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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 need a telescope would you help mee
Nauman Reply
i can only advice you to go to some museum
Gospel
wher is it?
Nauman
but before that.......... i wanna know where are you from and from which state...and if you are nearer to that of the museum or will be able to go then.,.. i donot have any more words.
Gospel
What Do Plan To Observe?
Adam
sorry . i am from asia
Nauman
Check on amazon.Celeston telescopes are good for beginners.
priyanshu
can yoy introduce your self
Nauman
can you
Nauman
Asia...? from which country you belong into ...,,is it India or where?
Gospel
Do You Want A Telescope With ALot Of Maintenance Or Zero Maintenance?
Adam
pakistan
Nauman
with alot offf
Nauman
Any second hand online stores in your area? great for unused Christmas gifts etc..
Collins
So You Want A Newtonian Reflector?
Adam
So what i would put for that answer ?
Abdulla Reply
The largest observatory complex in the world is on Mauna Kea, the tallest mountain on Earth. Whatare some factors astronomers consider when selecting an observatory site? Don’t forget practical ones.Should astronomers, for example, consider building an observatory on Denali (Mount McKinley) or MountEverest?
Abdulla Reply
i need help on the the second part
Abdulla
Should astronomers for example considered building an observatory on Denali (Mount Mckinley) or MountEverest ?
Abdulla
I would take into consideration. climate weather pattern.
basim
also, could an establishment be built and be able to sustain years of being beaten by the weather?
basim
so far Mount Everest to my understanding is a difficult mountain
basim
how far us milky way galaxy?
ShowsSpy Reply
oh man
madison
We are part of the Milky Way Galaxy. We are within it, so it's not really away from us. Our galaxy is 100,000 light years in diameter. The nearest big galaxy is Andromeda, around 2 million lightyears away.
Jignesh
its 200,000 light years
Gospel
Where is Rice University located?
Amanda Reply
it is in your vagina
world
that couldn't be any more wrong 😂😂
jay
what even is this conversation😂😂!?
Herrera
Show Some Class.....
Adam
Houston, Texas.....
Adam
Houston
Preston
what is the first step to take as far as getting into the field of astronomy?
john Reply
how are universe made up of?
madhu Reply
saksi
madhu
lanika
madhu
I think that your question is- What is the universe made up of?
priyanshu
Ever heard of matter?
priyanshu
Well,the universe is made up of matter and possibly antimatter
priyanshu
Actually,it's made up of matter's smaller units,fundamental particles
priyanshu
But for normal purposes,say that it's made of matter
priyanshu
If you're asking how was the universe made,
priyanshu
there are several theories,but the widely accepted one is the 'Big Bang Theory'.
priyanshu
In simple words,a veeeeeeery long time ago (20 billion years).
priyanshu
The universe was just inside a very small point.
priyanshu
Then,it started to expand.
priyanshu
It is still expanding today
priyanshu
But remember,it's just a theory,there is no hard and fast proof
priyanshu
oh d*mn😭😭😭😭 i cant do my astronomy quest
swagirl
up Down quarks atoms electrons proton neutrons
Blake
😭😭😭😭..
swagirl
usually in astronomy quest there's many math formula. i'm difficult on them
swagirl Reply
practice!
Collins
?
swagirl
what was the question? if you need any help CrashCourse/astronomy and physics channel on YouTube are a good help.
Collins
yeah like... you know venn diagram?
swagirl
yeah it's for sets
Vinit
***youtu.be/uR70knMr2Hg
Collins
this might help you understand Venn diagram. and yes they're sets of numbers.
Collins
tx for u
swagirl
why the dark side of moon never face us? because moon dont rotate? why tho
Rishabh Reply
im in 8th standard and my school teach us nothing about astronomy but i want to be an astronomer so i study from youtube and apps like this but sometimes i get language problems
Rishabh
dark**
Glory
side**
Glory
The Moon does actually rotate. It takes 27 days for the Moon to make a full rotation. Because it takes it so long and the Earth is rotating around the sun, it seems like the Moon is pretty much standing still. That's called "synchronous rotation".
steveh259
I ment to say "The Earth is orbiting around the sun"
steveh259
Tidal Locked.....
Adam
***svs.gsfc.nasa.gov/4442
Collins
thats cool
Gospel
ya gives you a look at how the moon orbits around us too and a perspective of how fast we are going. also gives the moon phases for the year! enjoy 😁
Collins
yes you are absolutely correct, it just like something like the milk of heaven
Subhasish
the far side of the moon is not the dark side of the moon.
Shawn
Tidal Locking?
priyanshu
what phases of the moon occurs when spring tide?
Florence Reply
the moon has a 28 day cycle. you can download a moon calander or follow the pattern and make your own calendar. 1st full moon of the year 21/22nd Jan, its also a blood moon and a wolf moon. if this info helps at all. 😊
Collins
***svs.gsfc.nasa.gov/4442
Collins
How to determine the velocity of light?
PARTHIB Reply
using this formula c=1/√absolute permeability×absolute permittivity of free space i.e. c=1/√8.854×10^-12×4π×10^-7~3×10^8ms^-1
Vidyashankar
what is mean by Big Bang
block Reply
What it means by Big Bang is how the world was made and came in to exist
Glory
The Big Bang Is The Best Explanation For What We See Around Us, And For The Origin Of The Universe, All Our Evidence Points To The Big Bang Theory..... And Was Proposed By A Belgian Priest.....
Adam
if the big bang theory is really true than i would like to know what exploded and where did it came from and where did the energy came from and where come from for the matter to expand into amd where did the organization come from and where did the information come from.........
Gospel
Some say that on the otherside of the singularity was a black hole. They say that at the other end of all black holes is another universe. Hence the multiverse. It's all theoretical at this point but less than a 100 years ago people believed that there was only 1 galaxy in the universe. So why
Rafael
should there be only 1 universe?
Rafael
universe.
Rafael
Big Bang is a theory on the origin of the universe.It says that the whole universe started as an infinitely dense and hot point,which expanded with time to form the universe in which we live today.
priyanshu
but if you study about string theory then you will come to know that before big bang there were string ( vibrating particles) which cause to form the conditions for big bang.
YOG
Hmm,actually,I dont know much about string theory,just a little.I just wanted to tell this guy what is the Big Bang Theory.
priyanshu
and according to string theory black holes are giant fuzzball of strings,there is no infinity dense singularity and no smooth horizon there is just only a ball of string
YOG
is astronomy a hard subject to learn because i want to be an astronomer
FNAF Reply
No it not it just takes time and effort to learn
Glory
if your already interested/observing, your already an astronomer ! The more you look and read the more you'll learn.
Collins
I want to become a scientist but my knowledge is very low. How I can improve my knowledge?
Wwe
any form of science courses, there's free crash course channel on you tube. I've learned alot from this channel. covers most topics, science: biology chemistry physics, astronomy, ranges to anatomy, history and many many more. it's a never ending subject!
Collins
suggest a channel as a example
Wwe
CrashCourse is the name of the channel and there is a picture of an apple. look through their play lists. They will have other related channels they follow also.
Collins
thank you so much
Wwe
your very welcome
Collins
Why are they so far away that their light takes hundreds of millions of years to reach us on earth?
LadyCatTM Reply
Because the universe is wayyyyyy bigger than the earth
priyanshu
You can't expect everything to form close to Earth.
priyanshu
what is comets , astroids ,
Sumit Reply
comets are broken pieces of dust frozen debris Asteroids are big soiled pieces of Ice that burst into flames when it in to earth's atmosphere
Glory
Glory,you aren't correct
priyanshu
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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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