Stellar Evolution

Life of a Star

H-R Diagram

• Named after Ejnar Hertzsprung and Henry Norris Russell– Independently founded

he use of such plots in the 1920s

• Graphs temperature and luminosity of stars– Luminosity: the amount

of radiation leaving a star in a given amount of time

H-R Diagram Continues• The main sequence

– Diagonal line– Highest temperature and luminosity top left and lowest to the bottom right– Temperatures range from 3000 K (spectral class M) to 30,000 K (spectral

class O).– The bottom right are 1/10 of the Sun and top left are 10 times the size of

the Sun• Mass of the fragment determines:

– where they will end up on the diagram– How long it will take to get to the main sequence

• Low mass = a long time (billions of years)– Nuclear fusion is slower

• High mass = short time (millions of years)– Nuclear fusion is rapid

Important thing to remember

• Heat/pressure and Gravity– The interstellar medium is balanced when

heat/pressure (outward force) equals gravity (inward force)

– when they are unbalanced the star tries to rebalance and changes happen in luminosity, density, temperature, size, position on the H-R Diagram, etc…

Star’s Life

Similar to our Sun

Stage 1

• Starts out as a dense cloud of cool gas and dust– Emission nebulae

• 1000s time more mass than the Sun • Temperature 10 K

Stage 1 Continue

• Part of the interstellar medium (dust and gas) collapses under its own weight and breaks into fragments– This happens many times before gravity wins over the

heat/pressure force– Produces a few dozen massive stars or collection of 100s of stars

similar or smaller in size than our Sun– Fragments don’t happen naturally, but some outside force cause

them• Shockwave from forming stars or an exploding star

• Fragments continues to contract until radiation can’t escape– Cause temperature and pressure to increase– Cause the fragmentation to stop

Stage 2

• Several 10s of thousands of years after, the fragment has shrunk to a gaseous sphere with a diameter roughly the size of our solar system

Stage 3

• Fragment resemble a star• Dense region is called a protostar• At the end of stage 3 a photosphere appears,

the surface

Stage 4

• Protostar is contracting • Both in the core and in the

photosphere the temperature and density are increasing

• Electrons and protons are ripped from atoms

• Core temperature: 1,000,000 K

• Size: Mercury’s orbit• Luminosity: 1,000 times

more than the Sun

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Stage 5

• Contraction slows down as heat from the core gradually comes to the cooler surface

• Violent surface activity• Core Temperature: 5,000,000 K• Surface Temperature: 4,000 K• Size: 10 times the size of the Sun• Luminosity: 10 times more than the Sun

Stage 6

• Surface Temperature: 4500 K• Core Temperature: 10,000,000K– Hot enough to start nuclear fusion– Protons begin to fuse into helium nuclei

• Star is born

Stage 7

• Next 30 million years the star contracts a little more and its density increase

• Star moves onto the Main Sequence• Pressure and gravity are now balanced• Core Temperature: 15,000,000 K• Surface Temperature: similar to the Sun• Takes 40 – 50 million years to form ( stages 1-7) and

will remain pretty much unchanged in temperature and luminosity for the next billion years

“Failed Stars”

• gravity and heat/pressure come into equilibrium before nuclear fusion can happen, thus they become cold fragments or unburned matter

Stage 8

• As hydrogen is consumed the balance is changed between gravity and pressure– Internal structure and its outward appearance begins to change– Star leaves the main sequence– Mass of the star determine how it will die

• Low = gentle death• High = catastrophic

• Core temperature are two low for helium to fuse• Hydrogen is gone in the core• Nuclear fusion moves outward leaving a non-fusing

helium core

Stage 8 Continues

• With out the core fusion gravity takes over and begins to contract the core.– Important to note fusion is still happening in the outer layers

• Outer shell increase in size due to the hydrogen fusion– Hydrogen fusion is happening very rapidly

• Luminosity: increases• Radius: 3 times that of the Sun• On its way to becoming a red giant– 100 million year process

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Stage 9

• Luminosity: 100 times more than the Sun• Radii: the size of Mercury’s orbit• Core: – 1/1000 of the entire star or a few times larger

than Earth– Very dense

Stage 10

• Nuclear Fusion begins again– Helium into Carbon

• Temperature: 108 K• Helium Flash– A sudden rise in temperature which cause the core to expand

rapidly– Cause the star to move off the red giant onto the horizontal

branch• Eventually balance is reached between gravity and

pressure• Luminosity: same

Stage 11

• Few 10s of millions years• Core:

– Carbon– Contracting further– No more fusion

• Outer Shell– Helium and hydrogen are

fuses quickly– Expanding

• Red giant for a second time• Luminosity and radius has

increased

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Stage 12

• Carbon core is dead and outer Hydrogen and Helium shells are fusing at a high rate

• Radiation pressure from within the star drives away the outer layers– Slow at first then more rapidly as the luminosity increases– Less then a million years

• Next the star is made of two parts: – hot and luminous core – the cold dust cloud

• planetary nebula

• Moving left on the H-R diagram• Central star cools and fades

Stage 12 Continue

• During the final stages of the red giant’s life nuclear reactions between carbon and unburned helium in the core creates oxygen, and in some cases even heavier elements, such as neon and magnesium

• “The evolution of low-mass stars is the source of virtually all of the carbon-rich dust observed throughout the plane of our galaxy” (McMillan 326).

• Core finally stops contracting

Stage 13 (white Dwarf)

• The carbon core is visible as a white dwarf

• Size of the Earth• Mass is ½ that of the

Sun

WhiteDwarf.gif

white_dwarf.jpg

Stage 13 Continues

• Novae (plural) Possible route for a white dwarf– Last three days– Can see 2 to 3 each year– Process

• Happens when a white dwarf undergoes a violent explosion on its surface. This results in a rapid temperature and luminosity increase.

• White dwarf pulls matter (hydrogen and helium) away from a near by star because of its gravity

• The matter starts to orbit around the white dwarf forming an accretion disk

• It builds up rapidly increasing in temperature, density, and luminosity until it begins to fuse Hydrogen into helium

• Fades as soon as the Hydrogen is used up• Then the process can happen again once things have returned to normal.

Nova

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Stage 14

• Continues to cool until it becomes a black dwarf– Cold, dense, burned-out ember in space– Temperature almost drops to absolute zero

Evolution of Massive Stars

Follow similar path as the Sun like stars

DifferencesLuminosity stays about the same through the life of a massive star.No helium flashCore temperature are high enough to fuse helium into carbon and then carbon into oxygen

oVery high mass stars can fuse: hydrogen, helium, carbon, oxygen, neon, magnesium, silicon, etc…

Doesn’t reach red giant before helium fusion beginsThey finally move off the main sequence to form red supergiant

oAt the end of the supergiant phase they form an iron core

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End of the road

Element formation TimeHydrogen: 10 million years Helium 1 million yearsCarbon 1,000 yearsOxygen 1 yearSilicon A weekIron Less then a day

Each nuclear reaction increases with speed

Core changes into iron

Super Nova

• Fusion involving iron doesn’t produce energy because their nuclei are so tight that energy can’t get out, thus fusion stops and the equilibrium is gone.

• Gravity overwhelms pressure and the star implodes (falls in on itself) – Super Nova

• If they don’t super nova then they become neon-oxygen white dwarfs

Super Nova What is happening in the core

• Iron atoms breaking down into protons, electrons, neutrons, and photons

• Lot of energy is produced• The core is getting smaller causing the proton and electrons

to crush together to form neutrons and neutrinos• Eventually stops contracting because the neutrons prevent it– Then it over corrects and expands rapidly sending out a

shock wave• Only takes a few seconds for this process to happen• The shock wave blast the outer layers including the

heavy elements outside the iron core into space

Super Nova

science.nasa.gov

astrophysics.gsfc.nasa.gov500 × 333 - Image of remnant from the type Ia Tycho supernova: X-ray: NASA/CXC/SAO, ...

Heavier Elements than Iron Formation

• “Low-mass stars are responsible for most of the carbon, nitrogen, and oxygen that make life on Earth possible. High-mass stars produced the iron and silicon that make up Earth itself, as well as the heavier elements on which much of our technology is based” (McMillan 341).– low massive stars at the end of their red giant phase

the second time around– super nova, where neutrons and protons are ripped

apart and crammed into other nuclei

Heavier Elements than Iron Formation Continue

• As these heavier elements are mixed into the stellar medium, new star formation contain more heavy elements then older stars

• Galaxies continuously recycle their matter• “Without the heavy elements synthesized in

the hearts of stars, life on Earth would not exist” (McMillan 341).