Illustration with black background. On the left side of the image is Earth and the Hubble Space Telescope. Hubble is pointing toward the right. A cone of light purple light starts at Earth and stretches to the right side of the image. At intervals along the cone are objects astronomers use to calculate distances. The first are Cepheid variable stars in the Large Magellanic Cloud (a satellite galaxy of our own Milky Way). Next are Cepheids and Type 1a Supernovas in galaxies. The cone next holds an illustration of how light is red-shifted due to the expansion of space. Finally, distant galaxies that host Type 1a supernovas mark the right side of the extended cone. Along the bottom of the illustration is a gauge in yellow that marks the distance of each object in light years: from Earth extending out to 1 billion light years.

Three Steps to the Hubble Constant

This illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All the steps involve building a strong "cosmic distance ladder," by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This "ladder" is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to make the most precise distance measurement to that galaxy. Astronomers compare the measurements of nearby Cepheids to those in galaxies farther away that also include another cosmic yardstick, exploding stars called Type Ia supernovas. These supernovas are much brighter than Cepheid variables. Astronomers use them as "milepost markers" to gauge the distance from Earth to far-flung galaxies. Each of these markers build upon the previous step in the "ladder." By extending the ladder using different kinds of reliable milepost markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values to measurements of an entire galaxy's light, which increasingly reddens with distance, due to the uniform expansion of space. Astronomers can then calculate how fast the cosmos is expanding: the Hubble constant.

Credits: NASA, ESA, and A. Feild (STScI)
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Image CreditNASA, ESA, and A. Feild (STScI)
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