16 The Sun – Basic Data (Contd.)

V. B. Bhatia

 

1.  Learning Outcomes

 

After studying this module, you should be able to

  • enumerate the various methods used to measure the solar surface temperature
  • discuss the Wien’s displacement law in physics and its use in measuring solar temperature
  • explain the application of Stefan-Boltzmann law to the solar surface temperature using the luminosity and the radius of the Sun
  • describe how the fitting of black body curve to the observed solar spectrum yields the solar surface temperature
  • derive the temperature of the Sun using its observed colour index
  • explain why the temperatures derived using various methods do not agree with one another
  • describe the layers of the solar atmosphere and explain why they are seen except when the solar disc is covered

 

2.  Introduction

 

In the last module, Module 20, we discussed some basic parameters which characterize the Sun. The first parameter to be discussed was the distance of the Sun from the earth, a quantity called Astronomical Unit. This distance plays a key role in estimating the mass and luminosity of the Sun.  We discussed the attempts made by ancient astronomers, using transits of planets in front of the solar disc to estimate the Sun’s distance from the earth.  We also discussed how they used a particular phase of the moon and moon’s distance from the earth for this purpose. Then we discussed the modern method of finding this distance.

 

Using the distance of the Sun, we could estimate the mass of the Sun by equating the centripetal force acting on the earth and the gravitational force of the Sun which holds earth in its orbit.

 

Another parameter that could be found using the distance was the radius of the Sun. It is easy to find the angular diameter of the Sun and multiplying it with the distance of the Sun, the size of the Sun could be accurately calculated.

 

Yet another important parameter, luminosity of the Sun, was also discussed.  The energy falling per second on a square meter of a surface placed outside the earth’s atmosphere normal to the rays of the Sun is called the solar constant. So, if we draw a sphere of radius equal to one AU centred on the earth, each square meter of its surface receives energy equal to the solar constant. Multiplying the total surface area of this sphere by the solar constant enables us to calculate the total energy emitted by the sun every second. That is the luminosity of the Sun.  Just to digress a bit, variations in the solar constant were also discussed since they may be important in connection with the weather and climate change on the earth and global warming we are witnessing today.

 

An important data for the Sun is its surface temperature, or effective temperature, which we now discuss.

 

 

Suggested Readings

  • http://www.eurastro.de/webpages/MRSPECT.HTM
  • http://kho.unis.no/Eclipse/index_Cor.html
  • http://cseligman.com/text/sun/sunatmosphere.htm
  • http://iopscience.iop.org/article/10.1086/301490/fulltext/
  • http://www.vendian.org/mncharity/dir3/starcolor/details.html

 

Ballestero (Ballesteros, F.J. (2012). “New insights into black bodies “. EPL (Europhysics Letters) 97 (2012) 34008)

  • https://apod.nasa.gov/apod/ap170907.html
  • https://apod.nasa.gov/apod/ap170823.html
  • https://navbharattimes.indiatimes.com/astro/religion–and–spiritualism/panchang/panchang–24–october–2017–tuesday/articleshow/61183632.cms
  • https://en.wikipedia.org/wiki/Panchangam
  • https://en.wikipedia.org/wiki/Hindu_calendar
  • https://hindunet.org/hindu_calendar/