Q8: How EXACTLY do astronomers measure the distance of a particular star to Earth? (In light years)

One cloudless night sky in July, you decide to take a look at stars. Six months later, work and responsibilities are getting the better of you so you decide to take another quick break from the world. You return to the same spot you observed the sky from last July and take a deep look at the night sky. Chances are, some of the stars you observed 6 months ago, have slightly changed their position when compared to the rest of the night sky. This apparent change in position is due to the Earth’s revolution around the sun. Think of it as looking at the star from a different angle. 

The above image helps illustrate parallax. Assume that the red dot in the image is a star while the January view and July view are the respective images of the night sky you observed. Obviously, the red dot is not in the same place in both views.

The image also contains the formula which shows the mathematical relationship between distance and parallax. In other words, the formula tells us how to compute for the distance given the parallax.

Alternate image to further help illustrate parallax and because who doesn’t like looking at images.

Going back to the above mentioned formula; p is the measure of the arc in seconds while d, which is the distance between the star and the Sun, is measure in parsecs. The average distance between the Earth and the Sun has its own unit which is known as 1 AU (astronomical unit). One parsec, to illustrate just how far a star can be, is equal to 206265 AU. Additionally, for those more comfortable with the measurement of lightyear (ly) which is the distance covered by light in one year, 1ly = 6.324 * 10⁴ AU.

The further away a star is from the sun, the greater the required displacement in space to obtain a discernible parallax. As such, the number of stars whose parallax can be observed simply due to the Earth’s rotation are limited. Satellites can help in this regard. They can take pictures of stars at various points in their exploration in space which astronomers on Earth can use to calculate the distances of those stars.

References and Images:

Department of Physics and Astronomy, Georgia State University. (2012). Parallax. Retrieved from: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html
European Space Agency. (2013). ESA Science and Technology: Stellar Distances. Retrieved from: http://sci.esa.int/education/35616-stellar-distances/
Institute of Astronomy, University of Cambridge. (n.d.). Stellar Distances – Parallax. Retrieved from: http://www.ast.cam.ac.uk/~mjp/calc_parallax.html

Prepared by: Manuel Christian Schuldes