April 2012 Sky from the Keeble Observatory
We promised last month that we would bring our discussion of neutrinos back around
to astronomy. It may be a tall order, but let me try!
Remember that these are ghostly particles, travelling essentially at the speed of
light, which interact so weakly that billions are traversing your body every second
as you read this. They were initially proposed as a means to save the ideas of energy
and momentum conservation in radioactive beta decays. They work admirably in that
respect, and we now have the technology to detect them, so they’re ghostly but still
real! In the original version of neutrino theory, they were massless particles which
travel exactly at the speed of light, but we now know that their masses aren’t quite
In the 1960s Raymond Davis installed a neutrino detector at the lowest levels of
the Homestake gold mine in Lead, South Dakota. He proposed to measure the flux of
neutrinos from the Sun, which should be produced at several stages of the nuclear
fusion reaction chain that powers our central star. Knowing the Sun’s luminosity,
we can calculate how many neutrinos should be captured in this experiment, and it
was hoped that the result would confirm our model of how the Sun works. However,
Davis recorded a persistent deficit of about one third fewer neutrinos than predicted.
Repeated trials and other experiments around the world confirmed the deficit, which
came to be known as the Solar Neutrino Problem. It’s a serious gaffe for
the theory, because we think we understand the physics of fusion – indeed, we understand
it well enough to make hydrogen bombs.
It turns out that a massless neutrino is the simplest possibility, but not the only
one. If they have just a little bit of mass, say one quarter millionth the mass
of an electron, they gain another property. They can spontaneously change from one
kind of neutrino to another. The electron neutrino can “oscillate” into a muon neutrino,
and Davis’s detector was not able to detect these. If the oscillation happened as
the neutrino travelled from the Sun, it would show up as a deficit. In 2001 a collaboration
at Canada’s Sudbury Neutrino Observatory measured the total of electron and muon
neutrinos from the Sun, and found that the total adds up to the predicted flux!
Thus, neutrinos help confirm our understanding of stellar physics.
Neutrinos should also be produced in supernova explosions. In 1987 the Japanese
neutrino experiment known as Super Kamiokande detected neutrinos from a relatively
nearby supernova in the Large Magellanic Cloud. Their energy, trajectory, and time
of arrival confirmed the basic model of a Type II supernova, i.e. that it results
from the core collapse of a massive star.
Neutrinos are also thought to be a component of the “dark matter” that makes up
nearly a quarter of our universe. So, having been “invented” to save one part of
physics, these elusive little particles may help to explain part of the overall
nature of the cosmos. Not bad for something that wasn’t supposed to be detectable!
Lunar phases for April: Full Moon on the 6th, at 3:19 pm; Last
Quarter on the 13th, at 6:50 am; New Moon on the 21st at 3:18
am; and First Quarter on the 29th at 5:57 am.
Predawn planet watchers will have to content themselves with watching Saturn settle
toward the west southwest. Maybe a good excuse to sleep in!
Evenings will continue to be putting on a show. Venus and Jupiter remain bright
to the west after evening twilight. Venus is now the upper planet, and will continue
to brighten through the month. Venus will be absolutely brilliant by month’s end,
when the combination of close approach and angle will maximize its brightness on
the 30th. Jupiter will drop below the horizon by then. Mars is high to
the southeast in the constellation Leo, below and left of Regulus. Saturn rises
about 9:15 early in April, and a little earlier each night. When it reaches opposition
on the 15th, it will rise as the Sun sets.
At midmonth, about 3 hours after sunset, your overhead view finds the faint constellation
Canes Venatici at zenith. Supposedly this asterism represents the dogs of Bootes,
the Herdsman; indeed, the name is Latin for Hunting Dogs. However, the constellation
is so faint that its stars were once assigned as part of Ursa Major. It became its
own constellation in the 17th Century in the star catalog of Johannes
Hevelius. To the southeast, about 40 degrees above the horizon, you’ll find Saturn
in the constellation Virgo, near the bright star Spica. A bit higher and toward
the east southeast is Arcturus, the brightest star in Bootes. Vega is low to the
northeast. High above the southwest horizon is Mars, near Regulus. We’ve seen the
last of Orion for the season, but Castor and Pollux, in Gemini are still visible,
about 30 degrees above the western horizon.