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Old 10-03-2012, 07:18 PM
 
Location: Texas
5,070 posts, read 9,076,032 times
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Spitzer's new measurement, which took advantage of long-wavelength infrared instead of visible light, improves upon a similar, seminal study from NASA's Hubble Space Telescope by a factor of three, bringing the uncertainty down to only three percent, a giant leap in accuracy for a cosmological measurement. The newly refined value, in astronomer-speak, is: 74.3 ± 2.1 kilometers per second per megaparsec (a megaparsec is roughly 3 million light-years).
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Old 10-04-2012, 01:59 AM
 
Location: Wasilla, Alaska
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Unfortunately, the article does not clear up the dispute that has been on-going concerning Cepheid Variables. While more common than type 1a supernovas, they are not a good "standard candle" for measuring distance. There are four types of Cepheid and Cepheid-like variable stars:
  • Classical Cepheids;
  • Type II Cepheids;
  • RR Lyrae variables; and
  • Delta Scutis.
All four of these types of Cepheid and Cepheid-like variable stars have the same pulsation rates, ranging from days to months, but they also have DIFFERENT absolute magnitudes. Cosmological distances are determined by knowing an object's absolute and apparent magnitudes. The apparent magnitude is easy, it is measured by what we can see. Absolute magnitude is trickier to determine. With regard to type 1a supernovas, they always have the exact same mass (1.44 solar masses) before they go supernova, therefore its absolute magnitude is always the same for every type 1a supernova. Type 1a supernovas can also be easily distinguished between type II supernovas. Type 1a supernovas contain no hydrogen. Type II supernovas are primarily hydrogen. That makes type 1a supernovas the perfect "standard candle" for measuring cosmological distances.

Unless they are absolutely certain which of the four different types of Cepheid and Cepheid-like variable stars they are viewing, they may incorrectly determine the absolute magnitude of the star and therefore incorrectly determine its distance. This is why the Hubble Constant is expressed as a range between 60 km/s/Mpc and 80 km/s/Mpc. It may not be as precise as 74.3 km/s/Mpc, but it is more accurate.

Last edited by Glitch; 10-04-2012 at 02:09 AM..
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