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Q. What is a Cepheid variable? — Lanny W.

Down by the station, early in the morning. See the little pufferbellies all in a row. See the stationmaster, pull the little handle. Puff, puff, toot, toot, off they go!

A. A Cepheid variable is a pufferbelly star.
Cepheid variables are stars that expand and contract, expand and contract. At one time, the star is deflated. At another, it is puffed up.

In the words of astronomers, a Cepheid variable oscillates between two states.

• In one state, the star is compact. Temperature and pressure inside the star increase, causing the star to expand.
  
  
• In the other state, when the the star is expanded, pressure inside drops and the star contracts to its compressed state.

Cepheid variable stars range from five to twenty times the size of our Sun. The more massive of those expand farther and give off more light.

For astronomers using telescopes on Earth, it has been difficult to see Cepheids in distant galaxies because our fluctuating atmosphere blended light from the variable stars into the general light of their home galaxies.

Local Group. The Large Magellanic Cloud and the Small Magellanic Cloud are satellite galaxies of our own Milky Way galaxy. They contain large numbers of Cepheid variables stars. The more distant Andromeda galaxy also contains Cepheid variables.

The two Magellanic Clouds, Andromeda, the Milky Way and others comprise the Local Group of galaxies in our neighborhood of the Universe.

Hubble Constant. Astronomers use Cepheid variable stars as cosmic yardsticks, measuring distances to galaxies to compute the Hubble Constant [learn more »»] used to estimate the size and age of the Universe. Recently, they have calculated values for the Hubble constant between 50 and 100 km/s/Mpc.

Those numbers are among the most important in cosmology. Cosmology is the study of the total physical universe in time and space, including its history, structure and dynamics.

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