Astronomers have discovered the secret Pleiades star family

The Pleiades star cluster has a secret stellar family scattered across the Milky Way

The “Seven Sisters” of the Pleiades are part of a much larger complex that can help reveal the deep history of our galaxy.

By Phil Plait edited by Lee Billings

The sun travels alone through the Milky Way; our star orbits the distant center of the galaxy without any stellar siblings. But it wasn’t always this way. The solar system most likely originated in a huge cloud of gas that also gave birth to thousands of other stars.forming a large, loosely linked stellar family called an open cluster.

These stars may have remained enveloped in this nebula for 10 million years or more, until their combined stellar winds and light pushed back the surrounding cloudiness. No longer drowned in gas, the cluster gradually disintegrated as its internal movements displaced some of its members. Add to that collisions with other enormous clouds of gas and dust, as well as interactions with the galaxy’s gravitational field, and the cluster’s fate was sealed: its stars scattered, eventually mixing with stars in the Milky Way’s background “field.”

Our solar parents may be long lost, but not all stars are. Some groupings are younger than the sun’s 4.6 billion years and are still eroding. Interestingly, we observe very loose affiliations of stars (called associations) all over the sky, and they often share similar characteristicslike age and speed, across the galaxy. Astronomers now suspect that many of these scattered associations formed in the same gas cloud, where they originated within a single, sprawling structure that has since broken up. But we might still see parts of these long-lost clusters that are still intact, because the stars in their tight cores should be tightly bound together by gravity and thus able to survive as a tight-knit group for much longer.

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It is important to find such surviving clusters; they can tell us when and where many stars in the Milky Way formed and how our galaxy’s stellar populations and structures have evolved over billions of years. In short, the study of 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 out on a clear winter night, you might notice a fuzzy cluster of stars not far from Orion. It’s the Pleiades (pronounced “PLEE-uh-deez”), a pretty cluster about 440 light years from Earth and located in the constellation Taurus. Six stars can be seen with the naked eye, although ancient myths of many cultures indicate that there are seven. It is possible that two stars have moved closer together in the sky over time, making it difficult to separate them; we always call these stars the Seven Sisters.

Binoculars reveal several dozen stars in the Pleiades, and hundreds can be seen in deep astronomical images. The cluster has no sharp boundary, but most of the stars are found within a volume about 40 light years across.

It’s about 125 million years old, give or take a few million years. If it was once part of a larger structure, this estimated age would match our expectation that the outer stars of this structure would have been shaken around. Could the Pleiades be the remaining core of a long-vanished cluster?

Finding distant stars once anchored in the Pleiades is no easy task. The sky is big, and the Pleiades band of the Milky Way also contains millions of other stars. However, astronomers have seized it and published their results on November 12 in the Astrophysics Journal.

They were smart, estimating that all the stars that frequented the Pleiades would have similar ages and chemical compositions, and would probably still have roughly the same motion through the galaxy, as the cluster’s current members. By crossing the known members of the Pleiades and a vast stellar database of the European Space Agency’s Gaia missionthe team identified a selection of stars crossing the Milky Way at less than five kilometers per second of the galactic speed of the Pleiades.

To confirm this link, the researchers then estimated the ages of these stars – a difficult task but made easier by knowing that younger stars tend to spin faster than older ones. (This effect exists because a star’s magnetic field can sweep away surrounding matter and act like drag on a parachute, slowing the star’s rotation.) A star’s rotation can be determined by looking for tiny variations in brightness in the dark. star spots— which resemble the sun’s sunspots but appear on other stars — rotate in and out of view. Use NASA data Satellite for studying transiting exoplanetswhich can accurately measure the brightness of stars, astronomers discovered stars that rotated with periods less than 12 days, which is roughly the expected rotation speed for stars as old as the middle member of the Pleiades.

Eventually, after applying various statistical techniques to search for other stars, the researchers came up with a list of more than 10,000 possible members of what they called the Great Pleiades Complex. By mapping these members in three dimensions, they discovered that the stars form a slightly elongated group with a size of about 1,600 by 2,000 light years. This includes stars belonging to at least seven already known associations. Among these associations, we find the AB Doradus groupwhich contains a few dozen stars located only about 70 light years from the sun. Another, called UPK 303, had been proposed as a “tidal tail” of the Pleiadesstars torn from the cluster by the gravity of the galaxy, and the new study supports this conclusion. In fact, by observing the movements of the stars and turning back the clock, astronomers discovered that they were all about 200 light years from the central Pleiades about 75 million years ago, which is consistent with the fact that all of these stars are part of a larger structure that is now well into the process of “evaporation” in the galaxy.

This delights and astonishes me. I’ve had my eye on the Pleiades ever since I started looking at the sky when I was little, and I can’t say how many times I’ve gaped at them through various telescopes and in spectacular images. If you go out from time to time to observe them – and I really hope you do – look at this small, narrow cluster and consider that there is still so much we don’t know about these stars – and that they can teach us so much more about the cosmos.