Scientific Background
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  Science Concept Background

The Hubble Law and Hubble Constant are the foundations of the Big Bang theory.  To test the theory, astronomers attempt to make reliable measurements of the expansion rate to determine the size and age of the Universe.  The evidence that stars and galaxies are moving away from us comes primarily from the red shift of the light from them.  These measurements are checked against independent estimates based on the ages of the oldest stars in the Universe.  Debate continues concerning the exact value of the Hubble Constant. 

Background Information

Variable star in galaxy M100The animation at the right is a sequence of photos, taken with the NASA Hubble Space Telescope, of a variable star in galaxy M100.  These variable stars help astronomers calculate distances to other galaxies.  Before 1910, Harvard astronomer Henrietta Leavitt began measuring the brightness of stars in a class known as Cepheid variables, which are bright, young stars with masses 5 to 20 times our own Sun.  She discovered that these stars reveal their true brightness by the way their light varies, and that this makes them reliable markers for measuring astronomical distances. 

Astronomer Edwin Hubble used these Cepheid variable stars to discover that the more distant a galaxy is from Earth the faster it moves away.  There is a proportionality between the distance to a galaxy and its recessional velocity.  This relationship is called the Hubble Law.  The ratio of velocity to distance is known as the Hubble Constant.  Hubble's observations led to the realization that in a uniformly expanding universe, galaxies would have been closer together in the past.  Early in the Universe, the density ( and temperature ) of matter would therefore have been very high.  This leads to a model for the evolution of the Universe called the Big Bang theory.  The theory says that the Universe began in an extremely hot and dense form and has been expanding ever since.  The age of the Universe is estimated to be equal to the inverse of the Hubble Constant. 

One of the primary uses of the Hubble Space Telescope has been to search for very faint Cepheid variables (as shown above) in distant galaxies and to use them to determine galactic distances in an attempt to improve our estimate of the Hubble Constant.  The current estimate of the Hubble Constant suggests that the Universe is between 8 and 15 billion years old. 
 
 
Extra On-Line Resources Hubble Space Telescope

Cross Curricular Connections