Light years, parsecs and AU’s explained

Measuring distances is very important to obtain the properties of an object, for instance, the width of a street, the height of a building, or the area of a lake. To measure distances on earth, man makes use of units of measurement, such as centimeters, meters, kilometers, miles etc. However, in the vast spaces between stars, such units are inadequate. To measure big distances in outer space, it is necessary to find other reliable methods. Astronomers utilize units of measurement, such as the astronomical unit (AU), the parallax method, the parsec, and the light year.

The astronomical unit (AU):

To measure distances within the solar system, astronomers make use of the astronomical unit (AU) which is the distance that separates the earth from the sun (159 million kilometers;). Mercury is 0.390 AU (58 million km) from the sun, Venus is 0.72 AU (108.2 million km) from the sun, Mars is 1.5 AU (228 million km) from the sun, Jupiter is 5.2 AU (778 million km) from the sun, Saturn is 9.5 AU; Uranus is 19 AU; Neptune is 30 AU; and Pluto is 39 AU from the sun. Eris, which is a dwarf planet, is approximately 68 AU from the sun.

The method of trigonometric parallax:

The method of trigonometric parallax is based on the fact that a small triangle can be related to a big triangle, thus using the AU as the base of the triangle, the angles which are formed can be measured. If one star is observed against the background of other stars on July the 14th and the same star is observed six months later (January the 14th) against the background of other stars when the earth will have traveled right to the other side of the sun, the star will seem to have moved with respect to the background stars in what is known as stellar parallax.

Using this movement and some trigonometry, the parallax angle, which corresponds to a small shift in apparent position in the celestial sphere, can be measured and the rules of trigonometry can be used to determine the distance to the star. Trigonometric parallax permits to measure distances directly by measuring the parallaxes of nearby stars. The parallax of Proxima Centaury is 0.742 arcs seconds = 278.46 AU. In the 1990´s the hipparcos satellite measured parallaxes with a precision of a thousand of an arc second for one hundred thousand stars.

The parsec:

Unfortunately, ground-based telescopes can only measure parallaxes for stars that are a few hundred light years. Telescopes in orbit around the earth are able to measure smaller parallaxes and consequently greater distances; nevertheless, the most distant stars for which parallax can be determined are just a few thousand light years away.

Another unit of distance derived from parallax is the parsec which is short for parallax second. The parsec if equal to 1/ parallax in arc seconds. When the parallax angle of a star measures one arc second, which is equivalent to one sixteenth of an arc minute which in turn equals one sixteenth of one degree, the distance to the star is one parsec. One parsec is equivalent to 206,265 AU. Proxima Centaury is 0.742 arcs seconds = 1.35 parsecs = 278,457 AU.

The light year:

The light year is the distance that the light travels in one year at a speed of 300,000 km/sec. One light year is equivalent to 63, 270 AU or 0.31 parsecs. A beam of light would take 1 ¼ of a second to cross the orbit of the moon; four hours to reach the planet Neptune; and 4.4 years to reach the nearest star (proxima centaury). To obtain the distance of a star in light years, it is necessary to first measure the parallax, turn it into parsecs and then multiply the parsecs by 3.26.

Fifty million light years separate the earth from the galaxy M87 in the Virgo cluster. The Milky Way galaxy is approximately 100,000 light years in diameter. The Milky Way galaxy is 100,000 light years across. The Virgo cluster is about 59 megalight years away. The distance from the earth to the edge of the visible universe is approximately 46.5 gigalight years.

The cosmic distance ladder (extragalactic distance scale) is a series of methods that astronomers use today to determine the distances of astronomical objects beyond the Milky Way galaxy, which are not easily obtained with conventional methods. Some of these methods make use of the properties of these objects, such as its luminosity. A direct distance measurement is only possible for objects that are within 100 parsecs, for objects that are further beyond, it is necessary to associate the methods that provide close distances with methods that make available larger distances.