Deep inside the galaxy

Scientists working with brand-new data from the European Space Agency’s Gaia space telescope have mapped the precise positions, distances, colours, and motions of more than 300,000 stars within 100 parsecs (about 330 light-years) of Earth, creating the most detailed map of our stellar neighborhood ever made. Using this, they have been able to create a 3D model of this region through which one can “fly”, as though piloting a starship. They have also projected the movements of these nearby stars millions of years into the future, seeing how they slowly spread out into streamers as the differences in their speeds send them on slightly different orbits around the center of the galaxy.

Until today, says Giorgia Busso, an astrophysicist at the University of Cambridge, UK, it was only possible to do this for fewer than 6000 nearby stars. But now, the number is up to 303,446, a more than 50-fold increase.

“It’s a massive increase in our understanding of [our] neighborhood,” adds Martin Bartstow, an astronomer and space scientist at the University of Leicester, UK.

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The density of stars from Gaia’s Early Data Release. Credit: ESA / Gaia / DPAC; CC BY-SA 3.0 IGO. A Moitinho, M Barros

The stars in Busso’s 3D map range from large, bright ones that we can see with the naked eye to ultra-cool dwarf stars barely big enough to be true stars. Some, she says, are even remnants of a dwarf galaxy that merged into the Milky Way billions of years ago – distinguishable from longer-term residents of our galaxy because they retain movement patterns left over from the demise of that ancient dwarf.

The Gaia space telescope was launched on 19 December 2013 on what is slated to be a 10.5 year observing mission. On 8 January 2014, it took up station at the L2 Lagrange point – a stable orbit about a 1.5 million kilometres farther from the Sun than the Earth. (NASA’s James Webb Space Telescope, scheduled to be launched next year, will eventually park in the same area.)

From this location, Gaia continually observes the sky, using a one-billion-pixel camera to pinpoint the locations, distances, and motions of 1.8 billion astronomical objects with extreme detail, refining its estimates with a series of detailed data releases, the most recent of which was announced via Zoom at a meeting hosted by the Royal Astronomical Society.

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Orbits of the nearby stars around the galaxy. Credit: ESA / Gaia / DPAC; CC BY-SA 3.0 IGO. S Payne-Wardenaar, . Jordan, . Reylé and Smart et al

The ultimate goal, Busso says, is to measure the positions of these stars with a precision akin to reading the title of a book on the Moon, all the way from Earth. But already, says Nicholas Rowell, of the Royal Observatory of Edinburgh, the new data release makes it possible to do the equivalent of seeing a coin from 200,000 kilometres.

This extreme accuracy allows astronomers not only to see precisely where distant stars are, but to track their motions by observing how their positions shift with time.

It is even possible to do so with stars in nearby galaxies such as the Magellanic Clouds, says Francesca de Angeli, a researcher at the University of Cambridge. 

By doing so, her team was able to better understand the rotation of these clouds, and even to map the motions of stars within the streamer connecting them. By doing this, she says, her team has confirming that, as astronomers have long suspected, these stars are indeed flowing from the Lesser Magellanic Cloud to the Greater Magellanic Cloud as the larger galaxy’s gravity strips gas and stars from the smaller one.

Closer to home, says Floor van Leeuwen, also of the University of Cambridge, it’s also possible to measure not just the motion of the Solar System (as the angles to more distant objects like quasars slowly shift as our point of view gradually changes) but to measure how that motion is changing over time.

Based on that, he says, his team has measured the degree to which the gravity of our galaxy is bending our orbit around its center, measuring it at 7.3 kilometres per second per million years. 

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A diagram of the two most important companion galaxies to the Milky Way, the Large Magellanic Cloud (left) and the Small Magellanic Cloud made using data from the Gaia satellite. Credit: ESA / Gaia / DPAC

Not that this means we are falling into the heart of the Milky Way; it’s just a way of calculating the acceleration of gravity felt by the Sun, this far out from the galactic center. But from that, says Gerry Gilmore, another Gaia researcher at the University of Cambridge, it should be possible to refine our estimates of the galaxy’s composition. 

“The immediate conclusion is that roughly half the weight interior to the Sun is dark matter,” he says. Otherwise, there wouldn’t be enough mass to produce the gravitational forces now seen bending our orbit. 

Meanwhile the new data release has been made public to any astronomer who wants to use it – something that many have already been doing with prior data releases. 

“More than 1500 papers a year are coming out of Gaia,” Gilmore says. 

“There is hardly a field of astronomy that isn’t revolutionised by the Gaia data,” adds another of his University of Cambridge colleagues, Dafydd Evans.

Related reading: AI and Gaia help identify 2000 young stars

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