The Sun and Rigel and their ages?

Question

if Rigel is extremely bright (40,000 times more than the sun at visible wavelengths) and burns with a blue-white light, why would this indicate that it is much younger than the Sun?

i thought young stars were cool, and that red stars were cool and blue ones were hot. if Rigel is shining blue then surely this should indicate that it is actually hotter and therefore older than the Sun?

Answer 1

A star's entire life is pretty well determined by its mass. The more massive a star is, the hotter its centre must be to balance the gravitational pressure. The hotter (and pressurised) the centre, the faster the rate of fusion. The faster the rate of fusion, the more energy must be radiated to conserve the balance.

A more massive star will be hotter, bluer and evolve faster (and not live as long).

A less massive star will be cooler, redder and evolve slower (and live much longer).

It is (in general) that simple.

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There may be minor differences due to the "metal content" (meaning anything else than hydrogen and helium) and the immediate neighbourhood, but the principle remains the same.

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The sun has been a main sequence star for almost 5 billion years. This is the time since the start of fusion at the core. Before that, as a proto-star, its energy came from the graviational energy released by the collapse of the cloud upon itself. Given its size and rate of fusion, the Sun should live a total of 10 billion years on the Main Sequence before entering the Red Giant phase of its life (at which time, it is no longer a "main sequence" star).

Rigel is 17 times the mass of the Sun. This gives a lot more pressure at the centre and, to compensate, Rigel's core must produce energy at 66,000 times the rate of our Sun.
Yes, 66 thousand!

So, even though it has 17 times as much fuel, it is burning it 66,000 times faster and will, therefore, run out of its primary fuel (hydrogen) faster than our sun.

10 billion * 17 / 66,000 = 2.5 million years.

The only reason we say that Rigel is younger than our Sun is simply that the total life-span of Rigel on the Main Sequence is less than the present age of the Sun. Therefore, Rigel (still on the Main Sequence) cannot be older than the Sun.

It is like saying that the oldest known cat is 20 years old and then asking, about any other cat:

Is this cat older than my grand-father?
Without checking out the other cat, I can be pretty sure that it is younger than my grand-father.

Answer 2

Hi Rigel is a blue giant star and therefore burns much more hydrogen and other fuels than the Sun. If it were older the fuel would have burned up already. http://www.answers.com/Rigel?cat=technology&gwp=13

Answer 3

Not always friend! In fact we are still discovering things about the incredible age of the stars! It is much more complicated than just saying by size, color radiation or something that we revealed! Good afternoon!