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So the star becomes simultaneously more luminous and cooler. On the H–R diagram, the star therefore leaves the main-sequence band and moves upward (brighter) and to the right (cooler surface temperature). Over time, massive stars become red supergiants, and lower-mass stars like the Sun become red giants. (We first discussed such giant stars in The Stars: A Celestial Census ; here we see how such “swollen” stars originate.) You might also say that these stars have “split personalities”: their cores are contracting while their outer layers are expanding. (Note that red giant stars do not actually look deep red; their colors are more like orange or orange-red.)

Just how different are these red giants and supergiants from a main-sequence star? [link] compares the Sun with the red supergiant Betelgeuse , which is visible above Orion’s belt as the bright red star that marks the hunter’s armpit. Relative to the Sun, this supergiant has a much larger radius, a much lower average density, a cooler surface, and a much hotter core.

Comparing a Supergiant with the Sun
Property Sun Betelgeuse
Mass (2 × 10 33 g) 1 16
Radius (km) 700,000 500,000,000
Surface temperature (K) 5,800 3,600
Core temperature (K) 15,000,000 160,000,000
Luminosity (4 × 10 26 W) 1 46,000
Average density (g/cm 3 ) 1.4 1.3 × 10 –7
Age (millions of years) 4,500 10

Red giants can become so large that if we were to replace the Sun with one of them, its outer atmosphere would extend to the orbit of Mars or even beyond ( [link] ). This is the next stage in the life of a star as it moves (to continue our analogy to human lives) from its long period of “youth” and “adulthood” to “old age.” (After all, many human beings today also see their outer layers expand a bit as they get older.) By considering the relative ages of the Sun and Betelgeuse, we can also see that the idea that “bigger stars die faster” is indeed true here. Betelgeuse is a mere 10 million years old, which is relatively young compared with our Sun’s 4.5 billion years, but it is already nearing its death throes as a red supergiant.

Betelgeuse.

Direct Image of the Star Betelgeuse. In this figure the H S T image of Betelgeuse is presented in the inset in the upper left of this image where the reddish, extended atmosphere surrounds the brighter, yellow core. Below the inset is a list of relative scales based on the image. At the top the “Size of Star” is indicated with a bar the width of Betelgeuse in the image. At the center the “Size of Earth’s Orbit” is shown with a much smaller bar. Finally, at the bottom, the “Size of Jupiter’s Orbit” is also shown with a bar. Both the orbits of Earth and Jupiter fit comfortably within the size of Betelgeuse. The right hand panel shows the full constellation of Orion, with Betelgeuse indicated at the upper left of the image.
Betelgeuse is in the constellation Orion, the hunter; in the right image, it is marked with a yellow “X” near the top left. In the left image, we see it in ultraviolet with the Hubble Space Telescope, in the first direct image ever made of the surface of another star. As shown by the scale at the bottom, Betelgeuse has an extended atmosphere so large that, if it were at the center of our solar system, it would stretch past the orbit of Jupiter. (credit: Modification of work by Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA)

Models for evolution to the giant stage

As we discussed earlier, astronomers can construct computer models of stars with different masses and compositions to see how stars change throughout their lives. [link] , which is based on theoretical calculations by University of Illinois astronomer Icko Iben, shows an H–R diagram with several tracks of evolution from the main sequence to the giant stage. Tracks are shown for stars with different masses (from 0.5 to 15 times the mass of our Sun) and with chemical compositions similar to that of the Sun. The red line is the initial or zero-age main sequence. The numbers along the tracks indicate the time, in years, required for each star to reach those points in their evolution after leaving the main sequence. Once again, you can see that the more massive a star is, the more quickly it goes through each stage in its life.

Evolutionary tracks of stars of different masses.

Evolutionary Tracks of Stars of Different Masses. In this plot the vertical axis is labeled “Luminosity (LSun)” and goes from 10-2 at the bottom to over 104 at the top. The horizontal axis is labeled “Surface Temperature (K)” and goes from 25,000 on the left to 4,000 on the right. The “Zero-age main sequence” is drawn as a diagonal red line beginning above L = 104 at the upper left of the image down to T ~ 4000 at the lower right. Six evolutionary tracks are drawn. Beginning at the top, a star of “15 solar masses” is plotted. It leaves the main sequence above L ~ 104 and T ~ 25,000. The track moves rightward across the top of the plot. The star maintains a relatively constant luminosity, but its surface temperature decreases with time. At “1.01 × 107” years its temperature is about 20,000 K. At “1.11 × 107” years it has fallen to about 15,000 K. At “1.19 × 107” years T is about 9000 K, and the track ends at “1.2 × 107” years near 4000 K. Next, a star of “5 solar masses” is plotted beginning near L ~ 103, where it leaves the main sequence. The star maintains a relatively constant luminosity, but its surface temperature decreases with time. At “6.55 × 107” years its temperature is about 12,000 K. but its surface temperature decreases with time. At “2.39 × 107” years it has fallen to about 5000 K. Then the luminosity rises slightly to the final plotted point at “7.02 × 107” years near 4000 K. Next, a star of “3 solar masses” leaves the main sequence near L = 102 and 15,000 K. After “2.21 × 108” years its temperature has fallen to near 11,000 K. After “2.46 × 108” years its temperature has dropped to near 6000 K. Then, its luminosity increases by about a factor of ten where its curve ends at “2.51 × 107” years and 5000 K. Next, a star of “1.5 solar masses” leaves the main sequence near L = 30 and 9000 K. After “1.55 × 109” years its temperature has fallen to near 7500 K. After “2.09 × 109” years, its temperature has dropped to near 5000 K. Then, its luminosity increases by about a factor of one hundred where its curve ends at “2.39 × 109” years and 4000 K. Next, a star of “1 solar mass” leaves the main sequence at L = 1 and 5700 K. After “7 × 109” years its temperature is nearly the same, but its luminosity has increased slightly. After “10.4 × 109” years, its temperature has dropped to near 5000 K, and its luminosity has increased about 20 times. Then, its luminosity steadily increases to where its curve ends at “11.4 × 109” years, L ~ 103 and T ~ 4000 K. Finally, a “0.5 solar mass” star is partially plotted. Its curve begins at L ~ 10-1 near T ~ 5000. Its curve is a short arrow pointing upward as its evolutionary timescale is too great for this diagram.
The solid black lines show the predicted evolution from the main sequence through the red giant or supergiant stage on the H–R diagram. Each track is labeled with the mass of the star it is describing. The numbers show how many years each star takes to become a giant after leaving the main sequence. The red line is the zero-age main sequence.

Note that the most massive star in this diagram has a mass similar to that of Betelgeuse , and so its evolutionary track shows approximately the history of Betelgeuse. The track for a 1-solar-mass star shows that the Sun is still in the main-sequence phase of evolution, since it is only about 4.5 billion years old. It will be billions of years before the Sun begins its own “climb” away from the main sequence—the expansion of its outer layers that will make it a red giant.

Key concepts and summary

When stars first begin to fuse hydrogen to helium, they lie on the zero-age main sequence. The amount of time a star spends in the main-sequence stage depends on its mass. More massive stars complete each stage of evolution more quickly than lower-mass stars. The fusion of hydrogen to form helium changes the interior composition of a star, which in turn results in changes in its temperature, luminosity, and radius. Eventually, as stars age, they evolve away from the main sequence to become red giants or supergiants. The core of a red giant is contracting, but the outer layers are expanding as a result of hydrogen fusion in a shell outside the core. The star gets larger, redder, and more luminous as it expands and cools.

Questions & Answers

hi, I am yamini, I am in class 8 but very much interested in astronomy and go to NASA, what are the subjects in which I can master and lead to NASA.
yamini Reply
Hello Yamini, Im Ken and I'm avery intrested in joining the NASA too. Is nice to meet you.
Ken
very*
Ken
nice to meet u ken
yamini
What I know about the subjects is that you have to be a master on science and math, also if you know about aviation is better too
Ken
I read that the Russian language is very important, is not a requirement but it's like an extra point!
Ken
yeah like physics, chemistry and maths, they are my most favorite.
yamini
than you can go free of cost
Arush
Yes, That's right!
Ken
how
yamini
Oh! Free of cost?
Ken
hello yamini nice to meet you
Burak
what is time
Abdul Reply
Time is relative
mrunal
pls elaborate
sakshi
the clear defination.I know that.
Abdul
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
It has been proven that there are water molecules on Mars but not enough that most lifeforms could thrive upon.
Ariana
There are huge amounts of water in the ice caps and under the surface. The surface and chemistry indicate that Mars had cosiderable amounts of water on its surface in the past.
Julius
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
Practice Key Terms 1

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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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