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Interstellar clouds converge on the Pleiades

credits: Astronomy.com
ackers
article given by ackers

I've allways loved this star cluster ever since I was a kid. I noticed it long before I knew anything about astronomy. I would look up at the stars when I was about seven years old, and wonder what special reason made those stars huddle together in this little pattern.

Then when I started looking in books on astronomy, I found the little groupe of stars. It was the Little Bear (Little Dipper). When I looked at the stars again, I found my little star cluster, but there was no sign of the Great Bear. In fact, non of the constellations that surrounded the little bear could be found. then one night, when I was nine, I was walking up Grange road (a street in Colburn, Catterick Garrison).

Grange road leads due north from Colburn lane. I looked up at the sky and saw the Plough (the Big Dipper) for the first time. It was huge. I didn't expect it to be so big. I immediately used the pointer stars to find the north star (Polaris). It worked. Which meant I also found the true little bear. Much larger than the little groupe of stars that I thought was the little bear.

So my little star group wasn't the little bear at all - which also explained why I couldn't see it during the summer. It wasn't even in the northern part of the sky. It wasn't long before I'd used one of the starmaps in my astronomy book to locate Taurus and, finally, identify my little star cluster as the Pleiades, the Seven Sisters.

ackers

The Pleiades — also known as the Seven Sisters or M45 — is a familiar sight in the autumn and winter sky of the Northern Hemisphere. Classified as an open star cluster, the Pleiades lies approximately 400 light-years away from Earth in Taurus and can be seen easily without optical aid.

With binoculars or a telescope, astronomers can observe faint but visible nebulosity in the region of the cluster. Long thought of as leftover material from the formation of the Pleiades, this gas scatters light from the cluster’s brightest stars like fog around a streetlamp. However, radio and infrared observations made in the 1980s showed that the relationship between this nebulosity and the stars is actually a chance encounter between the cluster and an interstellar cloud.

This discovery amazed astronomers at the time, and now, it seems there’s another chapter to add to the story. New data obtained at Kitt Peak National Observatory (KPNO) suggest the Pleiades actually are interacting with two clouds, giving rise to an extraordinary and rare occurrence: a three-body collision in the vast emptiness of interstellar space. “The idea of the Pleiades and one gas cloud in an interstellar train wreck already made this nearby cluster an especially interesting region for astronomers seeking to understand the details of physical and chemical processes in the interstellar medium,” says Richard White of

Smith College in Northampton, Massachusetts. “The presence of a second cloud interacting with the first cloud and with the cluster creates a situation more like a three-car crash in a demolition derby, which makes the Pleiades altogether unique as a natural laboratory.”

With students from Smith College and nearby Amherst College, White used KPNO’s 2.1-meter telescope to take high-resolution spectra of the Pleiades stars and to obtain this new perspective on the motion of interstellar gas near the cluster.

Sodium atoms in gas found between Earth and the Pleiades absorb two specific wavelengths of yellow starlight. This absorption produces dark lines in the spectra of the cluster’s stars. The lines reveal that there is a feature between Earth and the Pleiades moving toward the cluster with a line-of-sight velocity of about 6 miles (10 kilometers) per second. White associates this feature with a huge cloud of gas and dust known as the Taurus-Auriga interstellar cloud complex, the bulk of which lies about 40 light-years to the east of the Pleiades.

Looking toward several of the cluster’s stars, however, two or more absorption features were detected. The presence of an additional feature, primarily on the west side and moving toward the cluster at about 7 miles (12 km) per second, defies explanation unless a second cloud is converging on the Pleiades, White concludes.

White and students used the 2.1-meter telescope at Kitt Peak to study the Pleiades.AURA / NOAO / NSF

But don’t worry; these interactions won't change our view of the Pleiades. Interstellar collisions between the stars and gas clouds will unfold on a time scale of several hundred thousand years. “That is good news for those who enjoy the magnificent color images of the Pleiades that grace textbooks and coffee-table books, which suffer no danger of obsolescence,” White says. “It is bad news for those who would like to see celestial fireworks unfolding from year to year.”

Located about 400 light-years from Earth in the constellation Taurus, the Pleiades cluster includes six naked-eye stars (and one variable star that may have once been naked-eye as well) along with thousands of other, fainter stars. The young stars are surrounded by the glow of their own light reflected by clouds of interstellar gas.
John Chumack

curtesy Jack Holkheimer