The Pleiades, also known as the “Seven Sisters”, is the sky's most famous star cluster. Yet even the name poses a mystery. The Kepler Space Telescope has been used to measure the variability of their brightness, changing our understanding of some of them and opening up new methods for learning about other bright stars. Eventually, this work may help us explain an oddity in the way ancient cultures worldwide described the cluster.
The Kepler Space Telescope allows us to track the light curve of variable stars with a precision Earth-based telescopes cannot match. However, it is designed for looking at faint stars, and its pixels become saturated when looking at stars as bright as those in the Pleiades. Dr Tim White of Aarhus University in Denmark set out to find a technique to change that.
"The solution to observing bright stars with Kepler turned out to be rather simple," White said in a statement. "We're chiefly concerned about relative, rather than absolute, changes in brightness. We can just measure these changes from nearby unsaturated pixels, and ignore the saturated areas altogether."
It’s not quite as simple as White makes it sound, however. The team needed to create a method to weight the light from each pixel to remove errors in the detector and allow for subtle movements of the satellite.
In Monthly Notices of the Royal Astronomical Society, White and his co-authors report that “halo photometry” as they call their technique, can also be useful for ground based telescopes, since it allows measurements of stars’ brightness that use fewer pixels and therefore chew up less bandwidth when measuring many stars at once.
Using halo photometry on the seven brightest Pleiades, White confirmed six vary in brightness on a cycle of about one Earth-day. Some also show larger swings on longer time-scales. Despite this, the variability patterns are very diverse, from smooth to quite jumpy.
The most distinctive is Maia, the fourth brightest of the Pleiades, which has a much slower, but also more regular, 10-day rotation cycle. The authors attribute this to a large spot with a concentration of elements such as manganese. Ironically, when such accurate measures were not available, astronomers thought they saw much more short term fluctuations in Maia’s light, which led them to create a category known as Maia variables. Other stars have since been put in this class, but White noted: “Our new observations show that Maia is not itself a Maia Variable!"
The ancient Greeks described the cluster they named the Pleiades as seven nymphs pursued by Orion. Some Indigenous Australians had a remarkably similar story. Almost every culture worldwide had its own set of stories about these stars, but most agreed on their number. Which is strange, however, since there are not seven of them.
The Pleiades star cluster actually contains more than a thousand stars, but most are far too faint to see without a telescope. Oddly though, there is a substantial gap in brightness between the sixth brightest star, Taygeta, and the next three. Consequently, depending on people's eyesight and the seeing conditions, it's normal to be able to see six stars or nine, but not seven.
This fits with Japanese tradition, which describes them as six stars, now adopted as the logo of Subaru, but most other cultures described seven. Indeed some myths refer to one of the stars as hiding itself in shame, leading to speculation that one of the cluster’s fainter stars was once bright enough to win a place in oral tradition along with the current six.
So far, however, White’s work has shed no light on the possibility of a faded seventh bright star.
Kepler’s primary purpose is to find planets around other stars by watching for regular dips in brightness. Despite its abundant success in other cases, Kepler didn’t find planets around any of the Pleiades. Since these are young stars, just 125 million years old, there would certainly be no prospect of life on such planets even if they existed.
More significantly, however, White has shown that halo photometry is sensitive enough to detect planets crossing the face of very bright stars, including our near neighbors such as Alpha Centauri.