Star that changed the universe

Andromeda Galaxy with insets of star V1

Observations of a star in the Andromeda Galaxy that changes its brightness in a regular pattern, convinced astronomers that our cosmos was huge. Edwin Hubble's further studies of such stars showed that the universe is expanding.

THOUGH THE UNIVERSE IS FILLED with billions upon billions of stars, observations of a single star in 1923 altered the course of modern astronomy. And, at least one famous astronomer of the time lamented that the discovery had shattered his worldview.

The star goes by the inauspicious name of Hubble variable number one, or V1, and resides two million light-years away in the outer regions of the Andromeda Galaxy. V1 belongs to a special class of pulsating star called Cepheid variables, which can be used to make reliable measurements of large cosmic distances.

The star helped Edwin Hubble show that Andromeda lies beyond our galaxy. Prior to the discovery of V1 many astronomers, including Harlow Shapley, thought ‘spiral nebulae’, such as Andromeda, were part of our Milky Way Galaxy.

Others weren’t so sure. In fact, Shapley and Heber Curtis held a public debate in 1920 over the nature of these nebulae. But it took Edwin Hubble’s discovery just a few years later to settle the debate.

Hubble sent a letter, along with a light curve of V1, to Shapley telling him of his discovery. After reading the note, Shapley reportedly told a colleague, “Here is the letter that destroyed my universe.”

The universe became a much bigger place after Edwin Hubble’s discovery.

Andromeda Galaxy with an overlay of a Cepheid star light curve

Cepheid variable stars like V1 change their brightness with a regular pattern. This characteristic enables astronomers to use them to measure distances in the cosmos, by comparing their apparent brightness with their calculated theoretical brightness. Courtesy NASA, ESA, and Z. Levay (STScI), HHT (STScI/AURA), AAVSO. Acknowledgment: T. Rector (University of Alaska, Anchorage).

Cosmic distance ladder

In commemoration of this landmark observation, astronomers with the Space Telescope Science Institute’s Hubble Heritage Project partnered with the American Association of Variable Star Observers (AAVSO) to study the star.

AAVSO observers followed V1 for six months, producing a plot, or light curve, of the rhythmic rise and fall of the star’s light. Based on this data, the Hubble Heritage team scheduled Hubble Space Telescope time to capture Wide Field Camera 3 images of the star at its dimmest and brightest light levels.

“This observation is a reminder that Cepheid variables are still relevant today,” explains Max Mutchler of the Heritage team. “Astronomers are using them to measure distances to galaxies much farther away than Andromeda. They are the first rung on what astronomers call the cosmic distance ladder.”

Edwin Hubble's original photo of Andromeda

Edwin Hubble's original photo of Andromeda, showing three stars of interest marked 'N'. The one at the top became even more interesting when it was recognised as being variable (hence 'VAR'). This is Hubble's V1 star.

Copies of the photograph Edwin Hubble made in 1923 flew onboard space shuttle Discovery in 1990 on the mission that deployed Hubble. Two of the remaining five copies were part of space shuttle Atlantis’s cargo in 2009 for NASA’s fifth servicing mission to Hubble.

The most important star

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.

“V1 is the most important star in the history of cosmology,” says astronomer Dave Soderblom of the Space Telescope Science Institute, who proposed the V1 observations.

The space telescope that bears his name continues using Cepheids to refine the expansion rate of the universe and probe galaxies that were far beyond Edwin Hubble’s reach.

Adapted from information issued by STScI. Images courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: R. Gendler.

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  1. PCPete says:

    Excellent article!

    I first heard about Cepheids from reading Carl Sagan when I was a kid. But it was fascinating to see the original plate Hubble worked from… Now that shows the amazing dedication those guys had for astronomy. I mean, there must have been hundreds of similar photos that he had to work through to see the tiny changes – even the modern HST images take some close scrutiny to see the difference!

    Although I’d dearly love to work in an observatory, I don’t think my skills are anywhere near up to his! (I’m assuming it was Hubble who got the job because he was so good at getting excellent photos through the equipment, even though he wasn’t a ‘pro’? Or was that Milt Humason?)

    Anyway, great story with a really nice tie-in. I’m going to dig around and get a better copy of the original photo. I’m sure it’s in an online archive somewhere.

    Thanks Jonathan!