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SYFY WIRE Bad Astronomy

Don't panic, but we're 2,000 light years closer to the Milky Way's central black hole than we thought

By Phil Plait
In case you get lost, you’re 25,800 light years (or so) from the Milky Way’s center. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) & Phil Plait

A compilation of old and new observations out of Japan has revised a couple of important physical measurements of the Sun in the galaxy: According to the results, the Sun is orbiting the galactic center a bit faster and, more interestingly, is closer to the galactic center than previously thought.

Hey, there's a supermassive black in the Milky Way's center! Is that a problem?

Nope. The most recent distance measurement before this one put us something like 26,600 light years from the center. The new one is 25,800. That's only about 3% closer, and we're still a long way from the black hole.

If it makes you feel any better, that's about 260 quadrillion kilometers — 260,000,000,000,000,000. A bit of a hike. We're safe.

The new measurements come from a technique called radio astrometry — using radio waves to measure the distances to objects. In this case, they used parallax: the apparent motion of an object as the Earth orbits the Sun. As I've written before:

To synopsize it, as the Earth moves around the Sun, we see close by objects at a slightly different angle (like holding your thumb out and blinking first one eye then the other; your thumb seems to move back and forth relative to more distant objects). We know how big the orbit of the Earth is, so measuring the angles stars move gives us their distance if you apply a little trig.

I go into more detail in the "Distances" episode of Crash Course Astronomy, if you'd like.

This idea is simple in theory but difficult in practice, because as the Earth orbits the Sun, the Sun is orbiting the center of the galaxy, and other objects are orbiting it as well, all moving at different speeds. It's a bit of a mess to untangle, but it helps that at a certain distance from the galactic center, things tend to move at about the same speed. Combining that with where they are on the sky makes it possible to measure their distances and speeds; you need to account for where they are in their orbit and at what angle we see them. It's like watching racecars going around a track: Some seem to be headed toward you and some away depending on where they are, even if they're all traveling at the same speed.

You need extremely high resolution to make these measurements, and the astronomers in this case used VERA: the Very Long Baseline Interferometry (or VLBI) Exploration of Radio Astrometry. This is an array of four 20-meter radio dishes across Japan, using a very sophisticated technique to combine their power as if they are one telescope with a dish the size of their footprint across the island. They're spread out over 2,000 km, so this makes a big telescope.

Astronomers measured the locations and velocities of hundreds of objects in the Milky Way galaxy to triangulate the Sun’s position and velocity, finding that we’re 25,800 light years from the center and moving around it at 239 km/sec. Credit: NAOJ

They observed masers: like lasers, but these emit microwave energy instead of visible light. This is produced in gas clouds where stars are born. Molecules of water or methanol in the clouds get pumped full of energy by the young stars forming there, and re-emit that light in a very powerful manner. These objects can be visible clear across the galaxy, making them very useful for radio astrometry. The astronomers used 224 objects from old and new observations to make a map of all the objects' locations and velocities around the galactic core.

When all of this is done (aided by a physical model of the galaxy's rotation), the distance of the Sun from the galactic center can be found, as does its angular speed (like, how long it takes to go 360° around the galaxy, which is one orbit).

The distance calculated in the new work, 25,800 light years, is indeed closer than older measurements, though consistent with them given the uncertainties in the measurements. Most methods get around 26,000 light years.

Measuring the velocity of the Sun in its orbit around the Milky Way is more complicated (if that's even possible), but in the end they get a speed of 239 kilometers per second. That's pretty fast! But the gravity of the galaxy is strong, and it takes a zippy star to orbit it.

In case you get lost, you’re 25,800 light years (or so) from the Milky Way’s center. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) & Phil Plait

The International Astronomical Union, the Official Keeper of Astronomical Names and Numbers and Such™, lists the distance and velocity of the Sun as 27,700 light years and 232 km/sec. Those were adopted as standards in 1985, and things have moved on a bit since then. Most measured distances are smaller, though the velocities tend to hover around that value. But assuming the new work is correct, the Sun is actually nearly 2,000 light years closer to the Milky Way's center and moving 7 km/sec faster.

Mind you, the Sun isn't really closer and isn't really moving faster than before! We're just getting better measurements of what it's actually doing.

By the way, using these new numbers, it means it takes the Sun 219 million years to circle the galaxy once. This is sometimes called the Galactic or Cosmic Year. This number is smaller than previous estimates (ranging from 230 to 250 million years or so), because the new work gets a closer distance to the Milky Way center (and therefore a smaller orbital circle) and a slightly higher velocity.

That number is of limited scientific use, but it's interesting. Also, my friend and astronomer Jessie Christensen realized that this means that dinosaurs lived on the other side of the galaxy!

Actually it's a bit more complicated than this because the entire galaxy moves around at all different speeds, mixing things up. But conceptually this is pretty cool and gives you an idea of how long ago (and how long!) the dinosaurs were around, and how big the galaxy is.

And just how complicated apparently simple things can be.