AstroJargon of the Week: Parsec

This week I thought I’d scale back from such a rich topic as AGN and tell you a little bit about the parsec. (In this case, I do not mean the podcast awards!)

A parsec is a measure of distance. But, you are thinking, Nicole, don’t you astronomers already use the light year as a measure of distance? A light year is the distance that light travels in one year, so it is a convenient measure of distance to explain. (It’s about 5,878,630,000,000 miles, if you are curious.) However, astronomers actually use a unit of distance that makes more physical sense with respect to how we measure distances.

The name “parsec” is pretty self-explanatory once you know what two jargon-y words it’s made of. It is defined as the distance that an object is from Earth to create a parallax of one arcsecond.

Hold out your finger at arms length. No, really, do it. I’ll wait. I won’t laugh! Promise. Okay, now close one eye and note what your finger is in front of. Now, open that and close the other. Notice a difference? Compared to some background object, your finger has appeared to move, no matter how still you keep it. That is essentially parallax.

Now, take this system on a grander scale. Each one of your eyes was a point of view. Replace that with the orbit of the Earth, with one point of view 180 degrees around the orbit from the other. Your finger is now some star to which you would like to know the distance. The background of your room is the background of more distant stars and galaxies. A parallax observer takes pictures of their target star six months apart and measures the change in position against the background of stars that are too distant to show this effect. From that measurement, they can tell the distance thanks to a little geometry.

This picture is very much NOT to scale since the distance to any star is much, much greater than the size of the Earth’s orbit. So, we can use some handy approximations to calculate the distance. See that right triangle? We can pull out from our trigonometry background that

tan(p) = (distance from Earth to Sun) / (distance from Sun to star).

But with the “small angle approximation” and using the definition of the distance from the Earth to the Sun (Astronomical Unit, AU, approximately 93 million miles), we get

p = 1 AU / d

Using just the right units, and doing a little switching around, we get just

d = 1 / p

The unit of measure for p is arcseconds, which is a tiny, tiny angle. If you break a circle into 360 wedges, each one is one degree. If you break that wedge into 60 wedges, each is an arcminute. Split up one of those 60 ways, and you get an arcsecond. Another way to see how tiny that is…. hold up your index finger at arms length again. The width of your finger is about 1 degree. So an arcsecond is 1/3600 the width of your finger. Need I say… TINY! A star that measures a parallax of one arcsecond is now one parsec away.

So how far is a parsec really? If a one-arcsecond measurement is so tiny, then a distance of one parsec must be really far away. And it is, on human scales, but the nearest star is 1.3 parsecs away. The center of our Milky Way galaxy is almost 8000 parsecs away, or 8 kiloparsecs*. Cygnus A, one of my favorite radio galaxies, is 230 million parsecs (or 230 megaparsecs) away. Yeah, this stuff is FAR!

How do parsecs compare to the more-easily-explained light year? Well, one parsec is approximately 3.26 light years. If you ask me the distance to some celestial object, if I know it at all, I probably know it in parsecs, and will make a quick and dirty calculation in my head to light years before answering. And like its cousin the light year, the parsec has been mistaken for a measure of time.

Now, before you go off on your own parsec-scale adventure, check out the newest Carnival of Space #145 at Crowlspace.


*Edit: I should be a little more careful with my significant figures. We don’t know the distance all that accurately! Thanks @leifb 🙂

9 comments for “AstroJargon of the Week: Parsec

  1. March 16, 2010 at 14:34

    There’s more to it than just jargon: there’s something weird and cultural attached to the use of “parsec” rather than “light-year”. I’ve seen astronomers give new grad students such a look for using “light-year”; it’s as if they were using imperial units instead of metric. The implication was somehow that light-years are only used by popularizers, while real astronomers use parsecs for everything.

    I don’t know why this is; certainly we shouldn’t be looking down on popularizers, and the light-year is just as good a unit as the parsec. In fact, one could imitate the kilobyte and define a lightyear to be 10^18 cm exactly (instead of 0.95*10^18), making it a nice round metric unit. But for whatever reason, we don’t. We don’t even, mostly, take advantage of the fact that you can turn a parsec around: if you’ve got a nebula or something that’s a kiloparsec away and it’s got a feature an arcsecond across, that feature is 1000 AU across.

    • March 16, 2010 at 15:08

      Wow, really? That’s unfortunate. I’m sure we used a mix of light years and parsecs in undergrad, but now I’m just “used” to parsecs. But it’s certainly easier to explain light-years in a public setting, especially when the concept of parsec isn’t necessary to the story you are trying to tell.

      Also, I like your point about using AU more. It’s somewhat intuitive (as intuitive as astronomical distances can be, anyway.)

  2. Ryan
    March 16, 2010 at 15:46

    I think I understand what a parsec is, but I don’t really understand why it is. Is there any reason why this unit is used as opposed to light year, or is it just a matter of personal preference and one being more commonly used than the other?

    • March 16, 2010 at 17:26

      I’m curious about the history myself! “Why” parsecs makes sense to me because of the parallax programs of the early part of the 20th century which dutifully cataloged the distances of as many nearby stars as possible. Since the parsec is a natural measurement for parallax, then that was used more frequently in the journal when reporting on this. However, the term light-year predates parsec according to Merriam Webster (1888 vs 1913). Not knowing how astronomers talked about distances before parallax programs, I don’t know why parsec evolved to be more popular in general in the journals and light-year in popular writings. Maybe there’s some subsection of the History of Astronomy Division of the American Astronomical Society that looks at the evolution of astronomical jargon 🙂

      • March 16, 2010 at 20:58

        Could the issue have come from a poor measurement of the size of the AU? After all, if you don’t know how big an AU is, then converting a parallax distance into light-years loses you accuracy. (There’s actually a similar argument for giving the solar mass as 1.5 km rather than in kg, because the product GMsun is better known than G or Msun independently.)

        I’m not really convinced by this argument, though, since it’s not hard to get a good measurement of the AU, while it is hard to get an accurate parallax distance.

  3. March 16, 2010 at 22:49

    nice. thx

  4. March 17, 2010 at 10:31

    Anne, that kind of makes sense to me, since the AU wasn’t really well known until radar measurements of the distance to Venus, right? And that had to have been after WWII.

    (Sorry, I think we ran out of “reply” space on that thread.)

    • March 18, 2010 at 02:56

      Well, some reading shows that the word parsec didn’t appear in the astronomical literature until 1913, at which point the AU was known to within about 10%. On the other hand, in 1838 Bessel measured a stellar parallax to within about 10%, which suggests that parallax measurements may have been ahead of AU measurements. Certainly until we got a really solid measurement of the AU there’s a good reason to prefer parsecs to light-years. The IAU also alludes to some subtleties that could matter: there are several candidate definitions of “year”, for example. (They also say that astronomers use parsecs but popularizers use light-years.)

      On the plus side, the conversion factor between AUs and parsecs is roughly 200,000, which isn’t too hard to remember.

  5. April 1, 2010 at 13:21

    My understanding of it is that the parsec is something we can measure directly. (I did a little of this work in the early 1980s with the UVa Parallax Program.) Once you have the measurement in parsecs then you can calculate the distance in light years. So the parsec is more basic, more pure, if you will, based on first principals.

    The light year, though, is something the public can get a handle on more easily, so its use is more popular in that sphere. When I do my “What is Astronomy” talks at McCormick Observatory (usually to grade schoolers) I make a point of showing how fast the speed of light is by holding up seven fingers & then drumming them on the lectern in the span of a second to show how fast it is. Each tap represents one circuit of the Earth.

    Then I hold up (hidden in my hand) a little 1.25″ diameter super ball I keep in my pocket and tell the kids I have a scale model of the Milky Way galaxy in my hand. Then I show the ball and tell them that I have just the part that is our Sun. I then ask them (including the teachers or other adults present) how far away they think the closest star is at that scale. Only 1 person in 4 years has even come close. (She guessed St. Louis, while the answer is Chicago.)

    Then I tell them to imagine the ball in my hand is a light bulb in Chicago and that they can see it from here. With the naked eye.

    Space is vast & empty and stars are bright, huh?

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