Twenty-three years ago, on 23 February 1987 at 12:35 a.m. MST, detectors in the US, Japan and Russia observed a burst of 24 neutrinos. They came from a supernova in the Large Magellanic Cloud, a small satellite galaxy to the Milky Way. About 2 hours later, an experienced New Zealand amateur astronomer observed the area of the supernova. He noted nothing unusual.
The first visible sign of the supernova was captured on a photograph taken at a telescope in Australia about 3 hours after the neutrino burst. Since the neutrinos arrived two to three hours earlier than the light, does that mean that neutrinos travel faster than light? No, it means the neutrinos got a head start.
The processes inside stars and the events that lead to a supernova are detailed and complex. While the following description omits many important and interesting details, it has enough information to explain the neutrino head start.
A supernova is the explosive end of a massive star (the Sun is not big enough to explode as a supernova). Normal stars produce energy by fusing lighter elements into heavier ones deep in their cores. Energy is produced in the fusion process. The energy moves outward and eventually reaches the surface of the star, causing it to shine. This energy production also results in an outward pressure that balances the inward force of gravity. A supernova occurs when a star runs out of fuel in its core and the fusion reactions suddenly shut down. With the loss of outward pressure, gravity takes over and the core of the star collapses in a fraction of a second. The core of a massive star has enough gravity to squeeze the matter in it so tightly that protons and electrons combine to form neutrons. This transformation also produces an enormous number of neutrinos. The neutrinos are able to pass through the star’s outer layers and escape into space before the star shows any outward sign of trouble.
Meanwhile, deep within the star, the core collapse triggers a shock wave that moves rapidly outward. The shock wave takes several hours to reach the surface. When it does, the radiation released in the explosion can briefly outshine a galaxy. Astronomers predicted that neutrinos from a supernova would arrive before its light. So, the early arrival of neutrinos from supernova 1987A was evidence that astronomers have a correct understanding of what causes a massive star to go supernova.