15 The Sun – Basic Data

V. B. Bhatia

 

1.  Learning Outcomes

 

After studying this module, you should be able to

  • explain the study of the Sun is important for stellar astrophysics
  • describe the various ways in which the distance of the Sun has been determined
  • define angular diameter and explain how it can be determined
  • explain how matter is exchanged between the two components of the binary system
  • understand how Kepler’s third law leads to the determination of solar mass
  • define luminosity of the Sun
  • define solar constant and explain how it can be measured with pyroheliometer
  • appreciate that solar constant varies over short as well as long time scales
  • discuss the effect of variation of solar constant on the weather and climate of the earth

 

 

Summary

  • Sun is the star nearest to us. It is a main sequence star.
  • The Sun is an ordinary star, not too big, nor too sm It is neither too hot nor too cool.
  • We can observe Sun’s disc in great detail, and whatever we learn from these observations and their theoretical explanation, can apply to similar main sequence stars. In a way, the Sun acts as a laboratory for us.
  • The basic data for any study of the Sun is its distance from the earth, its size, its mass, luminosity and surface temperature.
  • In 17th century it was possible to build a scale model of the solar system using Kepler’s third law. Just one distance, say, of one planet from the Sun, or from another planet, could give all the distances in the solar system, including the distance of the Sun from the earth.
  • Observations of the transit of Venus in front of the Sun were used to determine its distance from the earth. Currently, the distance of Venus from the earth can be determined accurately by sending a laser beam and computing its time of journey to the planet and back. This, coupled with the observed maximum divergence of Venus from the Sun, gives the distance between the Sun and the earth.
  • The distance of the Sun from the earth is called astronomical unit (AU). It is a useful unit of distance within the solar system.
  • The size of the Sun is determined by the product of its angular diameter with 1 AU.
  • Luminosity of the Sun can be calculated using solar constant (energy per unit area per unit time on a surface placed normal to Sun rays) with the surface area of a sphere of radius 1 AU.
  • The solar constant varies in value sporadically and in sympathy with the solar cycle.

 

Suggested Readings

 

The major sources for the subject matter covered in this module are:

 

  • http://webpages.charter.net/darksky25/Astronomy/Articles/sun/sunindex.html
  • https://svs.gsfc.nasa.gov/2644https://science.nasa.gov/science-news/science-at-nasa/2003/17jan_solcon
  • https://www.cs.mcgill.ca/~rwest/link-suggestion/wpcd_2008-9_augmented/wp/s/Solar_variation.htm
  • https://solarsystem.nasa.gov/planets/sun/indepth
  • http://ircamera.as.arizona.edu/astr_250/Lectures/Lec12_sml.htm
  • http://www.exploratorium.edu/venus/question4.html
  • http://webpages.charter.net/darksky25/Astronomy/Articles/sun/sunindex.html
  • https://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate
  • https://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant
  • https://earthobservatory.nasa.gov/Features/SORCE/sorce_03.php
  • http://www.storyofmathematics.com/indian.html
  • http://www.phy6.org/stargaze/Shipprc2.htm