The ghostly particles known as neutrinos fill the universe. Trillions of them pass through your body every second. But you don't notice them because almost all of them pass through you, and the solid Earth, and even entire stars without effect. And because they almost never interact with other matter, they're hard to study.

In fact, astronomers have confirmed only two sources of neutrinos. One is the Sun. The nuclear reactions in the cores of stars produce neutrinos by the bucketful. But these neutrinos are relatively weak, so they're especially tough to detect.

The other confirmed source of neutrinos was a supernova in 1987. In that case, even though the neutrinos probably provided a lot of the "oomph" that caused the star to explode, detectors here on Earth caught just two dozen of them.

Those neutrinos were more energetic than the ones produced by "normal" stars. And even more-energetic neutrinos probably are produced by the clouds of debris that supernovae leave behind, and by the explosions known as gamma-ray bursts.

But astronomers have to be pretty clever to detect even these high-energy neutrinos. They've placed detectors at the bottom of the Mediterranean Sea, for example, and in the ice at the south pole, to look for flashes of light created when a neutrino hits a water molecule. But even though countless neutrinos stream by every second, scientists are hopeful that the detectors might see one neutrino event every year.

More tomorrow.

Script by Damond Benningfield, Copyright 2011