Where are we? We are cosmic in our home galaxy, commonly known as the Milky Way. The center of our galaxy is marked by a supermassive black hole that the Sun orbits at a distance of about 30,000 light-years. The official distance determined by the International Astronomical Union in 1985 is 27,700 light years. But a new study has confirmed that we are actually a little closer to the black hole.
It is difficult to know where we are in the galaxy. On the one hand, we are in the middle of everything and much of our view is blocked by a region of gas and dust known as the avoidance zone. We can’t just go outside our galaxy and map our location. The task is so difficult that it was not until a century ago that Harlow Chapley first discovered 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. This is 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 the motion of a star relative to us. In addition, the stars do not follow the same common orbit. Some have more circular orbits and others less circular. You need to measure enough to determine the total motion of the Sun relative to the galaxy as a whole. But it was 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 Basic Interferometry (VLBI). This is where an array of widely spaced radio antennas work together to monitor an object. As light takes time, the signal from an object reaches each antenna at a slightly different time. By synchronizing the signals, the team can determine the location of the object. In this case, the team uses VLBI Exploration of Radio Astrometry (VERA), which has antennas scattered throughout the Japanese archipelago. VERA can determine the location of a star with an accuracy of 10 micro-arc seconds, which is approximately equivalent to 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 this, the team determined that the Sun is 25,800 light-years from the galactic center. They also found that it orbits the galaxy at a speed of 227 km / s, which is slightly faster than the official value of 220 km / s. This is only the first message from the team, so we can expect their measure to become more accurate over time. VERA will also cooperate with the East Asian VLBI Network (EAVN), which has antennas in South Korea and China. From this, the team will be able to capture stars with an accuracy of 0.5 micro-arc seconds. So in the near future we will be a little closer to knowing exactly where we are.
Reference: VERA cooperation, etc. “VERA’s first astrometry catalog.” Publications of the Astronomical Society of Japan 72.4 (2020): 50.