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Astronomy’s Next 10 Years Kepler SOFIA mid-infrared image of part of the Orion star-forming region. GMT -- a 25m telescope TMT -- 30m effective aperture diameter JWST WFIRST – Understanding Dark Energy (formerly the “Joint Dark Energy Mission” [JDEM]) International X-ray Observatory New Astronomies – Gravitational Waves LISA “Astroparticle” Physics REVIEW How Far Away are the Stars? Earth-baseline parallax - useful in Solar System Earth-orbit parallax - useful for nearest stars Earth-orbit parallax using ground-based telescopes good for stars within 30 pc (1000 or so). Tiny volume of Milky Way galaxy. Other methods later. Our nearest stellar neighbors How Luminous are Stars? Remember, luminosity of the Sun is LSun = 4 x1033 erg/s Luminosity also called “absolute brightness”. How bright a star appears to us is the “apparent brightness”, which depends on its luminosity and distance from us: apparent brightness α luminosity / (distance)2 So we can determine luminosity if apparent brightness and distance are measured: luminosity apparent brightness x (distance)2 Please read about magnitude scale. Stellar Magnitudes (1) We measure the apparent magnitude of stars using a (logarithmic) scale. A difference of 5 magnitudes = 100 x in brightness. Astronomers also refer to a star’s absolute magnitude, which is related to its luminosity. The visible stars have magnitudes less than about 6. Larger magnitude = dimmer star. Smaller magnitude = brighter star. Brightest star : Sirius, magnitude (V) = -1.5 (Type = A1V) Stellar Magnitudes (III) • Apparent magnitude = magnitude we observe by eye, or measure at the telescope, here on Earth = dependent on luminosity, and proportional to 1/distance2 Denoted by lower-case letters, e.g., mV or mB • Absolute magnitude = apparent magnitude the star would have if placed at a standard distance (10 pc) from the Earth = dependent on luminosity only Variable Stars (brightness varies periodically) have Different Causes Intrinsic variables Luminosity changes periodically, usually associated with changes in size (pulsation), and color (spectrum) time Periods: hours to weeks, typically Eclipsing binaries -- example Binary star seen nearly (not completely) edge-on Shows changes in the total light due to the Partial eclipse of one star by another. Spectral Classes Strange lettering scheme is a historical accident. Spectral Class Examples O B A F G K M Surface Temperature 30,000 K 20,000 K 10,000 K 7000 K 6000 K 4000 K 3000 K Rigel Vega, Sirius Sun Betelgeus Further subdivision: BO - B9, GO - G9, etc. GO e hotter than G9. Sun is a G2. The Hertzsprung-Russell (H-R) Diagram Red Supergiants Red Giants Increasing Mass, Radius on Main Sequence Sun Main Sequence White Dwarfs A star’s position in the H-R diagram depends on its mass and evolutionary state. Star Clusters Open Cluster Globular Cluster Comparing with theory, can easily determine cluster age from H-R diagram. Star Clusters Two kinds: 1) Open Clusters -Example: The Pleiades -10's to 100's of stars -Few pc across -Loose grouping of stars -Tend to be young (10's to 100's of millions of years, not billions, but there are exceptions) 2) Globular Clusters - few x 10 5 or 10 6 stars - size about 50 pc - very tightly packed, roughly spherical shape - billions of years old Clusters are crucial for stellar evolution studies because: 1) All stars in a cluster formed about same time (so about same age) 2) All stars are at about the same distance 3) All stars have same chemical composition Gas Structures in the ISM Emission Nebulae or H II Regions Regions of gas and dust near stars just formed. The Hydrogen is almost fully ionized. Temperatures near 10,000 K Sizes about 1-20 pc. Hot tenuous gas => emission lines (Kirchhoff's Laws) Star Formation Stars form out of molecular gas clouds. Clouds collapse to form stars (remember, stars are ~1020 x denser than a molecular cloud). Probably new molecular clouds form continually out of less dense gas. Some collapse under their own gravity. Others may be more stable. Not well understood. Gravity makes cloud want to collapse. Outward gas pressure resists collapse, like air in a bike pump. How Long do Stars Live (as Main Sequence Stars)? Main Sequence stars fuse H to He in core. Lifetime depends on mass of H available and rate of fusion. Mass of H in core depends on mass of star. Fusion rate is related to luminosity (fusion reactions make the radiation energy). lifetime So, mass of core fusion rate mass of star luminosity Because luminosity (mass) 3, lifetime mass (mass)3 or 1 (mass)2 So if the Sun's lifetime is 10 billion years, a 30 MSun star's lifetime is only 10 million years. Such massive stars live only "briefly". Stellar Evolution: Evolution off the Main Sequence Main Sequence Lifetimes Most massive (O and B stars): millions of years Stars like the Sun (G stars): billions of years Low mass stars (K and M stars): a trillion years! While on Main Sequence, stellar core has H -> He fusion, by p-p chain in stars like Sun or less massive. In more massive stars, “CNO cycle” becomes more important. Final States of a Star 1. White Dwarf If initial star mass < 8-12 Msun . 2. Neutron Star If initial mass > 12 MSun and < 25 ? MSun . 3. Black Hole If initial mass > 25 ? MSun . Evolution of a Low-Mass Star (< 8 Msun , focus on 1 Msun case) - All H converted to He in core. - Core too cool for He burning. Contracts. Heats up. - H burns in hot, dense shell around core: "H-shell burning phase". - Tremendous energy produced. Star must expand. - Star now a "Red Giant". Diameter ~ 1 AU! - Phase lasts ~ 109 years for 1 MSun star. - Example: Arcturus Red Giant Helium Runs out in Core - All He -> C. Not hot enough -for C fusion. - - Core shrinks and heats up, as -does H-burning shell. - - Get new helium burning shell (inside H burning shell). - High rate of burning, star expands, luminosity way up. - Called ''Red Supergiant'' (or Asymptotic Giant Branch) phase. - Only ~106 years for 1 MSun star. Red Supergiant "Planetary Nebulae" - Core continues to contract. Never hot enough for C fusion. - He shell dense, fusion becomes unstable => “He shell flashes”. - Whole star pulsates more and more violently. - Eventually, shells thrown off star altogether! 0.1 - 0.2 MSun ejected. - Shells appear as a nebula around star, called “Planetary Nebula” (awful, historical name, nothing to do with planets). White Dwarfs - Dead core of low-mass star after Planetary Nebula thrown off. - Mass: few tenths of a MSun - Radius: about REarth - Density: 106 g/cm3! (a cubic cm of it would weigh a ton on Earth). - - - Composition: C, O. - White dwarfs slowly cool to oblivion. No fusion. Evolution of Stars > 12 MSun Low mass stars never got past this structure: Eventual state of > 12 MSun star Higher mass stars fuse heavier elements. Result is "onion" structure with many shells of fusion-produced elements. Heaviest element made is iron. Strong winds. They evolve more rapidly. Example: 20 MSun star lives "only" ~107 years. Neutron Stars If star has mass 12-25 MSun , remnant of supernova expected to be a tightly packed ball of neutrons. Diameter: 10 km only! Mass: 1.4 - 3(?) MSun Density: 1014 g / cm3 ! Rotation rate: few to many times per second!!! Magnetic field: 1010 x typical bar A neutron star over the Sandias? magnet! Please read about observable neutron stars: pulsars. Black Holes If core with about 3 MSun or more collapses, not even neutron pressure can stop it (total mass of star about 25 MSun ?). Core collapses to a point, a "singularity". Gravity is so strong that not even light can escape. RS for a 3 MSun object is 9 km. Event horizon: imaginary sphere around object, with radius RS . Event horizon Anything crossing the event horizon, including light, is trapped RS Effects around Black Holes 1) Enormous tidal forces. 2) Gravitational redshift. Example, blue light emitted just outside event horizon may appear red to distant observer. 3) Time dilation. Clock just outside event horizon appears to run slow to a distant observer. At event horizon, clock appears to stop. Do Black Holes Really Exist? Good Candidate: Cygnus X-1 - Binary system: 30 MSun star with unseen companion. - Binary orbit => companion > 7 MSun. - X-rays => million degree gas falling into black hole. The Three Main Structural Components of the Milky Way 1. Disk - 30 kpc diameter - contains young and old stars, gas, dust. Has spiral structure - vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer) 2. Halo - at least 30 kpc across - contains globular clusters, old stars, little gas and dust, much "dark matter" - roughly spherical 3. Bulge - About 4 kpc across - old stars, some gas, dust - central black hole of 3 x 106 solar masses - spherical New distance unit: the parsec (pc). Using Earth-orbit parallax, if a star has a parallactic angle of 1", it is 1 pc away. If the angle is 0.5", the distance is 2 pc. 1 Distance (pc) = Parallactic angle (arcsec) Closest star to Sun is Proxima Centauri. Parallactic angle is 0.7”, so distance is 1.3 pc. 1 pc = 3.3 light years = 3.1 x 1018 cm = 206,000 AU 1 kiloparsec (kpc) = 1000 pc 1 Megaparsec (Mpc) = 10 6 pc Stellar Orbits Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk. Bulge: similar to halo. Disk: rotates. Spiral Structure of Disk Spiral arms best traced by: Young stars and clusters Emission Nebulae Atomic gas Molecular Clouds (old stars to a lesser extent) Disk not empty between arms, just less material there. 90% of Matter in Milky Way is Dark Matter Gives off no detectable radiation. Evidence is from rotation curve: 10 Rotation Velocity (AU/yr) 5 Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian") 1 1 10 20 30 R (AU) observed curve Milky Way Rotation Curve Curve if Milky Way ended where radiating matter pretty much runs out. The Variety of Galaxy Morphologies Galaxy Classification Hubble’s 1924 "tuning fork diagram" bulge less prominent, arms more loosely wrapped increasing apparent flatness disk and large bulge, but no spiral Spirals Ellipticals barred unbarred SBa-SBc Sa-Sc E0 - E7 Irregulars Irr I "misshapen spirals" Irr II truly irregular I r r A further distinction for ellipticals and irregulars: Giant 1010 - 1013 stars 10's of kpc across Dwarf Elliptical NGC 205 Spiral M31 Dwarf Elliptical M32 vs. Dwarf 106 - 108 stars few kpc across How Far Away are Galaxies? For "nearby" (out to 20 Mpc or so) galaxies, use a very bright class of variable star called a "Cepheid". luminosity average luminosity time (days or weeks) Cepheid star in galaxy M100 with Hubble. Brightness varies over a few weeks. (average) From Cepheids in Milky Way star clusters (with known distances), it was found that period (days to weeks) is related to average luminosity. So measure period of Cepheid in nearby galaxy, this gives star's average luminosity. Measure average apparent brightness. Now can determine distance to star and galaxy. Has been used to find distances to galaxies up to 25 Mpc. Structures of Galaxies Groups A few to a few dozen galaxies bound together by their combined gravity. No regular structure to them. The Milky Way is part of the Local Group of about 30 galaxies, including Andromeda. About 20 dwarfs in Local Group found since 2004, most with Sloan Digital Sky Survey in NM! Clusters Larger structures typically containing thousands of galaxies. Center of Virgo Cluster of about 2500 galaxies Center of the Hercules Cluster Galaxies orbit in groups or clusters just like stars in a stellar cluster. Most galaxies are in groups or clusters. Superclusters Recognizable structures containing clusters and groups. 10,000's of galaxies. 50 Mpc The Local Supercluster consists of the Virgo Cluster, the Local Group and many other groups. Galaxy Interactions and Mergers Galaxies sometimes come near each other, especially in groups and clusters. Large tidal force can draw stars and gas out of them => tidal tails in spirals. Galaxy shapes can become badly distorted. Schematic of galaxy formation Subsequent mergers of large galaxies also important for galaxy evolution. Large galaxies continue to swallow small ones today: “galactic cannibalism”. Cosmology: The Study of the Universe as a Whole Given no evidence of further structure, assume: The Cosmological Principle On the largest scales, the universe is roughly homogeneous (same at all places) and isotropic (same in all directions). Laws of physics same. Hubble's Law might suggest that everything is expanding away from us, putting us at center of expansion. Is this necessarily true? (assumes H0 = 65 km/sec/Mpc) So if the CP is correct, there is no center, and no edge to the Universe! Best evidence for CP comes from Cosmic Microwave Background Radiation (later). The Big Bang All galaxies moving away from each other. If twice as far away from us, then moving twice as fast (Hubble's Law). So, reversing the Hubble expansion, all separations go back to zero. How long ago? H0 gives rate of expansion. Assume H0 = 75 km / sec / Mpc. So galaxy at 100 Mpc from us moves away at 7500 km/sec. How long did it take to move 100 Mpc from us? time = = = distance velocity 100 Mpc 7500 km/sec 13 billion years (Experts note that this time is just 1 ). H0 The faster the expansion (the greater H0), the shorter the time to get to the present separation. Big Bang: we assume that at time zero, all separations were infinitely small. Universe then expanded in all directions. Galaxies formed as expansion continued. If all distances increase, so do wavelengths of photons as they travel and time goes on. When we record a photon from a distant source, its wavelength will be longer. This is like the Doppler Shift, but it is not due to relative motion of source and receiver. This is correct way to think of redshifts of galaxies. The Cosmic Microwave Background Radiation (CMBR) A prediction of Big Bang theory in 1940's. "Leftover" radiation from early, hot universe, uniformly filling space (i.e. isotropic, homogeneous). Predicted to have perfect black-body spectrum. Photons stretched as they travel and universe expands, but spectrum always black-body. Wien's Law: temperature decreases as wavelength of brightest emission increases => was predicted to be ~ 3 K now. The Cosmic Microwave Background Radiation (CMBR) A prediction of Big Bang theory in 1940's. "Leftover" radiation from early, hot universe, uniformly filling space (i.e. isotropic, homogeneous). Predicted to have perfect black-body spectrum. Photons stretched as they travel and universe expands, but spectrum always black-body. Wien's Law: temperature decreases as wavelength of brightest emission increases => was predicted to be ~ 3 K now. That the CMBR comes to us from every direction is best evidence that Big Bang happened everywhere in the universe. That the temperature is so constant in every direction is best evidence for homogeneity on large scales. IF the Big Bang happened at one point in 3-d space: Later, galaxies form and fly apart. But radiation from Big Bang streams freely at speed of light! Wouldn't see it now. The Early Universe The First Matter At the earliest moments, the universe is thought to have been dominated by high-energy, high-temperature radiation. Photons had enough energy to form particle-antiparticle pairs. Why? E=mc2. pair production annihilation Primordial Nucleosynthesis Hot and dense universe => fusion reactions. At time 100-1000 sec (T = 109 - 3 x 108 K), helium formed. Stopped when universe too cool. Predicted end result: 75% hydrogen, 25% helium. Oldest stars' atmospheres (unaffected by stellar nucleosynthesis) confirm Big Bang prediction of 25% helium. Other Measurements Support this Model of the Universe In 1920's, Hubble used Cepheids to find distances to galaxies. Showed that redshift or recessional velocity is proportional to distance: V = H0 x D velocity (km / sec) (Hubble's Law) Distance (Mpc) Hubble's Constant (km / sec / Mpc) Or graphically. . . Current estimate: H0 = 65 -75 km/sec/Mpc If H0 = 70 km/sec/Mpc, a galaxy at 1 Mpc moves away from us at 70 km/sec, etc. The Expansion of the Universe Seems to be Accelerating The gravity of matter should retard the expansion. But a new distance indicator shows that the expansion rate was slower in the past! Redshift (fractional shift in wavelength of spectral lines) Type I supernovae: from ones in nearby galaxies, know luminosity. In distant galaxies, determine apparent brightness. Thus determine distance. Works for more than 3000 Mpc. From redshifts, they are not expanding as quickly from each other as galaxies are now. Taking this into account, best age estimate of Universe is 13.8 Gyrs. H0 was smaller in past (i.e. for distant galaxies) Current Theories : Universe is 70% + “Dark Energy” !! Supported by – CMBR measurements Supernovae Ia measurements Statistical studies of Galaxy Clusters Successes of the Big Bang Theory 1) It explains the expansion of the universe. 2) It predicted the cosmic microwave background radiation, its uniformity, its current temperature, and its black-body spectrum. 3) It predicted the correct helium abundance (and lack of other primordial elements). Is There Life Elsewhere in the Universe? Requirements for Life: - Elements to build cells (C, H, O most important) - Energy source to make chemical reactions happen to fuel metabolism - Liquid medium – water allows organic (C-based) molecules to dissolve and be carried into cells for metabolic reactions These conditions occurred on Earth, but it's not clear whether life was then inevitable. Also, how unique are they? Life Elsewhere in the Milky Way: To address question of whether other intelligent life exists in the Milky Way, Frank Drake formulated the Drake Equation. The equation is usually written: N = R* • fp • ne • fl • fi • fc • L Where, N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. R* =The rate of formation of stars suitable for the development of intelligent life. fp = The fraction of those stars with planetary systems. ne = The number of planets, per solar system, with an environment suitable for life. fl = The fraction of suitable planets on which life actually appears. fi = The fraction of life bearing planets on which intelligent life emerges. fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. L = The length of time such civilizations release detectable signals into space. Allen Telescope Array – radio SETI Optical SETI [Harvard U.] SETI League (Amateur Radio Astronomers) Extrasolar Planets More than 300 discovered. Main technique: detect Doppler Shift due to wobble of star caused by unseen planet. Biased – easier to detect heavier (Jupiter-class) planets. Second technique: detect eclipse (transit) of planet – Kepler mission. Third technique: detect wobble in star's position in sky due to unseen planet (astrometric method). Fourth technique: direct imaging of planet. Fifth technique: microlensing Future missions hope to detect Terrestrial planets Astrometric method http://planetquest.jpl.nasa.gov/science/finding_planets.cfm