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Seven Sisters Star Cluster Controversy Solved

Optical image of the Pleiades taken through ground-based telescope shows off the nebulosity these relatively young stars are still wrapped in.  CREDIT: NOAO/AURA/NSF
Optical image of the Pleiades taken through a ground-based telescope shows off the nebulosity these relatively young stars are still wrapped in.
Courtesy of NOAO/AURA/NSF

Good news for fans of that stargazer’s favorite, the Pleiades. Such star clusters are closer than astronomers supposed, most particularly when it comes to the iconic one known to many sky-watchers as the Seven Sisters.  

Not only does a new study, published in Science magazine by a team led by Carl Melis of the University of California, San Diego, put to rest a heated cosmic controversy over the distance to the Pleiades, but it may recast how we peg stellar distances.

More than 1,000 star clusters, groupings of thousands of similarly aged stars, litter the galaxy. The suggestion that the Pleiades was 10 percent farther away from us than long calculated had clouded study of these clusters. The discrepancies in distance left researchers scratching their heads for years, challenging  their basic understanding of how stars form and evolve.

Now though, Melis and colleagues report that a global network of radio telescopes has triangulated the distance to the star cluster.  The newly attained distance to the famous wintertime star cluster, located in the constellation Taurus, has been pinned down to 443 light-years.

Made up of scores of hot blue stars all around 100 million years old, the Pleiades is considered one of the closest star clusters to Earth and is therefore considered a great “cosmic laboratory” for helping us understand how they form and evolve.  Our own sun is believed to have been born in just such a cluster some 4.6 billion years ago.

Up until the 1990s, the distance to the Pleiades was estimated at 430 light-years, but the European stellar mapping satellite Hipparcos, launched in 1989, had made measurements of only about 390 light-years.

With parallax technique, astronomers observe object at opposite ends of Earth's orbit around the Sun to precisely measure its distance.  CREDIT: Alexandra Angelich, NRAO/AUI/NSF
With parallax technique, astronomers observe an object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance.
CREDIT: Alexandra Angelich, NRAO/AUI/NSF

“That may not seem like a huge difference, but, in order to fit the physical characteristics of the Pleiades stars, it challenged our general understanding of how stars form and evolve,” said Melis, in a statement. “To fit the Hipparcos distance measurement, some astronomers even suggested that some type of new and unknown physics had to be at work in such young stars,” he added.

The astronomers used their global radio astronomy system to observe several Pleiades stars over the course of about a year and a half. That allowed them to precisely measure the apparent shift in each star’s position caused by the Earth’s rotation around the Sun. Seen at opposite ends of the Earth’s orbit, a star appears to move slightly against the backdrop of more-distant cosmic objects. Called parallax, the technique is the most accurate distance-measuring method astronomers have, and it relies on simple trigonometry.

“Using these telescopes working together, we had the equivalent of a telescope the size of the Earth,” said Amy Mioduszewski, of the National Radio Astronomy Observatory (NRAO).

“That gave us the ability to make extremely accurate position measurements—the equivalent of measuring the thickness of a quarter in Los Angeles as seen from New York,” she added.

See for Yourself

While the Pleiades is traditionally considered a wintertime treat for sky-watchers, it is visible this time of the year too. You just have to be a night owl to see it.

This sky chart shows the Taurus constellation and the Pleiades star cluster after local midnight as seen in the Northern Hemisphere. Credit: SkySafari
This sky chart shows the Taurus constellation and the Pleiades star cluster after midnight in the Northern Hemisphere. Credit: SkySafari

After local midnight, face the eastern sky and you’ll find the constellation Taurus, the Bull,  charging up the horizon. Aldebaran, its lead orange star, marks the red eye of the bull and pins down the face of the beast, which is made up of a V-shaped cluster of stars. Scan up from its face, and you will find a faint fuzzy patch of light, visible even from city suburbs—that is the Pleiades.

This sky chart shows the moon pointing the way to the Pleiades on the night of September 14.  Credit: SkySafari
This sky chart shows the moon pointing the way to the Pleiades, as it will on the night of September 14. Credit: SkySafari

And mark September 14 on your calendar, because that’s when the moon will pose with the famed star cluster, making it even easier to track it down.

Happy hunting, sky gazers.

Follow Andrew Fazekas, the Night Sky Guy, on Twitter and Facebook.

 

Comments

  1. Steve nappic
    Franklin square ny
    October 18, 11:40 pm

    11:30pm the seven sister can be seen in the sky

  2. Christina Marie Brown
    United States
    August 28, 10:38 pm

    I knew that I was seeing something different about this cluster, if you see it now it is already pretty brilliant!

  3. Fin
    Los Angeles, CA
    August 28, 4:00 pm

    You state in the beginning of this article that such star clusters are closer than supposed.. except the whole article is about how the new measurement is FARTHER than the two previous measurements. How does this make any sense?