Common units used in astronomy.

While watching astronomical programs on TV or while reading news or article about any astronomical events, we always come across some of the different units of measurement. We don't use these measurement units in our everyday life since they represent very small or very big numbers which we hardly deal with in our normal day to day life. These units are quite simple to understand as they based on very simple concepts in geometry or physics. These units represents distances, time, brightness, temperature and many other entities in the gamut of astronomy.
So Let's see some of them one by one.

Parsec
The Parsec is another unit to measure the distances in astronomy other than AU and Light year. When astronomers use triangulation to determine distances, then Parsec unit is used to measure distances. Generally a parsec is 3.2616 light years or 30,857,000,000,000 km.

Now let us see, how Parsecs are calculated when Trignometry or Parallax is used to measure Distances.

Calculating Distance in Parsec

Now as shown in this figure the distance from the Earth to the Sun (i.e. 1 AU) is taken as the base of the right triangle, which is formed between three points
1. Earth
2. Sun
3. Object of Measurement (any distant star/ or Galaxy)

And the arcsecond is used as the angle. These angles have very small value, That is why it is measured in arcseconds. One Arcsecond is equal to 1/3,600th of a degree.

Arcsecond is calculated by astronomers in following way.
At some given time of the year, an astronomer can line up a distant star or galaxy with a nearby star of known distance from the Earth. Then, six months later, when the Earth is on the other side of its orbit around the Sun, the astronomer will line up the distant star with the known star. But because of the parallax effect, the distant star will seem to have moved and will be an a different angle from the Earth. By measuring that angle and making some calculations, the astronomer can determine the distance to the far-off star or galaxy.Generally measurements of arcsec angle are made in two parts of the Earth's orbit around the Sun, in order to get as great a distance for the base as possible.

Calculating the value of a parsec






calculating parsec

In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. Thus the distance ES is one astronomical unit (AU). The angle SDE is one arcsecond (1/3600 of a degree) so by definition D is a point in space at a distance of one parsec from the Sun. By trigonometry, the distance SD is



One AU = 149,597,870,691 m, so 1 parsec ≈ 3.085 678×10¹⁶ metres ≈ 3.261 564 light-years.

Parsec and Kiloparsec
Distances measured in parsecs include distances between nearby stars, such as those in the same spiral arm or globular cluster. A distance of one thousand parsecs is commonly denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to measure distances between parts of a galaxy, or within groups of galaxies. So, for example:

• One parsec is approximately 3.262 light-years.
• The nearest known star to the Earth, other than the Sun, is Proxima Centauri, 1.29 parsecs away.
• The center of the Milky Way is about 8 kpc from the Earth, and the Milky Way is about 30 kpc across.
• The Andromeda Galaxy (M31) is slightly less than 800 kpc away from the Earth.

Even if Parsec Unit is used frequently to measure the distance of distant star and galaxies, its not used that popularly by many scientist or other people.