A Cepheid Star in the Andromeda Galaxy

The arrow in this image points to a star in the Andromeda Galaxy that was observed by the Hubble Space Telescope (inset boxes). It is a special type of pulsating star called a Cepheid variable that can be used to make reliable measurements of large cosmic distances.

In 1923 this Cepheid variable star, known as V1 (for Hubble variable number one), altered the course of modern astronomy. The star helped astronomer Edwin Hubble show that Andromeda lies beyond our own galaxy. Prior to the discovery of V1 many astronomers thought spiral nebulas, such as Andromeda, were part of our Milky Way Galaxy.

Edwin Hubble's observations of V1 became the critical first step in uncovering a larger, grander universe. He went on to measure the distances to many galaxies beyond the Milky Way by finding Cepheid variables within them. The velocities of those galaxies, in turn, allowed him to determine that the universe is expanding.

The rate of expansion, called the Hubble constant, is an essential ingredient needed to determine the age, size, and fate of the cosmos. Before the Hubble Space Telescope was launched, the value of the Hubble constant was imprecise, and calculations of the universe’s age ranged from 10 billion to 20 billion years. Now, astronomers using Hubble have refined their estimates of the universe’s present expansion rate and are working to make it more accurate.

They do this by getting better galaxy distance measurements from Hubble and coupling these values with the best galaxy-velocity measurements obtained from other telescopes. Scientists measure distances by comparing the brightness of a known object in our galaxy (like a star or an exploded star) to that of a similar object in a distant galaxy. With Hubble’s refined distance values, calculations currently put the age of the universe as 13.8 billion years.

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: R. Gendler