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Author Topic: Can we map gravity for the Milky Way?  (Read 2323 times)

Offline Damien Huxley

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Can we map gravity for the Milky Way?
« on: 09/10/2012 00:01:31 »
Iíve been thinking about gravity maps and since we gravity mapped earth and the moon a though has occurred to me that can we map other things like
 
Goce gravity-mapping satellite
http://www.guardian.co.uk/science/video/2011/mar/31/gravity-map-earth-surface-goce [nofollow]
 
NASA's Grail spacecraft
www.space.com/14819-nasa-moon-gravity-probes-science.html [nofollow]
 
Can we detect the sun gravity pull from earth?
 
Can we detect Sagittarius A (Black Hole at centre of Our Galaxy) gravity from earth or space?
 
Can we gravity map the Milky Way galaxy? If we could then we could see all the object out there including other black holes stars and dark matter.
 
As the sun moves around in the spiral arms (Orion Arm) in the Milky Way Galaxy what are we rotating around in our local neighbourhood?
 
Sun's location and neighbourhood
http://en.wikipedia.org/wiki/Milky_Way [nofollow]


 

Offline evan_au

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Re: Can we map gravity for the Milky Way?
« Reply #1 on: 12/10/2012 11:03:47 »
To measure the gravity of an object, it is best to use a sensor fairly close to the object of interest.
If your sensor is too far away, the gravity measurement will be distorted by other, closer objects.
Unless you can physically place your sensor directly on the surface of the object of interest, your gravitational sensor will be in free-fall, so it is hard to gravitational fields directly.

Since we don't know how to travel faster than light (and even struggle to go further than the Moon), we usually have to observe from a distance; but we can get an estimate of the gravity field of an object by using:
  • Another object orbiting it closely:
    • We can see the effects of the Sun's gravity by the way it affects the annual orbit of Earth (and other planets), and by the way it produces tides on Earth.
    • We can estimate the mass of a planet by looking at the orbital period and orbital diameter of a moon circling it
    • One day, we will be able to estimate the mass of a star by taking images of its planets, and working out their orbits. However, this will take a new generation of large, space-based telescopes.
    • The mass of the galactic black hole by looking at the velocity of stars circling it. But it's hard to do in our own galaxy because there is a lot of dust in the plane of the galaxy
    • The velocity of stars towards us or away from us can be measured by using doppler shift of their light.
      The average velocity of stars orbiting a galaxy at different radii can be measured by taking a precise spectrum of a cross section of the galaxy; one side will be rotating towards us, and the other side away from us
      This was one piece of evidence that led to the idea of dark matter: http://en.wikipedia.org/wiki/Dark_matter_halo#Rotation_curves_as_evidence_of_a_dark_matter_halo
  • The colour/temperature of a star: The more massive a star, the faster it burns its fuel, and the hotter it is. By looking at a star's temperature, it is possible to estimate its mass
  • The oscillation period of some variable stars: http://en.wikipedia.org/wiki/Cepheid_variable
  • There are some other methods that work in particular cases:
    • If there are too many stars or galaxies to estimate individually, it is possible to assume an "average" brightness for a star or a galaxy, and come up with an estimate of the mass of a galaxy or supercluster.
    • Theory tells us the maximum mass of a white dwarf before it erupts as a supernova. But astronomers are trying to work out if there are some different ways that a white dwarf could reach that mass (and perhaps momentarily exceed it).
    • Gravitational lensing has been used to estimate the mass and distribution of unseen distant galaxies and free-roaming planets
    • Recently, variations in the X-Rays emitted by an accretion disk allowed some estimate of the mass of a black hole.
    • In theory, Einstein red-shift could be used to measure the mass of an object directly, but it is a subtle effect, and usually dominated by the Doppler effect.

« Last Edit: 12/10/2012 11:10:34 by evan_au »
 

Offline evan_au

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Re: Can we map gravity for the Milky Way?
« Reply #2 on: 12/10/2012 20:02:05 »
Using these and similar techniques, cosmologists have attempted to determine the mass of the observable universe, and its statistical distribution: http://en.wikipedia.org/wiki/Observable_universe#Matter_content

The sun is a plasma, and the temperature & density distribution of the interior can be studied with the aid of vibrations which travel through the Sun: http://en.wikipedia.org/wiki/Helioseismology
 

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Re: Can we map gravity for the Milky Way?
« Reply #2 on: 12/10/2012 20:02:05 »

 

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