How does a star become a red supergiant?

Answer 1

Short answer: as a star expands its outer most shell of gas is thinned and therefore cooler - rather like air at the top of a mountain is thinner and thus cooler than air at the base of a mountain. Cooler stars are red (such as red dwarfs).

Longer answer:

As a star burns up its hydrogen it creates in its center a core of innert helium with which it can (initially) do nothing. It lacks the mass to fuse helium into more massive elements such as carbon, etc. Eventually it uses up most of the hydrogen in its core. (The rest of the star still has plenty of hydrogen, but this is not near the core and so not under the pressures required for fusion.)

At this time explosive fusion weakens and crushing gravity takes over the balance of the star, causing the mass of the star nearest the core to rush inward. This creates increased pressure on the helium, finally passing the tipping point and allowing for helium fusion. This has several effects on the star.

First, more of the star's mass is concentrated at the core, for helium is denser than hydrogen. As a result, the rest of the star is somewhat less dense.

Second, as a result of the first, gravity gains a better grip, as it were, on the core of the star even as it loses some of its grip on the rest of the star. The mass remains part of the star, but due to gravity's weakened grip the star stars to expand.

Third, the increased pressure does not just jump start helium fusion - it is so great that the hydrogen *near* the core starts to fuse into helium. This volume of fusing hydrogen is vaster than the volume of the core of the star - and thus while the helium fusion in the core is less efficient than hydrogen fusion, the secondary hydrogen fusion outside the core (in addition to supplying more fuel for the core fusion, for a time) increases the luminosity of the star vastly.

The prior three have a fourth effect on the star. The concentration of mass nearer the core of the star *decreases* the density (relatively speaking) of the outer most reaches of the star. Furthermore, the increased luminosity - radiant energy - coming from (near) the core of the star, combined with the decreased gravitational hold on the outer mass of the star causes the expansion of the star to vastly and swiftly increase. A star like the sun, for example, could expand until its outer edge was nearly in earth's orbit. Mercury and Venus would not fare well, and there is evidence that the earth - after being scored dry and lifeless - would actually spiral into the sun. Mars would become the new Mercury, as it were.

What the final paragraph really means, however, is that while the inner core has become much more dense than what currently exists in the center of the sun, the outer envelope has become incredibly less dense - the same mass spread over literally hundreds of times the volume it priorly held. As described at the top, when gas is spread thin, it cools from its prior temperature. As shown, the outer mass of the sun has spread over an almost incomprehensibly vaster volume than was prior - resulting in a notable decrease of the 'surface' temperature of the star (despite its vastly increased luminosity).

Thus, having a cooler 'surface', it appears the color of a cooler star: red.

As for why cooler stars are red, that is another question.

Answer 2

• Stars similar in size to our own Sun swell up as their fuel supply gets low. They turn red and are known as red giants.

• Red giants are stars with radii tens to hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.
• They also undergo other fusion reactions.

• Red supergiants normally arise from stars much more massive than the Sun when they stop burning hydrogen in their cores.

Answer 3

Lack of hydrogen fuel

Usually happens after a long time, fuel runs out.

Answer 4

When a star begins to die, it uses up its last energy so quickly that it swells up, and then explodes.