Mass of Cosmic Bodies - How Are They Estimated?

[jimmydasaint]

As a Physics newbie, I am puzzled at how textbooks tend to come up with masses (and, I suppose) chemical composition of stars, comets and asteroids.

 

Also, If a star is converting mass into energy, isn't it losing mass (and therefore gravitational attraction)?

 

Any elucidation welcomed.

[ajb]

As a Physics newbie, I am puzzled at how textbooks tend to come up with masses...

 

In essence, as we know how a body of a given mass will move in a gravitational field we can match this to what is observed.

 

Assuming not too strong of a gravitational field we can use Newton's universal law of gravity and Kepler's laws of planetary motion.

 

In practice, this is not so easy as we have to solve a many body problem. Numerical methods are employed.

 

Also, If a star is converting mass into energy, isn't it losing mass (and therefore gravitational attraction)?

 

Yes, but the amount of mass lost is tiny compared to the over all mass of a given star. The production of the energy only occurs in a small region at the centre of a star and is governed by [math]E= mc^{2}[/math]. So, only a small amount of mass is needed to get a lot of energy as [math]c[/math] is large.

[jimmydasaint]

The answers are much appreciated ajb and have saved me a bit of reading. My problem would have occurred by the many body problem where many stars, objects would be attracting each other in a complex way. I did not understand the maths involved but got the idea that modelling can present a possible solution.

 

The mass problem was also clearly explained, and it makes sense that our Sun is estimated to last for another 5000 million years.

 

Gravity is a funny thing though isn't it? Is it true that humans also attract each other gravitationally and that gravity has no limit of distance?