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Where we are? Cosmically, we are in our home galaxy, typically known as the Milky Way. The center of our galaxy is marked by a supermassive black hole, on 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 we’re actually a little closer to the black hole.
It is difficult to understand where we are in the galaxy. For one thing, we’re in the thick of it all, and a good portion of our view is blocked by a region of gas and dust known as the avoidance zone. We can’t just get out of our galaxy and pinpoint our location on a map. The task is so difficult that only a century ago Harlow Shapley first established that the Sun is not near the center of the Milky Way.
The best way to determine our position is to measure the position and movement of many stars. This is easier said than done because motion 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. Furthermore, the stars do not follow the same general orbit. Some have more circular orbits and others less circular. It is necessary to measure enough to determine an aggregate motion of the Sun relative to the galaxy as a whole. But it is this kind of measurement that gave us the official IAU distance.
To get a better measure of our position, the team used a technique known as Very Long Baseline Interferometry (VLBI). This is where a series of widely separated radio antennas work together to observe an object. Since light takes time to travel, an object’s signal reaches each antenna at slightly different times. By timing the signals, the team can pinpoint 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 pinpoint the position of a star within 10 micro-seconds of arc, which is roughly 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 through the galaxy at a speed of 227 km / s, a little faster than the official value of 220 km / s. This is only the team’s first release of data, so we can expect their measurement to become more accurate over time. VERA will also work with the East Asian VLBI Network (EAVN), which has antennas in South Korea and China. From this, the team will be able to locate stars within 0.5 micro-seconds of arc. So in the near future we will be a little bit closer to knowing exactly where we are.
Reference: Collaboration VERA, et al. “The First VERA Astrometry Catalog.” Publications of the Astronomical Society of Japan 72.4 (2020): 50.
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