Imagine the Universe!
Imagine Home  |   Ask an Astrophysicist  |  
Ask an Astrophysicist

The Question

(Submitted June 09, 1997)

I wish to know how we go about calculating the mass of our Sun and other stars. Are there other ways to measure bodies in space rather than by gravitational means? Do we estimate the mass of our Sun by measuring it against the combined mass of all the planets in our solar system? I'm interested in the equations and procedures.

The Answer

You've asked about one of the fundamental issues in astronomy, namely determining the mass of objects such as the Sun and other stars. The short answer is that there is no other way to *directly* measure the mass of the Sun or any other star than by observing the gravitational effects of one object on another.

You can estimate the mass of the Sun one of two ways. Both use Newton's Laws of motion. The first way uses Newton's revision of Kepler's third law, which states that the period squared or any body orbiting the Sun is proportional to its average distance from the Sun cubed. Newton generalized this for all gravitating systems. In the case of the Sun, the equation we can use is:

Mass_of_Sun=((4*pi2)/G) (a3/P2), where pi=3.14159, G is a fundamental constant, a is the radius of the Earth's orbit about the Sun, and P is the orbital period of the Earth about the Sun.

Another method uses Newton's second law and gravitation. In this case, one starts with F=m*a (where F is the force on an object, m is the mass of the object and a is the acceleration of the object due to the force). Since the gravitational force can be expressed as

F = G(MSun)(Mearth)/(R2)

where MSun and Mearth are the masses of the Sun and Earth, and R is the distance between the two, and the acceleration for a circular orbit is equal to the velocity2/R, Newton's second law can be rewritten in this case to give


In both cases, putting in the values for Earth's orbital velocity, distance from Sun and the value for G gives a value of about 2 x 1030 kg for the mass of the Sun. (That's 2 with 30 zeroes after it.)

Since you are obviously very interested in astronomy, I encourage you to locate a textbook on introductory astronomy at the college level. It will have discussions of mass determination, and many of your other questions at about the same mathematical level as this answer. Some textbooks are not very mathematical at all. Try 'Astronomy The Cosmic Perspective' by M. Zeilik and J. Gaustad, or just look around your school or community library for astronomy texts. In any such text, you will find the values of the Earth's orbital parameters, and can go about determining the mass of the Sun on your own!


Padi Boyd
for the Ask an Astrophysicist

Previous question
Main topic
Next question

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index