What's Up July 2010: Interstellar clouds block the Milky Way

Jane Houston Jones: What's Up for July? Dark nebulae near the center of our Milky Way galaxy.

Hello and welcome! I'm Jane Houston Jones at NASA's Jet Propulsion Laboratory in Pasadena, California.
The Milky Way looks like a river of tiny diamond clusters surrounding mysterious dark islands.
Notice how the Milky Way divides into two streams overhead.
Between these two streams lies a dark band of starlight-obscuring dust.
Summer is the best time of year to observe these dusty areas.
The Milky Way thickens and brightens as it flows southward towards the horizon,
near the constellation Sagittarius.
Sagittarius is easy to see in the southern sky this month. The brightest stars look just like a teapot.

Title - Sagittarius
Jones: And the center of our galaxy looks like hot steam spewing from the teapot's spout.

Dr. E. E. Barnard made the first wide-angle photographs of our Milky Way at Lick Observatory in 1889, He saw dark regions visible among the mass of stars.

Title - Sketch of Milky Way by William Herschel (1785)
Jones: Earlier astronomers thought these dark regions were simply areas where there weren't any stars.
Barnard thought just the opposite. He thought that these empty areas were actually concentrations of matter blocking our view. He was correct. A dark nebula called "Barnard 86" is one of his discoveries.
A dark nebula is a kind of interstellar cloud so dense that the light from background stars, or from emission and reflection nebulae is blocked.

Title - Reflection nebulae
Jones: Like fog around a streetlamp, a reflection nebula shines only because the light from an embedded
source illuminates its dust. The nebula does not emit any visible light of its own.

Title - Emission nebulae
Jones: Emission nebulae are glowing clouds of interstellar gas which have been excited by some nearby energy source, usually a very hot star, causing them to emit light.
It takes radio or infrared astronomy to pierce these dark clouds and see beyond them.

Title - Trifid nebula
Jones: The three dark lanes of dust in the Trifid nebula can be seen in visible wavelength images.
Infrared images from the Spitzer Space Telescope reveal bright regions of star-forming activity.
There are many dark nebulae visible to the unaided eye. Look for the Pipe nebula, the Lagoon nebula

Title - Sagittarius
Jones: and the Great Dark Rift above the teapot of Sagittarius.

Title - NGC 4565. Andromeda Galaxy. Milky Way.
Jones: Then, when you look at the dust lanes within spiral galaxies, you'll be able to compare them to the Great Rift overhead in our summer Milky Way.

Title - Looking West, 8:30 pm. July 21-31. Saturn. Mars. Venus. Leo.
Jones: Towards the end of the month, don't miss the parade of planets low in the western sky.
Mars and Saturn march towards one another, readying for their August 1 conjunction.

Title - Learn about NASA missions at: www.nasa.gov
Jones: You can learn more about NASA missions at www.nasa.gov

Title - Thanks to the following for submitting images: Todd Boronson/NOAO/AURA/NSF. Stephane Guisard, European Southern

Observatories, La Silla, Chile. Morris Jones. Pete Lawrence. Jerry Lodriguss.
Jones: That's all for this month. I'm Jane Houston Jones.

Title - NASA Jet Propulsion Laboratory California Institute of Technology.