Which star is hotter, but less luminous, than
polaris?
(1) deneb
(2) aldebaran
(3) sirius
(4) pollux

The star which is hotter, but less luminous than Polaris is (3) Sirius.
This is because the luminosity of Polaris is 1,260, and its temperature is 6015K, whereas the same things for Sirius are 23.4 and 9,940 which makes this star less luminous, but hotter than Polaris.
Deneb is more luminous and hotter, Aldebaran is less luminous and less hot, and Pollux is less luminous and less hot as well. 

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen