Measuring the Distance of the LMC

How do astronomers measure the distances to the stars and other galaxies? That is a good question and has many answers. For nearby stars this can be done by measuring their apparent change in position with respect to more distant stars, as the Earth orbits the Sun - an effect known as parallax. Further away, we have to use standard candles such as Cepheids and RR Lyrae stars or cluster main sequence fitting. For these techniques, we assume the star to be emitting a certain amount of light and compare this to how much light we observe. Just as the light from a torch gets dimmer the further away it is, we can use the apparent brightness of the star to determine its distance.

For very distant galaxies, we can measure their velocity away from us via the Doppler effect due to the expansion of the universe. However, the velocities of nearby galaxies are largely determined by their own random motions. The Large Magellanic Cloud (LMC), a mid-sized irregular galaxy, is one of the closest to the Milky Way and is visible in the southern hemisphere. A future ESA space mission, GAIA, plans to map the structure of the LMC to the precision we currently have in the Milky Way. However, according to a paper on astro-ph (submitted to ApJ), another space mission could produce independent measurements on about the same time scale. That mission is the Laser Interferometer Space Antenna or LISA.

LISA is another joint mission between ESA and NASA but this doesn't take pretty pictures of stars. In fact LISA doesn't observe any part of the electromagnetic spectrum; this instrument will measure ripples in space-time itself. These ripples are called gravitational waves and are caused when massive objects are accelerated or disturbed. The ripples spread outwards, like the waves caused when you jump into a swimming pool, and can in theory be detected by the vibrations they create between widely separated objects. Lots of people are trying to detect gravitational waves from the Earth although none have been detected yet. That is because they are so small compared to all the other vibrations around us. LISA plans to get around these issues by going into space.

The paper claims that LISA could detect as many as 22 white dwarf binaries in the LMC, via gravitational waves. This could determine the distance of the LMC to an accuracy of around 5% which is pretty good going in astronomy.

Posted in astro blog by Stuart on Saturday 26th Feb 2005 (17:31 UTC) | Permalink
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