UH astronomers
map activity of
Pleiades star
They find an interstellar
dust cloud starting to fray
upon Merope's approach
By Helen Altonn
Star-Bulletin
University of Hawaii astronomers have detected the destruction of a dark interstellar cloud by one of the brightest stars in the Pleiades cluster.
George Herbig and Theodore Simon, with the Institute for Astronomy, obtained high-resolution images in September with NASA's Hubble Space Telescope.
Pleiades, a famous navigational signpost for Hawaiians, formed about 100 million years ago from interstellar clouds, Herbig explained.
The small group of bright blue stars is named for the Seven Sisters of Greek mythology. Easily seen in the night sky during winter months, the cluster resembles a small dipper, lying in the constellation Taurus about 380 light years from Earth.
"Moving through space, it shed all the material from which it was born, but if you look at it from photographs, it is covered with dusty clouds," Herbig said.
"Stars shine on the clouds and create luminosity.
"So it's kind of like a star seen through frosted glass. It's covered with smoky stuff, which is really dust illuminated by stars."
Not far from Pleiades is a big dark cloud of cold gas and dust, Herbig said. "It just happens that Pleiades, by pure accident, is passing through the edge of the dust cloud, like an airplane."
One little piece of cloud happens to be close to the bright Pleiades star Merope, which is reflecting light off the black clouds like a flashlight beam, Herbig said.
American astronomer E.E. Barnard discovered bright nebulosity next to Merope in 1890. "Barnard's Merope Nebula" is the brightest place in the conglomeration of dust around Pleiades, Herbig said.
"It just happens that the brightest reflection of nebula luminosity is not shown on ordinary photographs. This little glob of stuff ... that's what we studied."
The Hubble image doesn't show Merope itself but caught wispy tendrils of the interstellar cloud passing by the bright star.
No one has ever been able to measure the motion of dust clouds because they're so amorphous, Herbig said.
"But it turns out it's possible to associate a position and motion of this little glob of stuff, Merope nebula, with the motion of dark clouds nearby."
Thus, he said, he and Simon were able to map the approach of Pleiades to the unrelated mass of dust of interstellar material.
The star is so close and shining on the dust with such intensity that it is starting to fray and dissipate, Herbig said.
The same phenomenon is at work with Merope and the interstellar cloud as occurs with comet tails, he explained.
"We see dust tails of comets blown away from the comet's head by radiation pressure of the sun, because it exerts a force. It can drive the dust back in the opposite direction."
Space telescope images show radiation pressure from Merope also is destroying the cloud, Herbig said. In a couple of thousand years, as it gets closer to the star, the nebula may be blown apart completely, he said.
"Or, it may be just like comets that go past the sun repeatedly and lose dust."
People have always thought dust clouds were structureless masses of gas and dust, Herbig said. "This picture has shown us there is fine structure, all full of filaments and ridges and globs."
Like leaves of trees that have veins and structure, he said, "it looks like structure inside these interstellar clouds. This is something we didn't appreciate before."
