The Pleiades Star Cluster Has a Secret Stellar Family

The Pleiades Star Cluster Has a Secret Stellar Family Scattered across the Milky Way

By Phil Plait edited by Lee Billings

The sun wanders the Milky Way alone; our star orbits the galaxy’s distant center without any stellar siblings. But it wasn’t always this way. The solar system was very likely born in a tremendous gas cloud that also birthed thousands of other stars, forming a large, loosely bound stellar family called an open cluster.

These stars may have remained enshrouded in that nebula for 10 million years or more, until their combined stellar winds and light pushed away the surrounding nebulosity. No longer embedded in the gas, the cluster gradually disintegrated as its internal motions flung away some of its members. Throw in collisions with other huge clouds of gas and dust, as well as interactions with the galaxy’s gravitational field, and the cluster’s fate was sealed: its stars dispersed, eventually mixing with the background “field” stars in the Milky Way.

Our solar kin may be long lost, but that’s not the case for all stars. Some groupings are younger than the sun’s 4.6-billion-year age—and are still in the process of eroding away. Interestingly, we see such loose affiliations of stars (called associations) all over the sky, and they often share similar characteristics, such as age and velocity, through the galaxy. Astronomers now suspect that many of these scattered associations formed in the same gas cloud, where they were born as part of a single, sprawling structure that has since broken up. But we might yet glimpse parts of such long-lost clusters that are still intact because stars from their close-packed cores should be tightly bound to each other via gravity and thus able to survive as a tight-knit group for much longer.

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Finding such surviving clusters is important; they can tell us about when and where many of the Milky Way’s stars formed and how our galaxy’s stellar populations and structures evolved over billions of years. In short, studying these stellar relics can lead to a better perception of the galaxy as a whole.

So where are they?

If you’re in the Northern Hemisphere and go outside on a clear winter’s night, you might notice a fuzzy clutch of stars not too far from Orion. This is the Pleiades (pronounced “PLEE-uh-deez”), a lovely cluster that is about 440 light-years from Earth and located in the constellation Taurus. Six stars can be seen by the unaided eye, although many cultures’ ancient myths indicate there are seven. It’s possible two of the stars have moved closer together in the sky over time, making it hard to separate them; we still call these stars the Seven Sisters.

Binoculars reveal many dozens of stars in the Pleiades, and hundreds can be seen in deep astronomical images. The cluster doesn’t have a sharp boundary, but most of the stars are found in a volume that is about 40 light-years across.

It’s about 125 million years old, give or take several million years. If it was once part of a bigger structure, this estimated age would align with our expectation that that structure’s outer stars would have been shaken loose. Could the Pleiades be the leftover core of a long-gone cluster?

Finding any far-flung stars that once were anchored by the Pleiades is not an easy task. The sky is big, and the Pleiades’ swath of the Milky Way also contains millions of other stars. Astronomers took this on, however, and published their results on November 12 in the Astrophysical Journal.

They were clever, reasoning that any stars that used to hang out with the Pleiades would have similar ages and chemical compositions, and would probably still have close to the same motion through the galaxy, as the cluster’s current members. By cross-referencing known Pleiades members and a vast stellar database from the European Space Agency mission Gaia, the team pinpointed a selection of stars hurtling through the Milky Way within five kilometers per second of the galactic velocity of the Pleiades.

To firm up this connection, the researchers next estimated the ages of these stars—a difficult task but one that was made easier by the knowledge that younger stars tend to rotate faster than older ones. (This effect exists because a star’s magnetic field can sweep up surrounding material and act like drag on a parachute, slowing the star’s spin.) A star’s rotation can be determined by looking for tiny variations in brightness as dark star spots—which are like the sun’s sunspots but appear on other stars—rotate in and out of view. Using data from NASA’s Transiting Exoplanet Survey Satellite, which can precisely measures stellar brightness, the astronomers found stars that rotated with periods shorter than 12 days, which is roughly the expected spin rate for stars as old as the average Pleiades member.

Finally, after applying various statistical techniques to look for other stars, the researchers wound up with a list of more than 10,000 possible members of what they called the Greater Pleiades Complex. Mapping these members in three dimensions, they found that the stars fall in a slightly elongated grouping that is about 1,600 by 2,000 light-years in size. This includes stars that belong to at least seven previously known associations. Among these associations is the AB Doradus group, which contains a few dozen stars that are only about 70 light-years from the sun. Another, called UPK 303, had been proposed to be a “tidal tail” of the Pleiades, stars torn away from the cluster by the galaxy’s gravity, and the new study supports that conclusion. In fact, looking at the stars’ motions and running the clock backward, the astronomers found that they were all within about 200 light-years of the central Pleiades about 75 million years ago, which is consistent with all of these stars being part of a larger structure that is now well into the process of “evaporating” into the galaxy.

This delights and amazes me. I’ve had my eye on the Pleiades ever since I first started looking at the sky as a wee lad, and I cannot say how many times I’ve gawked at them through various telescopes and in dramatic images. If you go out sometime to look at them—and I really hope you do—gaze upon that tight little cluster and consider that there’s still so much we don’t know about these stars—and so much more they can tell us about the cosmos.

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