Stellar Evolution of Sun-Like Stars

Star BirthMain SequenceStar Death

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What are the raw materials from which stars are formed?

• Cold interstellar gas (H and He)• Cold complex molecules• Interstellar dust grains

Image Credit: Hubble Team, STScI, NASA

Interstellar Gas

• Low-density hydrogen and helium.

• 300,000 atoms per cubic meter (1 atom in every 3 cm3)

• About 70 K (-200oC), therefore atoms are very slow-moving.

• Even though the atoms are far apart, gravity slowly causes them to clump into nebulae (Latin for “clouds”).

• As the atoms draw closer together, the density of the cloud increases. This causes the atoms to speed up and to heat up.

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Emission Nebulae

• The gas heats up enough that it begins to emit light. The cloud “lights up” from the inside with an eerie reddish light.

• The most famous emission nebula is the Orion Nebula (but there are many others also.)

Credit: NASA, ESA, M. Robberto (STScI/ESA) et al.

• Emission nebulae also glow for a more important reason: hot, bright O & B class stars inside them.

• O & B stars emit much of their light in the X-ray and UV portions of the spectrum.

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• Near the star, the UV ionizes the H (ejects the electron from the atom).

• This is the HII (H-two) region.

• Farther from the star, the UV light merely excites the H gas, causing it to glow red.

• This is the HI (H-one) region.

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Credit: Cornell University

Credit: T. A. Rector, B. Wolpa, M. Hanna (AURA/NOAO/NSF)

• Astronomers use the visible red glow from nearby emission nebulae to locate the places where new stars are forming.

• Hydrogen also “glows” in another light: radio waves.

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H radio emissions

• When H atoms are excited, sometimes the electron will spin in the same direction as the proton (called parallel spins).

• When the electron flips back (anti-parallel), it gives off a radio wave with a wavelength of 21 centimeters.

Our galaxy in 21 cm radio waves.

Christine Jones/Smithsonian Astrophysical Observatory

Molecules in Interstellar Space• carbon monoxide (CO).

• water (H2O)

• carbon dioxide (CO2)

• methane (CH4)

• formaldehyde (CH2O)

• hydrogen cyanide (HCN)

• ethanol (CH3CH2OH)

• simple sugars• amino acids

These molecules areobserved by the

infrared (IR) light that they emit.

• Like much of the interstellar hydrogen, complex molecules exist in clumps known as molecular clouds. These giant molecular clouds can be up to 100 LY in diameter!

• The most famous & well-studied molecular cloud is near the Orion Nebula.

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The OrionMolecular Cloud

Interstellar Dust

• Dust forms thick clouds out in space which extinguish or completely block out star light. (The process is called extinction.)

• The Horsehead Nebula near Orion’s belt is a good example.

• So is the Eagle Nebula.

Credit: Nigel Sharp (NOAO), KPNO, AURA, NSF

Notice how the dustboth blocks andreddens starlight.

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• Dust also reflects starlight.

• The Pleiades star cluster is moving towards a dust cloud several light years behind the cluster. The dust reflects the bluish light from the bright O-type stars.

• The lit-up cloud is a reflection nebula.

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Credit: Hubble Team, STScI, NASA

What is dust?

• Interstellar dust isn’t anything like the dust under your sofa.

• Space dust is more like a tiny sand grain coated with tar.

• A dust grain has a core, mantle, and crust.

Most dust grains are much smaller than this,only 0.1 to 1.0 micrometers in diameter.

Image Credit: NASA Johnson Space Center

The core is made of silicates – rock.The mantle is made of frozen gases.

• Cosmic rays, UV light, and heat cause chemical changes in the dust grain’s mantle.

• The gases combine into more complex molecules, including sugars and amino acids.

• It appears that dust grains may be the chemical factories of molecular clouds and of some of the chemicals of life!

Evolution of Sun-like Stars

How are stars “born”?How do they go through their life cycles?

The Process of Starbirth

• Rotating, collapsing nebula

• Protostar

• Pre-main sequence star

• Main-sequence star

• Start with a nebula (gas, dust, molecules) several light years wide. The nebula is probably rotating slowly.

• Gravity begins to pull the nebula inward.

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• As the nebula contracts:– collisions between molecules increase in

frequency– pressure increases– temperature increases

• The interior of the cloud contracts the quickest because it’s closest to the center of gravity, so the cloud gets hottest in the interior.

• Material from the outer regions continues to pile onto the warm core, heating it further.

• Interior pressure rises enough that it balances the inward pull of gravity. The core stabilizes in size, with a temperature of a few hundred K, hot enough to emit IR, but not much visible light (yet).

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• This cloud of hot gas is opaque to visible light, which helps to trap heat and increase the temperature build-up.

• The hot cloud is several times larger and 10x to 1000x more luminous than the sun (in IR).

• The hot cloud is a protostar.

A protostar – glowing in IR. To our eyes, it would be a dark, hot cloud.

How does rotation affect all this?

• If the initial nebula is slowly rotating, the cloud particles don’t just fall inwards towards the center of mass.

• The gas & dust particles force one another into a disk around the equator of the protostar. (Think Saturn’s rings)

• If there’s enough “stuff” in this disk, it may clump up into planets, moons, comets, etc.

• For this reason, we call these dusty disks protoplanetary disks.

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• When the protostar starts blowing away the excess gas and dust around it, the disk at its equator confines most of this stellar wind to the star’s north and south pole.

• These high-velocity polar winds are called bipolar outflows.

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www.umanitoba.ca

• Any young star that shows bipolar outflows is called a T-Tauri star, after the first such star observed (star T in the constellation Taurus).

• One of the most famous T-Tauri stars is Beta-Pictoris, about 50 light years from earth.

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• Bipolar outflows seem to be a very brief part of a star’s evolution, lasting only about 10,000 years or so. This makes them literally a stellar “burp”.

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• Slowly, over millions of years, the protostar continues to contract and its interior grows slowly hotter.

• When the interior of the protostar reaches 8 million Kelvin, nuclear fusion “switches on”, and the star begins to shine with visible light.

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• Eventually, either all the extra gas and dust falls onto the surface of the protostar, or stellar wind blows it away.

• When it becomes visible, it’s called a pre-main-sequence star. It’s a little dimmer and cooler than it will be later in its life.

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Solar winds have begun toblow away the excess gas and dust.

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• As the nuclear fusion inside the pre-main-sequence star stabilizes, the star grows a little brighter and hotter. (Think of how a streetlight starts out orange and dim, and eventually grows brighter and hotter.)

• At this point, the star becomes a zero age main-sequence star.

• The whole starbirth process takes 10 to 100 million years.

Main Sequence Evolution

• The longest part of a star’s life is its middle age, where it normally fuses Hydrogen into Helium (proton-proton chain.)

• For a star like our sun, this stage lasts 8 to 9 billion years.

• During this time, the sun gradually brightens, possibly doubling in brightness. Life on earth ends.

Main Sequence stripMain Sequence strip

Sun starts its middle-agelife here.

9 billion years later,sun ends its middle-

aged life here.

End-Stage Evolutionof Small Stars

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• The evolution of any star is controlled by its mass.

• Mass controls gravity.• Gravity controls density.• Density controls how fast the star

uses up its available fuel.• Fuel availability controls when a star

goes through changes.

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• Every star is in a constant balance between 2 forces:– The inward pull of gravity.– The outward push of pressure caused by

the heat given off by nuclear fusion.

• This balance is called hydrostatic equilibrium.

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Hydrogen Fuel Runs Out!

• Eventually, the hydrogen fuel in the sun’s core runs out.

• The core no longer produces as much outward pressure, so it contracts.

• Contracting causes the core to heat up.

• Heat from the core causes the outer layers to expand. Sun becomes a red giant.

Red Giant Phase

• Sun expands to 100x its current size (80 million miles in diameter.)

• Mercury is consumed.• As the outer layers expand, they cool

to 3500 Kelvin & become red.• Sun begins to fuse Hydrogen to

Helium in outer layers.• This stage lasts about 1 billion years.

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Helium flash!• Core continues to shrink and grow

hotter until it reaches about 100 million Kelvin.

• Built-up Helium “ash” in the core suddenly ignites & begins fusing into carbon (triple alpha process).

• No sign of the flash is seen outside the star, however, because the energy is absorbed by the outer layers.

Triple-Alpha Process

Yellow Giant Phase• With fusion now going on in several

layers (H He in outer layer, andHe C in core) the sun grows hotter and turns yellow again.

• Horizontal Branch of the HR diagram.• This phase lasts 100 million years or

less.

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Fuel runs out again

• As the Helium fuel begins to run out in the core, the core begins to shrink a second time. Helium fusion slows down.

• Hydrogen fusion from the outer layers continues to dump waste Helium into the core.

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• Every so often, enough Helium builds up in the core to briefly start the triple alpha process again. The star pulses like a beating heart.

• These are called thermal pulses.

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Fuel is all gone…

• When all the sources of fuel are gone, the core contracts one last time and becomes intensely hot.

• The super-hot core causes the outer layers to expand. In the process of expanding, the outer edges of the outer layers cool and turn red.

• The sun is very briefly a red supergiant, larger than Mars’ orbit!

Fuel is all gone…

The proper name for a red supergiantstar in this stage is

an Asymptotic Giant Branch star orAGB Star.

Planetary Nebula

• Within just a few million years, the sun sheds its outer layers into the solar system.

• This expanding cloud of hot, glowing gas is called a planetary nebula.

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NGC 6543Cat’s Eye Nebulain Draco

Credit: HubbleTeam, STScI,NASA

NGC 6543Eskimo Nebulain Gemini,3000 LY away

Credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA

Credit: NASA, ESA, K. Noll (STScI) Acknowledgment: Hubble Heritage Team (STScI / AURA)

NGC 2440 in Puppis, 4000 LY away, 1 LY wide

The sun’s planetarynebula might looklike this from thesurface of Pluto!

White Dwarf• In a few thousand years, the

planetary nebula fades.

• The hot exposed core of the dead sun is now exposed. This is a white dwarf star, shining with residual heat.

• The core is about twice the size of the earth, and 100,000,000 Kelvin or hotter.

Black Dwarf

• Over tens of billions of years, the white dwarf cools off and no longer shines from left-over heat.

• The cinder is now a black dwarf. (There probably aren’t any black dwarfs yet – the universe isn’t old enough!)

A nearly dead black dwarf star.