Question

why do stars give out heat and light

Answer 1

This is a very good question. The standard answer is that the pressure and temperature at the core of the stars are so high that light elements like Hydrogen fuse there to produce Helium- 4, and such nuclear fusions release energy in the form of Neutrinos and Gamma rays having energy of several MeV.:While the neutrinos promptly leave the star imparting almost no energy to the star, the gamma rays strongly interact with the ionized stellar material, in particular the electrons by Compton scattering.Electrons gain energy by this process, and since protons and ions are electrostatically connected, the entire stellar material partakes of this gamma ray energy. But in each collision, the gamma rays lose only a small fraction of their huge energy. And they undergo millions and millions of collisions with stellar matter before they can escape the star. In fact photons perform random walk due to such multiple collisions:And for Sun, by the time those photons manage to escape, their energy degenerates into few eV (optical range) from initial MeV range. And this phase of energy general of the stars is known as MAIN SEQUENCE.

BUT THIS IS NOT THE FULL STORY: STARS CAN GENERATE ENERGY & RADIATE EVEN IN THE ABSENCE OF ANY NUCLEAR FUSION AT THEIR CORES:

A spherical star a self-gravitational (potential) energy ~ V∼−GM2RV∼−GM2R

It also has Heat Energy or internal Kinetic Energy : T. The relationship between V & T is given by the VIRIAL THEOREM:

2T+V=0, or T=−V2=+GM22R2T+V=0, or T=−V2=+GM22R (1)

Then the total Newtonian Energy of the star:

E=T+V=V−V2=V2=−GM22RE=T+V=V−V2=V2=−GM22R (2)

In reality, on the average, pre-main sequence, stars are contracting. As R decreases, it is seen from the above equation, the Energy of the star becomes more negative. This may be also seen from:

dEdt=+GM22R2dRdtdEdt=+GM22R2dRdt (3)

For contraction dR/dt <0, and dE/dt <0. Negative dE/dt means that the STAR IS RADIATING OUT ENERGY DURING CONTRACTION.

It is seen from Eq.(1) that, as R decreases the Heat Energy T Increases.

Therefore during self-gravitational contraction, a self-gravitating object MUST KEEP ON RADIATING even without any central nuclear fusion. Paradoxially, at the same time, it must get hotter and hotter.
Hope you get it.✌✌

Answer 2

stars give us light through nuclear reaction, using the energy stored in tiny nucleus at centre of atoms and it gives heat as our sun hydrogen is being converted into helium that heats star and thus,it give us heat