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Stellar Evolution of Sun-Like Stars Star Birth Main Sequence Star Death

Ohio University - Lancaster Campus slide 1 of 78 Spring 2009 PSC 100. Stellar Evolution of Sun-Like Stars Star Birth Main Sequence Star Death. What are the raw materials from which stars are formed?. Cold interstellar gas (H and He) Cold complex

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Stellar Evolution of Sun-Like Stars Star Birth Main Sequence Star Death

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  1. Ohio University - Lancaster Campus slide 1 of 78Spring 2009 PSC 100 Stellar Evolution of Sun-Like StarsStar BirthMain SequenceStar Death

  2. 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

  3. 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.

  4. Ohio University - Lancaster Campus slide 4 of 78Spring 2009 PSC 100 • 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.

  5. 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.)

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

  7. Ohio University - Lancaster Campus slide 7 of 78Spring 2009 PSC 100 • 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.

  8. Ohio University - Lancaster Campus slide 8 of 78Spring 2009 PSC 100 • Near the star, the UV ionizes the H (ejects the electron from the atom). • HII (H-two) region. • Farther from the star, the UV light merely excites the H gas, causing it to glow red. • HI (H-one) region.

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

  10. Ohio University - Lancaster Campus slide 10 of 78Spring 2009 PSC 100 • 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.

  11. 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.

  12. Christine Jones/Smithsonian Astrophysical Observatory Our galaxy in 21 cm radio waves.

  13. 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

  14. Ohio University - Lancaster Campus slide 14 of 78Spring 2009 PSC 100 • Like much of the interstellar gas, more complex molecules also exist in clumps, known as molecular clouds. • The most famous & well-studied molecular cloud is near the Orion Nebula.

  15. The OrionMolecular Cloud

  16. 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.

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

  18. Notice how the dust both blocks and reddens starlight. Credit: Hubble Team, STScI, NASA/ESA

  19. Ohio University - Lancaster Campus slide 19 of 78Spring 2009 PSC 100 • 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.

  20. Credit: Hubble Team, STScI, NASA

  21. 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.

  22. Image Credit: NASA Johnson Space Center Most dust grains are much smaller than this, only 0.1 to 1.0 micrometers in diameter.

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

  24. 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!

  25. Evolution of Sun-like Stars How are stars “born”? How do they go through their life cycles?

  26. The Process of Starbirth • Rotating, collapsing nebula • Protostar • Pre-main sequence star • Main-sequence star

  27. Ohio University - Lancaster Campus slide 28 of 78Spring 2009 PSC 100 • Start with a nebula (gas, dust, molecules) several light years wide. The nebula is probably rotating slowly. • Gravity begins to pull the nebula inward.

  28. 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.

  29. Ohio University - Lancaster Campus slide 32 of 78Spring 2009 PSC 100 • 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).

  30. 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.

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

  32. 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)

  33. Ohio University - Lancaster Campus slide 36 of 78Spring 2009 PSC 100 • 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.

  34. Ohio University - Lancaster Campus slide 37 of 78Spring 2009 PSC 100 • 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.

  35. www.umanitoba.ca

  36. Ohio University - Lancaster Campus slide 39 of 78Spring 2009 PSC 100 • 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.

  37. Ohio University - Lancaster Campus slide 41 of 78Spring 2009 PSC 100 • 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”.

  38. Ohio University - Lancaster Campus slide 42 of 78Spring 2009 PSC 100 • 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.

  39. Ohio University - Lancaster Campus slide 44 of 78Spring 2009 PSC 100 • 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.

  40. Solar winds have begun to blow away the excess gas and dust. Ohio University - Lancaster Campus slide 45 of 78Spring 2009 PSC 100

  41. 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 main-sequence star. • The whole starbirth process takes 10 to 100 million years.

  42. 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.

  43. Main Sequence strip 9 billion years later,sun ends its middle- aged life here. Sun starts its middle-age life here.

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