February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) Best way to reach me: Astronomy 350 Cosmology

February 18, 2003Lynn Cominsky - Cosmology A3502 Disks around stars  There is much evidence of disks with gaps (presumably caused by planets) around bright, nearby stars, such as Beta Pic

February 18, 2003Lynn Cominsky - Cosmology A3503 What makes a world habitable?  Select your top three candidates for life  Class votes: Earth (duh) Europa (25 votes) Titan (17 votes) Mars (16 votes) Io (13 votes) Callisto (12 votes)

February 18, 2003Lynn Cominsky - Cosmology A3504 The Nearest Stars Distance to Alpha or Proxima Centauri is ~4 x km or ~4.2 light years Distance between Alpha and Proxima Centauri is ~23 AU

February 18, 2003Lynn Cominsky - Cosmology A3505 The Solar Neighborhood Some stars within about 2 x km (~ 20 light years)

February 18, 2003Lynn Cominsky - Cosmology A3506 Distances to Stars Parallax : determined by the change of position of a nearby star with respect to the distant stars, as seen from the Earth at two different times separated by 6 months.

February 18, 2003Lynn Cominsky - Cosmology A3507 Calculating Parallax  Measure angle in radians: it is very small  The tangent and the sine of the angle are therefore about the same as the angle in radians  The Earth-Sun distance of 1 AU = 1.5 x 10 8 km  Distance to star = (Earth-Sun distance) / parallax parallax angle Parallax for Proxima Centauri is 0.76 arc-seconds

February 18, 2003Lynn Cominsky - Cosmology A3508 Parallax movie

February 18, 2003Lynn Cominsky - Cosmology A3509 Parallax, parsecs and light years  1 parsec is defined as the distance at which a star would have a parallax angle of 1 arc-second  1 arc-second = (1 degree/3600) = (1 degree/3600) (  radians/ 180 degrees ) = 4.85 x radians  1 parsec = (1.5 x 10 8 km)/(4.85 x ) = x km = 3.26 light years  1 light-year is the distance light will travel in one year  1 light-year = (2.998 x 10 8 m/s)(86400 s/d)(365 d/y) = 9.46 x km = 9.46 x m  A LIGHTYEAR IS A DISTANCE, NOT A TIME!

February 18, 2003Lynn Cominsky - Cosmology A35010 Absolute vs. Apparent magnitude  Apparent magnitude - How bright does the star appear (from the Earth)? Uses symbol “m”  Absolute magnitude - the apparent magnitude of a star if it were located at 10 pc. Uses symbol “M”  Absolute and apparent magnitude are related to the true distance “D” to the star by: m – M = 5 log (D/10 pc) = 5 log (D/pc) – 5 OR D = 10 pc * 10 ((m-M)/5)  Magnitudes seem backwards – the bigger the number, the fainter the star.

February 18, 2003Lynn Cominsky - Cosmology A35011 Classifying Stars Hertzsprung-Russell diagram

February 18, 2003Lynn Cominsky - Cosmology A35012 Classes of Stars  Bigger stars are brighter than smaller stars because they have more surface area  Hotter stars make more light per square meter. So, for a given size, hotter stars are brighter than cooler stars. White dwarfs - small and can be very hot (Class VII) Main sequence stars - range from hotter and larger to smaller and cooler (Class V) Giants - rather large and cool (Class III) Supergiants - cool and very large (Class I)

February 18, 2003Lynn Cominsky - Cosmology A35013 Properties of Stars  Temperature (degrees K) - color of star light. All stars with the same blackbody temperature are the same color. Specific spectral lines appear for each temperature range classification. Astronomers name temperature ranges in decreasing order as:  Surface gravity - measured from the shapes of the stellar absorption lines. Distinguishes classes of stars: supergiants, giants, main sequence stars and white dwarfs. O B A F G K M

February 18, 2003Lynn Cominsky - Cosmology A35014 Populations of Stars  Population I – young, recently formed stars. Contain more metals than older stars, as they were created from debris from previous stellar explosions.  Population II – older stars that have evolved and are almost as old as the Universe itself.  Population III – the original stars that were formed about 200 million years after the Big Bang. They should be nearly all H and He

February 18, 2003Lynn Cominsky - Cosmology A35015 Life Cycles of Stars

February 18, 2003Lynn Cominsky - Cosmology A35016 Life Cycles of Stars

February 18, 2003Lynn Cominsky - Cosmology A35017 The very first stars  Simulations by Tom Abel, Mike Norman and Greg Bryan  13 million years after the Big Bang, a piece of the Universe has collapsed due to a slightly higher density of dark matter. It forms a 100 million solar mass protogalaxy, and at the center of this protogalaxy, a star is born! Density movie Temperature movie

February 18, 2003Lynn Cominsky - Cosmology A35018 Life and death of the very first star  From The Unfolding Universe, directed by Tom Lucas, simulation by Tom Abel

February 18, 2003Lynn Cominsky - Cosmology A35019 Molecular clouds and protostars  Giant molecular clouds are very cold, thin and wispy– they stretch out over tens of light years at temperatures from K, with a warmer core  They are 1000s of time more dense than the local interstellar medium, and collapse further under their own gravity to form protostars at their cores Orion in mm radio (BIMA) Simulation with narration by Jack Welch (UCB)

February 18, 2003Lynn Cominsky - Cosmology A35020 Protostars  Orion nebula/Trapezium stars (in the sword)  About 1500 light years away HST / 2.5 light years Chandra/10 light years

February 18, 2003Lynn Cominsky - Cosmology A35021 Stellar nurseries  Pillars of dense gas  Newly born stars may emerge at the ends of the pillars  About 7000 light years away HST/Eagle Nebula in M16

February 18, 2003Lynn Cominsky - Cosmology A35022 Main Sequence Stars  Stars spend most of their lives on the “main sequence” where they burn hydrogen in nuclear reactions in their cores  Burning rate is higher for more massive stars - hence their lifetimes on the main sequence are much shorter and they are rather rare  Red dwarf stars are the most common as they burn hydrogen slowly and live the longest  Often called dwarfs (but not the same as White Dwarfs) because they are smaller than giants or supergiants  Our sun is considered a G2V star. It has been on the main sequence for about 4.5 billion years, with another ~5 billion to go

February 18, 2003Lynn Cominsky - Cosmology A35023 How stars die  Stars that are below about 8 M o form red giants at the end of their lives on the main sequence  Red giants evolve into white dwarfs, often accompanied by planetary nebulae  More massive stars form red supergiants  Red supergiants undergo supernova explosions, often leaving behind a stellar core which is a neutron star, or perhaps a black hole (more in later lectures about these topics)

February 18, 2003Lynn Cominsky - Cosmology A35024 Red Giants and Supergiants  Hydrogen burns in outer shell around the core  Heavier elements burn in inner shells

February 18, 2003Lynn Cominsky - Cosmology A35025 White dwarf stars  Red giants (but not supergiants) turn into white dwarf stars as they run out of fuel  White dwarf mass must be less than 1.4 M o  White dwarfs do not collapse because of quantum mechanical pressure from degenerate electrons  White dwarf radius is about the same as the Earth  A teaspoon of a white dwarf would weigh 10 tons  Some white dwarfs have magnetic fields as high as 10 9 Gauss  White dwarfs eventually radiate away all their heat and end up as black dwarfs in billions of years

February 18, 2003Lynn Cominsky - Cosmology A35026 Planetary nebulae  Planetary nebulae are not the origin of planets  Outer ejected shells of red giant illuminated by a white dwarf formed from the giant’s burnt-out core  Not always formed HST/WFPC2 Eskimo nebula 5000 light years

February 18, 2003Lynn Cominsky - Cosmology A35027 Variable stars  Most stars vary in brightness  Periodic variability can be due to: Eclipses by the companion star Repeated flaring Pulsations as the star changes size or temperature  Novae are stars which repeatedly blow off their outer layers in huge flares  Flare stars have regions which explode  Pulsating stars have an unstable equilibrium between the competing forces of gas pressure and gravity

February 18, 2003Lynn Cominsky - Cosmology A35028 Cepheid variables  Henrietta Leavitt studied variable stars that were all at the same distance (in the LMC or SMC) and found that their pulsation periods were related to their brightnesses L =K P 1.3 Polaris (the North Star) is not constant, it is a Cepheid variable!

February 18, 2003Lynn Cominsky - Cosmology A35029 Distances to Cepheids  Distance to closest Cepheid (Delta Cephei) in our Galaxy can be found using parallax measurements. This determines K in the period-luminosity relation (L = KP 1. 3 )  Cepheids are very bright stars – they can be seen in other galaxies out to ~10 million light years (with HST)  Since the period of a Cepheid is related to its absolute brightness, if you observe its period and the apparent brightness, you can then derive its distance (to within about 10%)

February 18, 2003Lynn Cominsky - Cosmology A35030 Pleiades Star Cluster  A star cluster has a group of stars which are all located at approximately the same distance  The stars in the Pleiades were all formed at about the same time, from a single cloud of dust and gas D = 116 pc

February 18, 2003Lynn Cominsky - Cosmology A35031 Open Star Clusters Open Cluster NGC 3293 d = 8000 c-yr  stars  diameter ~ 10 pc  young stars (Pop I )  mostly located in spiral arms of our Galaxy and other galaxies  solar metal abundance

February 18, 2003Lynn Cominsky - Cosmology A35032 Globular Star Clusters Globular Cluster 47 Tuc d=20,000 c-yr  stars  diameter ~ 30 pc  centrally condensed  old stars (Pop II )  galaxy halo  low in metals

February 18, 2003Lynn Cominsky - Cosmology A35033 Finding the age of star clusters  This graphing activity from the University of Washington allows you to figure out the age of 2 clusters of stars by plotting stellar data on color- magnitude forms of the H-R diagram 47 Tuc M45

February 18, 2003Lynn Cominsky - Cosmology A35034 Web Resources  Astronomy picture of the Day  Imagine the Universe  Ned Wright’s ABCs of Distance  National Geographic Star Journey index.html index.html  Zoom Star Types Site shtml

February 18, 2003Lynn Cominsky - Cosmology A35035 Web Resources  John Blondin’s supercomputer models  Cepheid variables  U Washington Star Age Lab color_mag.html  First star simulations  Molecular cloud - protostar simulations