Radio telescopes combine to map far side of our Milky Way Galaxy

How do you solve a problem from the inside? Nestled inside the flat disk of the Milky Way, we can’t just point telescopes at it to work out its shape. Instead, astronomers work out the structure of our Galaxy by calculating distances to other objects within it.

Twilight over radio telescopes at the Karl G. Jansky Very Large Array in New Mexico, which form part of the Very Long Baseline Array. Image credit: NRAO/AUI/NSF

That is not to say that working out the structure of our Galaxy is an easy task. Up until the 1990s, astronomers believed the Milky Way was a spiral galaxy, but today they think that our Galaxy is actually a barred spiral galaxy with four arms at its centre.

Now a team of astronomers has used the National Science Foundation’s Very Long Baseline Array (VLBA) to measure the distance to a star-forming region on the opposite side of the Galactic centre from our Sun, a key step towards mapping the Milky Way in more detail.

“This means that, using the VLBA, we now can accurately map the whole extent of our galaxy,” said Alberto Sanna from the Max Planck Institute for Radio Astronomy in Germany.

Astronomers directly measured the distance to a region on the far side of our Milky Way Galaxy, past the Galaxy’s centre. Image credit: Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA.

The team used trigonometric parallax to work out the distance from Earth to the star-forming region. To understand what that means, think about what happens when you put your finger to your nose and close one eye, then the other – your finger appears to change position. If you measured the apparent change in position and the distance between your eyes, you could work out the distance from your nose to your finger using trigonometry.

The same principle applies to measuring considerably larger distances. As the Earth orbits the Sun, distant objects appear to move around in the sky. Astronomers can measure these movements across the sky and calculate the distance between the object and the Earth.

Because these objects are so far away, they hardly appear to change position at all. This is where the VLBA comes in useful. It consists of ten radio telescopes scattered across North America, Hawaii and the Caribbean. This is effectively like having one huge, continent-sized radio telescope better equipped to detect shifts in position.

In reality, those shifts might be many kilometres across, but because they’re very distant, measuring the shifts from Earth is like trying to measure the size of a baseball bat left on the Moon.

The observations, made in 2014 and 2015, suggest that the star-forming region, called G007.47+00.05, is around 66,000 light-years away from Earth. For comparison, the Galactic centre is 27 000 light years away and until now, the largest distance measured using parallax was 36,000 light years, just over half the length of the measurement made with the VLBA.

Parallax technique determines distance by measuring the angle of apparent shift in an object’s position, as seen from opposite sides of Earth’s orbit around the Sun. Image credit: Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA.

“Most of the stars and gas in our galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the galaxy’s spiral arms and learn their true shapes,” said Sanna.

The measurement is impressively large – but it is still just one measurement. Ultimately astronomers hope to make more and more measurements like this to build up a more detailed picture of the Milky Way’s structure and perhaps even understand what our Galaxy looks like.

Related: Galaxy’s central bulge is shaped like a peanut

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