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Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the monthNovember 7, 2005 Dr. Uwe Trittmann Today’s Topics • Classification of Stars • The Night Sky in November Feedback! • Please write down suggestions/your interests on the note pads provided • If you would like to hear from us, please leave your email / address • To learn more about astronomy and physics at Otterbein, please visit – http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) – http://www.otterbein.edu/dept/PHYS/ (Physics Dept.) Classification of Stars • We can classify stars by many categories – – – – – – – – – – Name Position Constellation Distance Color Temperature Size Brightness Spectra Features: double stars, variable stars, … How Stars Got Their Names • Some have names that go back to ancient times (e.g. Castor and Pollux, Greek mythology) • Some were named by Arab astronomers (e.g. Aldebaran, Algol, etc.) • Since the 17th century we use a scheme that lists stars by constellation – in order of their apparent brightness – labeled alphabetically in Greek alphabet – Alpha Centauri is the brightest star in constellation Centaurus • Some dim stars have names according to their place in a catalogue (e.g. Ross 154) Positions of Stars The Celestial Sphere • An imaginary sphere surrounding the earth, on which we picture the stars attached • Axis through earth’s north and south pole goes through celestial north and south pole • Earth’s equator Celestial equator Celestial Coordinates Earth: latitude, longitude Sky: • declination (dec) [from equator,+/-90°] • right ascension (RA) [from vernal equinox, 0-24h; 6h=90°] Examples: • Westerville, OH 40.1°N, 88°W • Betelgeuse (α Orionis) dec = 7° 24’ RA = 5h 52m But: What’s up for you… Observer Coordinates • Horizon – the plane you stand on • Zenith – the point right above you • Meridian – the line from North to Zenith to south …depends where you are! • Your local sky – your view depends on your location on earth Constellations of Stars • About 5000 stars visible with naked eye • About 3500 of them from the northern hemisphere • Stars that appear to be close are grouped together into constellations since antiquity • Officially 88 constellations (with strict boundaries for classification of objects) • Names range from mythological (Perseus, Cassiopeia) to technical (Air Pump, Compass) Constellations of Stars (cont’d) Orion as seen at night Orion as imagined by men Constellations (cont’d) Orion “from the side” Stars in a constellation are not connected in any real way; they aren’t even close together! Distances to the Stars • Parallax can be used out to about 100 light years • The parsec: – Distance in parsecs = 1/parallax (in arc seconds) – Thus a star with a measured parallax of 1” is 1 parsec away – 1 pc is about 3.3 light years • The nearest star (Proxima Centauri) is about 1.3 pc or 4.3 lyr away – Solar system is less than 1/1000 lyr Our Stellar Neighborhood Scale Model • If the Sun = a golf ball, then – – – – – Earth = a grain of sand The Earth orbits the Sun at a distance of one meter Proxima Centauri lies 270 kilometers (170 miles) away Barnard’s Star lies 370 kilometers (230 miles) away Less than 100 stars lie within 1000 kilometers (600 miles) • The Universe is almost empty! • Hipparcos satellite measured distances to nearly 1 million stars in the range of 100 pc • almost all of the stars in our Galaxy are more distant Brightness • A measure of the apparent brightness • Logarithmic scale • Notation: 1m.4 (smaller brighter) • Originally six groupings – 1st magnitude the brightest – 6th magnitude the dimmest • The modern scale is more complex • The absolute magnitude is the apparent magnitude a star would have at a distance of 10 pc: 2M.8 Electromagnetic Spectrum Three Things Light Tells Us • Temperature – from black body spectrum • Chemical composition – from spectral lines • Radial velocity – from Doppler shift Peak frequency Black Body Spectrum (gives away the temperature) • All objects - even you emit radiation of all frequencies, but with different intensities Measuring Temperatures • Find maximal intensity Temperature (Wien’s law) Identify spectral lines of ionized elements Temperature Wien’s Law • The peak of the intensity curve will move with temperature, this is Wien’s law: λ T = const. = 0.0029 m · K So: the higher the temperature T, the smaller the wavelength λ, i.e. the higher the energy of the electromagnetic wave Luminosity and Brightness • Luminosity L is the total power (energy per unit time) radiated by the star • Apparent brightness B is how bright it appears from Earth – Determined by the amount of light per unit area reaching Earth – B L / d2 • Just by looking, we cannot tell if a star is close and dim or far away and bright Measuring the Sizes of Stars • Direct measurement is possible for a few dozen relatively close, large stars – Angular size of the disk and known distance can be used to deduce diameter Sizes of Stars • Dwarfs – Comparable in size, or smaller than, the Sun • Giants – Up to 100 times the size of the Sun • Supergiants – Up to 1000 times the size of the Sun • Note: Temperature changes! Star Systems: Binary Stars • Some stars form binary systems – stars that orbit one another – visual binaries – spectroscopic binaries – eclipsing binaries • Beware of optical doubles – stars that happen to lie along the same line of sight from Earth • We can’t determine the mass of an isolated star, but of a binary star Visual Binaries • Members are well separated, distinguishable Spectroscopic Binaries • Too distant to resolve the individual stars • Can be viewed indirectly by observing the back-and-forth Doppler shifts of their spectral lines Eclipsing Binaries (Rare!) • The orbital plane of the pair almost edge-on to our line of sight • We observe periodic changes in the starlight as one member of the binary passes in front of the other 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 white yellow orange red Examples Rigel Vega, Sirius Canopus Sun, Centauri Arcturus Betelgeuse Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me Spectral Lines – Fingerprints of the Elements • Can use spectra to identify elements on distant objects! • Different elements yield different emission spectra Origin of Spectral Lines • 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) • The energy of the electron depends on orbit • When an electron jumps from one orbital to another, it emits (emission line) or absorbs (absorption line) a photon of a certain energy • The frequency of emitted or absorbed photon is related to its energy E=hf (h is called Planck’s constant, f is frequency) Hertzsprung-Russell-Diagram • 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. K5III 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 K0 is very close to G, K9 is very close to M. Constructing a HR-Diagram • Example: Aldebaran, spectral type K5III, luminosity = 160 times that of the Sun L 1000 160 100 Aldebaran 10 1 Sun (G2V) O B A F G K M Type … 0123456789 0123456789 012345… The HertzprungRussell 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 Hertzsprung-Russell diagrams … of the closest stars …of the brightest stars 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 Stellar Lifetimes • From the luminosity, we can determine the rate of energy release, and thus rate of fuel consumption • Given the mass (amount of fuel to burn) we can obtain the lifetime • Large hot blue stars: ~ 20 million years • The Sun: 10 billion years • Small cool red dwarfs: trillions of years The hotter, the shorter the life! Preview: Stellar Lifecycle • Next Starry Monday: – – – – How stars are born and die What makes stars “shine” Planetary nebulae are dead stars! …and much more The Night Sky in November • Back to standard time -> earlier observing! • Autumn constellations are up: Cassiopeia, Pegasus, Perseus, Andromeda, Pisces lots of open star clusters! • Mars at opposition • Saturn is visible later at night Moon Phases • Today (New Moon, 36%) • 11 / 8 (First Quarter Moon) • 11 / 15 (Full Moon) • 11 / 23 (Last Quarter Moon) • 12/ 1 (New Moon) Today at Noon Sun at meridian, i.e. exactly south 10 PM Typical observing hour, early November Mars Uranus at meridian Neptune Moon SouthEast Plejades Mars at its brightest in Aries West The summer triangle is still hanging on … Due North Big Dipper points to the north pole High up – the Autumn Constellations • W of Cassiopeia • Big Square of Pegasus • Andromeda Galaxy Andromeda Galaxy • “PR” Foto • Actual look SouthEast High in the sky: Perseus and Auriga with Plejades and the Double Cluster SouthWest • Planets – Uranus – Neptune • Zodiac: – Capricorn – Aquarius Mark your Calendars! • Next Starry Monday: January 9, 2005, 7 pm (this is a Monday • Observing at Prairie Oaks Metro Park: – Friday, November 18, 7:30 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. It’s Nuclear Fusion ! • 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 Could We Use This on Earth? • Requirements: – High temperature – High density – Very difficult to achieve on Earth! Nuclear Fission • Problems: limited fuel supply, dangerous byproducts, expensive technology, limited lifetime of power plant due to radiation The Solar Neutrino Problem • 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” Homework: Luminosity and Distance • Distance and brightness can be used to find the luminosity: L d2 B • So luminosity and brightness can be used to find Distance of two stars 1 and 2: d21 / d22 = L1 / L2 (since B1 = B2 ) Stars II - Lifecycle The Fundamental Problem • 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? Star Formation • A star’s existence is based on a competition between gravity (inward) and pressure due to energy production (outward) • Stage 1: Contraction of a cold interstellar cloud – Lasts about 2 million years – Central temperature about 10 K – Size ~ tens of parsecs Star Formation (cont’d) • Stage 2: Cloud contracts/warms, begins radiating; almost all radiated energy escapes – Duration ~ 30,000 years – Temperature ~ 100 K at center, 10 K at surface – Size about 100 times that of the solar system • Stage 3: Cloud becomes dense opaque to radiation radiated energy trapped core heats up – Duration ~ 100,000 years – Temperature ~ 10,000 K at center, 100 K at surface – Size ~ the solar system Example: Orion Nebula • Orion Nebula is a place where stars are being born Orion Nebula (M42) 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 Stage 5 (T Tauri): • Violent surface activity • high solar wind blows out the remaining stellar nebula – Duration ~ 10 million years – Temperature ~ 5106 K at core, 4000 K at surface • Still too low for nuclear fusion – Luminosity drops to about 10 the Sun – Size ~ 10 the Sun Possible T Tauri Stars Jets from T Tauri Stars 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 – Duration ~ 30 million years – Temperature ~ 10 million K at core, 4500 K at surface – Size ~ slightly larger than the Sun • 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 Path in the Hertzsprung-Russell Diagram • The new-born stars ‘hops’ onto the main sequence 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 100 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 • Temperature ~ 50 million K (core) to 4000 K (surface) • Size ~ several Suns Stage 9: The Red Giant Stage • Luminosity huge (~ 100 Suns) • Core contains 25% of the star’s mass and continues to shrink • Strong stellar winds eject up to 30% of stars mass from surface • Duration: 100,000 years • Temperature: 100 106 K (core) to 4000 K (surface) • Size ~ 70 Suns (orbit of Mercury) Lifecycle • Lifecycle of a main sequence G 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 enough for atoms to recombine with electrons, and becomes opaque; solar radiation Duration: 100,000 years pushes it outward from the Central Temperature: 300 106 K star Surface Temperature: 100,000 K • A planetary nebula is formed Size: 0.1 Sun Planetary Nebulae “Eye of God” Nebula “Cat’s Eye” Nebula “Wings of the Butterfly” Nebula The Ring Nebula (M57) “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 • Temperature: 100 106 K at core; 50,000 K at surface • 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/m3; 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