Cepheid variables can help us to determine distances to celestial objects. Cepheids are referred to as "cosmic mileposts" or "standard candles" of the universe because they help us measure interstellar distances.Photo. The Relative Size of a Cepheid Variable. This photograph shows us a part of the M100 galaxy -- its bright center is cut off on the left-hand side. To put the sizes of cepheid variables into perspective, the cepheid variable in this image is in the center of the white box in the upper right-hand corner. From this view, it is a dot so small that we cannot even see it. Courtesy Space Telescope Science Institute, NASA.
Cepheids were first found in Cepheus, near Cassiopeia, in the northern sky. Cepheids get bright rapidly...then dim gradually over periods of time ranging from a few days to a few months. The period of time a Cepheid takes to complete one pulsation is directly related to the star's intrinsic brightness.
Its intrinsic brightness helps us find the distance to the star. If we know how bright the star really is and we can see how bright the star appears to be, then we can measure its distance.
If a Cepheid appears brighter than it should, it's closer to us.
If it appears dimmer than it should, it's farther away.
Use the Hubble Constant, measure the intrinsic brightness of a star, and plug them into a formula. Voilà! The age of the universe will be right there.
A galaxy designated M100 is in the Virgo Cluster of an estimated 2500 galaxies. That's where the latest Cepheid variables have been discovered. The Virgo Cluster can be seen as a faint pinwheel-shaped fuzzy area in the spring constellation of Coma Berenices.
Photo. The M100 Cepheid Variable. At a much greater magnification, this is the tiny cepheid variable (marked by the white arrow) in the M100 galaxy. Even with Hubble, this object is so small that the telescope must be zoomed in so far that the image becomes blocky. Courtesy Space Telescope Science Institute, NASA.
Using Cepheid variables, we find the distance to M100 is 56 million light years -- give or take 6 million light years. Using this information to determine the value of the Hubble constant, we estimated the universe to be between 8 and 12 billion light years old.
M100 is the farthest galaxy in which these special stars have been discovered, but HST needs to find more than 20 Cepheids to get a precise age and size for the universe.