The size and temperature of a star can be understood by its colour at this stage.
Smaller and relatively cooler stars are bright red in colour with surface temperatures
thousands of degrees lower than our sun. Moderate stars, like our sun are yellowish.
But the massive stars are white, blue or even ultraviolet. They have surface
temperatures of up to 45,000 degrees and are 10,000 times more luminous than our
sun.
Credit: "Evolution of a sun-like star" by Lithopsian (Wikimedia
Commons)
But situations begin to change. After burning brightly for a few billion years the star
starts to run out of its main fuel, Hydrogen. In order to survive it then tries to confine
itself to a smaller area and the core begins to shrink. The temperature of the core
soars, till it reaches a point where Helium itself begins to be utilised as fuel. Helium
fuses together to form Carbon, Oxygen and the next heavier elements of the periodic
table. As it does so, it creates more energy than the Hydrogen fusion and the
equilibrium is disturbed. Gravity takes the lower hand as the outer layers of the star
begin to expand, cooling slightly as they do so. Gradually the star transforms into a
red giant (as the star cools it heads towards red). As the layers drift further and further,
the gravitational pull gets weaker and weaker, causing the star to now become a
planetary nebula. The only remnant of the star at this point is its core, called a ‘white
dwarf’ at this stage. This extremely dense object is white in colour, hence its name
and a teaspoonful of it would weigh as much as a Steamroller on Earth. It eventually
cools to become a black dwarf, the dead remains of a once bright star.