Jagpal Singh October 2013 ~ All About Astronomy

Tuesday 15 October 2013

Astronomical Distance Scales

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Some Common Distance Units:
  • Light Year: the distance that light travels in one year (9.46 x 10^17 cm).
  • Parsec (pc): 3.26 light years (or 3.086 x 10^18 cm).; also kiloparsec (kpc) = 1000 parsecs and megaparsec (Mpc) = 1,000,000 parsecs.
  • Astronomical Unit (AU): the average separation of the earth and the sun (1.496 x 10^13 cm).
Some Common Distance Units
Some Representative Distances:
  • The Solar System is about 80 Astronomical Units in diameter.
  • The nearest star (other than the sun) is 4.3 light years away.
  • Our Galaxy (the Milky Way) is about 100,000 light years in diameter.
  • Diameter of local cluster of galaxies: about 1 Megaparsec.
  • Distance to M87 in the Virgo cluster: 50 million light years.
  • Distance to most distant object seen in the universe: about 13 billion light years (13 x 10^9 light years).
Logarithmic scale

 
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Monday 14 October 2013

Hubble's constant (Hubble's Law)

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The Hubble constant H is one of the most important numbers in cosmology because it may be used to estimate the size and age of the Universe. Hubble constant indicates the rate at which the universe is expanding. Although the Hubble "constant" is not really constant because it changes with time (and therefore should probably more properly be called the "Hubble parameter"). The Hubble constant is often written with a subscript "0" to denote explicitly (clearly) that it is the value at the present time, but we shall not do so. 

Hubble's Law


The Hubble Expansion Law


In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. This phenomenon was observed as a redshift of a galaxy's spectrum. This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth. The Hubble constant is given by
H = v/d
where v is the galaxy's radial outward velocity, d is the galaxy's distance from earth, and H is the current value of the Hubble constant.

Redshift & Blueshift
(Note -  In physics, redshift happens when light or other electromagnetic radiation from an object moving away from the observer is increased in wavelength, or shifted to the red end of the spectrum. In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength – equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light. Redshifts are an example of the Doppler effect.)


Determining the Hubble Constant


Obtaining a true value for H is complicated. Two measurements are required. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity.
Determining the Hubble Constant
The second measurement, the most difficult value to determine, is the galaxy's precise distance from Earth. The value of H itself must be derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small (these are called peculiar motion, and they represent deviations from the Hubble Law).

Units for Hubble's Constant


The units of the Hubble constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. For example, if the Hubble constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc would have a redshift corresponding to a radial velocity of 500 km/s.


Current Value of the Hubble Constant


The value of the Hubble constant initially obtained by Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances. For the past three decades, there have been two major lines of investigation into the Hubble constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for H around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate H to be around 100 km/s/Mpc.

Current Value of the Hubble Constant


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