Abstract
To span the great distance from our solar system to the farthest clusters of galaxies in our universe, astronomers break down the distance indicators into steps. The nearest ones are measured geometrically, with what is known as parallax. Among the objects whose distances are thus directly measured are a rare type of star known as a Cepheid variable. Henrietta Leavitt about a hundred years ago discovered, by looking at a number of such stars in the Small Magellanic Cloud, that the brighter the star, the longer its period. This secondary indicator can be used in nearby galaxies to calibrate as a standard candle a specific type of supernova (Type Ia, from explosions of white-dwarf stars), whose peak intensities are so bright that they can be seen throughout the observable universe.
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