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Astronomers track hypervelocity stars

New video shows 6 hypervelocity stars, zipping from Milky Way center to outskirts. An interaction with our galaxy’s central, supermassive black hole might have kicked them into fast motion.

Gaia is a space observatory of the European Space Agency – launched in 2013 – whose job is measuring the positions, motions, and magnitudes of stars. On June 26, 2017, ESA released this video, which is based on Gaia data, and which shows movements of stars in our Milky Way galaxy over the past million years. In particular, it shows six stars zipping at high speed from the center of the Milky Way to its outskirts. Scientists call them hypervelocity stars, and ESA said they:

… could provide key information about some of the most obscure regions of the Milky Way.

That’s because the high velocities of these stars – several hundred kilometers/second, much faster than most Milky Way stars move – is thought to be the result of past interactions with the four-million-solar-mass black hole at the center of the Milky Way.

The video starts by showing the positions of stars in the sky 1,035,000 years ago. The positions were calculated using data from the Tycho-Gaia Astrometric Solution, a tool professional astronomers began using after the first Gaia data release. The video follows the evolution of stellar positions until the present day, ending with a view of the sky as measured by Gaia between 2014 and 2015.

The trajectories of the six hypervelocity stars are highlighted in yellow.

A simplified illustration of a gravity assist – or gravity brake – caused by the interaction of a less massive body with a more massive one. Essentially, the smaller body passes in front of the larger one to brake and behind it to accelerate. Image via Kerbal Space Program Wiki.

ESA said its scientists spotted the six speeding stars from the TGAS data set of two million stars with the help of an artificial neural network – software that mimics a human brain. It said:

These stars owe their high speeds to past interactions with the supermassive black hole that sits at the center of the Milky Way …

One of the six stars (labelled 1 at the end of the video) seems to be speeding so fast, at over 500 km/s, that it is no longer bound by the gravity of the galaxy and will eventually leave. The other five stars are somewhat slower (over 400 km/s for the stars labelled 2, 3, 4 and 6, and 360 km/s for the star labelled 5) and are still bound to the galaxy.

These slightly slower stars are perhaps even more fascinating, as scientists are eager to learn what slowed them down – the invisible dark matter that is thought to pervade the Milky Way might also have played a role.

Elena Maria Rossi from Leiden University in the Netherlands, who presented Gaia’s discovery of six new such stars in late June at the European Week of Astronomy and Space Science in Prague, Czech Republic, said:

These are stars that have travelled 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 center to its outskirts.

Artist’s concept of 2 hypervelocity stars, speeding from the center of our Milky Way galaxy to its outskirts. Image via ESA.

Bottom line: New ESA video – based on data from the Gaia astrometric satellite – shows six hypervelocity stars, or stars that move much faster through space than average, zipping from the center of our Milky Way to its outskirts.


Deborah Byrd