The Pleiades star cluster contains a secret stellar family scattered throughout the galaxy
The “seven sisters” of the Pleiades are part of a much larger complex that may help uncover our galaxy’s deep history
A view of the star cluster Pleiades, about 440 light years from Earth.
Robert Gendler/Science Photo Library/Getty Images
The sun wanders alone in the galaxy; Our star orbits the distant center of the galaxy without any stellar siblings. But it was not always like this. The Solar System was probably born in a massive gas cloud that also gave rise to thousands of other stars, forming a large, loosely bound stellar family called an open cluster.
These stars may have remained hidden in that nebula for 10 million years or more, until their combined stellar winds and light pushed the surrounding nebula away. No longer contained in gas, the cluster slowly disintegrated as its internal motions threw some of its members away. Collisions with other giant clouds of gas and dust occurred, as well as interactions with the galaxy’s gravitational field, and the cluster’s fate was sealed: its stars scattered, eventually merging with the background “field” stars in the galaxy.
Our solar kin may have been extinct long ago, but that is not the case for all stars. Some clusters are younger than the Sun’s age of 4.6 billion years—and are still in the process of being destroyed. Interestingly, we see such loose associations (called associations) of stars all over the sky, and they often share similar featuresSuch 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, giant structure that has since broken up. But we may still see parts of such long-lost clusters that are still intact because the stars in their close-packed cores should be held tightly to each other through gravity and thus they would be able to survive as a tight-knit cluster for long periods of time.
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It is important to find such surviving groups; They can tell us 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 remnants can lead to a better understanding of the galaxy as a whole.
So where are they?
If you’re in the Northern Hemisphere and go outside on a clear winter night, you might be able to see a faint cluster of stars not far from Orion. This is the Pleiades (pronounced “plee-uh-dees”), a beautiful cluster that is about 440 light years from Earth and located in the constellation Taurus. Six stars can be seen with the unaided eye, although ancient myths from many cultures indicate that there are seven stars. It is possible that over time the two stars may have moved closer to each other in the sky, making them difficult to distinguish; We still call these stars the Seven Sisters.
Telescopes 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 in a volume of about 40 light-years.
It is about 125 million years old, let’s say several million years old. If it were part of a larger structure, this estimated age would be consistent with what we would expect the outer stars of that structure to have become loose. Could the Pleiades be the leftover core of a long-gone cluster?
Finding any distant star that was once anchored by the Pleiades is no easy task. The sky is big, and there are millions of other stars in the Pleiades region of the galaxy. However, astronomers accepted that published its results on 12 November Astrophysical Journal,
They were clever, reasoning that any stars orbiting the Pleiades would have a similar age and chemical composition, and would probably have close to the same motion through the galaxy as the current members of the cluster. By cross-referencing a vast stellar database of known Pleiades members and the European Space Agency mission Gaia, the team pinpointed a selection of stars moving through the galaxy to within five kilometers per second of the Pleiades’ galactic velocity.
To solidify this connection, researchers estimated the ages of these stars – a difficult task but one made easier by the knowledge that younger stars rotate faster than older stars. (This effect exists because a star’s magnetic field can blow up material around it and act like drag on a parachute, slowing the star’s rotation.) The rotation of a star can be determined by observing small changes in brightness as it darkens. star spots– which are like sunspots but visible on other stars – move in and out of the scene. Using data from NASA’s Transiting Exoplanet Survey Satellite, which can precisely measure stellar brightness, astronomers found stars that spin with a period less than 12 days, which is the expected spin rate for stars as old as the average Pleiades member.
Ultimately, after applying various statistical techniques to look for other stars, the researchers compiled a list of more than 10,000 possible members of what they call the Greater Pleiades Complex. By mapping these members in three dimensions, they found that the stars fall into a slightly elongated cluster measuring about 1,600 by 2,000 light years. It includes stars that belong to at least seven previously known associations. Among these unions is AB Doradus GroupWhich contains a few dozen stars that are only about 70 light years away from the Sun. The second, called UPK 303, It was proposed to be the “tidal tail” of the Pleiades.The stars were torn apart from the cluster by the galaxy’s gravity, and the new study supports that conclusion. In fact, by looking at the motions of the stars and turning the clock backward, 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 in the process of “evaporating” into the Milky Way.
It pleases and surprises me. I’ve had my eye on the Pleiades ever since I first started looking at the sky as a little boy, and I can’t tell you how many times I’ve seen them through various telescopes and in dramatic images. If you go out to look at them sometimes – and I really hope you do – look at that tight little group and consider that there is still a lot we don’t know about these stars – and they can tell us a lot more about the universe.
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