that sits at the center of our galaxy. These stars are extremely important to the study of the overall structure of the Milky Way, as they are indicative of the kinds of events and forces that have shaped its history.

As Elena Maria Rossi, from Leiden University in the Netherlands and one of the co-authors on the paper, explained in an ESA press release:

These are stars that have traveled great distances through the Galaxy but can be traced back to its core – an area so dense and obscured by interstellar gas and dust that it is normally very difficult to observe – so they yield crucial information about the gravitational field of the Milky Way from the centre to its outskirts.

Artist’s impression of stars speeding through the Galaxy. Credit: ESA

Finding such stars is no easy task, mainly because their velocity makes them extremely difficult to spot in the vast and crowded disk of the Milky Way. As a result, scientists have relied on looking for young, massive stars (2.5 to 4 Solar masses) in the old stellar population of the Galactic. Basically, their young age and high masses are indications that they might not have originated there.

Combined with measurements of their past speeds and paths, this method has confirmed the existence of hypervelocity stars in the past. However, only 20 hypervelocity stars have been spotted to date, and they have all been young and massive in nature. Scientists believe that many more stars of other ages and masses are also being accelerated through the Milky Way, but were previously unable to spot them.

To address this, the European team – led by from Tomasso Marchetti of Leiden University in the Netherlands – began considering how to use Gaia‘s vast dataset to optimize the search for more hypervelocity stars. After testing various methods, they adopted the artificial neural net approach – i.e. using a machine learning algorithm – to search through the star census data Gaia is in the process of gathering.

Beginning in the first half of 2016, the team began developing and training this program to be ready for the first release of Gaia data – which occurred a few months later on Sept. 14th, 2016. As Tommaso Marchetti, a PhD student at Leiden University, described the process:

“In the end, we chose to use an artificial neural network, which is software designed to mimic how our brain works. After proper ‘training’, it can learn how to recognize certain objects or patterns in a huge dataset. In our case, we taught it to spot hypervelocity stars in a stellar catalogue like the one compiled with Gaia.”

Artist’s impression of a hypervelocity star that was detected using the ESO’s Very Large Telescope. Credit: ESO

In addition to a map with the positions of over a billion stars, this first data release included a smaller catalogue with the distances and motions for two million stars. This catalog – which is known as the Tycho-Gaia Astrometric Solution (TGAS) – combined data from both the first year of the Gaia mission and with data from the Hipparcos mission, and is essentially a taste of what’s to come from Gaia.

On the day of the catalog’s release, Marchetti and his team ran their algorithm on the two million stars within the TGAS, which revealed some interesting finds. “In just one hour, the artificial brain had already reduced the dataset to some 20 000 potential high-speed stars, reducing its size to about 1%,” said Rossi. “A further selection including only measurements above a certain precision in distance and motion brought this down to 80 candidate stars.”

The team then examined these 80 stars in more detail, and compared the information about their motions to data from other catalogues. Paired with additional observations, they eventually found six stars which appeared to be moving faster than 360 km/s. One even appeared to be exceeding 500 km/s, which means that it is no longer bound by the gravity of our Milky Way and will eventually leave it altogether.

But perhaps the sot significant aspect of this find is the fact these stars are not particularly massive like the previous 20 that had been discovered, and were comparable in mass to our Sun. In addition, the 5 slower stars are likely to become a focal point of study, as scientists are eager to determine what slowed them down. One possible explanation is that interaction with the galaxy’s dark matter might have been responsible.

Gaia’s first sky map. Credit: ESA/Gaia/DPAC. Credit: A. Moitinho & M. Barros (CENTRA – University of Lisbon), on behalf of DPAC.

Much as the TGAS has been merely an early indication of the vast and valuable data Gaia will eventually provide, this study showcases the kinds of discoveries and research that this data will enable. By with not just 2 million, but a billion stars to study, astronomers are sure to reveal many new and exciting things about the dynamics of our Milky Way and the kinds of forces that have shaped it.

For this purpose, Marchetti and his team are upgrading their program to handle the much larger data set, which is scheduled to be released in April of 2018. This catalog will include distance and motions for over a billion stars, as well as velocities for a specific subset. From this, the team may find that fast-moving stars which are being booted out of the Milky Way are a lot more common than previously thought.

And be sure to enjoy this video that shows the paths of these six newly-discovered fast-moving stars, courtesy of the ESA:

Further Reading: ESA

‘Runaway’ Star Cluster Breaks Free from Distant Galaxy

We’ve discovered dozens of so-called “hypervelocity stars” — single stars that break the stellar speed limit. But today astronomers multiplied the number of these ‘runaway’ stars by hundreds of thousands. The Virgo Cluster galaxy, M87, has ejected an entire star cluster, throwing it toward us at more than two million miles per hour.

“Astronomers have found runaway stars before, but this is the first time we’ve found a runaway star cluster,” said lead author Nelson Caldwell of the Harvard-Smithsonian Center for Astrophysics, in a press release.

About one in a billion stars travel at a speed roughly three times greater than our Sun (which clocks in at 220 km/s with respect to the galactic center). At a speed that fast, these stars can easily escape the galaxy entirely, traveling rapidly throughout intergalactic space.

But this is the first time an entire star cluster has broken free.

What would cause an entire cluster — hundreds of thousands of stars packed together a million times more closely than in the neighborhood of our Sun — to reach such a tremendous speed?

Single hypervelocity stars have puzzled astronomers for years. But by observing their speed and direction, astronomers can trace these stars backward, finding that some began moving quickly in the Galactic Center. Here, an interaction with the supermassive black hole can kick a star away at an alarming speed. Another option is that a supernova explosion propelled a nearby star to a huge speed.

Caldwell and colleagues think M87 might have two supermassive black holes at its center. The star cluster wandered too close to the pair, which picked off many of the cluster’s outer stars while the inner core remained intact. The black holes then acted like a slingshot, flinging the cluster away at a tremendous speed.

The star cluster is moving so fast it should soon by sailing into intergalactic space. It may already be, but its distance remains unknown.

Velocity distribution of objects toward Virgo, includ- ing all confirmed GCs, all Hectospec velocities, and galaxies (from Rines & Geller 2008). The distinct stellar and GC distributions are clear, as is the broader galaxy distribution (dotted and shaded magenta). HVGC-1 is the marked extreme left outlier. Image Credit: Caldwell et al.
Velocities of stars, globular clusters and galaxies toward Virgo. HVGC-1 is the marked extreme left outlier.
Image Credit: Caldwell et al.

The team found the globular cluster — dubbed HVGC-1 — with a stroke of luck. They had been analyzing 2,500 globular cluster candidates for years. While a computer algorithm automatically calculated the speed of every cluster, any oddity was analyzed by hand.

Over 1,000 candidates have measured velocities between 500 and 3000 km/s. These speeds are typical for Virgo Cluster members. But HVGC-1 has a radial velocity of -1026 km/s. “This is the most negative, bulk velocity ever measured for an astronomical object not orbiting another object,” writes Caldwell.

“We didn’t expect to find anything moving that fast,” said coauthor Jay Strader of Michigan State University.

Future measurements pinpointing the exact distance to the globular cluster will help shed light on its exact origins.

The paper will be published in The Astrophysics Journal Letters and is available for download here.