Parsec

• 1. Introduction

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  The parsec is an astronomer's unit of length, and the nearest star, Proxima Centauri, sits about 1.3 of them from the Sun. Seen from that star, the whole gap between Earth and the Sun would look slightly less than one arcsecond wide. That tiny angle is the secret behind the unit's size. How can a measure of distance be built out of an angle? Who gave it such an odd name? How far can instruments reach before the method fails, and how does one unit stretch from the edge of the Solar System to the edge of the observable universe? The answers begin with an old trick: looking at one star from two different places.

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• 2. Two Seasons, One Star

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  In 1838, the German astronomer Friedrich Wilhelm Bessel published the first successful direct measurement of the distance to an object between the stars. His target was 61 Cygni, which he placed 3.5 parsecs away. The method he used was already among the oldest ways to size up a star.

  An astronomer records the star's position in the sky once, then waits about half a year. By then Earth has swung to the opposite side of the Sun, and the star is measured again. The two observing positions lie twice the Earth-Sun distance apart.

  Between the two readings, the star seems to have shifted against the sky. That shift equals twice the parallax angle, which is the angle formed at the star by lines running to the Sun and to Earth. Put another way, parallax is the angle that Earth's orbital radius would span if seen from the star itself. With the angle and the baseline known, trigonometry delivers the distance.

  The sky handed Bessel a number in arcseconds, and that number turned out to hold the distance almost ready-made.

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• 3. A Skinny Triangle

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  Picture a right triangle stretching out into space, with one short leg measuring exactly one astronomical unit, the average distance from Earth to the Sun. The angle at the far vertex, opposite that short leg, measures one arcsecond. The long leg beside that angle is one parsec. That is the entire definition.

  In familiar figures, one parsec comes to about 206,265 astronomical units, or roughly 30.9 trillion kilometres. It also equals about 3.26 light-years. A mirror-image statement holds as well. A parsec is the distance from which a disc one astronomical unit across would appear exactly one arcsecond wide.

  A parallax of one arcsecond puts a star one parsec from the Sun. A parallax of half an arcsecond puts it two parsecs away, and so on. The distance in parsecs is simply the reciprocal of the parallax in arcseconds. No trigonometric functions are needed, because the angles involved are so small that the skinny triangle can be solved by approximation.

  That tidy reciprocal rule is a large part of why the unit needed a name, and in 1913 it was offered three.

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• 4. A Name and an Exact Number

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  In 1913, the Astronomer Royal, Frank Watson Dyson, wrote in an astronomical publication that this unit of distance badly needed a name. He proposed astron himself. He also noted that Carl Charlier had suggested siriometer, and that the British astronomer Herbert Hall Turner had offered parsec.

  Turner's word, a contraction of a distance corresponding to a parallax of one arcsecond, is the one that stuck. The term may have been used earlier, but this was its first mention in an astronomical publication. The unit was meant to simplify turning raw observations into distances. Partly for that reason, astronomers and astrophysicists prefer it, while popular science and everyday speech still lean on the light-year.

  In August 2015, the International Astronomical Union passed Resolution B2 as part of standardizing a scale for absolute and apparent bolometric magnitude. The resolution cited an existing explicit definition of the parsec, built on the small-angle approximation. Because the astronomical unit had been fixed in 2012 at exactly 149597870700 metres, the parsec became an exact length too. To the nearest metre, it is 30856775814913673 metres.

  The older inverse-tangent definition differs from this one only after the 11th significant figure.

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• 5. Beating the Atmosphere

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  Ground-based telescopes can measure parallax only for stars no more than about 100 parsecs away. The reason is the air itself. Earth's atmosphere blurs the almost-pointlike image of a star into a seeing disc, and the disc's position can be pinned down only roughly.

  Space-based telescopes escape that blur. Between 1989 and 1993, the European Space Agency's Hipparcos satellite measured parallaxes for about 100000 stars. It produced accurate distances for stars as far as 1000 parsecs away.

  The agency's Gaia satellite launched on the 19th of December 2013 with a far bolder goal: measuring one billion stellar distances. Its target was errors of just 10% even for objects as far off as the Galactic Centre, about 8000 parsecs away in the constellation of Sagittarius. All of this work rests on parallax, which remains the fundamental calibration step for measuring distance in astrophysics.

  Beyond the Galactic Centre, counting in single parsecs gets clumsy, and astronomers reach for a prefix worth a thousand.

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• 6. From Probes to Galaxies

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  Voyager 1, the most distant space probe, is only 0.0007897 parsecs from Earth. The Oort cloud, estimated at about 0.6 parsecs across, is larger still but remains part of the Sun's own neighbourhood. Fractions of a parsec generally describe objects inside a single star system.

  Whole parsecs suit the spaces between nearby stars, such as those in the same spiral arm or globular cluster. The open cluster known as the Pleiades lies 130 parsecs from us, or 420 light-years, according to Hipparcos parallax measurements.

  A kiloparsec, written kpc, is 1000 parsecs. Astronomers use it for distances between parts of a galaxy or within groups of galaxies. The centre of the Milky Way is more than 8 kpc from Earth, and the Milky Way is roughly 34 kpc across. ESO 383-76, one of the largest known galaxies, has a diameter of 540.9 kpc.

  The Andromeda Galaxy, also catalogued as M31, lies about 780 kpc away, which is where the next prefix takes over.

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• 7. Megaparsecs and Beyond

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  A megaparsec, written Mpc, is one million parsecs, or about 3.26 million light-years. Astronomers use it for neighbouring galaxies and galaxy clusters. The nearest large galaxy cluster, the Virgo Cluster, is about 16.5 Mpc from Earth. The galaxy RXJ1242-11, observed to have a supermassive black hole core like the Milky Way's, is about 200 Mpc away.

  Sometimes galactic distances appear as Mpc/h, as in 50/h Mpc. Here h is the dimensionless Hubble constant, a value between 0.5 and 0.75 that reflects uncertainty in the rate of the universe's expansion. The Hubble constant matters whenever an observed redshift must be converted into a distance.

  One gigaparsec, written Gpc, is one billion parsecs, making it one of the largest units of length in common use. Astronomers apply it to large-scale structures such as the CfA2 Great Wall, to distances between galaxy clusters, and to the distance of quasars. Since November 2013, the largest known structure in the universe has been the Hercules-Corona Borealis Great Wall, a galaxy filament about 3 Gpc across.

  The particle horizon, the boundary of the observable universe set by the cosmic microwave background radiation, has a radius of about 14 Gpc.

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• 8. Cubic Counting

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  The Sun is currently the only star in its own cubic parsec. In globular clusters the picture is very different, and stellar density can begin at 100 stars in the same volume.

  Volumes of cubic kiloparsecs are how astronomers estimate the number of stars in the Milky Way. They select volumes in various directions, count every star inside, and determine the total statistically. The number of globular clusters, dust clouds and interstellar gas is found in the same way.

  Cubic megaparsecs do the same job for superclusters. Astronomers pick volumes, classify and tally the galaxies inside, and then estimate the overall total. The huge Boötes Void is measured in cubic megaparsecs. Physical cosmology goes larger still, using cubic gigaparsecs to chart how matter is distributed through the visible universe and to count galaxies and quasars.

  Even the gravitational wave interferometers LIGO and Virgo state their observational volume in cubic megaparsecs, essentially the effective distance cubed.

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