After Centuries Of Observations, Astronomers Still Puzzle Over The Pleiades
Known as the Seven Sisters, the Pleiades are among the most iconic group of stars in the night sky; a captivating collection of massive, hot blue stars readily visible with the naked eye. Their deep blue hues have fascinated observers since time immemorial.
Thought to have been first depicted in a Northern German bronze age artifact known as the Nebra sky disk, dating to some 1600 B.C., this famous collection of B-spectral type stars is only the tip of the iceberg in a young open cluster of stars in the Northern constellation of Taurus.
In fact, the Pleiades is home to some 2200 stars which run the spectral gamut from the hottest to the coolest and least massive. They include solar type G and K stars, many of which provide astronomers with a veritable real-life stellar laboratory in which to take a virtual peek into what our own sun may have looked like when it was much younger.
Because the cluster lies 440 light-years distant, the stars are bright, and were some of the first to be studied in detail. They give us a still image or snapshot of the life cycle of stars, Marc Pinsonneault, an astronomer at The Ohio State University in Columbus, told me.
It’s our closest way of studying what stars like the Sun would have been like when it was only a 100 million years old, says Pinsonneault.
The current accepted age of the Pleiades cluster is only some 125 million years old, extraordinarily young by astrophysics standards. By comparison, our own yellow dwarf Sun is already 4.568 billion years old.
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In some cases, like in the Orion Nebula, newly-formed stars are loosely packed and move away from one another, says Pinsonneault. Our own Sun was probably born in an environment like that, he says. In other cases, like the Pleiades, the stars are close enough to one another to be held together by their mutual gravity, he says. This makes the Pleiades a gravitationally-bound star cluster, he notes.
One major Pleiades puzzle involves the amount of lithium detected in the cluster’s plethora of solar type stars. In the Pleiades, stars of the same mass appear to have different abundances of lithium, says Pinsonneault. Since we are very confident that they were born with the same lithium abundance, then there has to be some reason this happened, he says.
Lithium is fragile and easily destroyed in the interiors of stars, says Pinsonneault. The fact that some stars in the Pleiades have a lot of lithium and others have very little means that when they were 10 million years old, something really changed so that their cores stopped having strong nuclear reactions, he says.
Thus, the most magnetically active stars in the Pleiades appear to have burned less lithium in their early evolution than their more peaceful counterparts. Part of this difference could stem from star spots and very strong magnetic fields lines that appear to cover some 85 percent of these young stars’ surfaces.
In fact, these magnetic fields are so strong that they are actually physically changing the size of the star, says Pinsonneault. Magnetic fields concentrated into spots can suppress turbulence, causing feedback that puffs these stars by about 10 percent, he says.
Pinsonneault says researchers still puzzle over why only some of the stars in the Pleiades have this kind of inflation. And why some of these stars appear to have had a much more dramatic effect from these magnetic fields than others?
As for how this magnetic activity might affect life?
A habitable planet puffed up due to magnetic activity would absolutely have a much tougher time in developing life, says Pinsonneault. Associated with the starspots, you tend to have very strong emission of x-rays and ultraviolet light, he says.
“So, that’s a serious question for us; whether these x-rays are so persistent and these effects are so dramatic that they make these red dwarfs inhospitable, if not hostile to life,” said Pinsonneault.
As for those beautiful blue Seven Sisters that have so captured the human imagination through the millennia?
Most likely, they would have been in the same positions, since the cluster itself only reshuffles its structure on million-year time frames, says Pinsonneault.
So, we can rest assured that our distant ancestors had the same beguiling view of these stellar gems as we do today.