Hubble reveals individual stars in the galaxy M81, including Cepheid variables that were used to determine that the Universe is 13.8 billion years old © NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

Hubble’s greatest discoveries: the age of the Universe

The Cepheid stars discovered by the Hubble Space Telescope has helped us pinpoint the age of the Universe.

The Spiral galaxy, M81, was the first of many galaxies observed by Hubble to find the expansion rate, and therefore the age, of the Universe.


“Before the launch of Hubble, there was a heated debate over whether the Universe was 10 or 20 billion years old,” says Prof Wendy Freedman, an astronomer at The University of Chicago.

Freedman set out to measure Cepheid variable stars – pulsating stars, whose brightness increases and decreases over a timescale of days to months. By determining the relationship between a Cepheid’s brightness and its pulsation rate, it is possible to estimate its distance.

Cepheids are the most accurate way of measuring the distances to galaxies, and for setting the expansion rate of the Universe. The high resolution of Hubble’s instruments meant that the team was able to discover over 800 Cepheids in 24 nearby galaxies.

The Hubble Space Telescope measurements helped to determine that the age of the Universe is 13.8 billion years.

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