# The Naked Scientists Forum

### Author Topic: How do scientists calculate the masses of celestial objects?  (Read 7549 times)

#### abihilder

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##### How do scientists calculate the masses of celestial objects?
« on: 27/05/2009 16:39:16 »
What are the physical parameters that astronomers use to find the masses of celestial objects, for example the sun, planets in our solar system, stars other than the sun and galaxies???
Thanks!!

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« Last Edit: 28/05/2009 04:06:51 by chris »

#### Soul Surfer

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##### Re: How do scientists calculate the masses of celestial objects?
« Reply #1 on: 27/05/2009 23:23:49 »
Physicists can determine the gravitational constant on earth by measuring the attraction of two massive objects in a sensitive torsion balance. This also allows you to calculate the mass of the earth from the acceleration due to gravity at the earth's surface.

The distances of the moon the planets and some of the nearer stars can be measured by parallax either from the earth or by using the earth's orbit.  For the solar system they can also be measured using radar and space probes.

Once you know the gravitational constant and you find a double star or any objects orbiting each other it is possible from measurements of orbital periods and distances to determine the masses of these objects.

Most stars are very simple objects (main sequence stars) and their colour and brightness depends only on their mass.  This can be proved by the measurements of the masses of local stars and confirmed by measuring stars in star clusters.

Therefore once you have seen the star measured its spectrum and established that it is a reasonably standard star you can calculate its mass and its distance.

There are also ways of estimating the masses of more exceptional stars by more indirect methods using the standard atars.
« Last Edit: 27/05/2009 23:35:18 by Soul Surfer »

#### RD

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##### Re: How do scientists calculate the masses of celestial objects?
« Reply #2 on: 27/05/2009 23:25:20 »
It is possible to calculate the mass of a planet from its distance from the sun and its period ... http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion#Nonzero_planetary_mass
« Last Edit: 27/05/2009 23:26:51 by RD »

#### Bored chemist

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##### How do scientists calculate the masses of celestial objects?
« Reply #3 on: 28/05/2009 07:07:42 »
It is possible to calculate the mass of a planet from its distance from the sun and its period ... http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion#Nonzero_planetary_mass
To do that you need to know G so one of the most important experiments in astronomy (Cavendish's experiment) was done in a basement without looking at anything outside the room.

#### relic1974

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##### How do scientists calculate the masses of celestial objects?
« Reply #4 on: 20/11/2011 19:33:55 »
is there a way to determine the mass of a celestial object knowing it's period, and the mass of the body that the object is orbiting?  thank you in advance.

#### Soul Surfer

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##### How do scientists calculate the masses of celestial objects?
« Reply #5 on: 20/11/2011 22:49:29 »
In general looking at a pair of bodies orbiting each other it is possible to calculate the total mass of the pair of bodies.  If it is possible to measure the orbits of both bodies it is then possible to calculate the ratio of the masses of the two bodies.  however if the second body is too small and the motion of the heavier cannot be detected the mass of the smaller body cannot be calculated

#### The Naked Scientists Forum

##### How do scientists calculate the masses of celestial objects?
« Reply #5 on: 20/11/2011 22:49:29 »