Since Hubble's seminal work establishing galaxies as separate "island universes" astronomers now know that galaxies are not isolated but actually interact with each other on a vast cosmic scale. It is now known that galaxies exist not only in clusters but also superclusters. From our standpoint in the Milky Way the closest giant agglomeration of galaxies is the Virgo cluster. Over 2500 individual galaxies reside in the Virgo cluster and together with our local group of galaxies, make up the Virgo supercluster. By virtue of its enormous mass the supercluster creates an attractive force drawing its members towards a center point near the mammoth elliptical galaxy M87. Our local group has experienced the tug of the cluster in the form of a general slowing of our expansion velocity over millions of years. Many of Virgo's giant ellipticals are curiously located at the center of the cluster supporting the belief that elliptical types are the end result of intergalactic collisions and mergers.
The Virgo galaxies have played a key role in defining the so called "Hubble constant" which defines the expansion rate of the universe. Refining the Hubble Constant remains one of the central problems in cosmology and will ultimately allow astronomers to narrow down the true age of the universe. In order to calculate the Hubble constant astronomers require two values; the recessional speed of a galaxy and the precise distance to that galaxy. Establishing precise distances to remote galaxies has been one of the primary goals of the HST. In 1994 the HST studied "Cepheid Variable" stars in M100 to establish with high precision the distance to that galaxy, the brightest member of the Virgo cluster. The distance of 56 million light years allowed an estimation of the Hubble Constant, later refined in 2003 to be 71+/-4 kilometers/second/million parsecs. The age of the universe based on these values is between 13 and 14 billion years old. These numbers will surely change as methods become further refined.
Astronomers depend heavily on "standard candles" which allow the precise determination of distance to remote objects. Because of their high luminosity, Cepheid variable stars have been used as "standard candles" to estimate distances potentially out to 100 million light years. Cepheid variables are radially pulsating giant and supergiant stars up to 10,000 times more luminous than our sun. They have pulsation periods of 1 to 100 days and can brighten up to 2.0 magnitudes but usually show amplitudes of 0.6 to 1.3 magnitude. In 1912, the American astronomer Henrietta Leavitt reported on the fixed period-luminosity relationship of Cepheid variables which she studied in the nearby small magellanic cloud, a small satellite galaxy of the Milky Way. She found that by determining the intrinsic brightness of a Cepheid from its period, we could estimate with good precision the distance to the star. One year after Leavitt's work, Ejnar Hertzsprung used independent methods to measure the distance to several nearby cepheids establishing a precise calibration of the Cepheid distance scale.