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Question 1

each starts as a contracting protostar, then lives most of its life as a stable main-sequence star, and eventually moves off the main sequence toward the red-giant region.

Answer 1

As we have seen, the pace at which each star goes through these stages depends on its mass, with more massive stars evolving more quickly. But after this point, the life stories of stars of different masses diverge, with a wider range of possible behavior according to their masses, their compositions, and the presence of any nearby companion stars.

Question 2

we carefully used the term ……….. ………. of stars because, as we will see, stars can lose quite a bit of mass in the process of aging and dying.

Answer 2

initial masses

Question 3

Remember that red giants start out with a ………… core where no energy generation is taking place, surrounded by a shell where hydrogen is undergoing fusion.

Answer 3

helium

Question 4

The core of a red giant, having no source of energy to oppose the inward pull of gravity, is ………. and ……. ……

As time goes on, the temperature in the core can rise to much hotter values than it had in its main-sequence days.

Answer 4

shrinking / growing hotter

Question 5

Once the temperature core of a red giant reaches a of ………. …….. K (but not before such point), three helium atoms can begin to fuse to form a single ……….. …………

Answer 5

100 million / carbon nucleus

Question 6

This process ( three helium atoms can begin to fuse to form a single carbon nucleus) is called the …………….. ……….., so named because physicists call the nucleus of the helium atom an ………… particle.

Answer 6

triple-alpha process / alpha

Question 7

When the triple-alpha process begins in low-mass (about 0.8 to 2.0 solar masses) stars, calculations show that the entire core is ignited in a quick burst of fusion called a ………. …………

Answer 7

helium flash

(More massive stars also ignite helium but more gradually and not with a flash.)

Question 8

why the next major step in nuclear fusion in stars involves three helium nuclei and not just two.

Answer 8

Although it is a lot easier to get two helium nuclei to collide, the product of this collision is not stable and falls apart very quickly.

It takes three helium nuclei coming together simultaneously to make a stable nuclear structure.

Question 9

Given that each helium nucleus has two positive protons and that such protons repel one another, you can begin to see the problem. It takes a temperature of 100 million K to slam three helium nuclei (six protons) together and make them stick. But when that happens, the star produces a carbon nucleus.

Answer 9

r 2