The Parsec

Some days ago I've read a bit of news about the discovering of a better approximation of the Hubble constant, useful to measure the age and the expansion of the Universe.

The text said "...Spitzer took advantage of long-wavelength infrared light to make its new measurement. It improves by a factor of 3 on a similar, seminal study from the Hubble telescope and brings the uncertainty down to 3%, a giant leap in accuracy for cosmological measurements. The newly refined value for the Hubble constant is 74.3 ± 2.1 km per second per megaparsec. ..."

Spitzer is a NASA's telescope that studies the infrared space sinse 2003, and with which the scientists have achieved to reduce the uncertainty of the Hubble constant to around the 3%, a value necessary to estimate the expansion of the Universe according the Hubble's Law.

I'm not going to write about the Hubble's Law or Hubble's Constant (although some scientist says that is a variable: "Hubble's parameter") in this entry, only want to explain a unit meusure used in the value of that constant, the parsec.

The parsec (pc) is a length measure unit used in astronomy which name derives of "parallax of one arc second", and measures the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond.

In a different post I've explained the astronomical unit (AU) as the measure unit approximately the mean of the Earth–Sun distance. Well, the parsec is normally refered in function of astronomical units, or light-years (ly). 

The parallax method is the fundamental calibration step for distance determination in astrophysics. The Earth’s atmosphere limits the sharpness of a star's image, that implies that the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01 arcseconds, and thus to stars no more than 100 pc distant. On the other hand, space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations.

 Equivalences with other measure units:

(Astronomical units): 1 parsec = aprox. 206.26×10³ AU = aprox. 3.26156 ly

(SI units): 1 parsec = aprox. 30.857×10¹² km = aprox. 30.857×10¹⁵ m

(US customary / Imperial units): 1 parsec = 19.174×10¹² mi = 101.24×10¹⁵ ft

References:

   - http://www.rdmag.com/news/2012/10/nasa-precisely-measures-expansion-universe

   - Hubble's Law

   - http://en.wikipedia.org/wiki/Parsec

   - http://es.wikipedia.org/wiki/Parsec