FOR RELEASE: 9:20 AM CST, January 8, 1999

Astronomers Measure Distant Molecular Cloud Approaching Earth

Austin, TX -- An international group of astronomers has recently constrained the distance, size, and mass of a mysterious cometary molecular cloud that appears to be falling into the plane of our galaxy. The astronmers, led by Professor Bryan Penprase of Pomona College in Claremont, California, have presented the first evidence of stellar light from multiple background stars being absorbed by the cloud during the Winter 1999 AAS Meeting in Austin, Texas. The findings are especially important as they provide the best data available for determining the distance and size of the cloud, by measuring absorption in multiple background stars by the object known as the Draco Molecular Cloud. The enormous cloud extends for over 100 light years in diameter at a distance of 3200 light years and is getting closer to the Earth at a rate of 13 miles per second.

"This would mean ‘Deep Impact’ times several million if the cloud were to hit Earth," states Dr. Penprase, an associate professor of physics and astronomy at Pomona College, who notes that this cloud, "is composed of literally billions of comet-like clumps and condensations which spread out over 100 light years." However, there is no need to purchase impact insurance right away, since the Draco Cloud won't impact the plane of the Milky Way (the Earth is in or near the plane) until about 44 million years from now. And, Penprase adds, "By then the rotation of the galaxy, and a transverse component in the Cloud's motion, will put Earth in what will likely be a safe location." The galaxy rotates about once every 200 million years.

The observations, presented at the American Astronomical Society meeting in Austin TX, report the first mulitple detections of the Draco Molecular cloud using stars to detect the faint wisps of dust and gas which loom above the galaxy. The results are the culmination of a campaign of observations which began in 1994, using the 1.5-meter (60-inch) telescope at Mt. Palomar Observatory in California, the 2.5-meter (100-inch) Nordic Optical Telescope at La Palma, Spain, and most recently the 1.0-meter (40-inch) Pomona College telescope in Wrightwood California. With collaborators Jason Rhodes (Princeton University), Eirik Harris (Pomona College), Eliana Palazzi (CNR, Bologna, Italy), Elvira Covino (Osservatorio Capodimonte, Naples Italy) and Hugo Schwartz (NOT, La Palma Spain), Dr. Penprase has studied the stars surrounding an area in the sky near the constellation Draco (near the North Celestial pole, or near the star Polaris) looking for slight changes in the brightness and color of the stars which would indicate the presence of the otherwise invisible molecular cloud. Such color changes, or reddening, indicate the absorption from vast regions of dust and gas within the cloud. Dr. Penprase, and his (then) student Jason Rhodes, studied the reddening of the Draco cloud for several years, and detected the first evidence of reddening from the cloud. These photometric results indicated that the Draco cloud is at a distance of approximately 1,100 parsecs, or 3,500 light years. Their results have been submitted for publication in the journal Astronomy and Astrophysics.

More recently, the international collaboration involving astronomers in Italy, Spain and the United States began to study a sample of the stars for more subtle clues of the cloud’s location using a larger telescope and spectrograph. These results indicated absorption from Sodium atoms 3,200 light years away, which provided the first set of data which detects such absorption in multiple background stars. "We use the stars as light bulbs," states Dr. Penprase, "and these light bulbs shine through the cloud, which leaves fingerprints of its composition by absorbing light at very specific wavelengths, such as those which Sodium atoms absorb. The Sodium atoms allow for a more accurate detection of the cloud, since the atoms in the cloud are Doppler-shifted to a very precise wavelength which can be measured by the spectrograph. Then, by carefully determining the distance to the background stars, the distance to the cloud can be measured." "Multiple detections" means the study was made by simultaneously observing the absorbing effects of the Draco Molecular Cloud over several (many) lines of sight through the Cloud. This method is much more informative than the occasional single detection against one background star, as done in the past.

Dr. Penprase and his collaborators are refining a new technique of measuring stellar distances, and the preliminary results indicate a distance of 1,000 parsecs, or 3,200 light years for the Draco Molecular Cloud. Once the distance is known, the size of the cloud can also be calculated by studying maps of the cloud in infrared or radio wavelengths to map out the angular extent of the cloud.

While the present distance estimates are still being refined and may be revised as improved stellar measurements are obtained, the group’s results show the Draco cloud to be the most distant known of a class of high galactic latitude molecular clouds, extending across hundreds of light years of space. The Draco Cloud is especially unusual since it appears over 1,800 light years above the plane of the galaxy, well above the typical ‘ceiling’ for molecular clouds which is approximately 300 light years. Using the infrared and radio data, Dr. Penprase calculates that the Draco Cloud contains hundreds of clumps in sizes ranging from two to 20 light years across. By contrast, a typical comet in our solar system is 10 km across, or less than one trillion times smaller.

"We are not sure how dense or small the clumps are in the Draco Cloud since we still need better infrared and radio images of the cloud to resolve its smallest structures," states Dr. Penprase, who also notes, "the Draco Cloud is like a swarm of many millions of comets surrounded by a vast expanse of gas and dust." The spectroscopic results have been presented at the Workshop on High Velocity Clouds, in Canberra, Australia, and will be published in the conference proceedings. An additional paper is being prepared for publication in the Astrophysical Journal.


The image shows a schematic drawing of the Milky Way Galaxy, showing the approximate location of the Sun and the Draco Molecular Cloud. The cloud appears over 1800 light years above the plane of the galaxy, and is a mysterious "cometary" molecular cloud falling into our galaxy at a rate of 13 miles per second. The size of the Draco cloud is over 100 light years, based on new observations of stars behind the cloud, and it is expected to impact the galaxy sometime in the next 40 million years.







For More information:

Dr. Bryan E. Penprase (909) 621-8727

Jason Rhodes (609) 258-9387


The full color schematic drawing is available on the Internet via

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