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What is a parsec?

Astronomers typically speak of distance in the universe in parsecs, not light-years. One parsec is about 30 trillion kilometers, or just over three light-years.

If you ever listen to astronomers talk among themselves, you won’t hear much talk of light-years. The concept of light-years is a great way to think about distance scales of the universe. But light-years are not very useful when it comes to actually measuring those distances. Parsecs—a unit of distance equal to about 30 trillion kilometers—are much more closely related to how we actually go about the business of figuring out the size of the universe.

To find the distance to a star, astronomers use a method called triangulation. You can try this right now. Hold your finger in front of your face, focus on something in the distance, and close one eye. Now switch which eye is open. If you alternate eyes, you’ll notice your finger appears to dance back and forth in front of your face. The motion is, of course, an illusion. Your finger isn’t moving. Each eye sees your finger from a different angle so the finger’s location, relative to stuff in the background, looks different.

This apparent shift is called parallax, from the Greek word for “alteration.”

Stellar parallax

Parsecs are based on the movement of Earth in orbit around the sun. As Earth moves from one side of its orbit to the other, a nearby star appears to shift position relative to the more distant stars. The amount of shift depends on how far away the star is. Image via Wikipedia user Rasbak.

If you measure the angle over which your finger appears to move, you can figure out how far your finger is from your face. Likewise, astronomers measure angles to find the distances to stars. Rather than blink their eyes, however, astronomers move the Earth. Or rather, we use the fact that Earth moves around the sun.

Schematic of a parsec

One parsec is the distance to a star that shifts by one arcsecond from one side of Earth’s orbit to the other. One parsec equal to 30 trillion kilometers or just over three light-years. Image via Wikipedia user Srain.

For example, if we observe a star in January, and then look at it again in July, the Earth will have gone half way around its orbit. We’re looking at the star from two locations approximately 300 million kilometers apart. If the star is reasonably close, it will appear to move ever so slightly. The parallax angle, combined with the size of Earth’s orbit, lets astronomers calculate the distance to the star.

These angles are miniscule. They’re too small for degrees to be a practical unit of measurement. Parallax angles are typically measured in arcseconds. There are 3,600 arcseconds in one degree. To provide some perspective: one arcsecond is equivalent to the width of an average human hair seen from 20 meters away.

And here’s how we arrive at parsecs as a unit of distance: one parsec is the distance to an object whose parallax angle is one arcsecond.

After first appearing in a 1913 paper by English astronomer Frank Dyson, the term stuck. If you see a star with 1/2 arcsecond of parallax, it is two parsecs away. At 1/3 arcseond, three parsecs. And so on.

Basically, astronomers liked it because it made the math easier!

One parsec is approximately 30 trillion kilometers or 19 trillion miles. That’s a bit over three light-years. The Voyager 1 probe, launched in 1977, is the most distant manmade object from Earth. It is a mere six ten-thousandths of a parsec away. The nearest star to the sun, a small red dwarf named Proxima Centauri, is just over one parsec from us. That is actually fairly typical in our neck of the galaxy – one star for every cubic parsec – but it’s not typical everywhere. In the cores of globular clusters, the density can reach well over a hundred stars per cubic parsec!

Alpha, Beta, and Proxima Centauri

The circle shows the location of Proxima Centauri. At just over one parsec away, it is the closest star to our sun. Sitting in the southern constellation Centaurus, the Centaur, it is most likely gravitationally bound to the bright star on the right: Alpha Centauri. The other very bright star is Beta Centauri, about 100 parsecs (300 light-years) from Earth. Credit: Wikipedia user Skatebiker.

The center of the galaxy lies just over 8,000 parsecs from us in the direction of the constellation Sagittarius. The Andromeda Galaxy, the closest spiral galaxy to our own, is nearly 800 kiloparsecs away, or one thousand parsecs.

At larger scales, astronomers start to talk of megaparsecs and even gigaparsecs. That’s one million and one billion parsecs, respectively. These are generally reserved for the largest structures in existence. The Virgo Cluster, a conglomeration of thousands of galaxies towards which our own Local Group is falling, lies 16 megaparsecs from home. It would take 54 million years to reach it traveling at the speed of light.

The Virgo Cluster

The Virgo Cluster, containing over 1,000 galaxies, sits 17 megaparsecs away. That’s about 54 million light-years or over 500 trillion kilometers. The black holes in this image are actually nearby stars that have been removed from the image.  Via Chris Mihos (Case Western Reserve University)/ESO.

The edge of the visible universe—the cosmic horizon—is the limit of our vision. And it is 14 gigaparsecs away. That’s 46 billion light-years. The universe isn’t old enough for light beyond this region to have yet reached us. We don’t know what lies beyond. More of the same? Or is our bubble of existence different from the rest of creation? We probably won’t know until we get out there.

And with Voyager 1, we are 40 quadrillionths of the way there!

Bottom line: One parsec is defined as the distance to a star that shifts by one arcsecond from one side of Earth’s orbit to the other. One parsec is about 30 trillion kilometers, or just over three light-years.

Christopher Crockett