"Optically, it's like trying to look through a velvet cloth-black as black can be", says Thomas Dame, Director of the Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and Senior Radio Astronomer at the Smithsonian Astrophysical Observatory.
Distance measurements are crucial for understanding the structure of the Milky Way.
For astronomers trying to map it, suggests Scientific American, "the effort is a bit like learning the anatomy of a human body from the perspective of a single skin cell somewhere on a forearm". He and his team pinned down the source's location to the Scutum-Centaurus arm of the galaxy, probably one of the Milky Way's two major arms. "What does our cosmic home actually look like, viewed from another nearby galaxy?" It was first used in 1838 to measure the distance to a star in the constellation Cygnus. It's impressive, and these astronomers said their achievement almost doubles the previous record for distance measurement within the galaxy. For the measurement across the width of the Milky Way, they used the continent-wide VLBA.
Previous attempts to observe and accurately map the opposite side of the Milky Way have mainly failed because of interloping interstellar dust in the galactic plane, which blocks optical light from reaching us.
"The idea that you could be doing this for more objects on the far side of the galaxy is really exciting", says Robert Benjamin at the University of Wisconsin, Whitewater. Since we are inside the galaxy, it is hard for us to see its actual shape and structure, because that would require travelling hundreds of thousands of light-years, at the very least, to a distance from where the whole of the galaxy is visible face-on.
The object that Dame and his colleagues measured is so far away that to determine its parallax, they had to measure it over the course of a whole year, taking observations at either side of Earth's orbit around the sun.
Measuring the angle of an object's apparent shift in position this way allows astronomers to use simple trigonometry to directly calculate the distance to that object. That's because, the greater the distance, the smaller the observed shift. In this case, the measurement was roughly equal to the angular size of a baseball on the Moon. The most precise measurement yet of an object on the far side of the galaxy's centre is paving the way for a definitive map of the other side of the Milky Way. The last record for a parallax-based direct measurement was 36,000 light-years. Most of the stars and gas in our galaxy are within this newly-measured distance from the sun. Also, star-forming regions are rich in water and methanol, both of which amplify the radio signals that VLBA detects, acting as natural masers. This effect makes the radio signals bright and readily observable with radio telescopes.
"The Milky Way has hundreds of such star-forming regions that include masers, so we have plenty of "mileposts" to use for our mapping project, but this one is special. We're looking all the way through the Milky Way, past its center, way out into the other side", said the MPIfR's Karl Menten. Although we can peer along its edge while embedded in its disk, we cannot see what it looks like face-on.
Within the next 10 years, we should have a fairly complete picture.The observations were made in 2014 and 2015 and the findings were published in journal Science.