How is heat energy transferred?
Heat is transferred by three means : Conduction, convection and lastly by means of radiation.
Conduction involves transfer of heat between adjacent particles as in a metallic strip. if one of the sides are heated, the constituent particles present their absorb heat and start supplying it to the adjacent particles. Soon, we find that the other end of the strip becomes hot. When finally the two ends attain the same temperature (they attain thermal equilibrium), heat transfer stops altogether.
It is noted that in conduction, there is no movement of particles from their mean positions. But, in case of fluids, when one side gets heated, the particles heat up and start moving faster, colliding with others and allowing colder particles to take up heat. This is the process of convection. This happens when we heat up a saucepan of water on a stove : The water at the bottom heats up and finally we find the entire water is boiling, not just the bottom part.
In the previous two means of heat transfer, we see that constituent particles are involved. A natural conclusion to draw from this is that for conduction and convection to happen, some material medium is present. But, we know that the universe is a free space (vacuum), then how does the sun's heat reach us, is a question that we still have left unanswered.
To explain this, we have to understand that the sun (and other stars as well) emit Electromagnetic radiation which are oscillating electric and magnetic fields propagating through space. They don't necessarily need a material medium. You might be familiar from your studies on electricity and magnetism that an electric field and a magnetic field store energy of energy densities