Parsec In Astronomy

In our previous article, we have discussed the three major cosmic distance scales which are Astronomical Unit (AU), Light Year (LY), and Parsec (Pc), and their significance in astronomy. You came to know how these units ease up our process of studying and understanding this Amazing Universe.

In this article, we will solely discuss one of these units which is Parsec. By the end of this article, you will be able to understand its derivation, definition, significance, and how the small difference between light year and parsec creates a huge difference in the world of astronomy. 

An artist’s impression of 14 galaxies that are in the process of merging.(NRAO/AUI/NSF; S. Dagnello)

So before we proceed, let's have a brief overview of all these units.

• Astronomical Unit i.e. AU, is the distance from the center of the Sun to a point at which a particle of almost negligible mass revolves in a stationary orbit with an orbital period of 365.2568983 days (One Gaussian Year). 1 AU is numerically equal to 150 million kilometers or 93 million miles.

• Light Year is defined as the product of a Julian Year(365.25 days) and the speed of light (299,792,458 m/s). It is the distance that a photon of light travels in the period of one year. 1 light-year is equal to 9.46 trillion Km.

• A Parsec is a distance at which the length of 1 AU subtends an angle of 1 arc second. Its numerical value is nearly equal to 19 trillion miles and 3.26 Light-Year.

To know more about these units, visit our blog page The Cosmic Distance Scale.

Now here comes the question that if there is less difference between a light year and a parsec, what is the need for a different unit? Let us see how it works...

Parallax Method

In astronomy, the parallax method is one of the oldest methods of determining the distance to a star. This method records the difference in angle between two measurements of the star's position in the sky. The first observation is made from the Earth on one side and the second six months later, when the Earth is on the opposite side of the Sun.

To determine the distance to a nearby star, astronomers use the triangulation method. Now you can test it yourself. Keeping your finger in front of you, focus on something far away and close one eye first, then close the other. When you change your eyes, you will see that your finger seems to be jumping back and forth in front of you. Motion, of course, is an illusion. Your finger doesn't move. Each eye looks at your finger from a slightly different angle. So the finger position looks different from the ones in the background. This apparent change is called parallax, from a Greek word meaning change. 

Measurement of Unit

 By measuring the angle at which your finger seems to be moving, you can get an idea of ​​how far away your finger is from your face. Astronomers also measure angles to determine the distances to the stars. However, instead of flashing, astronomers are moving the earth. 

Or rather, we take advantage of the fact that the earth revolves around the Sun.

The distance between the two positions of the Earth in the two measurements is twice the distance between the earth and the sun. The angle difference between the two measurements is twice the parallax angle formed by the lines from the sun and the earth to the star at the distant vertex.

Now, look carefully at the figure below.

 

(Parallax Method)

If we look at a star in December and then look at it again in June, the Earth will have half-orbited its orbit. We consider the star of two places that are about 300 million kilometers apart. When the star is reasonably close, it appears to move very easily from one side of Earth's orbit to the other.

So the parallax is half the angular distance that the star appears to move across the sky. This is equivalent to the angle of the semi-major axis of the Earth's orbit from the point of view of this star. It is easy to see that the star (D), the sun (S), and the earth (E) form a right triangle.

(Mathematical Representation)

Then we know the angle SDE and the length SE (1 AU). From this information, we can use trigonometry to determine the SD distance and therefore the DE. If the angle is now 1 arc second, then the distance to the star is 1 parsec.

And here’s how we arrive at parsecs as a unit of distance: 1 parsec is the distance to an object whose parallax angle is one arcsecond. 

(Overview of Parsec)

Difference Between Light Year And Parsec?

On the one hand, the Light-year is generally used to measure the distance between the earth and relatively closer heavenly bodies like stars, moons, meteorites, asteroids, etc. While on the other hand, Parsec, on the broader terms used to measure the distance between distant galaxies and stars.

For example, The center of the galaxy is just over 8,000 parsecs from us in the direction of the constellation Sagittarius. 

The Andromeda Galaxy, our closest spiral galaxy, is nearly 800 kiloparsecs away. One kiloparsec is one thousand parsecs. 

On a larger scale, astronomers are beginning to talk about megaparsecs and even gigaparsecs. That's a million and a billion parsecs respectively. They are usually reserved for the largest structures that exist. The Virgo Cluster, a conglomerate of thousands of galaxies into which our own Local Group falls, is 16 megaparsecs from home. It would take 54 million years to get there at the speed of light.

As observed by the Hubble Space Telescope, the astrophysical jet erupting from the active galactic nucleus of M87 subtends 20″ and is thought to be 1.5 kiloparsecs (4,892 ly) long (the jet is somewhat foreshortened from Earth's perspective).