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2013 North Carolina Science Olympiad Coaches’ Institute Astronomy (C) Don Warren Physics Department NC State University State event leader Event Rules Description: Students will demonstrate an understanding of the basic concepts of mathematics and physics relating to stellar evolution and variable stars. Team size: Up to 2 Event parameters: Each team is allowed two laptops, two 3-ring binders (any size), or one of each. Each member may bring a programmable calculator. No internet access! Scoring: Each question is worth a set amount of points. Highest score wins. Ties broken by score on selected questions. Check NCSO website regularly for changes/updates! My Philosophy The purpose of science is not acquisition of facts. Science is about understanding the Universe with the simplest theories possible. My Philosophy My tests require more than just rote memorization! Understand the concepts, and know how to read graphs and charts. Q1: What is the most common element in the Universe? A. Helium B. Hydrogen C. Carbon D. Iron vs. Q2: Stars near which letter have the smallest radius? My Philosophy Finally, Can’t do astronomy without physics and mathematics, so students should expect to have to do math at some point! Brief Outline I. How stars work II. Stellar evolution III. Filling out the stellar zoo IV. Astronomical concepts V. List of objects I. How stars work A star (like the Sun) is a mass of incandescent gas (really, a miasma of incandescent plasma) They’re HUGE Earth I. How stars work Two competing forces perpetually in near perfect balance Gravity pulls star inward (Thermal) Pressure from burning fuel through nuclear fusion II. Stellar evolution All stars begin as cloud of gas (nebula), which collapses to form a protostar Protostars contract under gravity until fusion begins Single most important attribute for a star: its mass at birth II. Stellar evolution Stellar life cycle splits into three paths based on mass at birth • Low mass stars (d 0.5 MSun) • Medium mass stars (0.5 MSun – 8 MSun) • Massive stars ( t 8 MSun) Many, many more low mass stars than high mass stars! II. Stellar evolution Low Mass Stars (d 0.5 MSun) Fuse hydrogen to helium at core (Slowly!) Convert all fuel, and eventually dim as fuel runs out (Not very interesting, eh?) No dead low-mass stars exist: Universe not old enough II. Stellar evolution Medium Mass Stars (0.5 MSun – 8 MSun) Fuse hydrogen to helium at core Convert core to helium, and contract due to gravity Have inert core of helium inside shell of burning hydrogen II. Stellar evolution Medium Mass Stars (0.5 MSun – 8 MSun) Star becomes a red giant – increased energy production pushes outer layers away from star; outer layers cool down II. Stellar evolution Medium Mass Stars (0.5 MSun – 8 MSun) Core of star continues to contract, reaching helium burning temperatures Burning rate increases, until pressure is blowing away outer layers of star (i.e. pressure wins battle vs. gravity) II. Stellar evolution Medium Mass Stars (0.5 MSun – 8 MSun) Outer layers become planetary nebula. What about the core? II. Stellar evolution Medium Mass Stars (0.5 MSun – 8 MSun) Carbon core (C + O, but sometimes O + Ne) becomes white dwarf About size of Earth, about mass of Sun: incredibly dense! (Very!) Slowly radiates heat, cooling into black dwarf (Again, none exist yet) II. Stellar evolution Massive Stars (t8 MSun) Fuse hydrogen to helium at core Convert core to helium, and contract due to gravity Have inert core of helium inside shell of burning hydrogen II. Stellar evolution Massive Stars (t8 MSun) Star becomes a red supergiant – increased energy production pushes outer layers away from star; outer layers cool down II. Stellar evolution Massive Stars (t8 MSun) Core of star continues to contract, reaching helium burning temperatures Sound familiar? These stars too massive to blow off outer layers. Instead… II. Stellar evolution Massive Stars (t8 MSun) Core fusion continues until iron Can’t fuse iron and gain energy, so no more thermal pressure – only gravity Entire star collapses under own weight II. Stellar evolution Massive Stars (t8 MSun) Rebounds off of core, becomes (core-collapse) supernova Explosion (roughly) 10,000,000,000 times brighter than Sun – briefly outshines entire host galaxy II. Stellar evolution Massive Stars (t8 MSun) Outer layers expand as a supernova remnant Detectable tens, hundreds, to thousands of years later. What about the core? II. Stellar evolution Massive Stars (t8 MSun) Core becomes either neutron star or black hole. Neutron star: ≈ 2 MSun in ≈ 10 km radius Very dense, very strong magnetic field, very fast rotation Supported by strong nuclear force II. Stellar evolution Massive Stars (t8 MSun) Black hole: so dense nothing opposes collapse Nothing – even light – can escape after getting too close (“event horizon”) Can’t be directly observed – must be inferred from presence of accretion disk and/or jet II. Stellar evolution Summary: • Low-mass stars: slowly burn all fuel • Medium mass stars: burn H, become red giant, separate into planetary nebula & white dwarf • Massive stars: burn H all the way to Fe, explode in supernova, leave behind remnant and either neutron star or black hole III. Filling out the stellar zoo T Tauri variables Protostars surrounded by gas cloud in which they formed Dusty blobs may become planets Mira variables Red giants just beginning to transform into planetary nebulae (Sun will be one in 5 billion years) III. Filling out the stellar zoo RR Lyrae variables Regular (periodic) variable star Low mass stars (≈ 0.8 MSun) nearing end of life (like Mira vars) Much brighter than Sun (≈ 40x) All stars in this class roughly the same brightness Used as distance indicator III. Filling out the stellar zoo (Classical) Cepheid variables Regular (periodic) variable star Massive stars (4-20 MSun) near death Much brighter than Sun (up to 1,000,000 times brighter) Strong relation between period of pulsation and peak brightness M V = −2.43log10 P − 1.62 Used as distance indicator III. Filling out the stellar zoo S Doradus variables Irregular variables Also called Luminous Blue Variables Massive stars (up to 150 MSun), with brightness > 1 million times the Sun So massive it’s permanently unstable LBV Very short lifetimes, very violent ends Sun III. Filling out the stellar zoo Magnetars Neutron stars with very intense magnetic fields (up to 1015 times stronger than Earth’s) Pulsars Rapidly spinning neutron stars Emit light from magnetic pole As beam sweeps across Earth, we see regular pulses of light III. Filling out the stellar zoo (Classical) Novae White dwarf with companion star Companion donates matter, which forms layer on surface of WD Eventually, layer dense enough to start nuclear burning, briefly shining like Sun does Burning finishes, star dims, but is slightly heavier Cycle repeats until… III. Filling out the stellar zoo Type Ia Supernova White dwarf becomes too heavy to support its own weight Explodes in runaway thermonuclear event Briefly outshines entire host galaxy (i.e. 1010 times brighter than Sun) Uniform peak magnitude Used as distance indicator III. Filling out the stellar zoo Type II Supernova Explosion after massive star burns all the way to iron in core Leaves behind remnant, and neutron star or black hole X-ray binary system Compact object (NS or BH) with a companion, visible in X-rays Could be two NSs, or NS+BH IV. Astronomical concepts Stellar temperature At birth, heavy stars hotter Surface temps from 3000 K to over 50,000 K Easy to accurately measure because light curve only depends on temperature Temperature determines color of star IV. Astronomical concepts Spectral class Temp also determines spectral class of star Easy way to group similar stars IV. Astronomical concepts Luminosity Stars release a lot of energy Solar luminosity: 3.9 x 1026 Joules/sec (93,000,000,000 Megatons of TNT each second) Other stars measured in units of LSun, not Joules/sec IMPORTANT: Luminosity related to temperature and radius of star 2 L1 R1 T1 = L2 R2 T2 Cooler star can be brighter if radius large enough! 4 IV. Astronomical concepts Hertzsprung-Russell diagrams Way to show information about groups of stars Each star plotted by temp and luminosity Many, many possible questions using HR diagrams. KNOW HOW TO READ & USE THEM IV. Astronomical concepts The distance problem Everything in astronomy depends on distance from Earth: • Size & age of Universe • History & fate of Universe, Sun, stars, Milky Way, etc. • Size of Milky Way galaxy • Nearby cosmic neighborhood • Etc., etc., etc. Problem: Nearest stars are light-years away. Nobody makes rulers long enough How to determine distance, then? IV. Astronomical concepts Parallax Use motion of object against distant background to get distance New unit of measurement: 1 parsec = distance at which parallax is one arcsecond (1 arcsecond: size of ping pong ball from 5 miles away) Distance-parallax relation: Dpc = 1 parcsec IV. Astronomical concepts Magnitude Another way to measure brightness 2 kinds: apparent magnitude and absolute magnitude Apparent magnitude: how bright star appears from Earth (depends on distance and type of star) Lower number means brighter star (i.e. magnitude 1 brighter than magnitude 6) Absolute magnitude: how bright star would be at 10 parsecs from Earth IV. Astronomical concepts Distance modulus Relates apparent (m) and absolute (M) magnitudes to get distance (d) to star IV. Astronomical concepts Light spectrum Know where radio, infrared (IR), visible/optical, ultraviolet (UV), and X-ray fall on the spectrum V. List of objects Mira Star at right of UV image Namesake of Mira variable class of stars Moving through space, trailing outer layers (so no planetary nebula) V. List of objects W49B Supernova remnant Pictured in radio, IR & X-ray Barrel shape suggests SN was gamma-ray burst ≈ 1000 years old ≈ 26000 ly from Earth V. List of objects Tycho’s SNR Type Ia supernova remnant (seen here in IR and X-ray) SN seen in 1572 (so 440 years old), observed by Tycho Brahe ≈ 9000 ly from Earth Expanding at t5000 km/s V. List of objects Vela SNR Type II supernova remnant (seen here in optical) Old SNR (≈ 12,000 yrs) Close to Earth (800 ly) Left behind pulsar, which was first confirmation of supernova/pulsar relationship Huge in night sky (16 times wider than full Moon) V. List of objects G1.9+0.3 Type Ia supernova remnant (shown in X-ray) Youngest remnant in Milky Way (just 140 years old) 25,000 ly away, toward galactic center V. List of objects Eta Carinae Luminous blue variable (a.k.a. S Doradus variable) Surrounded by nebula ejected in 1841 outburst Binary system, with primary ≈ 100 MSun and ≈ 5x106 LSun Shown in visible light Strong contender for next galactic supernova V. List of objects SS Cygni Dwarf nova (recurring bursts due to accretion disc, not burning on WD) Outbursts every 7-8 weeks Goes from 12th to 8th mag ≈ 370 ly away Close binary system (stars orbit every 6.5 hrs, d 100,000 miles separation) V. List of objects T Tauri Not a star yet: powered by gravitational contraction, not fusion Protostar surrounded by dusty disk Shown in optical Strong wind coming off star, heavy accretion of disk matter onto star (hasn’t reached birth mass yet) Trinary+ system, & T Tauri may have been ejected V. List of objects GRS 1915+105 X-ray binary with star & black hole Observed in X-ray as well as radio (pictured at right) Famous for “faster-than-light” expansion of jets About 40,000 ly distant V. List of objects 47 Tucanae Globular cluster Millions of stars, of multiple populations Many pulsars, but no (observed) planets ≈ 17,000 ly distant Size of full moon, but very difficult to see from northern hemisphere – impossible from Europe V. List of objects The Trapezium Open cluster in Orion nebula (false color visible image here) 5 primary stars, all of which are O-B stars, > 15 MSun Only 1600 ly distant, but partially obscured by nebula Very compact – telescope needed to resolve individual stars in cluster V. List of objects T Pyxidis Recurrent (but irregular) classical nova with WD & companion WD estimated to be near max. allowed mass, so may become Type Ia supernova soon Normally 15.5 mag, bursts to 7 (2500 times brighter) V. List of objects Abell 30 Planetary nebula (shown in visible light) Very rare “two-stage” expulsion of nebula material ≈ 5500 ly distant Spherical shell marks interaction between two stages of nebula emission V. List of objects RX J0806.3+1527 WD-WD X-ray binary system Orbital period 320 sec, with 50,000 mile separation Radiating energy in form of gravitational waves, moving inwards at 60 cm/day Only 1600 ly distant V. List of objects V1647 Orionis Very new protostar of T Tauri type Rapidly accreting matter from surroundings, so very variable brightness Characterized by large outburst in 2004 that lasted for 18 months V. List of objects M31 V1 Cepheid variable star in nearby Andromeda galaxy (a.k.a. M31) Identified by Edwin Hubble Used to prove that Andromeda did not lie within Milky Way, i.e. was an entirely different galaxy V. List of objects NGC 1846 Globular cluster in Large Magellanic cloud, satellite galaxy of Milky Way As with 47 Tucanae, more than one population of stars visible Suggests that many globular clusters have rich history of interaction with galaxies & molecular clouds V. List of objects NGC 3132 Planetary nebula lit by new white dwarf at center Central WD over 100,000 K but only size of Earth About 2000 ly away 0.4 ly across, expanding at 14 km/s, so about 8000-9000 years old