February 2007 Sky from the Keeble Observatory
Two months ago we bade farewell to the Mars Global Surveyor. This month we say our
goodbyes to another premium piece of astronomical observing equipment.
Professional photographers can spend a lot of money on their cameras, much of it
on the lenses alone. Without quality lenses, there’s nothing the film (or the CCD
in modern electronic cameras) can do to overcome bad image quality. Terms like astigmatism,
coma, spherical aberration describe various distortions which can be introduced
by an imperfectly ground lens. But, the quality of the film, or the number, density,
and quality of the pixels on the camera chip also affect the outcome of a particular
Telescopes are the “lenses” of astronomy, though virtually all astronomical telescopes
capture light with mirrors instead of lenses. The expense of grinding a mirror to
precisely the right shape is one reason that observatory telescopes cost millions
of dollars. Hubble Space Telescope (HST) was launched with an imperfectly ground
mirror – more accurately, it was ground with high precision to the wrong shape.
The outer edge of the 2.6 meter primary mirror was too high by a little less than
the thickness of a human hair. Corrective optics were installed by the first shuttle
servicing mission, and the telescope has performed impressively ever since, with
three additional visits by astronauts to repair and upgrade the orbiting observatory.
(Another reason for the cost is size, since larger and larger telescopes are used
to observer fainter and fainter objects, probing farther and farther into the universe.)
The “film” of the astronomer is the instrumentation deployed on the telescope to
capture and analyze the light from the distant stars and galaxies. One of the instruments
on HST, the Advanced Camera for Surveys (ACS), has now stopped working. On 27 January
controllers noted that HST had gone into automatic safe mode – a pre-programmed
response to a wide range of anomalies, designed to protect the telescope and its
instruments. The particular anomaly that triggered this mode (“safing” in NASA speak)
was a sudden rise in pressure in the instrument compartment (known as the Aft Shroud)
on the telescope. Note that the telescope orbits in vacuum, and that it is not designed
to be air tight – so where does this sudden burst of gas come from? It turns out
that the ACS developed a short circuit and blew a fuse! The short circuit had sufficient
energy to vaporize the fuse!
The good news is that there are redundant electronics, known as Side 1 and Side
2. The blown fuse is on Side 2. The bad news is that the short circuit seems to
have permanently damaged most of the camera itself. If it is determined that Side
1 can be safely restarted, some limited capability may be restored. The scheduled
2008 servicing mission, which will be Hubble’s last before it is retired in the
next decade, will probably replace the ACS with a new camera.
Lunar phases for February: Full Moon at 12:45 am, on the 1st; Last Quarter on the
10th, at 4:51 am; New Moon on the 17th, at 11:14 am, and First Quarter on the 24th,
at 2:56 am.
If you are an early morning sky watcher, you’ll note that Jupiter is about 30 degrees
off the south-southeast horizon as the Sun rises. That red star about 5 degrees
below and to the right is Antares - meaning “rival of Mars.” The real Mars is below
and to the left of Jupiter, about 16 degrees above the southeast horizon. Mercury
will return to predawn visibility at month’s end. Saturn sets at sunrise.
As the Sun sets, you’ll see Mercury and Venus both to the west-southwest, emerging
from the twilight. Mercury will move lower as the month advances before disappearing
into the solar glare and shifting to predawn rising by the end of the month. Shortly
after sunset, Saturn rises to the east and is visible all night as it swings across
the southern sky.
At mid-month, an overhead view about three hours after sunset finds the Milky Way
dividing the sky northwest to southeast. Directly overhead is the constellation
Auriga. Its brightest star is Capella, about 10 degrees north of zenith. Elnath,
about the same displacement to the southeast, is sometimes associated with Auriga.
However, it’s actually part of Taurus. Notice that Elnath is an Arabic name, meaning
“Butting One” – appropriate for the tip of the Bull’s horn. Elnath is a blue giant
star, about 300 times brighter than the Sun and about 130 light years distant. Castor
and Polluz, in Gemini, lie about 30 degrees east of zenith, above Leo rising. That’s
Saturn just above Leo.
To the south is Orion’s familiar asterism. Last month we noted the red star on the
upper left of this familiar constellation, Betelgeuse, and promised to look in detail
at some of the other bright stars in Orion. Perhaps the most familiar feature of
Orion are the three aligned stars forming the Hunter’s belt. These are blue super-giant
stars, all about 1500 light years distance, and each more than 20,000 time brighter
than the Sun. The western most star (to the right) is Mintaka, from an Arabic word
meaning “belt.” The eastern most star is Alnitak, another Arabic word meaning “girdle.”
Alnilam, the middle star is named for a “belt of pearls.” Below and to the right
of the belt region is the bright supergiant Rigel. It’s about the same distance
and brightness. Current thinking is that these stars all formed about 10 million
years ago on the visible edge of the Orion Molecular Cloud Complex. We get a peek
into this molecular cloud when we look at the Orion Nebula – in the Hunter’s sword
hanging below the belt. This is an active star-forming region, with the young Trapezium
Cluster only a few million years old.
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.