Measured distance within the Milky Way gives clues to what our galaxy looks like

illustration of Milky Way

FROM THE INSIDE OUT  The best images of the Milky Way are artist’s impressions like this one, as it’s difficult to map the galaxy from our position inside it. But new measurements will give us more direct clues to what the Milky Way actually looks like.

R. Hurt, ESO, JPL-Caltech/NASA

For the first time, astronomers have directly measured the distance to a spot clear across the galaxy. The established but challenging technique promises a new way to map the structure of the Milky Way.

This technique, called parallax, has measured distances to stars since the 1830s. But because of galactic dust in the way, it has been difficult to use parallax on stars on the opposite side of the galaxy. Other ways to measure distance are saddled with assumptions and uncertainties.

Alberto Sanna of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and his colleagues used the Very Long Baseline Array of radio telescopes in New Mexico to track a star-forming region in the outer Scutum-Centaurus spiral arm, which is on the opposite side of the Milky Way from the local arm where the sun resides. The scientists report in the October 13 Science that the region is more than 66,500 light-years away.

The team observed the distant spot from March 2014 to March 2015, and drew an imaginary triangle between it and two points in Earth’s orbit. They then used trigonometry to measure the distance.

“Our measurement is essentially the same as a surveyor uses to locate points on the Earth,” says study coauthor Mark Reid of the Harvard-Smithsonian Center for Astrophysics. “It requires no model or assumptions.”

Applying the same technique to other regions of the Milky Way will help astronomers figure out what our galaxy looks like from the outside and compare it to other spiral galaxies.

“We predict that, within the next 10 years, we will be able to answer the question: What does the Milky Way look like?” Sanna says.

Lisa Grossman

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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