February 2011 Sky from the Keeble Observatory
Last month we followed the evolution of stars like the Sun, and those less massive.
The sequence followed a similar outline: after core hydrogen exhaustion, the star
becomes a red giant, eventually expelling the outer envelope a leaving behind a
white dwarf remnant. There are some differences, depending on mass. For example,
the Sun will become a carbon rich white dwarf, while a star of less than half a
solar mass will never fuse helium, leaving a helium rich white dwarf. Also, the
time lines for these events scale inversely with the square of mass. Compare the
Sun’s 10 billion year main sequence lifetime with a star half that mass: it “lives”
40 billion years before becoming a red giant.
Massive stars will follow a track which is similar, but not quite the same. A five
solar mass star will remain on the main sequence for only 400 million years. At
ten solar masses, the main sequence lifetime is “only” 100 million years. The other
difference is in the fusion processes available at the core. At 5 solar masses,
there is sufficient density and pressure to begin fusing carbon without blowing
the star apart. Above 7 – 10 solar masses, we can fuse elements all the way to iron,
leaving the star with a structure somewhat resembling an onion. Each successive
fusion process lasts less and less time, releasing less and less energy.
Iron is the end of the line. No more energy can be released by making heavier elements,
and once the iron core becomes massive enough it will collapse catastrophically.
When the density reaches a critical value, protons and electrons will combine in
a process called inverse beta decay, leaving the core as a ball of neutrons,
accompanied by the release of a huge flux of neutrinos. The infalling envelope will
reach sufficient density to capture the neutrinos (which would normally simply escape
into space without interacting at all). This is equivalent to suddenly slamming
into a wall. The envelope will “bounce” and be ejected in a massive explosion known
as a Type II supernova, which will for a few months outshine an entire galaxy. The
core may even be crushed into a black hole – an object so dense that even light
cannot travel fast enough to escape. We’ll say more about these and a possible end
to the white dwarf remnants of low mass stars next month.
Lunar phases for January: New Moon on the 2nd, at 9:31 pm; First
Quarter on the 11th, at 2:18 am; Full Moon on the 18th at
3:36 am; Last Quarter on the 24th, at 6:26 pm.
Our overhead view in the middle of February, about two hours after sunset, finds
the constellation Auriga (the Charioteer) at zenith. Its brightest star, Capella,
lies about 10 degrees north of zenith. The bright star at 10 degrees south of zenith
is Elnath, which is in the constellation Taurus. Facing east and casting our vision
down from zenith, we encounter the bright pair of Castor and Pollux in Gemini, about
60 degrees above the horizon. About 10 degrees lower, in the constellation Cancer,
you can find the Beehive Cluster, which makes a nice target for binoculars. Leo
is rising, and you’ll see bright Regulus about 20 degrees above the horizon. Unmistakable
to the south is the familiar and magnificent Orion. The bright red star on Orion’s
“shoulder” is Betelgeuse, which is not only bright but fairly close at 425 light
years. That bright blue star to the left of Orion is Sirius, the “dog star” and
the brightest star visible other than the Sun. Above and to the right of Orion,
the bright star is Aldebaran, in Taurus. Following a line from Betelgeuse through
Aldebaran will bring you to the familiar asterism of the Pleiades, another nice
binocular target. Turning to the northwest we see the familiar shape of Cassiopeia,
here tipped to look like an upper case Greek letter sigma. With binoculars, if you
follow the imaginary line through the upper pair of stars back toward the west,
you should find the Andromeda galaxy, the nearest large spiral to our own home Galaxy,
the Milky Way.