In determining the Hreaction for formation of MgO, why didn’t we directly burn the Mg metal in air and measure the heat produced?

for MgO(s) is difficult because the combustion of Mg metal

Explanation:

The formation are often difficult to measure directly in the reaction, it tends to evolve too much heat or form toxic substances.

no, the red giant is the hottest with 2.00.000.000 degrees celsius. while the white dwarf only 25,000 degrees celsius.