What is a kiloparsec?

(What is a kiloparsec, and what do we use it for?)

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Kiloparsecs are another unit used to measure huge distances between objects in space, and if the ‘kilo’ part seems familiar from having seen words like kilogram and kilometer, you’d be exactly right - the prefix here just means 1000 of the part that follows. In this case, the base unit is a parsec, instead of a gram or a meter.

The parsec (regardless of what you remember from Star Wars’ infamous line re: the Kessel Run) is a unit of distance that’s equivalent to 3.26 light years. A light year, as we’ve talked about before, is quite a long distance, but once you get away from the immediate group of stars surrounding our solar system, it’s less useful as a ruler, since everything is so far away. The distance from us to the center of our galaxy is measured in tens of thousands of light years.

Unlike the light year, which is based in the physics of light, a parsec was determined geometrically using the size of the orbit of the Earth around the sun, and the apparent motion of nearby stars. The actual term “parsec” has a somewhat fun origin - it’s actually a mashup of the words “parallax” (the apparent motion of the stars) and “arcsecond” (how far they move). Arcseconds are frequently abbreviated as “arcsec”, so parallax-arcsec got shortened into parsec.

Parallax, as we just mentioned, is describing the apparent motion of a star. But more generally, parallax is the term for the relative motions of any two objects at different distances. The most common example is looking out the window of a moving vehicle - the shrubs, trees, and telephone poles near the road move very quickly past your window, but things further away - any mountains or distant buildings will appear to move at a much slower pace from your perspective. Applying this idea to the stars, stars which are quite close to us will appear to shift ever so slightly relative to other stars in the sky which are at much greater distances, which will appear fixed.

Arcseconds tell us how far these stars appear to move. They are a unit of size on the sky - sixty arcseconds go into an arcminute, sixty arcminutes go into a degree, and 360 degrees makes a full circle (no one has ever said that astronomers use sensible units). To give you some scale, the full moon is half a degree across - thirty arcminutes. Venus, the bright morning or evening star, tends to be a few tens of arcseconds in the sky.

One parsec is defined geometrically as the distance from the Sun where the motion of the earth around the Sun causes a parallax of one arcsecond. (The image at the top shows a diagram of this.) As our planet moves from one extreme in our orbit to the other extreme, it has moved by 2 astronomical units side to side - 186 million miles. And after moving 186 million miles, we are looking for a star which has moved by less than one tenth the size of Venus in the sky. If the star moved by exactly 1 arcsecond, it was defined to be at a distance of precisely one parsec from the sun. Most of the stars in the sky are too distant to make these measurements directly, but we can still use this geometric definition as a ruler.

This unit was useful back in the day when the speed of light was not so precisely known. Since the distance from the earth to the Sun was well known, and the rest of the calculation is just geometry, the parsec could be used as a distance measurement without depending on the speed of light. Nowadays, we have the speed of light measured to phenomenal precision, so we can use either unit of distance, depending on which one is more convenient.

Kiloparsecs (1000 parsecs, or 3260 light years) are the unit of choice for measuring distances when galaxies are involved. Rather than saying that the Earth is about 30,000 light years from the center of our galaxy, it’s easier to say that it’s a little over 8 kiloparsecs. The distances between galaxies can also be given in kiloparsecs, although unless something exciting is happening, or they’re in a very particular region of space, the distances between galaxies are often several hundred kiloparsecs or more. Andromeda, our galactic neighbor, is 780 kiloparsecs away. That’s about 2.5 million light years.

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