Neutrino Astronomy
An exciting development involving neutrinos is the birth of neutrino astronomy. In the fusion reaction that powers our sun, where four hydrogen nuclei (protons) end up as a helium 4 nucleus, the weak interaction and the β decay process comes into play in the conversion of two of the protons into the neutrons of the helium nucleus. Thus the emission of neutrinos must accompany the fusion reaction, and the neutrinos themselves must carry off a significant amount of the energy liberated by the fusion reaction.
In a star like the sun, the fusion reaction takes place down in the core of the star where the temperatures are highest. Any light emitted by the fusion reaction should take the order of about 10,000 years to work its way out. Thus if the fusion reaction in the sun were shut off today, it would be roughly 10,000 years before the sun dimmed.
Neutrinos, however, escape from the core of the sun without delay. If the fusion reaction stopped and we were monitoring the neutrinos from the sun, we would know about it within 8 minutes. As a result there is considerable incentive to observe the solar neutrinos, for that gives us a picture of what is happening in the sun’s core now.
To study solar neutrinos, and do other experiments like look for decay of the proton, several large neutrino detectors have been set up around the world. Solar neutrinos have been monitored fairly carefully for over a decade, and there is an unexplained, perhaps disturbing result. Only about one third as many neutrinos are being emitted by the sun as we expect from what we think the fusion reaction should produce. Perhaps we are not detecting all we should, but the detectors are getting better and the number remains at 1/3. This is one of the major puzzles of astronomy.
That neutrino astronomy is really here was dramatically illustrated with the supernova explosion of 1987. On the average, supernovas occur about once per century per galaxy. Kepler saw the last supernova explosion in our galaxy 400 years ago. In 1987 a graduate student spotted the sudden appearance of a bright star in the large Magellanic cloud, a close small neighboring galaxy. This was the first supernova explosion in the local region of our galaxy in 400 years. In a supernova explosion, huge quantities of neutrinos should be emitted. In fact a fair fraction of the energy of the explosion should be carried out by neutrinos.
Theoretical models of supernova explosions suggest that light should take about three hours to work its way out through the expanding envelope of gas before it starts its trek through space at the speed c. Neutrinos, on the other hand, should escape without being slowed down, and have a three hour head start on the light. If neutrinos have no rest mass, and therefore travel at the speed of light, they should have reached the earth about three hours before the light. Two of the major neutrino detectors, one in the US and one in a tunnel in the Alps, detected significant pulses of neutrinos about three hours before the flare-up of the star was seen. (This was determined by a later analysis of the neutrino data.) That event marks the birth of neutrino astronomy on a galactic scale.