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  Fall 2005 ASTR 1120-001 General Astronomy: Stars & Galaxies: Weekly Summaries

Week 1 (Aug 23)

• Be ready for a 10 minute in class group discussion on "What is the Scientific Method?" at the end of class on Thursday. The aim is for each group of 3 or 4 to come up with 5 possible statements of what is the scientific method. Chapter 3 of "Cosmic Perspective" has some comments on the subject.
• Register your clicker online.
• Read Chapter 1 of "Cosmic Perspective".
• Appendix C of Cosmic Perspective reviews powers of ten, scientific notation, units, and ratios, all of which are needed to do the homeworks. Or check out Fran Bagenal's Math Reminder.
• Did you know that Baseline Road in Boulder is at exactly longitude 40° North? The view at 40°N 105°W.
• The Sun is a star.
• An Astronomical Unit is the distance between the Earth and the Sun. First measured reliably by Cassini (1672), using a measurement of the parallax of Mars.
• The nearest star beyond our Sun, Proxima Centauri, is almost 10⁴ times more distant than Pluto, the ninth planet. Bessel (1838) was the first to measure successfully a parallax of a star, 61 Cyg, and hence deduce a reliable measurement of its distance.
• The center of the Milky Way is almost 10⁴ times more distant than the nearest star.
• Our Galaxy, the Milky Way, is the second largest member of the Local Group of about 40 galaxies. The largest member of the Local Group is M31, the Andromeda Galaxy. Edwin Hubble first measured a reliable distance to M31 in 1923, using the method of Cepheid variables discovered by Henrietta Leavitt around 1907.
• The Local Group is on the outskirts of the Local Supercluster of galaxies, containing several thousand galaxies. At the center of the Local Supercluster is the Virgo cluster of galaxies.
• The Virgo cluster is part of a "Cosmic Web" of large scale structure.
• The redshifts (Doppler shifts) of the spectra of galaxies indicate that galaxies are moving away from us: the Universe is expanding (Hubble 1929). More distant galaxies are receding faster, suggesting there was a Big Bang. Raisin cake model of the Universe (Applet on Cosmic Perspective CDROM).
• Because light has a finite speed, as we see deeper in space, we see further back in time.
• The deepest optical image of the Universe so far taken by human beings is the Hubble Ultra Deep Field.
• The deepest image of the Universe at any wavelength of the electromagnetic spectrum is the Cosmic Microwave Background.
• Read Chapters 4 & 6 of Cosmic Perspective in preparation for Week 2.

Week 2 (Aug 30)

• Last week ...
• Here is a distillation of your answers to the project about the Scientific Method.
• Astronomers cannot do experiments: they can only observe. What they observe is light.
• NASA Goddard Space Flight Center's SkyView generates images of any part of the sky at wavelengths from radio to gamma-rays.
• Stars have different colors, according to their surface temperature.
  Mitchell Charity's What color are the stars?
• Temperature worksheet (pdf); (Postscript).
• Rob Scharein's Blackbody spectrum applet.
  Or, Wien's Law: Blackbody spectrum applet (Applet on Cosmic Perspective CDROM).
• Some webpages on color that I wrote a few years ago.
• Light.
  • Light is an electromagnetic (Maxwell 1855) wave (Young 1800)
    • Physics 2000 Experiments on Wave Interference.
    
    

  • Light moves at the speed of light c = 299,792,458 m/s
    

    c  =  l   f Speed of light  =  wavelength  ×  frequency

    
    
  • Light is both a particle and a wave
    

    particle property       wave property E = h   f Energy  =  Planck's constant  ×  frequency

  
  
• Atoms.
  • H (hydrogen) is the simplest of all elements: its nucleus consists of just a single proton.
    Neutral hydrogen has one electron in orbit around the proton.
  • Electrons are also both particle and wave.
  • The wavelike nature of electrons means they can only occupy certain orbitals in an atom.
    Electron probability cloud (Applet on Cosmic Perspective CDROM).
  • Every atom has a characteristic spectrum of lines, corresponding to transitions between different orbitals. These are some of the transitions in a hydrogen atom. The Lyman lines, connecting to the ground (n = 1) state, are in the ultraviolet.
    

    Emission		Absorption

  • The emission and absorption lines of an atom occur at the same characteristic wavelengths (energies)
    See Production of absorption lines (Applet on Cosmic Perspective CDROM).
  • The Rosette nebula glowing in the pink light of Ha.
  • Spectra:
    • A hot, dense substance emits a blackbody (or Planck, or thermal) spectrum
    • A hot, low density gas produces an emission line spectrum
    • A cold, low density gas seen against a bright background produces an absorption line spectrum.
• Read Chapter 15 of Cosmic Perspective in preparation for Week 3.

Week 3 (Sep 6)

• National Solar Observatory/Sacramento Peak's Current images of the Sun.
• Images of the Sun at various wavelengths.
• Sir Arthur Eddington's "Internal Constitution of the Stars".
  • The interior of the Sun is a compressible, ionized plasma of electrons and nuclei.
  • Gas pressure balances gravity.
  • Eddington estimated the temperature inside the Sun from
    

    mean particle velocity = gravitational escape velocity

    The gravitational escape velocity at the surface of the Sun is 620 km/s.
    This yields an estimate T = 10⁷ K for the temperature of the interior of the Sun.
  • More detailed calculations indicate T = 1.5 × 10⁷ K for the temperature at the center of the Sun.
  • Eddington did not understand how the Sun generates energy, but speculated it might come from conversion of mass to energy.
• Physics 2000: Temperature and Absolute Zero applet showing a box of particles at different temperatures.
• The Sun has a fairly sharp edge in visible light - the photosphere.
  • The photosphere is the boundary between (partially) ionized and (mostly) neutral gas.
• Gravity is the perpetual motion machine that drives the Universe.
  • When you remove energy from a gravitating system, it contracts, and gets hotter (particles move faster).
• Examples of gravity power in astronomy (we'll look at all these in due course):
  • A protostar contracts, heats up to the point where it can fuse H (hydrogen).
  • When a main sequence star exhausts H at its center, the He (helium) core contracts and heats up, enabling H to burn in a shell around the core, and causing the star to bloat into a luminous red giant.
  • When the Fe (iron) core of an evolved massive star collapses, the gravitational energy released powers a supernove.
  • The gas in an accretion disk around a neutron star or black hole gets faster and hotter as it spirals inward. Near the central compact object, the disk reaches relativistic temperatures, and emits x-rays.
  • Twin jets, often relativistic, emerge from the vortical opening along the spin axis of the accretion disk.
  
  

• Rob Scharein's Doppler effect applet.

• The Sun is a star.
  • A star is an object that is undergoing nuclear fusion at its core.
  • The smallest mass that can achieve nuclear fusion is about 0.08 solar masses.
  • Jupiter is not a star: it is not undergoing nuclear fusion.
    Jupiter's mass is about 0.001 solar masses.

• The Sun has three principal layers.
  1. A core, in which hydrogen is fusing into helium.

  2. A radiative zone, in which photons carry energy outward from the core. It takes about 10⁷ years for a photon to diffuse from the core through the radiative zone.
     

  3. A convective outer zone, in which large scale convective motions carry energy bodily upward, like water boiling in a pot.

     Animation (click on image for movie at original site)

• From the surface of the Sun upward are three regions.

  © 1996-2001 Wendy Carlos - All Rights Reserved

  1. The photosphere is the visible surface of the Sun. It forms a fairly sharp edge. It has a temperature of about 6,000 K.
  2. Just above the photosphere is the chromosphere ("color sphere"). It is the principal source of the Sun's ultraviolet radiation, and has a temperature around 10,000 K.
  3. Extending upward from the chromosphere to large distances is a hot, low density corona, with a temperature of about 10⁶ K.
     Movie of total solar eclipse of 1994.

• The Sun fuses hydrogen through a three-step process called the proton-proton chain (Bethe 1938).
  

• The first reaction converts a proton into a neutron. In so doing it releases a neutrino, a weakly interacting particle that passes freely through the entire Sun.

The Super-Kamiokande neutrino observatory being refurbished.

An image of neutrinos from the Sun observed by the Super-Kamiokande neutrino observatory.

• The Solar neutrino problem: Only about 1/3 as many neutrinos are observed from the Sun as are predicted by solar models.
• The remarkable solution to the problem is that the electron type neutrinos emitted by the Sun are oscillating into the two other types of the neutrino, the muon and tauon type neutrinos. This is a deep insight into fundamental particle physics that is still not well understood.
• Convective motions in the Sun's convective zone act like a dynamo, generating magnetic fields. These magnetic fields are responsible for much of the action observed at and above the Sun's surface.
  • Solar granulation delineates convective cells. Hot gas wells up at the center of a cell, and cooler gas descends at the edge.
    
  • Bubbles of magnetic field bursting from the surface of the Sun cause flares, which accelerate particles to high speed, and heat gas to high temperatures.
    
    Movie of flare observed by Big Bear Observatory, 1971 Oct 10 from Solar Flares.
  • The release of magnetic field energy is what heats the corona.

    Movie of the Sun in soft x-rays, observed by the Yohkoh satellite.

  • Charged particles follow paths that spiral along magnetic field lines, so that hot gas is constrained to move only along magnetic field lines.
    
  • The result is beautiful prominences and loops.

    Animation (click on image for movie at original site)	Animation (click on image for movie at original site)
    Animation (click on image for movie at original site)

• Read Chapter 16 of Cosmic Perspective in preparation for Week 4.

Week 4 (Sep 13)

• Stars have different colors and luminosities (intrinsic brightnesses).
  • Young open star clusters
  • Old globular star clusters

• Astronomers classify stars mainly by two observable quantities: their Luminosity L (energy emitted per unit time), and their surface Temperature T.
  Women astronomers, with Edward Pickering, at the Harvard College Observatory in 1913.

• Williamina Fleming (1890) originally classified stars according to the strength of their H lines
  ¬ stronger H lines   A B C ...   weaker H lines ®

• Rearranged by Annie Jump Cannon (1900) into a sequence of spectral types:
  ¬ higher Temperature    OBAFGKM   lower Temperature ®

  Famous mnemonic: Oh Be A Fine Girl, Kiss Me

  Image credit:NOAO/AURA/NSF

• Spectral types were recognized as a sequence of surface Temperature by Cecilia Payne-Gaposhkin,

• who also showed (1925) that stars are composed mainly of

  H (Hydrogen) 75% by mass He (Helium) 25% by mass

• A star's surface Temperature T can be measured either from its spectral type, or from the peak wavelength l_max of its blackbody spectrum using Wien's Law (Cosmic Perspective p. 162):

  T (Kelvin)  =  2,900,000 nm lmax

  
  
• A star's Luminosity L can be deduced from its measured apparent brightness, or flux F, and distance d using (Cosmic Perspective p. 523):

  L  =  4p d2   F Luminosity  =  4p (distance)2  ×  Flux

  
  
• For nearby stars, a star's distance d can be measured from its parallax:

  distance (parsec)  =  1 parallax angle (arcsec)

  See
  • Parallax of a nearby star (Applet on Cosmic Perspective CDROM).
  • Parallax angle versus distance (Applet on Cosmic Perspective CDROM).
    

    Stereo image illustrating parallax.

• Given the star's luminosity L and surface temperature T, its radius R can be deduced from the Stefan-Boltzmann law (Cosmic Perspective p. 162):

  L  =  4p R2   s   T4 Luminosity  =  Area  ×  Stefan-Boltzmann constant  ×  Temperature4

  
  

  The HR Diagram

• The Hertszsprung-Russel (HR) diagram (Ejnar Hertzsprung, Henry Norris Russell 1912) was key to understanding the nature of stars. The HR diagram is a plot of Luminosity versus surface Temperature (or Spectral Type).
  

• The HR diagram shows that stars fall into four main categories:
  • Main sequence stars - from cool dim to hot bright;
  • Super giants - the most luminous stars;
  • Red giants - cool bright;
  • White dwarfs - hot dim.

• The Sun is a main sequence star.

• Plotting a star on the HR diagram requires knowing its Luminosity, which can be deduced from measures of its Flux (apparent brightness) and distance. For nearby stars, the star's distance can be measured from its parallax. Historically, only parallaxes of nearby stars, within about 50 parsec, could be measured.

• The European Space Agency (ESA) Hipparcos satellite obtained parallaxes for 118,218 stars during 1989-1993.
  

• Star clusters played an important role in extending the HR diagram, because the stars in a cluster are all at about the same distance. The stars in a cluster formed out of the same cloud of interstellar gas and dust, and therefore have approximately the same:
  • Distance;
  • Age;
  • Initial composition.

• HR diagram of the Pleiades, an open star cluster:

• HR diagram of Palomar 3, a globular cluster:

• The radius of a star on the HR diagram can be deduced from the Stefan-Boltzmann law:
  • Main sequence stars - 0.1 to 10 times the radius of the Sun;
  • Red giants - from a few to 1000 times the radius of the Sun, about the orbit of Jupiter;
  • White dwarfs - 0.01 the radius of the Sun, or about the radius of the Earth.

• Eddington argued that the main sequence was a sequence of stars of different Mass:
  • Hot bright - more massive, up to about 100 solar masses;
  • Cool dim - less massive, down to about 0.08 solar masses.
  (Hertzsprung & Russell had originally suggested, incorrectly, that the main sequence was an evolutionary sequence, with stars evolving from hot bright to cool dim.)

• Eddington's arguments were confirmed by measurements of Masses of stars in binary systems, such as Sirius A-B. Masses can be deduced from the orbital dynamics of the binary.
  

• The age of a star on the HR diagram can be estimated from its Mass and Luminosity:

  Age  =  Energy available Luminosity  =  Mass × c2 × efficiency Luminosity

  
  

• The age of a star cluster can be deduced from the "main sequence turnoff" point in its HR diagram - the age of the most massive stars still remaining on the main sequence.

• Summary. This week you learned something about:
  • How astronomers estimate various properties of stars:
    • Distance;
    • Composition;
    • Surface Temperature;
    • Luminosity;
    • Mass;
    • Age.
  • The history of how these properties were learned from hard observations and hard thought.
  • How the HR diagram played a central role in understanding the stars. In particular, the HR diagram shows that stars come in four main categories:
    • Main sequence stars;
    • Super giants;
    • Red giants;
    • White dwarfs.

• Read Chapter 17 of Cosmic Perspective in preparation for Week 5.

Week 5 (Sep 20)

• On Tuesday there was a Star Talk at the Fiske Planetarium. You made connections between what you have been learning in class, and stars and star patterns that you have known on the sky your entire life. You met:
  • the Big Dipper, Cepheid variable stars, the summer triangle;
  • the Milky Way;
  • Messier objects, star forming regions, globular clusters;
  • Algol and Andromeda;
  • the winter hexagon, the Orion star-forming nebula, and open clusters like the Hyades and the Pleiades.
• On Thursday, Prof. Phil Armitage gave a guest lecture on Stellar Evolution (pdf, 6.4MB).

Week 6 (Sep 27)

• Eddington's paradox: "If a gravitating system, whenever it runs out of energy, contracts and heats up, how can it ever cool down?"
  White dwarf stars, with a density of 100 tons per cubic inch, were a mystery to Eddington.
• Mystery of white dwarfs solved by Subrahmanyan Chandrasekhar (1930), aged 19, who realized that white dwarfs were held up by electron degeneracy pressure.
  • For the delightful history of what happened, read Ch 4 of Kip Thorne's prize-winning book "Black Holes & Time Warps: Einstein's Outrageous Legacy".
• What is electron degeneracy pressure (Cosmic Perspective S4.5, page 486)? It's a quantum mechanical effect.
  • Electrons cannot be pushed closer than their wavelengths.
  • The shorter the electron wavelength, the higher the electron energy.
  • When electrons are squashed smaller than an atom, they become free (have enough energy to be ionized).
  • Squashed electrons retain this quantum zero-point energy even at zero absolute temperature.
• What does electron degeneracy pressure do?
  • It's what holds up white dwarfs, and the centers of red giants.
  • The outer electrons of solid metals are electron degenerate, giving metals their special properties: high thermal and electric conductivity, high reflectivity.
• More massive white dwarfs have smaller radii
  
  
• The Chandrasekhar (1931) limit: if a white dwarf (an electron degenerate object) exceeds 1.4 solar masses, the Chandrasekhar limit, it will collapse.
  • Why? Because the squashed electrons become so energetic that they become relativistic (move at almost the speed of light). Since they cannot move faster than light, they cannot exert enough additional pressure to withstand collapse.
• Evolution of a red giant
  • "At the core of a red giant is a white dwarf" (the core of a red giant star is electron degenerate).
  Evolution of a 1 solar mass star
  
  
• Helium flash.
  • Nuclear fusion of electron degenerate matter is explosive, because when the temperature increases, the matter does not expand, so the temperature sky-rockets.
  • Helium is consumed in about 1 second. But the star does not explode.
  • Helium burns to carbon in the "triple alpha" reaction. This is where the carbon for life was created.
    
    
  
  
• The red giant develops a powerful wind.
• In the last stages of losing its envelope, the star appears as so-called planetary nebula (nothing to do with planets).
• The envelope dissipates into space, the core cools, leaving a white dwarf.

  Supernovae

• Supernova. A star that for a while becomes comparable in brightness to its parent galaxy.

• History.
  • 1054. Chinese astronomer Yang Wei-te reported a "guest star" (supernova) in constellation Taurus, which was recorded in the Annals of the Sung Dynasty.
  • 1931. Fritz Zwicky points out that some novae (new stars) are exceptionally bright. He calls them "supernovae". He begins a solo campaign to discover supernovae.
  • 1932. James Chadwick discovers the neutron.
  • 1933. Baade & Zwicky make their famous prediction that supernovae produce neutrons stars.
  • 1940s. Baade and others measure the expansion rate of Messier 1, the Crab nebula, and show that it must have originated in an explosion around 1054. They conclude that the Crab nebula is the remnant of the Supernova of 1054.
  • 1967. Jocelyn Bell discovers a pulsar, with a precise period of 1.337301 seconds, then three more. The first few pulsars are given the name "LGM" (Little Green Men). Observations that pulsars pulse over a wide range of radio frequencies ruled out the possibility of the source being intelligent life.
  • 1968. American astronomers discover a 0.033 second pulsar in the Crab nebula. The period is too short and too regular to be anything other than a rotating neutron star.
    
    
  • Today. More than 1300 pulsars are now known in our Galaxy, with periods ranging from 0.00156 to several seconds.

• A pulsar is a rotating, magnetized neutron star.
  • Pulsar animation by Paul Bourke, Swinburne Centre for Astrophysics and Supercomputing.
  
  
  

• Listen to the sounds of pulsars.

• There are two kinds of supernova:

  Type:	Core collapse supernova	Thermonuclear supernova
  Spectra:	Show H lines	Show no H lines
  Where:	In star-forming regions, spiral arms	Anywhere in a galaxy
  Ž	Young, massive star (> 8 solar masses)	Old white dwarf

  
  
  

  Core collapse supernova

• Evolution of a massive star
  
  
  

• Eventually the core burns all the way to iron, which contains no more nuclear energy.
  
  

• The iron core is held up by electron degeneracy pressure. When it reaches the Chandrasekhar limit, 1.4 solar masses, it collapses, in a fraction of a second.

• Where can the squashed electrons go? Electrons and protons are pressed together into neutrons, releasing a burst of neutrinos.
  
  

• The envelope bounces off the core, producing a supernova.
  • The supernovae is powered by the gravitational energy released by the collapse of the core.
  • computer simulation by Adam Burrows

• The core left behind is a neutron star, or a black hole if the star is very massive (> 25 solar masses).

  Thermonuclear supernova

• A carbon-oxygen white dwarf accretes matter from a companion star, probably a red giant.

• The accreted hydrogen-rich gas forms an electron degenerate layer on the white dwarf. From time to time the hydrogen explodes, producing a nova (new star), as luminous as the most luminous stars (a million solar luminosities).
  
  

• Eventually the mass of the carbon-oxygen white dwarf may build up to the Chandrasekhar limit, 1.4 solar masses. At this point, the white dwarf begins to collapse. The high density causes carbon at the center to start to fuse. Because the gas is electron degenerate, the burning is explosive.
  
  

• In a fraction of a second, the center of the white dwarf is incinerated to ⁵⁶Ni. The white dwarf explodes, leaving no remnant.
  • The supernova is powered by nuclear energy from the fusion of carbon (C) and oxygen (O) to nickel (Ni).

• Nickel-56 is radioactive, decaying to cobalt (Co) then to iron (Fe):

  6 days   111 days   56Ni ® 56Co ® 56Fe

  The energy released by this radioactive decay is what produces the characteristic light curve of the supernova.

Week 7 (Oct 4)

• A week of review and midterm.

Week 8 (Oct 11)

Fall break. We needed that!

• On Tuesday there was a talk on "Black Holes and Relativity" at the Fiske Planetarium.
• You learned some of the history about how black holes were discovered, first as theoretical predictions, then as actual objects in the sky.
• To find black holes, look in x-rays or gamma-rays. X-rays and gamma-rays are the most energetic kinds of electromagnetic radiation; if you want to find something violent going on in the cosmos, then look in high energy radiation.
• Astronomers identify two distinct types of black hole:
  • Stellar sized black holes (three to several tens of solar masses), seen in x-ray binary systems.
  • Supermassive black holes (millions to billions of solar masses), seen at the centers of galaxies.

• Read Chapter S2 of Cosmic Perspective, on Special Relativity, in preparation for Week 9.
  Also check out my Special Relativity website.

Week 9 (Oct 18)

Special Relativity

• Special Relativity website.

• Postulates of Special Relativity
  1. Spacetime is a 4-dimensional continuum (3 dimensions of space, 1 of time).
  2. There exist "globally inertial" spacetime frames: frames with respect to which unaccelerated objects move in straight lines at constant velocity.
  3. The speed of light is the same in any inertial frame.
  4. The Principle of Special Relativity: the laws of physics are the same in any inertial frame (in other words, there is no absolute spacetime).

• How can the speed of light be the same in any frame?
  • Isn't this a paradox?
  • Einstein's solution to the paradox is that observers in relative motion measure space and time differently.

    Are these boxes the same?

• How a scene actually appears seen at close to the speed of light
  • Aberration: ahead appears fisheyed, behind zoomed.
  • Color: blueshift (higher energy photons) ahead, redshift (lower energy photons) behind.
  • Brightness: brighter ahead, dimmer behind.
  • Time: faster ahead, slower behind.

    The rules of 4-dimensional perspective: An observer moving to the right sees the "celestial sphere" distorted into a "celestial ellipsoid" with self at the focus of the ellipse.

• Nothing can move faster than light:
  • It would take an infinite energy to accelerate to faster than the speed of light.
  • If you could travel faster than light, you could also travel backwards in time.

• Nevertheless, time dilation allows you to travel effectively "faster than light".
  • A relativistic trip across the Universe.

Black Holes in General Relativity

• No hair theorem: An isolated black hole rapidly (on few light crossing times) evolves to a state characterized by just 3 properties:
  • Mass
  • Electric charge
  • Angular momentum (spin)
  It loses its "hair" by gravitational radiation.
• Falling into a Black Hole: approach; orbit; singularity
  • What happens when you watch someone else fall into a black hole?
  • What happens when you fall into a black hole?
• The River Model of Black Holes.

  The fish upstream can make way against the current, but the fish downstream is swept to the bottom of the waterfall.

Week 10 (Oct 25)

Observational evidence for Black Holes

• Astronomers see evidence for two kinds of black hole
  • Stellar-sized black holes (3 to 20 solar masses) in x-ray binary systems
  • Supermassive black holes (10⁶ to 10¹⁰ solar masses) at the centers of galaxies
• X-ray binary systems.
  • J. Orosz's (2002) Inventory of Black Hole Binaries.
  • X-ray binaries consist of a low or high mass main sequence losing gas on to an accretion disk around a neutron star or black hole.
  • Simulation of a binary star produced from Rob Hynes' binsim sofware.
  • If the mass of the neutron star or black hole is more than 3 solar masses (the maximum theoretical mass of a neutron star), then it is inferred to be a black hole. Cygnus X-1 is the most famous example.
  • In some case the x-ray binary emits a jet; such x-ray binaries are called microquasars.
  • John Hawley's magnetohydrodynamic computer simulation of a jet from an accretion disk.
  • Summary: why do astronomers think that some x-ray binaries contain black holes?
    The most direct evidence is:
    • More than 3 solar masses in a small space (inferred from orbital dynamics of the binary).
    Supporting evidence is:
    • A large luminosity (10⁴ suns)
    • coming out in energetic radiation (x-rays)
    • typical variable (down to timescales of 1/100 second)
    • sometimes there are jets.

• Quasars are the most extreme (luminous) form of Active Galactic Nuclei
  • 3C273, the nearest bright quasar, the first quasar discovered, by Maarten Schmidt in 1963.
  • Name quasar shortened from quasi-stellar-object - an unresolved, star-like source with a non-stellar spectrum and a large redshift.
  • Large redshift, from the expansion of the Universe, indicates that quasars are at cosmological distances.
  • Intensely luminous, as much as 1000 times the brightness of their parent galaxy.
  • Typically variable, in some cases on timescales of days; this indicates that they are less than lightdays in size.
• Active Galactic Nuclei - a generic term for a variety of phenomena seen at the centers of galaxies
  • Bright.
  • Variable - on timescales of days to years.
  • Non-stellar spectrum - radio, x-rays, gamma-rays; optical emission lines.
  • Jets, often relativistic - superluminal motion, one-sided.
• John Kormendy's 2001 census of Supermassive Black Holes in Galactic Nuclei
  • Observations of the centers of nearby galaxies indicate the presence of a large unseen gravitational mass in a small region - it must be a black hole.
  • Every galaxy large enough to have a bulge appears to have a central black hole.
  • M87, the huge galaxy at the center of the Virgo cluster, at the center of the Local Supercluster of galaxies, contains the most massive black hole known in the local Universe, 3 × 10⁹ solar masses.
  • Observations of the motions of stars, both angular motions on the sky, and radial motions from spectroscopy (redshifts and blueshifts), indicate the presence of a 4 × 10⁶ solar mass black hole at the center of our own Galaxy, the Milky Way.
• Summary: why do astronomers think that the centers of (most) galaxies contain supermassive black holes?
  The most direct evidence is:
  • A lot of mass (inferred from velocities of stars or gas)
  • in a small space (inferred from imaging).
  Supporting evidence is:
  • Active Galactic Nuclei.

Week 11 (Nov 1)

History of understanding of our Galaxy, the Milky Way

On Tuesday there was a Fiske Planetarium show on the Milky Way. The show was mostly about the history of the discovery that the Milky Way is a galaxy of stars, and that there are many other galaxies in the Universe.

• My favorite book on the subject is Leila Belkora's "Minding the Heavens: The Story of Our Discovery of the Milky Way"
• The ancients had various imaginative ideas about the Milky Way. Our word galaxy comes from Greek galaktikos = milky.
• Galileo was the first person to use a telescope in astronomy. He found (1610) that parts of the Milky Way resolved into numerous stars.
• Philosopher Kant proposed (1755) that some nebulae were "Island Universes", comparable to the Milky Way in size, rotating according to Newton's laws of gravity and motion.
• William Herschel, discoverer of the planet Uranus, with the aid of his sister Caroline, undertook a laborious "star-gauging" enterprise to map the size of the Milky Way with his 19 inch telescope. He concluded (1785) that the Milky Way was roughly elliptical, with the Sun at the center.
• William Parsons, 3rd Earl of Rosse, built a huge telescope with a 72 inch speculum mirror. With this he discovered (1845) spiral structure, whence the term "spiral nebulae".
• J. C. Kapteyn, during 1906-1920, organized a major international co-operative effort to repeat Herschel's star-gauging exercise using observations from 40 observatories. He concluded (1922), sadly incorrectly, that the Milky Way is elliptical with the Sun at center.
• Henrietta Leavitt discovered (1907-1912) the Period-Luminosity relation for Cepheid variable stars, thereby providing a reliable way to measure distances to other galaxies.
• Vesto Slipher successfully obtained spectra of spiral nebulae (1912-1914), and discovered that most galaxies were redshifted, some by as much as 1000 km/s, an unprecedentedly large redshift.
• Harlow Shapley argued (1920) that the system of globular clusters marked the center of the Milky Way in Sagittarius, and that the Sun was a long way from the center.
• Edwin Hubble discovered (1923-1924), with the 100 inch telescope, Cepheid variables in the Andromeda Nebula, thereby establishing an accurate distance that demonstrated that Andromeda was a galaxy well outside the confines of the Milky Way.
• Following theoretical work by B. Lindlbad, J. Oort (1927), by measuring redshifts of stars in the Galactic neighbourhood of the Sun, demonstrated that the Galaxy is rotating about a center in Sagittarius.

Our Galaxy, the Milky Way

• Structure of the Milky Way, a barred spiral galaxy
  • bulge
    • Barred bulge
    • A black hole at the center
  • disk
    • Gas
    • Stars
    • Spiral arms
  • halo
    • Globular clusters
    • Dark Matter
    
    

    A model of the Milky Way, reconstructed by Bissantz et al. from near infrared observations from the Cosmic Background Explorer (COBE) satellite.

• Computer simulation of the evolution of the Milky Way. The Milky Way today is not quite as stately as you may have thought!

• Satellite galaxies in orbit around the Milky Way
  • The Large and Small Magellanic Clouds - each about 1/100 of the mass of the Milky Way
  • About 10 "dwarf spheroidal" galaxies - each about 1/10,000 of the mass of the Milky Way

    Sculptor dE, the first dwarf elliptical satellite of the Milky Way discovered, by Silvia Mussels in 1937 (not by Harlow Shapley as commonly stated) © the Royal Observatory Edinburgh and the Anglo-Australian Observatory

    
    

    Sagittarius dE, 5° × 10° on the sky, but discovered only in 1994 by the Automatic Plate Measuring machine. © David Martinez-Delgado (MPIA) & Gabriel Perez (IAC)

• Globular clusters
  • Gravitationally bound clusters of 10,000 to a million stars
  • About 120 in the halo of our Galaxy
  • Concentrated on the sky broadly towards Sagittarius, the first clue that the center of our Galaxy is in Sagittarius
  • The oldest objects in the Galaxy - at 12-14 billion years (from their HR diagrams), almost the age of the Universe
  • Almost primordial composition, 75% Hydrogen, 25% Helium, with almost no heavier elements (whereas the Sun contains about 2% heavy elements, produced by nucleosynthesis in stars and supernovae)

• Most of the rest of the visible (at any wavelength) Galaxy is in a disk
  • The Milky Way at many wavelengths

    1-4 µm near infrared emission from stars penetrates most interstellar gas.

    1-5 nm X-ray emission from shock-heated hot gas. Point sources are mostly x-ray binaries.

    21 cm radio emission from warm diffuse atomic hydrogen gas.

    2.6 mm radio emission from carbon monoxide reveals cool dense molecular gas.

• Star-gas-star cycle.
  ® stars
   ® supernovae and stellar winds
    ® hot gas
     ® warm diffuse gas
      ® cool dense molecular gas
       ® stars
  • Notice the driving role of gravity

• "We are star stuff" (Carl Sagan).
  • The heavy elements (C, N, O, Si, Fe, ...) of which you and the terrestrial planets are made were nucleosynthesized in stars and supenova explosions, and thrust back into space by supernovae and stellar winds.
  • Carbon, the element of life, was synthesized in He burning (the triple alpha reaction)
  • Iron, and all the very heavy elements, were produced by thermonuclear supernovae

• Spiral structure
  • Star forming regions and hot young stars are observed to be concentrated in spiral arms
  • Older stars are more uniformly distributed in a disk
  • The spiral galaxy M81 at many wavelengths

    Ultra-violet shows hot young stars, in star-forming spiral arms

    3.6 µm near infrared shows stars distributed throughout the galaxy

• The rotation of the Galaxy
  • Modern measurements of Galactic rotation use precision measurements of the redshift of 2.6 mm carbon monoxide radio emission

Velocity map of carbon-monoxide in the Milky Way.

Week 12 (Nov 8)

Galaxies

• The Hubble expansion of the Universe. Probably the single most important observational result about galaxies.

  Hubble's original (1929) diagram.

  
  
  • Hubble law:

    v  =  H0   d Recession velocity  =  Hubble's constant  ×  distance

  • Indicates that there was a Big Bang.

  • Yields the age of Universe = 1/H₀ = 14 billion years.

• The recession velocity v of a galaxy is measured, relatively easily and accurately, from its redshift z

  z  =  lobs - lem lem redshift  =  observed wavelength - emitted wavelength emitted wavelength

  • The recession velocity is related to its redshift by (for velocities much less than the speed of light)

    v  =  c   z Recession velocity  =  speed of light  ×  redshift

• The distance d to a galaxy is much harder to measure precisely. The principal method is to use "standard candles", objects whose luminosities are known precisely. There is no perfect standard candle in astronomy, but the best are:
  • Cepheid variable stars (visible out to the Virgo cluster)
  • Thermonuclear supernovae (visible to cosmological distances)

• Hubble diagram of thermonuclear supernovae (Perlmutter & Schmidt 2003)

• The Hubble expansion is not perfect. Small ripples in the initial smooth distribution of matter in the Universe grew by gravity, collapsing into galaxies and groups of galaxies.
  • Redshift surveys of galaxies reveal a "cosmic web" of structure.
  • The large scale structure shows a characteristic scale of about 100 Mpc (300 million lightyears). This is the imprint of the scale of the horizon at Recombination.
  • A zoom into the Millenium simulation (Volker Springel, using Gadget computer code).

• The Local Group of galaxies is the local region of the Universe that has turned around from the general Hubble expansion, and is beginning to collapse for the first time.
  • The Local Group is about 1 Mpc across.
  • The Local Group contains about 40 galaxies:
    • M31, the Andromeda galaxy;
    • The Milky Way;
    • M33, the Triangulum galaxy;
    • various smaller irregular and dwarf spheroidal galaxies.
  • Computer simulation of the Local Group of galaxies by Klypin et al. 2003.
  • M31 is approaching the Milky Way at 300 km/s, and we may collide in 5 billion years time. Simulation.

• The Local Supercluster of galaxies is our piece of the cosmic web.
  • The Local Supercluster is about 100 Mpc across.
  • It contains thousands of bright galaxies.
  • The Local Group is on its outskirts.
  • At the center of the Local Supercluster is the Virgo cluster of galaxies.
  • Computer simulation of the Local Supercluster of galaxies by Klypin et al. 2003.

• Galaxies routinely collide.
  • When galaxies collide, they feel each other's gravity, but the stars do not collide.
  • Computer simulation of the Antennae galaxies. Simulation.
  • Computer simulation of the Cartwheel galaxy. Simulation.

Dark Matter