Welcome to Starry Monday at Otterbein Astronomy Lecture
Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month. February 6, 2006 Dr. Uwe Trittmann
Today’s Topics • Lifecycle of Stars • The Night Sky in February
On the Web • To learn more about astronomy and physics at Otterbein, please visit – http: //www. otterbein. edu/dept/PHYS/weitkamp. a sp (Observatory) – http: //www. otterbein. edu/dept/PHYS/ (Physics Dept. )
The Lifecycle of the Stars
Reminder: Hertzsprung-Russell-Diagrams • Hertzsprung-Russell diagram is luminosity vs. spectral type (or temperature) • To obtain a HR diagram: – get the luminosity. This is your y-coordinate. – Then take the spectral type as your x-coordinate. This may look strange, e. g. K 5 III for Aldebaran. Ignore the roman numbers ( III means a giant star, V means dwarf star, etc). First letter is the spectral type: K (one of OBAFGKM), the arab number (5) is like a second digit to the spectral type, so K 0 is very close to G, K 9 is very close to M.
Reminder: Spectral Classification of the Stars Class O B A F G K M Temperature 30, 000 K 20, 000 K 10, 000 K 8, 000 K 6, 000 K 4, 000 K 3, 000 K Color blue bluish white yellow orange red Examples Rigel Vega, Sirius Canopus Sun, Centauri Arcturus Betelgeuse Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me
Constructing a HR-Diagram • Example: Aldebaran, spectral type K 5 III, luminosity = 160 times that of the Sun L 1000 160 100 Aldebaran 10 1 Sun (G 2 V) O B A F G K M Type … 0123456789 012345…
The Hertzprung -Russell Diagram • A plot of absolute luminosity (vertical scale) against spectral type or temperature (horizontal scale) • Most stars (90%) lie in a band known as the Main Sequence
Mass and the Main Sequence • The position of a star in the main sequence is determined by its mass All we need to know to predict luminosity and temperature! • Both radius and luminosity increase with mass
The Fundamental Problem in studying the stellar lifecycle • We study the subjects of our research for a tiny fraction of its lifetime • Sun’s life expectancy ~ 10 billion (1010) years • Careful study of the Sun ~ 370 years • We have studied the Sun for only 1/27 millionth of its lifetime!
Suppose we study human beings… • Human life expectancy ~ 75 years • 1/27 millionth of this is about 74 seconds • What can we learn about people when allowed to observe them for no more than 74 seconds?
Theory and Experiment • Theory: – Need a theory for star formation – Need a theory to understand the energy production in stars make prediction how bight stars are when and for how long in their lifetimes • Experiment: observe how many stars are when and for how long in the Hertzsprung-Russell diagram • Compare prediction and observation
Nuclear Fusion is the energy source of the Stars • Atoms: electrons orbiting nuclei • Chemistry deals only with electron orbits (electron exchange glues atoms together to from molecules) • Nuclear power comes from the nucleus • Nuclei are very small – If electrons would orbit the statehouse on I-270, the nucleus would be a soccer ball in Gov. Bob Taft’s office – Nuclei: made out of protons (el. positive) and neutrons (neutral)
Nuclear fusion reaction – 4 hydrogen nuclei combine (fuse) to form a helium nucleus, plus some byproducts – Mass of products is less than the original mass – The missing mass is emitted in the form of energy, according to Einstein’s famous formulas: E= 2 mc (the speed of light is very large, so there is a lot of energy in even a tiny mass)
Further Reactions – Heavier Elements Start: 4 + 2 protons End: Helium nucleus + neutrinos Hydrogen fuses to Helium
Fusion is NOT fission! • In nuclear fission one splits a large nucleus into pieces to gain energy • Build up larger nuclei Fusion • Decompose into smaller nuclei Fission
Check: Solar Neutrinos • We can detect the neutrinos coming from the fusion reaction at the core of the Sun • The results are 1/3 to 1/2 the predicted value! • Possible explanations: 1. Models of the solar interior are incorrect 2. Our understanding of the physics of neutrinos is incorrect 3. Something is horribly, horribly wrong with the Sun • #2 is the answer – neutrinos “oscillate”
Theory of Star Formation • A star’s existence is based on a competition between gravity (inward) and pressure due to energy production (outward) Gravity Heat Gravity
Star Formation & Lifecycle • Stage 1: Contraction of a cold interstellar cloud • Stage 2: Cloud contracts/warms, begins radiating; almost all radiated energy escapes • Stage 3: Cloud becomes dense opaque to radiation radiated energy trapped core heats up
Example: Orion Nebula • Orion Nebula is a place where stars are being born
Orion Nebula (M 42)
Protostellar Evolution • Stage 4: increasing temperature at core slows contraction – Luminosity about 1000 times that of the sun – Duration ~ 1 million years – Temperature ~ 1 million K at core, 3, 000 K at surface • Still too cool for nuclear fusion! – Size ~ orbit of Mercury
The T Tauri Stage 5 (T Tauri): • Violent surface activity • high solar wind blows out the remaining stellar nebula – Duration ~ 10 million years – Temperature ~ 5 106 K at core, 4000 K at surface • Still too low for nuclear fusion – Luminosity drops to about 10 the Sun – Size ~ 10 the Sun
Path in the Hertzsprung-Russell Diagram Stages 1 -5
Observational Confirmation • Preceding the result of theory and computer modeling • Can observe objects in various stages of development, but not the development itself
A Newborn Star • Stage 6: Temperature and density at core high enough to sustain nuclear fusion • Stage 7: Main-sequence star; pressure from nuclear fusion and gravity are in balance – Duration ~ 10 billion years (much longer than all other stages combined) – Temperature ~ 15 million K at core, 6000 K at surface – Size ~ Sun
Mass Matters • Larger masses – higher surface temperatures – higher luminosities – take less time to form – have shorter main sequence lifetimes • Smaller masses – – lower surface temperatures lower luminosities take longer to form have longer main sequence lifetimes
Failed Stars: Brown Dwarfs • Too small for nuclear fusion to ever begin – Less than about 0. 08 solar masses • Give off heat from gravitational collapse • Luminosity ~ a few millionths that of the Sun
Main Sequence Lifetimes Mass (in solar masses) Lifetime 10 Suns 10 Million yrs 4 Suns 2 Billion yrs 1 Sun 10 Billion yrs ½ Sun 500 Billion yrs Luminosity 10, 000 Suns 1 Sun 0. 01 Sun
Why Do Stars Leave the Main Sequence? • Running out of fuel
Stage 8: Hydrogen Shell Burning • Cooler core imbalance between pressure and gravity core shrinks • hydrogen shell generates energy too fast outer layers heat up star expands • Luminosity increases • Duration ~ 100 million years • Size ~ several Suns
Stage 9: The Red Giant Stage • Luminosity huge (~ 100 Suns) • Surface Temperature lower • Core Temperature higher • Size ~ 70 Suns (orbit of Mercury)
Lifecycle • Lifecycle of a main sequence G star • Most time is spent on the main-sequence (normal star)
The Helium Flash and Stage 10 • The core becomes hot and dense enough to overcome the barrier to fusing helium into carbon • Initial explosion followed by steady (but rapid) fusion of helium into carbon • Lasts: 50 million years • Temperature: 200 million K (core) to 5000 K (surface) • Size ~ 10 the Sun
Stage 11 • Helium burning continues • Carbon “ash” at the core forms, and the star becomes a Red Supergiant • Duration: 10 thousand years • Central Temperature: 250 million K • Size > orbit of Mars
Stage 12 • Inner carbon core becomes “dead” – it is out of fuel • Some helium and carbon burning continues in outer shells • The outer envelope of the star becomes cool and opaque • solar radiation pushes it outward from the star Duration: 100, 000 years Central Temperature: 300 106 K • A planetary nebula is formed Surface Temperature: 100, 000 K Size: 0. 1 Sun
Planetary Nebulae “Eye of God” Nebula
“Cat’s Eye” Nebula
“Wings of the Butterfly” Nebula
The Ring Nebula (M 57)
“Eskimo” Nebula
“Stingray” Nebula
“Ant” Nebula
Stage 13: White Dwarf • Core radiates only by stored heat, not by nuclear reactions • core continues to cool and contract • Size ~ Earth • Density: a million times that of Earth – 1 cubic cm has 1000 kg of mass!
Stage 14: Black Dwarf • Impossible to see in a telescope • About the size of Earth • Temperature very low almost no radiation black!
Evolution of More Massive Stars • Gravity is strong enough to overcome the electron pressure (Pauli Exclusion Principle) at the end of the helium-burning stage • The core contracts until its temperature is high enough to fuse carbon into oxygen • Elements consumed in core • new elements form while previous elements continue to burn in outer layers
Evolution of More Massive Stars • At each stage the temperature increases reaction gets faster • Last stage: fusion of iron does not release energy, it absorbs energy cools the core “fire extinguisher”
Neutron Core • The core cools and shrinks • nuclei and electrons are crushed together • protons combine with electrons to form neutrons • Ultimately the collapse is halted by neutron pressure – Most of the core is composed of neutrons at this point • Size ~ few km • Density ~ 1018 kg/m 3; 1 cubic cm has a mass of 100 million kg! Manhattan
Formation of the Elements • Light elements (hydrogen, helium) formed in Big Bang • Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae – Condense into new stars and planets – Elements heavier than iron form during supernovae explosions • Evidence: – Theory predicts the observed elemental abundance in the universe very well – Spectra of supernovae show the presence of unstable isotopes like Nickel-56 – Older globular clusters are deficient in heavy elements
Review: The life of Stars
The Night Sky in February • Long nights, early observing! • Winter constellations are up: Orion, Taurus, Gemini, Auriga, Canis Major & Minor lots of deep sky objects! • Saturn at its brightest
Moon Phases • Today (Waxing Gibbous, 67%) • 1/ 12 (Full Moon) • 1 / 21 (Last Quarter Moon) • 1 / 27 (New Moon) • 3 / 6 (First Quarter Moon)
Today at Noon Sun at meridian, i. e. exactly south
10 PM Typical observing hour, early January Saturn Mars Moon
South. West Plejades Mars in Aries / Taurus
Due North Big Dipper points to the north pole
West – the Autumn Constellations • W of Cassiopeia • Big Square of Pegasus • Andromeda Galaxy
Andromeda Galaxy • “PR” Foto • Actual look
Zenith High in the sky: Perseus and Auriga with Plejades and the Double Cluster
South The Winter Constellations – – – Orion Taurus Canis Major Gemini Canis Minor
The Winter Hexagon • • • Sirius Procyon Pollux Capella Aldebaran Rigel
East • Saturn near Praesepe, an open star cluster
Mark your Calendars! • Next Starry Monday: March 6, 2005, 7 pm (this is a Monday • Observing at Prairie Oaks Metro Park: – Friday, May 5, 9: 00 pm • Web pages: – http: //www. otterbein. edu/dept/PHYS/weitkamp. asp (Obs. ) – http: //www. otterbein. edu/dept/PHYS/ (Physics Dept. ) )
Mark your Calendars II • • Physics Coffee is every Wednesday, 3: 30 pm Open to the public, everyone welcome! Location: across the hall, Science 256 Free coffee, cookies, etc.
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