1999 September Sky from Keeble Observatory

The troubled launch of Columbia on July 23^rd carried into orbit the latest of NASA’s "great observatories." These will provide a constellation of observing tools, which can span the electromagnetic spectrum, observing above the interference and distortion of Earth’s thick atmosphere. The first of these was the Hubble Space Telescope, useful from the near infrared to the ultraviolet (just a bit longer and shorter wavelengths than your eye can see). The Compton Gamma Ray Observatory is sensitive to short wavelength, energetic photons emitted in nuclear processes. Not yet deployed is the Space Infrared Telescope Facility (SIRTF), which will observe in the infrared.

Astrophysical X-rays are produced by extremely hot gas, with temperatures in excess of tens of millions of degrees. Such gas is produced briefly by a supernova, or in the accretion disk of a black hole or quasar. Surprisingly, it is also found in the tenuous halo surrounding the Milky Way and other galaxies. Fortunately for us, the Earth’s atmosphere absorbs virtually all of this radiation, which could do great harm to life on our planet. Unfortunately for the astronomer, this means that to effectively observe in the X-ray regime, it is necessary to place the telescope in orbit above the atmosphere. Columbia deployed successfully the Chandra X-Ray Observatory, once dubbed the Advanced X-Ray Astronomy Facility (AXAF). This is the largest satellite ever deployed from the shuttle. Its orbit is an eccentric ellipse, reaching more than 1/3 of the way to the Moon. This will permit observations unobscured by the Earth itself. One of its key missions will be an attempt to discern the nature of the "dark matter" known to make up nearly 90% of the total mass of the universe. Another will be the study at high resolution of quasars and black holes.

Of course, since X-rays easily penetrate most matter, we cannot use conventional lenses or mirrors. Chandra uses a nested set of "grazing incidence" reflectors, which look like a series of cones of different sizes. The mirrors themselves are a technical marvel – if Colorado were as smooth as Chandra’s mirror surfaces, Pike’s Peak would be less than one inch tall. The precision and size of these mirrors will permit resolving power equivalent to reading a stop sign at a distance of about 12 miles. Another surprise, considering that this telescope is designed to produce images in some of the most energetic radiation in the universe, is that the total electrical power to operate it is about 2 kilowatts – roughly that consumed by a hairdryer!

For more about Chandra , check out their web site.

Lunar phases for September: Last quarter at 6:17 pm on the 2^nd; New Moon at 6:02 pm on the 9^th; First quarter at 4:06 pm on the 17^th; Full Moon on the 25^th at 6:51 pm.

September evenings begin with Mars to the south-southwest. At the beginning of the month, it will be about 10 degrees west (that’s to the right!) of Antares. By mid-month these two objects – one a nearby planet, the other a distant star – will pass within 3 degrees on the sky. Look for them close to the Moon on the 15^th. At the end of the month look for Mars about 10 degrees above and to the left of Antares. Jupiter rises about 2 ˝ hours after sunset on the 1^st, only about an hour after sunset by the 30^th. Saturn rises about 40 minutes after Jupiter. Venus has returned to the morning skies, rising before the Sun in the predawn twilight. It will be about 5 degrees from the horizon when the Sun comes up at the beginning of the month, almost 30 degrees by the end.

The stars which marked our summer skies are all but gone for the season. Our overhead view at mid-month, shows the Milky Way dividing the sky at about 10:00 pm. Directly overhead is the constellation Cygnus, with bright Deneb marking the tail of the Swan. The head of the figure (or the foot of the "northern cross") is the beautiful binary Albireo – look for this with binoculars to see the distinct color (i.e. temperature) difference between its component stars. Vega to the west and Altair to the south, mark the other corners of a triangle. The line through Deneb and Altair roughly marks the plane of the Galaxy. To the northeast of Deneb, about half-way to the horizon, you can find the Andromeda Galaxy. A bit higher and almost due east you can trace out the Great Square of Pegasus. The Big Dipper, properly Ursa Major is low to the north-northwest.