Observations to decide the primary source of energy of a star is hydrogen or helium?

Question

What observations could you make of a star to determine whether its primary source of energy is hydrogen or helium?

Answer 1

For helium there is a white line around the star

Answer 2

spectroscopy.

Look for the emmission lines for hydrigen and helium to see which is stronger.

Large telescope, slit, defraction grating, comparator (lamp source), and detection unit (CCD).

Answer 3

The evidence that the Sun burns hydrogen is indirect.

Fusion of hydrogen is the only reaction we know that can provide the energy produced (which we can see and measure) over timescales consistent with the age of the oldest rocks on Earth.

Any chemical reaction (imagine the Sun made of coal for instance), would consume the Sun in a mere 200,000 years or so.

The last twenty years we have also found direct evidence in the form of neutrinos, particles that are produced in those fusion reactions and that escape the Sun more or less unharmed, and are detected on Earth.