Mapping the Milky Way

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^This artist’s impression shows Gaia mapping the stars of the Milky Way. Credit: ESA/ATG medialab/ESO/S. Brunier

The Milky Way Galaxy is our home, but we know surprisingly little about it. Last week, on September 14, the European Space Agency (ESA) released a wealth of data gathered by the space probe Gaia (named after the ancient Greek goddess Gaia). The probe pinned down the precise position and brightness of over a billion stars. As massive as this dataset seems, it represents just a taste of what is to come from Gaia, which promises to revolutionize the study of our galaxy in the next several years.

The Milky Way contains the sun, Earth, and other objects in our solar system. It also includes hundreds of billions of stars besides the sun. Huge clouds of gas and dust lie throughout the galaxy, and they constantly form new stars. The Milky Way is so massive that about 10 smaller galaxies orbit it like satellites revolving around a planet.

Because our solar system is located within the Milky Way, vast clouds of dust and dense swaths of stars prevent astronomers from determining the galaxy’s exact structure. It’s a bit like trying to figure out what the outside of one’s home looks like while being trapped inside. Until now, astronomers had to guess what the Milky Way looks like by observing other galaxies with similar traits—the equivalent of looking at other houses from a window. Gaia, however, is like a drone taking photos around the neighborhood. Launched in 2013 by the ESA, Gaia traveled to a point behind Earth and began measuring the positions of over a billion stars as it orbited the sun.

Gaia measures stellar distances using a technique called parallax. It first images a star against a background of other stars. Halfway around the sun, it takes another picture of the same star. Because the two pictures are taken many millions of miles apart, the star’s position changes slightly in relation to its background. This is the same effect that causes an object at arm’s length to appear to change positions when viewed from one eye or the other at a time. Because scientists know how far away the two star pictures were taken, and how much the star appeared to move, they can calculate its distance from Earth.

Gaia’s first map shows a two-dimensional plot of star density in the Milky Way. As Gaia orbits the sun, it will gather information on more and more points of light. It will then be able to refine its data to better define star positions. Astronomers are confident that future data from Gaia will allow them to create extremely accurate, three-dimensional maps of the galaxy.

Furthermore, as Gaia’s mission continues, its portrait of the Milky Way will extend into four dimensions (including time). As it observes over the years, Gaia will be able to detect the speed and direction in which stars are moving through space. This information will allow astrophysicists to see what the galaxy looked like in the recent past and to predict what it will look like in the near future.

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