March 2012 Sky from the Keeble Observatory
Last month we were describing the Standard Model, which is the current framework
for understanding the “zoo” of subatomic particles. Recall that we identified two
kinds of quark and the electron as constituents of the material world around us,
and then two more families of quarks and associated leptons to explain the rest.
(Lepton is a label applied to light particles which are not subject to strong
nuclear forces.) The table below identifies the components of this model.
The electron is the most familiar of the leptons. (The word electron comes
from the Greek word for amber, and is the same root as electricity. Supposedly,
static electricity was first noticed by Thales of Miletus, who reported that when
amber was rubbed with wool it would attract lint.) Their motion through wires subject
to an electric potential difference (i.e. a “voltage”) is the current which causes
the lights to come on. They are stable, fundamental particles which do not decay
into anything. Electrons also have a mirror “antiparticle” called the positron,
which has the same mass but opposite electric charge. If an electron meets a
positron they annihilate and vanish in a flash of gamma radiation. As we noted last
time, the muon was discovered in the 1940s, but nobody knew what to make of it.
We now see it as a heavier version of the electron – about 200 times more massive
– but it’s unstable. The muon decays into an electron and two neutrinos.
OK, you ask – what’s a neutrino, and where does it fit into the Standard Model?
Recall last month’s observation that we needed a third participant to explain beta
decay, in which the neutron decays into an electron and a proton and something else.
That something else is the neutrino, originally “invented” as a neutral particle
which couldn’t be detected, but whose participation in beta decay would preserve
energy and momentum conservation. Neutrinos are extremely light particles which
travel at virtually the speed of light. They interact only weakly with matter, and
would typically travel thousands of light years before interacting with anything.
It turns out that each lepton has an associated neutrino, so there are electron
neutrinos, muon neutrinos, and tau neutrinos. Each also has an antineutrino! When
the muon decays it produces an electron and an electron antineutrino, thus preserving
the “count” of particles in the first family. Since the muon has disappeared, the
count of second family particles is preserved by emitting a muon neutrino. The tau
is even more massive. 34% of the time it decays into either a muon or an electron
with its appropriate antineutrino, plus a tau neutrino. It can also decay into a
pion plus a tau neutrino.
You may have seen in the news a report that researchers may have found neutrinos
travelling faster than light. Although those results are now being questioned (apparently
two cables were not properly connected!), the fact that it made the news should
tell you that neutrinos are indeed part of “standard” physics. Next time we’ll try
to pull this back to astronomy.
Lunar phases for March: Full Moon on the 8th, at 4:39 am; Last
Quarter on the 14th, at 9:25 pm (also Albert Einstein’s birthday!); New
Moon on the 222nd at 10:37 am; and First Quarter on the 30th
at 3:41 pm.
I hope you’ve been enjoying the evening show, assuming you like to look for bright
planets. Four of the brightest have been gracing our evening skies. To the west,
you can’t have missed Jupiter and Venus. You may have missed Mercury low to the
horizon. To the east, that red “star” is Mars. And we’ve seen the Moon wander from
west to east.
Saturn has been rising after 10:00 pm, and is still above the western horizon at
By month’s end you’ll still see Venus and Jupiter as the first to emerge from evening
twilight. Venus is the brighter of the two, and it will be above Jupiter after they
come within 4 degrees on the 12th. Mars is rising earlier, so you’ll
see it higher to the east and crossing to the south as the evening advances.
An overhead look at mid-March reveals a mostly empty sky at zenith. The likely unfamiliar
constellations of Lynx and Leo Minor are in that direction, but there are no bright
stars to grab your attention. Brighter and more familiar asterisms lie tens of degrees
below zenith. Gemini are high (about 70 degrees) to the southwest, marked by the
“twins” Castor and Pollux. Below Gemini we bid farewell to Orion and all the familiar
bright stars in that constellation and those around it. To the south-southeast we
find the sickle shape of Leo, with bright blue Regulus joined by the red planet
Mars. Ursa Major (including the Big Dipper asterism) is high to the northeast, and
Capella marks the constellation Auriga to the northwest.