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Billion-star map of Milky Way set to transform astronomy

After a feverish wait, astronomers around the world have an ocean of new information to throw themselves into. On 25 April, the European Space Agency’s (ESA) Gaia mission published its first fully 3D map of the Milky Way.


The data haul includes the positions of nearly 1.7 billion stars and the distances, colours, velocities and directions of motion of about 1.3 billion of them. Together, they form an unprecedented live movie of the sky, covering a volume of space 1,000 times larger than that captured by any previous survey (see ‘Gaia’s gold’). “In my professional opinion, this is crazy awesome,” says Megan Bedell of the Center for Computational Astrophysics in New York City, one of the many astronomers who will conduct studies based on the data set. “I think the whole community is eager to dive in.”

Milky Way mapper: 6 ways the Gaia spacecraft will change astronomy

Within hours of the catalogue going online, 3,000 users around the world had already started downloading the data, ESA said in a tweet.

“We’re very curious to see what the community will do with it,” says Anthony Brown, an astronomer at the Leiden Observatory in the Netherlands who chairs Gaia’s data-processing collaboration.

At an event to present the Gaia catalogue at the Royal Astronomical Society in London, astronomer Gerry Gilmore of the University of Cambridge, UK, presented a striking video that extrapolated from the Gaia data to simulate the future motion of millions of stars. “Everything moves,” he said.

The 2-tonne Gaia spacecraft, part of a €1-billion (US$1.2-billion) mission, launched in late 2013 and began collecting scientific data in July 2014. Gaia is in stable orbit that remains fixed relative to both the Sun and Earth. It makes repeated measurements to estimate the distances of stars — and other celestial objects — using a technique called parallax (see ‘The parallax effect’).

Alongside its 551-gigabyte database, the Gaia team also released a number of scientific papers. The main goal of these was to describe quality checks the researchers did on the data and demonstrate how they can be used; the mission’s policy is to make the catalogue immediately available to the broader community, rather than reserve it for the team’s own science studies first.

Still, the Gaia papers describe a wealth of original findings, said Floor van Leeuwen, another senior Gaia scientist from Cambridge, at the press briefing. He shows, for example, how Gaia proved for the first time that certain star clusters puff up at the same time as large stars sink to their centres. “We weren’t allowed to make discoveries, but we couldn’t avoid making them,” he said.

One of those findings has implications far beyond the Milky Way. Some astronomers are especially eager to see Gaia’s measurements of certain types of variable star that are used as ‘standard candles’ of cosmology. Knowing the precise distances of these stars in the Milky Way makes them useful as yardsticks for measuring distances to galaxies much farther away. In particular, astronomers use standard candles to estimate how fast the Universe is expanding, but in recent years, measurements based on this technique have been in apparent contradiction — or “tension”, as scientists say — with predictions made using maps of the cosmic microwave background (CMB), the afterglow of the Big Bang. A preliminary look at the data shows that Gaia has improved the precision of the standard-candle measurements, Gilmore said at the press briefing. But, he adds, “at face value, the tension is still there”.

Dozens of preprints are likely to appear in the next few days, Gilmore says, as teams around the world download Gaia data and run them through algorithms honed for years in preparation. For example, researchers will be able to test models of how the Milky Way formed through mergers of smaller galaxies; measure the distribution of dark matter; and refine their theories for how stars evolve as they burn through their reserves of nuclear fuel.

Denis Erkal, an astronomer at the University of Surrey in Guildford, UK, and his collaborators plan to use Gaia data to weigh the Large Magellanic Cloud, the largest of the dwarf galaxies orbiting the Milky Way. They will do so by measuring tidal motions in our Galaxy’s stars that are caused by the dwarf galaxy — a bit like weighing the Moon by measuring its effects on Earth’s oceans.

Gaia released a preliminary catalogue in 2016, but at that time, it had not yet gathered enough data to directly measure the distances of many stars. Further data releases will contain more and more information and will enable entirely new kinds of studies (the next release will be in 2020). Some researchers expect to discover tens of thousands of exoplanets by watching stars wobble under their planets’ gravitational pull — but the probe must collect several years’ more data for these motions to become apparent. Others will inspect similar wobbles in search of evidence of the passage of gravitational waves. In addition to tracking stars, the probe has monitored asteroids and will help scientists to monitor bodies in the Solar System that might look to be on a collision trajectory with Earth.

A technical glitch in February temporarily sent Gaia into ‘safe mode’, but the probe is in overall good health, says project scientist Timo Prusti at ESA’s European Space Research and Technology Centre in Noordwijk, the Netherlands. If nothing breaks down and ESA continues extending the mission, Gaia has enough fuel to keep operating until 2024, for a total of 10 years, he says.

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Where is the edge of the universe?

When Galileo Galilei pointed his first telescope to the heavens in 1610, he discovered “congeries of innumerable stars” hidden in the band of light called the Milky Way. Our cosmos grew exponentially that day. Roughly three centuries later, the cosmic bounds exploded once again when astronomers built telescopes big enough to show the Milky Way is just one of many “island universes.” Soon they learned the universe was expanding, too, with galaxies retreating from each other at ever-accelerating speeds.


Since then, ever-larger telescopes have shown the observable universe spans an incomprehensible 92 billion light-years across and contains perhaps 2 trillion galaxies. And yet, astronomers are still left wondering how much more universe is out there, beyond what they observe.

“The universe has always been slightly larger than what we can see,” says Virginia Trimble of the University of California, Irvine, an astronomer and expert in the field’s history.


Building bigger telescopes won’t help extend the cosmos anymore. “Telescopes only observe the observable. You can’t see back in time further than the age of the universe,” explains Nobel Prize-winning cosmologist John Mather of NASA’s Goddard Space Flight Center, who’s also chief scientist for the James Webb Space Telescope. “So we are totally limited. We’ve already seen as far as you could possibly imagine.” At the edge, we see the leftover glow from the Big Bang — the so-called cosmic microwave background radiation (CMB). But this isn’t some magical edge of the universe. Our cosmos keeps going. We just may never know how far.
In recent decades, cosmologists have tried to solve this mystery by first determining the universe’s shape, like the ancient Greek mathematician Eratosthenes calculating Earth’s size using simple trigonometry. In theory, our universe can have one of three possible shapes, each one dependent on the curvature of space itself: saddle shaped (negative curvature), spherical (positive curvature) or flat (no curvature).

Few have championed a saddle-shaped universe, but a spherical cosmos makes sense to us earthlings. Earth is round, as are the sun and planets. A spherical universe would let you sail into the cosmos in any direction and end up back where you started, like Ferdinand Magellan’s crew circumnavigating the globe. Einstein called this model a “finite yet unbounded universe.”

But starting in the late 1980s, a series of orbiting observatories built to study the CMB made increasingly precise measurements showing that space has no curvature at all. It’s flat to the limits of what astronomers can measure — if it is a sphere, it’s a sphere so huge that even our entire observable universe doesn’t register any curvature.

“The universe is flat like an [endless] sheet of paper,” says Mather. “According to this, you could continue infinitely far in any direction and the universe would be just the same, more or less.” You’d never come to an edge of this flat universe; you’d only find more and more galaxies.

That’s all well and good with most astronomers. A flat universe agrees with both observation and theory, so the idea now sits at the heart of modern cosmology.

The problem is that, unlike a spherical universe, a flat one can be infinite — or not. And there’s no real way to tell the difference. “What could you look for to see whether there’s an infinite universe?” Trimble says. “Nobody quite knows.”

So instead, astronomers hope an answer can come from theory — a model that could offer indirect proof one way or the other. For example, the Standard Model of physics predicted the existence of numerous particles, like the Higgs Boson, years before they were actually discovered. Yet physicists still presumed those particles were real.

“If you have a good description of everything you’ve observed so far and it predicts something is true, then you expect it is,” Trimble says. “That’s how most scientists think about how science works.”

This story originally appeared in the December issue of Discover magazine as “Is the Universe Infinite?” Support our science journalism by becoming a subscriber.

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Personalised Star Maps

Let’s face it, it can be quite a difficult task to choose gifts for your loved ones. You always want to go for something unique, yet it should also have a personal touch to it. A gift that reminds them of you and that special day or time you spent together.

If you’re currently looking for a stunning, unique, and intimate gift, you have come to the right place. We at Blue Horizon Prints, design exquisite star maps for your special days, events, and celebrations. You can create your own personalized star constellation map artworks. People are sometimes bored with the usual posters and paintings hanging on the walls. Star Maps are a meaningful and fascinating way to decorate your wall while adding a personal touch to it.

Death Star Star Wars Personalised Map Art

Star Maps are fully custom made according to the day and time provided by you. Every moment happens under a special sky. You can always cherish that sight of the sky by designing a Star Map for your special day. Whether it’s a birthday, a wedding day, the day you met your special one, the day you kissed them for the first time, the first hangout with a friend, or any other day. We guarantee to make it more exclusive with our star maps.

We have various themes and styles for your star maps including, Mandala Sky Map, Custom Blue Watercolour Star Map, Once Upon A Star, and the forest Star Map. All made to accuracy and perfection for the important days of your life. For all the Star Wars lovers out there, we also have a Death Star Bespoke Star Map.

After your order is placed, we send you a proof for you to approve the art piece and request tweaks, if any. When you’re 100% satisfied with the results, your order is delivered. It can all be totally customized according to you, the words, dates, time, place, etc.

We use the best astronomy software to prepare the most precise night sky for the given day, time, and location. The printed sky that you see is an exact recreation of the sky that was above you on a special day.

Take your special day and recreate it into an entire horizon hanging on the wall of your room. Our Star Maps are not only unique and highly appealing but, they are also very meaningful and a gift that you’ll cherish and remember for years ahead.

If you are looking to buy art, other than star maps, we have a beautiful collection of wall art, available to buy on our sister site Canvas Prints Australia. Browse through a collection of artwork ranging from Australian themed art in a wide variety of formats, beautiful classical prints for artists such as Van Gogh and Monet, or for something more modern we have a popular collection of Banksy Prints.