September 2002 Sky from Keeble Observatory
NASA has another embarrassment to deal with - the CONTOUR spacecraft we highlighted
in August has apparently been lost. After the scheduled burn of the solid booster,
the spacecraft's signal was not recovered. Telescopes identified what appear to
be three fragments of the spacecraft on its predicted trajectory. We'll have more
to say once the inquiry is completed.
Speaking of embarassment, how embarrassing would it be if you couldn't find most
of the money you knew was in your bank account? That is somewhat the state of modern
cosmology, which has wrestled in recent decades with what was once known as the
missing mass problem. We had determined from orbits of stars in our own and other
galaxies that the total mass of stars, gas, and dust in those galaxies was insufficient
to account for the observed paths of the stuff we could see. Also, the inferred
and observed motions of galaxies within clusters also indicated that there was not
enough stellar and interstellar material to account for the observed gravity - i.e.
much of the mass was missing. Actually, it's not missing, but it doesn't emit or
absorb electromagnetic radiation - i.e. light - so it is better called dark matter.
Indeed, the necessary mass would be almost ten times the observed luminous matter.
Some of it, though not nearly enough to solve the problem, has been detected in
the form of brown dwarfs or free-floating planets, too cool to emit detectable radiation,
but which sometimes block the light from more distant stars.
To make the situation even more embarrassing, there are firm theoretical arguments
to suggest that there couldn't be that much normal matter in the universe. But,
the total mass of normal matter both observed and inferred is only about 20% of
what theory tells us to expect. A substantial part of what the universe is made
of has to be in the form of exotic dark matter - curious particles hinted at by
some versions of the standard model of fundamental particles. Even more curious,
most of the universe seems to consist of a mysterious dark energy which is accelerating
the universal expansion. We'll say more about these next month.
Recall that I said above that we hadn't been able to observe all the normal matter
that our theories told us to expect. The good news is that researchers using the
Chandra X-Ray Observatory have found the missing normal matter. Where is it? It's
not in galaxies, but in the vast space between them. And, it's very hot - 300,000
to 5 million degrees Kelvin. Observing distant quasars, Chandra observers from the
Harvard-Smithsonian Center for Astrophysics (CfA) and Ohio State measured the absorption
of x-rays by the hot gas, primarily looking for absorption by highly ionized oxygen
and other heavier elements. The amount of absorption at specific wavelengths provides
a probe of the temperature and density of the intervening material. Extrapolating
results from a handful of quasars to the whole sky tells us that the intergalactic
medium is filled with filaments of this hot gas - with more mass than all the galaxies
Other observers from the University of Michigan flipped the procedure around, using
the absorption of x-rays emitted by the hot intergalactic gas absorbed by passing
through intervening galaxies. Analysis of their results gives similar answers for
the distribution and temperature of the gas filaments.
Lunar phases for September: New Moon on the 6th, at 11:10 pm; First Quarter on the
13th, at 2:08 pm; Full Moon on the 21st, at 9:59 am; Last Quarter on the 29th, at
Evening planet watchers will have to be content with Venus, brilliant but low above
the southwest horizon for about an hour after sunset. Mercury sets even earlier,
and is not likely to be visible through clutter and haze. Jupiter and Saturn are
visible in the pre-dawn sky, east to east-southeast.
An overhead view about an hour after sunset finds Vega, in the constellation Lyra,
almost at zenith. During the first and last weeks of the month, there should be
little moonlight to interfere with seeing the Ring Nebula in the same constellation
with binoculars or a small telescope. Near Vega you can also resolve the double
double as a binary with binoculars. A modest telescope and good seeing will permit
you to notice this as a pair of binaries. The systems are actually gravitationally
bound - four stars in an elegant celestial dance. To the east of Vega, note the
familiar cross shape of Cygnus. The bright star at the tail of the Swan is Deneb.
The star at the other end of the pattern is Albireo - a small telescope reveals
this as a binary, as well, with the two stars of strikingly different colors (one
blue, one orange, which means they have different surface temperatures). A bit to
the south we find Altair, in the constellation Aquila (the Eagle). The three bright
overhead stars (Vega, Deneb, Altair) are sure signs that autumn is coming. On the
23rd the Sun crosses the celestial equator heading south, an event marked as the
Autumnal Equinox. Only on this day does the Sun actually rise due East and set due
West. As the year continues toward December's Winter Solstice, the sunrise and sunset
points will drift to the south, making fewer hours of daylight and longer nights.
Returning attention to Deneb, follow the Milky Way to the northeast. You'll notice
a crooked W shape above the horizon - that's Casseiopeia. A larger triangle of bright
stars is to the west. Again anchored by Vega, sweep your vision west through Hercules
to bright Arcturus, in Bootes. To the southwest is bright red Antares, in Scorpio.
The northwest horizon is marked by Ursa Major, the familiar Big Dipper. Another
binary is found in the middle of the handle - the Mizar/Alcor system.
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.