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

Stars in your little finger

Stop reading for a moment and look at your little finger. It’s full of carbon atoms because carbon is a fundamental chemical building block for life on Earth. Each of those carbon atoms was once inside a red giant star and was fused from helium nuclei in the triple-alpha process. All the carbon on Earth—in you, in the charcoal you use for barbecuing, and in the diamonds you might exchange with a loved one—was “cooked up” by previous generations of stars. How the carbon atoms (and other elements) made their way from inside some of those stars to become part of Earth is something we will discuss in the next chapter. For now, we want to emphasize that our description of stellar evolution is, in a very real sense, the story of our own cosmic “roots”—the history of how our own atoms originated among the stars. We are made of “star-stuff.”

Becoming a giant again

After the helium flash, the star, having survived the “energy crisis” that followed the end of the main-sequence stage and the exhaustion of the hydrogen fuel at its center, finds its balance again. As the star readjusts to the release of energy from the triple-alpha process in its core, its internal structure changes once more: its surface temperature increases and its overall luminosity decreases. The point that represents the star on the H–R diagram thus moves to a new position to the left of and somewhat below its place as a red giant ( [link] ). The star then continues to fuse the helium in its core for a while, returning to the kind of equilibrium between pressure and gravity that characterized the main-sequence stage. During this time, a newly formed carbon nucleus at the center of the star can sometimes be joined by another helium nucleus to produce a nucleus of oxygen—another building block of life.

Evolution of a star like the sun on an h–r diagram.

Evolution of a Star like the Sun on an H–R Diagram. In this plot the vertical axis is labeled “Luminosity (LSun),” and goes from 1.0 near the bottom to 10,000 near the top. The horizontal axis is labeled “Surface Temperature (K),” and goes from 9000 on the left to 3000 on the right. The main sequence is drawn as a diagonal red line beginning at L ~ 40 on the left down to T ~ 4000 at the bottom. The evolutionary path of the star is drawn as a black line. Beginning at L = 1 and T = 5500, the line moves upward away from the main sequence. This portion of the line is labeled “(a),” and is described in the caption. The line continues upward to L ~ 1000 and T ~ 3000 to point “(b),” labeled “Core helium flash.” From point b, the line (now dashed) moves downward to L ~ 100 and T ~ 5000 and labeled “(c),” and is described in the caption. From c, the line moves upward again. This portion of the line is labeled “(d),” and is described in the caption. The line culminates in a series of waves near L = 5000 and T ~ 3500 and is labeled “Helium shell flashes.”
Each stage in the star’s life is labeled. (a) The star evolves from the main sequence to be a red giant, decreasing in surface temperature and increasing in luminosity. (b) A helium flash occurs, leading to a readjustment of the star’s internal structure and to (c) a brief period of stability during which helium is fused to carbon and oxygen in the core (in the process the star becomes hotter and less luminous than it was as a red giant). (d) After the central helium is exhausted, the star becomes a giant again and moves to higher luminosity and lower temperature. By this time, however, the star has exhausted its inner resources and will soon begin to die. Where the evolutionary track becomes a dashed line, the changes are so rapid that they are difficult to model.

Questions & Answers

In planet mars there the life exits or not and is there water there
Eshwarsa Reply
see till now nothing can be found as u know that the curiosity rover has struck in mars
Maya
what is your opinion about the theory of Vedas about modern physics..
Manish Reply
i think in some ways vedas are also correct but not everytime
Maya
I agree
sakshi
hmm even I agree
Samuel
Is there any patened theory about time relativitg in growth and development?
donot Reply
some astronomer's says that there is no alien exist but why search for extra terrestrial intelligence center is established
Eshwarsa Reply
No One Knows That For Absolute Fact, The Universe Is Too Huge To Have Any Type Of Idea About What Exist In The Far Reaches Of Our Universe.....
Adam
Check Out The Drake Equation.....
Adam
their should be aliens as like ours there would be another planet
Maya
which could have existed life on it
Maya
adam i want to ask a question
Maya
can kepler 1st law be applied on all the planets of the universe
Maya
hello, anyone home?
Denise
sjskskfhjkkktewqqw and try?
Lanika Reply
what is this ?
Samuel
hi I am Samuel from India mumbai
Samuel
nice to meet you
Samuel
thats my question, what is this?
penzias and wilson's a discovery of the cosmic microwave background is a nice example of scientific serendipity-something that is found by chance but turns out to have a positive outcome
Jacqueline Reply
how should I make my carrier in astronomy
Ayush Reply
I think that Newton's third law is not appropriate if any also thinks like this please reply me
Ayush
Can you explain your reasoning
Huh
why u think so
joseph
yes Ayush u are right
Yoganshu
I think when we apply force to a object it start moving but , a/c to Newton's third law every action has equal and opposite reaction,so object should also exert equal force on us and it should not move due to balanced force
Ayush
if I am not right then reply me
Ayush
no
Zack
because of friction that opposes that force and help us to move ahead
Manish
but this is not satisfied as third law say another thing
Ayush
you are telling why object moves
Ayush
you have to think a/c to third law
Ayush
its because of its mass
Maya
because it is applying equal and opposite force but also our mass is also less in comparison to the object
Maya
which is why we cant move the object but it can make move us
Maya
manish is too correct in his place because we need to apply force which would overcome the frictional force
Maya
My dear friends, can u plz tell me that among u guys who are in the field of cosmology
Madhav Reply
😢I am not there in cosmology
Samuel
Just A Science Fan.....
Adam
Adam even I am 😥😅😅😂😂
Samuel
I am also not in cosmology but I am just a fan or we can say science and part of NASA is my dream
Yoganshu
yoganshu Arya same here
Samuel
you are from which country
Yoganshu
hi yoganshu
Samuel
India
Samuel
which state
Samuel
I am also from India
Yoganshu
from delhi
Yoganshu
and u...?
Yoganshu
I am from Maharashtra
Samuel
from which state?
Yoganshu
You are a ASTRONOMER ...
Yoganshu
or a scientist..
Yoganshu
or just a member
Yoganshu
What is time...? not about Newton= time is constant..... that all scientists openions n point of view I m knowing. . what can be the Perfect Definition of Time
Madhav Reply
time is what clock reads
Ayush
Who is the best astronomer of India at present time
Gian Reply
Jayant Narlikar, Proponent Of Steady State Cosmology.....
Adam
What is the real colour of sun rays
Gian Reply
white.. so white it becomes violet.. so violet it become ultraviolet
Tom
white and red and yellow
Bianca
Vibgyor
Samuel
the real colour of sunlight is White
Madhav
the Sun's has a variety of waves all throughout the elextromagnetic spectrum.
Jacie
we only see it as a few bc of how some of them get redshifted (? can that term be applied for something so local?) by some particles in our upper atmosphere
Jacie
Vibgyor will be when, the white light will pass through the clouds ( prism ) then Refraction phenomenon leads us to 7 colours splitting from a single colour "White " light
Madhav
so the sun rays r of White colour
Madhav
Taurus in astronomy and horoscope?
Yasser Reply
how to put E=MC2
Gospel Reply
What Do You Mean By How To Put?
Adam
What Do You Mean By How To Put?
Adam
What Do You Mean By "How To Put E=MC2?
Adam
yep
Gospel
Hi guys
Samuel
i mean how NASA came to know the mass and diameters of Stars.how?
Gospel
how did they do using E=MC2
Gospel
thats my questioning
Gospel
that's easy formula's derivation
Madhav
why all planets revolving orbits are nearly in equal inclination?
Kartik Reply
***medium.com/starts-with-a-bang/ask-ethan-82-why-are-the-planets-all-in-the-same-plane-4470245c8743
rishabh
dependent on the mass
Madhav
no
Janak
I think because of the Suns gravity pull ,😕😕
Samuel
Samuel,. it's but obvious
Madhav
what hubbel's law. and examples on it.?
Kavyanjali Reply
Hubbel's Law is a Expanding Universe. The distant galaxies we see in all directions are moving away from the Earth, as evidenced by their red shifts. Hubble's law describes this expansion.
Роза
okk. sir/madam, i studied that all 3 laws.
Kavyanjali
i m solving examples on "p" square directly proportional to "a"cube
Kavyanjali
Practice Key Terms 3

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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