Days of hot and sticky weather, often during the high summer months of July and August, are commonly referred to as the dog days of summer. This term is thousands of years old and is a reference to Sirius, the Dog Star.
Sirius is the brightest star visible in the sky (not including the moon, the sun and the other planets). It is seen more easily from latitudes closer to the tropics where, coincidentally, there is also more hot weather.
Ancient Greeks as well as others believed the hot weather during late summer was due to heat from the sun combining with heat from Sirius, because the bright star could be seen rising and setting in tandem with the sun for a few weeks during the hottest part of the year. In fact, no measurable heat can reach Earth from any star in the sky other than the sun. It is estimated that if the sun were not there and Earth produced no internal heat, radiation from all the stars in the sky would barely keep our planet above absolute zero.