March 2011 Sky from the Keeble Observatory
In February we discussed the end state of a massive star: iron as the dead end of
energy production, followed by core collapse and the detonation of the star in a
“type II supernova – SN2.” This violent death of a star can outshine an entire galaxy
for several months – we’re lucky that they are rare, because it would not be healthy
to be within even a few tens of light years. The gamma radiation alone would kill
everything on Earth! The most recent nearby SN2 was observed in 1987 in the Large
Magellanic Cloud, one of several satellite galaxies orbiting the Milky Way, and
that one was 160,000 light years away. By the way, it was the first nearby supernova
seen since the invention of the telescope, which should give you further evidence
for their rarity.
Recall that during the core collapse the electrons and protons combine via inverse
beta decay to become neutrons. For a moderate mass progenitor, the core becomes
a neutron star, roughly the mass of the Sun, but only 15 – 20 kilometers across.
For a more massive core, the neutrons get crushed so close together that even the
repulsive “degeneracy pressure” which sustains the neutron star is overcome. The
collapse proceeds to a geometric point of infinite density. Both classical and relativistic
calculations give us a value for the distance from this singularity where the escape
speed is equal to the speed of light. Any closer than this so-called “event horizon”
and not even light can escape. The stellar core has effectively fallen out of the
external universe! This bizarre object is known as a black hole, a term coined by
physicist John Archibald Wheeler. We can take some comfort from the fact that these
are not, as often portrayed in bad science fiction movies, giant cosmic vacuum cleaners.
Objects outside the event horizon can orbit and escape, although they will notice
distortions in space and time when comparing to more distant observers. The size
of the event horizon scales directly with the mass. For one solar mass, the event
horizon radius is about 3 kilometers. For ten solar masses, it’s about 30 km. However,
the Sun will not end as a black hole, because it’s not massive enough to undergo
core collapse. Rather, it will end as a white dwarf (about the size of Earth).
But, if the Sun were in a binary star system, the end could be even more violent
than SN2. There is an upper limit to the mass of a white dwarf, beyond which it
will collapse. Unlike the iron core of a massive star (which leads to a neutron
star or a black hole), this collapse will initiate a catastrophic episode of fusion.
Any residual helium will fuse to carbon; carbon will undergo explosive detonation,
leading to the complete destruction of the remnant. This explosion is known as a
Type Ia supernova, and (perhaps surprisingly, since it’s the remnant of a lower
mass star) it will exceed the luminosity of even a Type II supernova. Next month,
we’ll see how the Type Ia provides us a “standard candle” for measuring the size
of the Universe.
Lunar phases for March: New Moon on the 4th, at 3:46 pm; First
Quarter on the 12th, at 6:45 pm; Full Moon on the 19th at
2:10 pm; Last Quarter on the 26th, at 8:07 am.
This March is not a great month to look up if you are only interested in planet
watching! Venus begins the month low on the southeast horizon at sunrise, brilliant
but getting lower as we move into spring. By the end of March, it will be lost in
horizon clutter. Jupiter is low to the west at sunset, and you may miss it if you
wait for total darkness.
Saturn is your best bet, rising to the east at about 9:00 pm and visible essentially
all night. It will be due south around 3:00 am.
Looking overhead at midmonth, we find the faint constellation Lynx at zenith, but
you will probably notice the bright “twins” in Gemini close to zenith, marked by
its two brightest stars, Castor and Pollux. Castor is officially “alpha Geminorum,”
which should mean that it’s the brighter of the two (Pollux is beta Gem), but Pollux
is actually brighter. A modest telescope should resolve Castor as a binary. To the
west of zenith, about 60 degrees above the horizon you will see the bright star
Capella in the constellation Auriga.
To the east, that bright star about 50 degrees above the horizon is Regulus, in
the constellation Leo. Imagine a line from Regulus to the belt of Orion … that bright
star midway is Procyon, in Canis Minor. Above Procyon (near the Moon on the 15th)
your binoculars will let you explore the Beehive Cluster.
Turning to the northeast, we see the familiar shape of the “big dipper” in Ursa
Major. The middle star in the “handle” is the binary system of Mizar and Alcor.
This pair is resolvable without binoculars or telescope. With a telescope, you can
see that each of these is also binary. Closer to the horizon, at the end of the
handle is Alkaid … and about a degree above and to the right, your binoculars may
enable you to see the spectacular “Whirlpool Galaxy” – also designated M 51.