Surprise in the seven-star

Kepler telescope maps astonishingly large variability in the Pleiades

The brightest stars of the Pleiades can be seen as seven-pointed star in the sky with the naked eye © NASA / ESA, AURA / Caltech, Palomar Observatory
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Astonishingly variable: for the first time astronomers have been able to observe the brightness variations of the seven-pointed star more closely. It showed that six of the seven brightest stars in the Pleiades pulsate amazingly differently: some oscillate very fast, others show rather slow pulses. Particularly surprising was the star Maia: He does not belong in the originally created especially for him class of variable stars.

The seven-star was probably already in the Stone Age as an important calendar scoreboard in the sky. For the rising of this open star cluster in the constellation of Taurus marked the beginning of spring millennia ago. The Pleiades could also be depicted on the Bronze Age Sky Disc of Nebra. Today it is known that the Pleiades comprise more than 1, 000 stars, but only the seven brightest are visible to the naked eye.

Kepler telescope mapped Pleiades

This seven-star has a special feature: Almost all of its stars belong to the variable stars of the B-Class. Their brightness fluctuates. But the exact frequency and amplitude has been difficult to determine, because the stars are so bright that they simply outshine most sensitive telescope optics.

A solution has now found Tim White from the University of Aarhus and his team. They targeted the Pleiades with the Kepler Space Telescope and evaluated not the supersaturated pixels of the image sensor, but the changes of the immediately adjacent pixels. "This allows us to capture the relative changes in the brightness of these stars, " explains White. He and his colleagues christened their new halo-photometry method.

Time course and amplitude of the brightness fluctuations of the seven brightest Pleiades stars. © Aarhus University / T. White

Big differences

The observations revealed surprisingly large differences between the seven Pleiades stars: "Although these stars are of the same cluster and therefore have a similar age and composition, they show a wide range of variability, both in terms of amplitude and frequency Pulse ", so the astronomers. display

So Merope, Pleione and Atlas oscillate very fast, and with a relatively high amplitude. Alcyone, Electra and Taygeta pulsate a little slower, but also more irregular. Some of these differences can be explained by the properties of the stars: Four of them rotate very fast and therefore carry rings of spun material around their equatorial region. Atlas, Pleione and Taygeta know that they are part of multiple systems.

Maia is not a Maia-changeable one

A special surprise, however, was provided by Maia, the fourth brightest star of the Pleiades. Several decades ago, astronomers believed that this star was indicative of rapid pulsation with a period of about two hours. Because the star features did not fit any known kind of changeable one, they created their own class: the Maia-changeable ones. Also some other stars seemed to belong to this new class.

Maia is the fourth brightest star of the Pleiades. He is pulsing much slower than previously thought. NASA / ESA, AURA / Caltech, Palomar Observatory

But the new data now shows that of all people Maia herself does not belong in this class. "Maia is a changeable star, but she is not a Maia variable, " White and his colleagues emphasize. Because Maias brightness fluctuates very regularly in a period of ten days and thus much slower than previously thought.

Gigantic star spot as cause

And one more thing the observations revealed: "Maia's brightness fluctuations go hand in hand with changes in manganese absorption on the surface of the stars, " says White's colleague Victoria Antoci. "We conclude that the variations are caused by a large chemical stain on the surface of the star."

Due to the rotation of the star, this dark star spot regularly appears in the field of view of the telescopes every ten days, changing Maia's brightness. Because Maia definitely does not belong to the "Maia-changeable" with fluctuations in brightness in the hour range, the astronomers plead for renaming this whole class in the future. "Otherwise, this will increasingly cause confusion, " says White and his colleagues. (Monthly Notices of the Royal Astronomical Society, 2017; doi: 10.1093 / mnras / stx1050)

(Royal Astronomical Society (RAS), 28.08.2017 - NPO)