Question

PHYS 122 Explain or justify all answers Astronomy: Stars & Galaxies HOMEWORK Unit III 1) How are the p-p chain and the CNO cycle similar? What kinds of stars have each as their main fusion reaction? 2) What is the underlying physical reason that there is a mass-luminosity relation? Explain. 3) Why does a star's life expectancy depend on its mass? EXPLAIN 4) What are the main sequence lifespans of (a) a 25 solar mass star, and (b) a 5 solar mass star? 5) Explain how & why the turnoff point on the H-R diagram reveals a star cluster's age. 6) What will cause the Sun to turn into a red giant 5 billion years from now? P ere to search

please help me solve these

Answer 1

1)

p-p chain and CNO cycle are similar in a way that both convert four hydrogen nuclei into one helium nucleus. However, CNO cycle needs carbon to be present therefore, CNO cycle needs higher temperature.

The proton-proton chain reaction is found in stars having size of the Sun or smaller and CNO cycle comes when the size is much bigger than the sun.

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2)

the physical reason is that more massive and bigger main sequence stars are hotter and more luminous than the low-mass main sequence stars. More bigger size means they shed their energy faster

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3)

larger the mass, shorter the life because with larger size comes large requirement to burn core hydrogen so star will run out of hydrogen earlier

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4)

equation to find life span

T = 10¹⁰ ( mass of sun / mass of star in solar masses) ^2.5

so,

for M = 25 solar masses

T = 3.2 million years

and

for M = 5 solar masses

T = 178.9 million years

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