November 2010 Sky from the Keeble Observatory
In August we described how astronomers measure the masses of stars by observing
binary systems, in which a pair of stars are orbiting a common center of mass. In
so-called spectroscopic binaries, we observe Doppler shifts in the wavelengths
of light from those stars as they alternately move toward and away from us in their
orbits. Observing orbital periods and deducing the size of the orbits from the measured
speeds, we can calculate the total mass of the pair. The ratio of orbital speeds
for the two stars allows us to calculate the mass ratio. Coupled with the total
mass, this gives us the masses of the individual stars.
Perhaps it is not a surprise, but planets orbiting a star can also give rise to
Doppler shifts, though they are typically much smaller than those measured in binary
stars. Using these and related techniques, astronomers have detected over 400 of
these extrasolar planets. Admittedly, the technique is biased toward finding
massive planets orbiting close to the primary, so most of the known extrasolar planets
are even greater than the mass of Jupiter. Alternatively, the technique can also
find planets of moderate mass orbiting low mass stars.
One system which has been extensively studied for more than a decade is a small
red dwarf star, unglamorously named Gliese 581. (It gets its name from being the
581st entry in the 3rd edition of the Gliese Catalog of Nearby
Stars, which lists all known stars within 25 parsecs of the Sun.) Last September
29th, researcher Steven Vogt of the University of California, Santa Cruz,
announced the discovery of two more planets orbiting this star (which was previously
known to have four planets). One of these planets, roughly 3 to 4 times the mass
of Earth and designated Gliese 581g, orbits within the “habitable zone” of the star.
Its orbital period is only 39 days, but the star itself is much smaller and cooler
than the Sun, so a close orbit is required to be in the habitable zone.
What does this mean? We know from our own solar system that planets closer to the
Sun are warmer, while those farther from the Sun are colder. Mercury and Venus are
both extremely hot – so hot that liquid water is impossible there. Mars is very
cold, with most of its water tied up in ice. Distant Pluto, though no longer classified
as a planet, is essentially a big ice ball. Earth sits in the middle of those temperature
extremes. Here we can find water as vapor, as liquid, and as ice. And here we find
the only known life. Gliese 581g lies close to its dim red dwarf star, but not too
close. In the so-called “Goldilocks zone” the likely temperature range for this
planet seems to allow liquid water. And, maybe life. What we know about our own
home world is that anywhere we find liquid water, we also find life in abundance.
However, before anyone gets too excited about looking for ET, we should admit that
we don’t know if this planet even has an atmosphere. Unfortunately, its orbit does
not carry it in front of the primary, so we have no way to measure the presence
of an atmosphere. Indeed, it’s close enough to its parent star that it could be
tidally locked, with one face always toward the star and the other in perpetual
darkness, just as our Moon always keeps one hemisphere toward Earth. Also, the discovery
has not yet been confirmed independently by other observers. Dr. Vogt stands by
his analysis. Curiously, the official NASA “artist’s conception” shows a blue and
green planet illuminated by a red star … which can’t happen, since blue and green
would both look black in that light!
Lunar phases for November: New Moon on the 6th, at 12:52 am; First
Quarter on the 13th, at 11:39 am; Full Moon on the 21st at
12:27 pm; Last Quarter on the 28th, at 3:36 pm.
Predawn planet watchers will have to content themselves with Saturn in early November.
Look for it rising about 2 hours before sunrise; it will be about 25 degrees above
the east-southeast horizon at dawn. Things get better as the month advances. Venus
returns to the southeast morning twilight, and by the end of the month you’ll see
it shining brightly above the southeast horizon at month’s end. Saturn climbs through
the month, and will be at 45 degrees to the south-southeast.
Jupiter emerges from twilight after sunset, low to the east-southeast early in the
month. Mars begins low to the southwest, near Antares, and sets a few hours later.
By the end of November, look for Jupiter higher to the southeast, beginning about
35 degrees above the horizon and moving in a high arc toward the southwest. Mercury
returns to the southwest as the “evening star,” but very low to the southwest. Without
an uncluttered horizon, you may not catch it before it sets.
At mid-month, our overhead view about 2 hours after sunset finds the faint constellation
Lacerta (the Lizard). This was never one of the constellations to the ancients,
but is a modern creation. It contains no truly bright stars, so don’t be surprised
if it doesn’t impress! It lies on the edge of the Milky Way, which arcs from northeast
West of zenith you will encounter Deneb in Cygnus, and below that the bright star
Vega in Lyra. These two stars form a triangle with Altair, in Aquila, which is to
the left of Vega, but at about the same altitude above the horizon. Carefully sweeping
binoculars from Vega to Altair, you may encounter the Ring Nebula in Lyra.