A giant underground experiment has given researchers their first glimpse into the heart of the sun and the subatomic particles that shine down on Earth everyday.

Scientists have long theorized how these particles, called neutrinos, are formed in the solar inferno, but direct proof has been hard to come by. Neutrinos can give scientists a priceless glimpse into the inner workings of the sun because they arrive on Earth virtually unchanged from when they left the sun's interior.

Princeton researchers, working at the underground Gran Sasso National Laboratory in Italy, have made the first real-time observations of low-energy solar neutrinos, fundamental particles that are created by the roiling nuclear reactions inside the sun and that stream in vast numbers from the sun's core.

In stars about the size of the sun, most solar energy is produced by a complex chain of nuclear reactions that convert hydrogen into helium. These reactions can take several different routes, but they all end in the same product: sunshine.

Steps along two of the routes require the presence of the element beryllium, and physicists have theorized that these steps are responsible for creating about 10 percent of the sun's neutrinos.

Until now, technological limitations have made it hard to detect neutrinos because they rarely interact with other forms of matter.

The Gran Sasso lab's huge Borexino detector, located more than 0.62 miles (1 kilometer) underground, overcame the limitations and observed the low-energy neutrinos. The results confirmed the two nuclear steps that involve beryllium, showing that physicists have been on target at least about those routes to neutrinos.

However, that confirmation makes scientists more certain that they are also correct about how the other processes that create sunlight work, said Princeton physicist Frank Calaprice, principal investigator of the team.

"Our observations essentially confirm that we understand how the sun shines," Calaprice said. "Physicists have had theories regarding the nuclear reactions within the sun for years, but direct observations have remained elusive. Now we understand these reactions much better."