It might surprise you to learn that scientists don't know exactly what makes sunshine. They know that the engine behind it is fusion, in which the nuclei of atoms are forced together in the Sun's core, new substances are formed, and tremendous energy is released.

But the mix of elements that give up their identity to become something else has never been pinned down. The Sun hides its secrets, and no amount of probing by fancy telescopes has shone much light on what happens at the core.

A new study of invisible particles called neutrinos, which emanate from the center of the Sun and zoom right through Earth, provides the best answer to date, though it still leaves a pretty wide gap between theory and firm knowledge.

Interestingly, scientists estimate that it takes a few hundred thousand years for photons, the basic units of light, to escape the Sun's core and reach the surface. They arrive at Earth about 8-and-a-half minutes later, where we can bask in the warm glow of light waves initiated long before sunscreen was invented.

Based on measurements of elements at the surface of the Sun, theorists have long assumed that 98.5 percent of the fusion at the core involves the lightest elements, mostly hydrogen and helium. The remaining 1.5 percent of the fusion involves nitrogen and oxygen, according to theory. But these assumptions have never been well tested by observations.

And while the Sun is like millions of stars in the Milky Way Galaxy, there are plenty of other stars that generate their energy via a different mix.

"Stars just slightly heavier than the Sun shine primarily by reactions involving nitrogen and oxygen," explains John Bahcall of the Institute for Advanced Study in Princeton, New Jersey.

The new study, which Bahcall led, puts a somewhat loose lid on the role nitrogen and oxygen play in the Sun's fusion. The cap of their involvement is now set at 7.3 percent -- still significantly above the theoretical estimate of 1.5 percent. The as-yet unpublished results will be detailed in the journal Physical Review Letters.

Further study will likely bring the figure down, but theory may ultimately have to give way to reality. "I think it is possible that [nitrogen and oxygen account] for significantly more than the 1.5 percent of the luminosity we estimate," Bahcall told SPACE.com.

To examine the core of the Sun, scientists have had to get creative.

Bahcall and his colleagues, Carlos Pena-Garay of the Institute for Advanced Study and Concha Gonzales-Garcia from Stony Brook University, reached their conclusion by examining results from several recent solar neutrino detection efforts and laboratory reactor experiments.

"Neutrinos interact so weakly with matter that they can easily get out of the center of the Sun and reach the Earth," Bahcall explained. "Physicists use neutrinos to look inside the Sun in much the same way that your doctor uses ultrasound or X-rays to look inside your body and diagnose what is going on there."