New measurement brings the Sun 2,000 light years closer to the center of the Milky Way

Where are we? Cosmically, we are in our home galaxy, commonly known as the Milky Way. The center of our galaxy is marked by a supermassive black hole, which the Sun orbits at a distance of about 30,000 light years. The official distance, set by the International Astronomical Union in 1985, is 27,700 light years. But a new study has confirmed that we are actually a bit closer to the black hole.

It is difficult to know where we are in the galaxy. On the one hand, we’re in the middle of it all, and much of our view is blocked by a region of gas and dust known as the Avoidance Zone. We cannot just step out of our galaxy and locate our location on a map. The task is so difficult that it was only a century ago that Harlow Shapley first determined that the Sun was not near the center of the Milky Way.

The best way to determine our location is to measure the position and motion of many stars. It’s easier said than done because the movement is relative. As nearby stars orbit the Milky Way, so does the Sun, and we can only measure a star’s movement in relation to us. In addition, the stars do not follow the same general orbit. Some have more circular orbits and others less circular. You need to measure enough to determine an overall motion of the Sun relative to the galaxy as a whole. But it is this type of measurement that gave us the official distance from the IAU.

To get a better measure of our location, the team used a technique known as Very Long Baseline Interferometry (VLBI). This is where an array of widely separated radio antennas work together to observe an object. Since light takes a long time to travel, an object’s signal reaches each antenna at slightly different times. By timing the signals, the team can identify the location of the object. In this case, the team used the VLBI Exploration of Radio Astrometry (VERA), which has antennas scattered throughout the Japanese archipelago. VERA can locate a star within 10 micro-seconds of an arc, which is roughly the width of a penny on the lunar surface.

VERA has measured the position and motion of nearly a hundred stars in our galactic neighborhood. From there, the team determined that the Sun is 25,800 light years from the galactic center. They also found that it orbits through the galaxy at a speed of 227 km / s, which is a little faster than the official value of 220 km / s. This is only the team’s first data release, so we can expect their measurement to become more accurate over time. VERA will also collaborate with the East Asian VLBI Network (EAVN), which has branches in South Korea and China. From there, the team will be able to pin the stars at 0.5 arc second. So in the near future, we’ll be a little closer to knowing exactly where we are.

Reference: VERA collaboration, et al. “The first VERA astrometry catalog.” Publications of the Astronomical Society of Japan 72.4 (2020): 50.