Physics 3250: An Introduction to Astrophysics....Spring 2008

• Monday 14 January at 4 p.m. in 1110 Rood Hall - I will be giving the Physics Department Colloquium, Quasars and the Birth and Evolution of Galaxies; refreshments served in Bradley Commons (2202 Everett Tower) at 3:30 p.m. You are all invited to attend (optional, of course).
  
• The first meeting of the Cosmic Cafe is Friday 18 January, right after class, at the Coffee Works.
• Monday 21 January: MLK Holiday, no classes.
• A public, cosmic presentation by yours truly, Friday 1 February at the Kalamzoo Astronomical Society meeting (free and open to all), 4th floor of the Old Central H.S. on Vine St. and Westnedge. Use the Dutton St. entrance and walk up the stairs.
• First exam covering Chapters 2,3,5 (and the main ideas from assigned readings of Chapter 7, although nothing specific from the radial velocity plots from Chapter 7 - to be covered on the Final Exam) will be on Wednesday, 6 February. You need a few pencils, an eraser, and a calculator.
• Monday 11 February at 4 p.m. in 1104 Rood Hall - The Mars Exploration Rover, by Dr. Ray Arvidson of Washington University in St. Louis. He will discuss the findings of the Spirit and Opportunity Mars rover missions.
  
• The second meeting of the Cosmic Cafe is Friday 15 February, right after class.
• February 20 - a total lunar eclipse! For details, see this webpage.
  
• Friday 29 February - Sunday 9 March: "Spring" Break
• Second exam covering Chapters 8,9,11.2 (through the top of p. 370) will be on Friday 21 March. You need a few pencils, an eraser, and a calculator.
• The third meeting of the Cosmic Cafe is Friday 28 March, right after class.
  
• Harlow Shapley Lecturer, Christine Jones from Harvard Center for Astrophysics, to give a colloquium on Monday 24 March at 1 p.m. (1110 Rood Hall) and a public lecture on Tuesday 25 March at 7 p.m. (1104 Rood Hall).
  
• The final meeting of the Cosmic Cafe (aka Cosmic Pizza at Bilbo's Pizza) is Friday 18 April (last day of class), right after class.
  
• Final Exam: Tuesday 22 April, 12:30-2:30 p.m. It will be comprehensive, with emphasis placed on material covered in the last unit. You need a few pencils, an eraser, and a calculator.

On-line Unix help

• Project CLEA homepage: laboratory and virtual observatory exercises in astronomy; freeware for your PC
• Stellar Evolution of 0.1-7 solar mass stars here, just plug and chug...And here too, covering 0.1-100 solar mass stars and several abundance sets. Feel free to ask me about them, if you're interested.
  
• Some really cool JAVA animation demos in astronomy are here & here.
  Here is a JAVA animation on eclipsing binaries (note: it doesn't get the light curve right for eclipsing stars of different temperatures when the angle is set to 0 degrees.), and here is another one demonstrating spectroscopic binaries (radial velocity curves).
• Here is another set of really neat JAVA animations of astrophysical phenomena: Dynamical Astronomy JAVA lab
  

Here below, I've collected some of the latest images from our advanced telescopes (ground and space-based), plus a few animations and some really cool computer simulations to illustrate cosmic phenomena. These are for your casual use and perusal to help your appreciation of the cosmos - nothing below is required knowledge for this course. You'll note that I also include stuff about galaxies and cosmology. Some day, I hope to offer a second course in astrophysics covering those topics. Any how

``Listening'' to the story of the cosmos

• Overhead view of Yerkes Observatory in Williams Bay, WI, and the 40-inch refractor here and here
• The Cerro Tololo Interamerican Observatory (Chile) 4-meter dome and beautiful night sky
• The Very Large Telescope Array (Chile) and one of its 8.2-meter telescopes
• A cartoon of the twin 10-meter (segmented mirror) Keck I & II Observatories, atop Mauna Kea, Hawaii, and a photograph of these two observatories plus the 8-meter Subaru - note the ocean of clouds lying below
• The new MMT telescope (6.5-meter) in southern Arizona
• The Very Large Array Radio Observatory in New Mexico
• Three photos of the Hubble Space Telescope: in the Shuttle bay for repairs, in action, and why astronauts cannot suffer from vertigo, the HST within the Shuttle bay for repairs, 600 km above the western coast of Australia
• A movie of speckle images of the bright star Betelgeuse, 30 msec per frame, demonstrating atmospheric "twinkling"
• A movie from the Keck Telescope showing the Galactic Center before and after the adaptive optics is turned on, removing the effects of atmospheric "twinkling". Here is another.
• Optical interferometery techniques allow astronomers to measure accurate diameters of hundreds of stars. Here observations of some of the smallest known stars is compared to theoretical predictions of radius vs. mass for stellar ages 400 million years (red dashed) and 5 billion years (black solid). Jupiter - perhaps akin to a failed star (brown dwarf ) - is shown for comparison (solid triangle). The brown dwarf stars occupy the region of the diagram where the curves are roughly horizontal. Here is the press release with some futher details. Speaking of brown dwarf stars, this is an illustration comparing our Sun, an M dwarf star 75x mass of Jupiter, L and T class brown dwarf stars 65x and 35x the mass of Jupiter, and Jupiter itself. The same sequence is shown here, as they would appear (roughly) to eyes sensitive to IR wavelengths.
  

Star birth, life, & death

• as it appears at visual wavelengths: the photosphere, and here
• an extreme close-up of the Sun's photosphere: sunspot and granules
• a gif movie of a sunspot complex and granules in motion
• sunspots in motion, showing the Sun's rotation; here is another movie
• this is a plot showing the sunspot cycle: monthly average number vs. time from 1750 to the present.
• the Sun's upper atmosphere observable during eclipse
• observing the chromosphere in the light of hydrogen-alpha (electron hops from the third to the second energy level of hydrogen), 656.3 nm here and here
• chromospheric spicules, in the light of hydrogen-alpha
• the chromosphere in the light of ionized calcium, 393.4 nm
• A gif movie showing a series of narrow band filter images from the Dutch Open Telescope that show how a region near a sunspot changes with height, from the photosphere to over 1000 km above into the chromosphere.  Explanation: The light of different absorption lines in the solar spectrum escapes at different heights in the solar atmosphere. This property enables us to observe solar phenomena simultaneously in multiple layers. The three images that constitute this morph were taken in the blue G band (wavelength 430.5 nm, due to CH molecules), the violet Ca II H line (396.8 nm, once-ionized calcium atoms), and the red Balmer Halpha line (656.3 nm, neutral hydrogen atoms). They show three slices of solar active region AR10675 on September 29, 2004. The G band shows the photospheric solar surface covered by convective granules and tiny bright magnetic elements between these, as well as a sunspot about as large as the Earth (inset). The strong Ca II H line samples the low chromosphere (a few hundred kilometers higher up) in which the granulation appears reversed intensity and magnetic elements appear considerably brighter. The Halpha line shows fibrils in the high chromosphere (at a few thousand km in height) that are obviously controlled by magnetic fields. Many field lines originate in the spot and the magnetic elements and connect elsewhere.
• the transition zone, in the light of ionized helium, 30.4 nm
• the outer corona, in visible light (during eclipse)
• the corona in the light of three emission lines: 8x,9x ionized iron (17.1 nm), 11x ionized iron (19.5 nm), plus 14x ionized iron (28.4 nm)
• coronal loops in extreme UV light, also here
• the corona in continuous X-ray light
• an mpeg movie showing images of our Sun across the electromagnetic spectrum, from the photosphere up into the corona. These images were taken at approximately the same time. Notice the brightest active regions in the Sun's hot upper atmosphere correspond to the darker/cooler sunspots that lie beneath in its photosphere.
  
• a cartoon showing the major outer structures
• large solar prominence in the light of ionized helium, 30.4 nm here
• a solar flare event in the light of ionized helium, 30.4 nm
• an mpeg movie of our Sun's active corona (November 2000); here is another during October-November 2004 in gif format in the light of 17.1 nm 8-9x ionized iron; and this is another (very large mpeg movie) in the light of 11x ionized iron (19.5 nm) during October-November 2003 in which historically enormous coronal mass ejection and X-ray flare events took place.
• the magnetic carpet: what powers the Sun's chromosphere and corona
• the changing Sun: X-ray images of the Sun's corona over the 11 year magnetic activity cycle
• a cartoon of the solar wind interacting with Earth's magnetic field
• some pictures of aurorae: 1, 2, 3, 4, and here in SW Michigan (October 28, 2000)
• a photo of an aurora taken by astronauts on-board the International Space Station; note the thin blue veil that is the Earth's lower atmosphere (troposphere and stratosphere), and that aurorae occur Earth's tenuous upper atmosphere. The white circular feature on Earth's surface is a snow-filled 212 million year old impact crater in northern Canada.

• links to pages on aurorae (and predictions thereof) and other Sun-Earth interactions
• links to pages containing information, high resolution pictures and movies of our Sun
  

• a cutaway view of the Sun's interior
• and another one...
• the Sun rings like a bell. A discussion of how astronomers use this to probe the interior of our Sun, and how we know how stars work is found here.

• A temperature sequence of stellar spectra O,B,A,F,G,K,M
• The spectrum of Arcturus, a K1 giant star
• Our Sun's spectrum, a G2 main sequence star, and an exploded view.  Another representation of our Sun's spectrum. The vertical axis measures the observed intensity just above Earth's atmosphere, the horizontal scale measures wavelength in nanometers (nm). The ultraviolet (UV), visible and infrared (IR) portions of the Sun's spectrum are marked. Note the many absorption lines (narrow downward 'spikes') formed in its photosphere and lower atmosphere, where atoms and ions absorb light at particular wavelengths (photons of particular energies). Note also the overall shape of the Sun's spectrum is a good match to the spectrum of a simple thermal radiator of temperature T = 5777 K (in green). The equivalent in total energy of this thermal radiation spectrum is emitted within the Sun's photosphere.
• The spectrum of Procyon, a F5 main sequence star

• Our Galaxy, the Milky Way
• A dark cloud toward the center of our Galaxy
• Another dark, cold (10 K), molecular cloud, some 520 light years away and about 0.65 light year across, is partially transparent at infrared wavelengths. This is demonstrated in a dramatic way in this image, which shows the same image through each of the 6 broad-band filters separately - from 440 nm (blue visible light), to 550 nm (green visible light), and then 900 nm, 1250 nm, 1650 nm and 2160 nm in the infrared (the filter central wavelengths are given in units of microns = 1000 nm in the figure). This is a map measuring the extinction of light through the cloud. The first and outermost contour represents a loss of visible light at 550 nm by a factor of 40. Each contour inside is a step up in visible light extinction by a factor of 6.2. The innermost contour represents a factor of 10¹⁴ loss of visible light!
  
• a tiny grain of cosmic dust, just 10 microns across (size of a human white blood vessel)
• lots of molecules are found in the cosmos, and many of these are the organic building blocks of life
• The constellation of Orion.

• The Orion Nebula, nearest stellar nursery, at visual wavelengths, and also here. The Orion star forming complex is just ~1 million years old, and 1500 light years away.
  
• Hubble Space Telescope visible and near-IR image of the Orion Nebula. This view is 13 light years across.
  
• The central Trapezium region of the Orion Nebula at infrared wavelengths, and a broader view at infrared wavelengths; here's another infrared  image of the Orion starforming complex.
  
• The Orion Nebula's Trapezium, visual vs. infrared, as observed by the Hubble Space Telescope. Note the large numbers of stars that come into view at infrared wavelengths - infrared light is not as easily blocked by dusty gas as is visible light. The dimmest points of light in the infrared image are "Brown Dwarfs", not massive enough to become stars.
• Two image galleries (1, 2) of solar systems forming around new stars in the Orion Nebula, and a cartoon. The dark, dusty disks might be forming planets.
• The Horsehead dark nebula in Orion; the bright star on the left is the leftmost star of Orion's belt - seen here in a spectacular style. The three bright stars of the "belt of Orion" are Alnitak, Alnilam, Mintaka.
• Short wavelength (blue) light emitted by stars scatters easily from dust grains within surrounding gas clouds - these dusty clouds reflect starlight, especially blue light, and are known as reflection nebulae.
  

• The Eagle Nebula, as imaged at visible wavelengths by the Hubble Space Telescope, and a close-up of one of the "pillars" showing the photo-evaporating gaseous globules, and a broad view of the surrounding region, including the main star cluster, at near-infrared wavelengths - note the near transparency of the Eagle Nebula at these wavelengths. Here is an unbelievably beautiful visible light, 20 light year wide, view of the molecular cloud complex surrounding the Eagle Nebula. Narrow band green, blue and red filters were used to sort out emission from hydrogen atoms, twice-ionized oxygen atoms, and singly-ionized sulphur atoms, respectively. This star forming region is 6500 light years away.
• Here is the Cone Nebula star forming complex, 2500 light years away. Note the newly formed stars emerging from the cone-shaped molecular gas near the bottom of the diagram. A close-up from the Hubble Space Telescope is here; the "cone" is about 1 light year across.
• Just 2 light years across and perhaps just a few hundred thousand years old, a brand new stellar nursery lying in the constellation Cygnus, as seen in near-infrared light
• 50 light years across and extremely young, a newly emerging cluster of very young, massive stars in the nearby dwarf galaxy, the Large Magellanic Cloud. And here is NGC 346, 200 light years across and 210,000 light years distant in the Small Magellanic Cloud, 3-5 million years old..
  
• A gargantuan star formation region & star cluster (some 1500 light years across) in a nearby spiral galaxy called M33. An even better view is here.
• Perhaps the largest star forming complex in the local universe, in the nearby dwarf galaxy the Large Magellanic Cloud 160,000 light years away, the Tarantula Nebula Star forming complex.  Shown is just the central region; images of the whole complex lie in the "Star Clusters" section, below. If it were at the distance of the Orion Nebula (1500 light years), it would be visible in the day time and span about 1/4 of the sky!
• The Trifid Nebula, an emission (red part) and reflection (blue part) nebula
• The Rosette Nebula; here's another view
• The Keyhole Nebula in Carina
• A young stellar cluster in a star forming region (NGC 3603): VLT near-infrared view
• Young stars with dusty gas disks around them: 1,2,3,4,5. When the disks are observed from the side, they are dark when viewed at visible wavelengths and bright when viewed at infrared wavelengths. Why might that be?

• Near the end of a medium mass star's life, unstable hydrogen and helium fusion occur within two concentric shells surrounding a shrinking inert core. The result is that the star gradually ejects its outer envelope in multiple ejection episodes (note the concentric expanding spherical shells), leaving behind a dense remnant known as a white dwarf (the point of light at the center).
  
• Hubble Space Telescope image of newly emerging white dwarf star and planetary nebula
• 20 years before this image was taken by the Hubble Space Telescope in 1996, no planetary nebula was present in Henize 1357 (now dubbed the Sting Ray Nebula). 18,000 light years away, the nebula spanned 130 solar system diameters in 1996 and continues to expand today.
• Along with collaborators, George Jacoby and Gary Ferland, I did research on this planetary nebula, lying about 7000 light years away. A hot (150,000 K) white dwarf lies at the center of this star's former envelope - now an expanding spherical shell 5.5 light years across (a corresponding angular diameter of 160 arc seconds).....a death shroud. This is our Sun's fate, 7 billion years hence. Spiral galaxies at enormous distances may be seen lying in the background of this image.
• Hubble Space Telescope image of planetary nebulae: the Ring Nebula and NGC 6369. This deep exposure image of the Ring Nebula, taken by a ground-based telescope, shows the multiple ejection episodes that the central star underwent to become the white dwarf (at the center of the ring).  Note the distant galaxies in the background.
  
• Here are some more pictures of the death shrouds of low/medium-mass stars: the Dumbbell Nebula from the VLT and WIYN, Helix Nebula, Spirograph Nebula, and the hour-glass shaped Ant Nebula. The cores of the dead stars (aka white dwarfs) lie at or near their centers.

• Hubble Space Telescope image of the red supergiant Betelgeuse
• Hubble Space Telescope image of massive star blowing a hot, ionized wind
• Hubble Space Telescope image of the dying supermassive star, Eta Carinae, and its surroundings at visible and infrared wavelengths. An explosion in the 1846 produced the twin lobes of expanding gas, and temporarily made it one of the brightest stars in the sky.
• Here is a computer simulation of an exploding star or "supernova"
  
• VLT image of the supernova remnant, the Crab Nebula in the constellation of Taurus. The expanding glowing gas is the former envelope of a star that exploded in the year 1054 (as observed on Earth), and lies 6300 light years away. The star's remnant core is the dimmer of the pair of stars near the center of the nebula, and lies at the center of this extreme close-up shown in this composite image from the Hubble Space Telescope (visible light: red) and Chandra X-ray  Observatory (X-rays: blue). It is a rapidly rotating (30 times per second!) neutron star, called a pulsar. Here is another spectacular image from the Hubble Space Telescope, and a composite image showing X-ray (Chandra X-ray Observatory: blue), visible (Hubble Space Telescope: green), and infrared emission (Spitzer Space Telescope: red).
• This star, called Cassiopeia A, exploded in 1680 (as observed on Earth); it is shown here  in three X-ray "color" (energy) bands, and it lies 10,000 light years away. A several million degree neutron star lies at the center of the explosion. A second observation, with an alternative color scheme, is shown here.
  
• Pages containing images of Supernova 1987A, the most recent nearby example of an exploding star.
• the Veil Nebula: interstellar gas shocked (compressed/heated) by the blast wave of a star that exploded 5000 years ago, 1440 light years away in the constellation of Cygnus
• Also in Cygnus, but 6000 light years away, seen in 25 and 60 microns (mid-infrared) and 21 and 74 cm (radio) - the cosmic dance of star birth and death and the recycling of the interstellar medium. The larger hollow shells of gas are supernovae ejecta (expanding exploded star envelopes), including the brownish one in the lower left. The bright white knots are stellar cocoons of star formation. The red dots are some immensely distant, yet enormously luminous, quasars (we're not covering these objects in this course).
• A drawing of an accretion disk surrounding a black hole, and another one here.

• WWW links to sites tutoring in relativity, neutron stars, black holes, gravitational lenses, etc

• The ages of several open (or galactic) star clusters in the Milky Way galaxy determined by two independent means: main sequence "turn-off" and the time it takes white dwarf stars to cool to a particular temperature. The straight line indicates an equality between the methods. It is important to note that the types of stars used in and the physics involved in the two methods to determine the star cluster age are completely different.
  
• At the center of the enormous Tarantula Nebula star forming complex, 30 Doradus: a dense cluster of newly born massive stars (just 2-3 million years old) in the Large Magellanic Cloud (a dwarf irregular galaxy nearby the Milky Way). Another spectacular view...This is one of the largest (more than 1000 light years across) and most active stellar nurseries known in the local universe.
• Lying near to 30 Doradus in the Large Magellanic Cloud, Hodge 301: a young, 20 million year old, open star cluster
• Visible to the naked-eye (just 430 light years away), a 100 million year old open star cluster: the Pleiades contains well over a thousand stars, and another view (courtesy of Matthew T. Russell). The "spikes" of light surrounding the bright stars are due to the diffraction of light at the secondary mirror supports, and their appearance has been enhanced in these two images for artistic effect.
  
• Here is another open star cluster 5000 ly distant and 250 million years old: M11
• Ancient globular clusters M80,  M10, M13, and giant 47 Tucanae, with 100,000 to a few million stars! Typically, globular clusters lie 15-50,000 light years away in the "halo" of the Milky Way. Here is  M3, 34,000 light years from Earth and 180 light years across, it contains a half million stars.
• The core of the most massive globular cluster belonging to our Galaxy: Omega Centauri. This view spans just 13 light years and contains over 50,000 stars. The full cluster contains 10 million stars, is 150 light years across, and lies 15,000 light years away. It may actually be a captured dwarf elliptical galaxy.
• The "nearby" Andromeda galaxy has open and globular star clusters, too. Why do you think the globular clusters of the Andromeda galaxy have angular diameters that are on average 100 times smaller than the globular clusters belonging to our Galaxy?

Galaxies and Cosmosolgy

• The Milky Way as it appears from the northern and southern hemisphere skies
• NGC 1288: a good match to the shape and size of the Milky Way
• spiral galaxy NGC 4565, observed edge-on
• very dusty, edge-on spiral NGC 891
• near-infrared all-sky image of the Milky Way. See any similarities to the above two galaxies?
• An artistic yet scientific rendition of our Galaxy, also showing the relative location of the Sun (illustration courtesy: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)).
  
• An infrared view toward the Galactic Center, and 1 million stars in the direction of the Galactic Center
• A view of the nucleus of our Galaxy, at 90 cm radio wavelengths
• Two young star clusters with VERY massive stars near the Galactic center, at infrared wavelengths. The Pistol Star (brightest orange one in right panel) is at least 100 solar masses with a luminosity of 10 million Suns! It may have lost nearly half its mass through an enormous wind.
• Comparisons of a piece of our Sun's spectrum (at very high spectral resolution) with those of two ancient stars of the Milky Way with extremely low heavy element abundances
• a gif animation demonstrating the observational properties of Cepheid variable stars; here is a movie of RR-Lyrae variable stars changing their intrinsic brightnesses as their outer layers pulsate, all lying within the globular cluster known as M3. All RR-Lyrae stars have the same average luminosity of about 45x the luminosity of the Sun, while the more luminous Cepheid variable stars follow a Period - Avg. Luminosity relationship. Both are used to determine distance within the Milky Way and beyond.
• A graph illustrating the effects of a "dark matter halo" within which the Milky Way galaxy and most galaxies are embedded.
  

• more than 2 million light years distant the nearest spiral galaxies: Andromeda (M31) and the smaller M 33. The central bulge of M31 is just visible to the unaided eye in good viewing conditions, and the whole of the galaxy's visible disk spans approximately 190 arcminutes (11,400 arcseconds, about 3 degrees or 6 widths of the Moon) in the sky. This is what the Andromeda galaxy looks like in the light of dust grains with gas clouds heated by the absorption of visible and UV light of stars - that is, thermal radiation at a wavelength of 24 microns (24,000 nm).  And finally, the same galaxy looks like this in ultraviolet wavelengths. This light is dominated by hot, luminous, massive main sequence stars in regions of active star formation. 
  
• a nearly "perfect," grand-design spiral, M74, at a distance of 30 million light years.
• some more beautiful spiral galaxies: M 51 or the "whirlpool galaxy" (here and here - the large red areas are star-forming regions with hot main sequence stars emitting lots of energetic UV light that excites/ionizes nearby gas to produce emission lines - in this case the red line of the hydrogen Balmer series, and here is a side-by-side comparison of M51 at visible vs. infrared wavelengths: "blue" = starlight, "red"=mainly hot dust within gas clouds), NGC 2997, NGC 2997 from the VLT, M100, M101 from the ground and from the Hubble Space Telescope 25 million light years distant, NGC 1232,  M104 - a massive spiral galaxy 30 million light years away - from the VLT and another view from the Hubble Space Telescope and yet another in infrared light from the Spitzer Space Telescope (blue = stars, red = star-heated warm dust inside gas clouds), a barred spiral M83, M83 from the VLT, and distinctly "barred" spiral galaxies NGC 7424 and NGC 1300. The last of these lies 70 million light years away.
• A member of the Local Group, dwarf elliptical Leo 1
• The giant elliptical galaxy: M87 and accompanying globular clusters in the Virgo Cluster.
• The Milky Way's two neighbors, dwarf irregulars: Large Magellanic Cloud at 169,000 ly distant and Small Magellanic Cloud at 200,000 ly distant
• another dwarf irregular NGC 6822 in the Local Group, 1.5 million ly distant
• another irregular in the nearby M81 Group: M82; large numbers of newly formed stars in the galaxy's center blow a "superwind" (shown in red, as glowing hydrogen gas in the previous image), producing a lot of dust
• two members of a nearby small group of galaxies: the M81 group, 12 million light years away (M82 is the one on the right)
• About 50 million light years away, the most nearby cluster of galaxies: the central regions of the Virgo cluster, and the view of a much more distant cluster called Coma, 320 million light years away.

• 2 colliding spiral galaxies, 65 million light years away, also as observed by the Hubble Space Telescope, and also here at infrared wavelengths which penetrate the dusty star forming regions. Note the brand new super star clusters, which formed within colliding gas clouds. Here is a an MPEG movie of a computer simulation of their collision (credit: John Dubinski, CITA). Note that this is NOT a cartoon. It is an actual simulation of what happens when two spiral galaxies collide; all of the masses follow the laws of motion and gravity.
• more galaxian collisions: the Tadpole (420 million light years away; tail of stars is 280,000 ly long) and Mice galaxies (300 million light years away).There are over 4000 background galaxies in the Tadpole image that spans an area in the sky equal to the angular area spanned by a dime at a distance of about 53 feet. The two colliding galaxies in the `Mice' pair were nearly identical spiral galaxies that began their dance roughly 160 million years ago. Here is a computer simulation spanning several hundred million years showing how it all happened and the fate of these two clashing titans (courtesy Josh Barnes & John Hibbard). If you've a high speed internet connection, you might give this monster (23 Mbyte) simulation of the same collision a try. Note the many stellar arcs that form around the central, mostly spherical, star concentration. This is reminiscent of the bizarre giant elliptical galaxy, Centaurus A just 13 million light years away; see the star rings here.
  
• the result of two colliding galaxies: the polar ring galaxy
• two more galactic wrecks, 500 and 600 million light years away: the Cartwheel galaxy (and a multiwavelength view: purple X-rays, blue ultraviolet, green visible and red mid-infrared) and Hoag's galaxy
• 2 MPEG movies (1, 2) representing computer simulations of a possible collision between the Milky Way and Andromeda Galaxies, 3 billion years hence (credit: John Dubinski, CITA). A higher resolution (10.7Mbyte) version is here.
  
• And if you just can't get enough of watching 2 spiral gladiators destroy themselves (and become an elliptical galaxy), here is another MPEG movie, courtesy of the Max Planck Institute for Astrophysics.
• These simulations are set to music!
  

• A spiral galaxy NGC 3370 in the constellation of Leo, lying 98 million light years away. Its angular diameter is approximately 200 arc seconds, whereas the Milky Way's neighboring spiral galaxy M31 (linked above) spans 11,400 arc seconds at a distance of 2.5 million light years. Note also the many galaxies of much smaller angular diameters - more distant still. NGC 1309 is another beautiful spiral galaxy, 100 million light years away, with very distant galaxies visible in the background.
  
• giant elliptical NGC 4881 and a spiral galaxy in the Coma Cluster, 320 million light years away
• Two dense clusters of galaxies Abell 2218 and Abell 1689. Both are about 2 billion lyrs distant and demonstrate the warping of space due to the masses of these galaxy clusters - gravitational lensing, as predicted by Einstein's theory of General Relativity. Here is another demonstration of this strange phenomenon.
  
• The distant dense galaxy cluster, Cl0024+1654 (4 billion light years away) is shown in red. Astronomers used the distorted images of roughly 7000 background (more distant) galaxies to construct a map of the dark matter  distribution, shaded in blue. The dark matter in this cluster represents 80-85% of the total matter! These distortions are caused by the warping of spacetime (gravitational lensing) due to the presence of the huge amount of mass in this galaxy cluster. This shows that the dark matter is (1) highly concentrated toward the center of this cluster, and (2) that it follows closely the visible mass distribution of the galaxies themselves. The area colored light blue near the cluster's center is the hot X-ray emitting gas as observed by the Chandra X-ray Observatory (the nucleus of this enormous galaxy cluster in visible light is shown here; note the blue arcs of light: gravitationally distorted images of more distant galaxies). The full image represents an area of the sky equal to that of the full Moon.
  
• A very distant (7 billion lyrs) galaxy cluster, showing many galaxies are interacting. Galaxy interactions on this scale are rare in the local universe (here and now). 

• The 2dF galaxy survey: a census of 1/4 million galaxies within two thin wedges in the sky (each dot = 1 galaxy, but on the scale of this map galaxies are physically smaller than the dots). We lie at the intersection of the two wedges.
• The Sloan Digital Sky Survey:  the positions of roughly 200,000 galaxies within two thin wedges in the sky plotted with distance in megaparsecs (Mpc) (each dot = 1 galaxy, as above), within 2.8 billion light years of Earth located at the intersection of the two wedges.  Here is same, but showing the scale in units of lookback time (how long ago light we observe now left the source) in years. The final survey will include approximately 1 million galaxies and their redshifts, resulting in a 3-dimensional census of galaxies covering approximately one-quarter of the sky out to several billion light years.
• The 2MASS infrared all-sky survey of 1.6 million galaxies. This snapshot of the sky was taken through 3 infrared filters: blue, green and red have been coded to represent light at wavelengths of 1.2, 1.6, and 2.2 microns. This image shows the same kind of structure of walls and voids, as the 2dF and Sloan galaxy surveys just above, except that the galaxies appear flat against the sky (distance information has been suppressed). The "blue" band running through the middle is infrared light from stars within the disk of our Galaxy.
• A map of our "local" universe, showing galaxy superclusters within 1 billion light years of the Milky Way Galaxy whose home supercluster is called "Virgo", located at the center of this map.
  

• Hubble Deep Field North: This image taken by the Hubble Space telescope in December of 1995 is one of the most important ever taken of the cosmos. It looks back through 1-12 billion years in cosmic history - more than 2000 galaxies lie within this "blank" area of the sky equal to the angular area spanned by a dime at a distance of about 74 feet! A similar image acquired from the southern hemisphere sky in 1998 appears (in part) here. 
• Look at this image taken by a ground-based telescope of our nearest large neighbor, the spiral Andromeda galaxy (aka M 31; note also the two dwarf satellite galaxies), 2.5 million light years away.  Note the tiny green box with a couple of stars visible inside of it. It spans an area of the sky equivalent to that spanned by a dime at a distance of about 53 feet. Here is what the ACS camera on-board the Hubble Space Telescope recorded after collecting light over an 84 hour period within that tiny green box. There are hundreds of thousands of stars (most of which lie in the halo of M 31; a few of the brightest stars belong to our Milky Way Galaxy). There are also thousands of very distant galaxies, many of them spirals just like M31 and the Milky Way. Snapshot closeups of 6 representative areas of the image are found here. A bright globular cluster belonging to M 31 appears in the lower right panel. Finally, here is an mpeg pan across the image. 
• The Hubble Ultra Deep Field: From September 2003 to January 2004, the Hubble Space Telescope collected light through several broad-band filters across the visible and near infrared (centered on 4350, 6060, 7750, 8500 Angstroms, ACS camera) into the infrared (1.1 and 1.6 microns, NICMOS camera). Here is the image from its ACS camera - with 10,000 galaxies in a view looking back between 1 billion and 13+ billion years of time. This view spans an area in the sky equivalent to that spanned by a dime at 53 feet. Higher resolution versions of the image and further explanations may be found at links from here. This mpeg movie shows a zoom-in to the empty spot in the sky in the constellation of Fornax, and this mpeg movie pans across the image. Ned Wright's web site allows you to flick the ACS and NICMOS images back and forth.

• Images of representative, high redshift (z = 3, lookback time 12 billion years) galaxies, and a close-up of another one, observed at infrared wavelengths. Here are 18 separate galaxy building blocks, each a super star cluster spanning 2000-3000 lyrs across, and lying at the same high redshift (z = 2.4, looking back 11 billion years in time) within 2 million lyrs of each other within the Hubble Deep Field (North) image. Most of these likely later merged to form a single galaxy, just as we see these protogalaxies doing here. How do these galaxies compare to the ones found in the local universe (two sections above)? This series of images shows direct comparisons between the way representative galaxies appear at different stages in the history of the universe (here and now on the left; high redshift long ago on the right).
• The "spiderweb galaxy", at a redshift of 2, as it was assembling itself some 10.6 billion years ago. The width of the expanded view is approximately 300,000 light years.
• 28 protogalaxies as they were in a universe approximately 900 million years old (z ~ 6), from the Hubble Ultra Deep Field; each box spans about 30,000 light years on a side.
• a gravitationally lensed protogalaxy, as it appeared 13 billion years ago: just 500 lyrs across, containing just a few million solar masses of stars.
• A sketch showing two possible histories of star and galaxy formation in the universe (note that where it says "Milky Way galaxy forms" refers to when the present disk began organizing - the spheroidal component is older still), and an artististic rendition of the birth of stars and protogalaxies beginning perhaps 200 million years after the Big Bang (corresponding to a redshift z = 20). Astronomers are just now observing faint, irregularly shaped smudges of light at redshifts of 5-7. Future technology will allow for more detailed studies of the first gargantuan star clusters that served as the building blocks for protogalaxies and galaxies. Maybe they'll look like this nearby starbursting irregular galaxy, 13 million lyrs away. This dwarf blue compact galaxy is just 59 million light years away, yet a large fraction of its stars are less than 1 billion years old (it also contains an old population of stars some 10 billion years in age).
  

• The evolution of large scale structure in the universe as governed by the force of gravity - all of the simulations presented below start at very large redshifts (z = 20-50) with very small fluctuations in the matter density. All are displayed in co-moving coordinates, i.e., those that expand with the universe and so this expansion is not displayed in the simulations.
• An MPEG movie showing the simulation of the gravitational clumping of matter (dominated by dark matter), within a co-moving volume of space spanning 140 million light years on a side. Note the filamentary structures that develope forming the walls of bubbles. It is along the interesections of the dark matter filaments that large galaxy clusters form.
• An MPEG movie of a simulation of the formation of a galaxy cluster within a co-moving volume of space spanning 14 million light years on a side. Note the merging of small structures to form larger structures.
  
• An MPEG movie of a simulation of the formation of a very large galaxy supercluster (the box spans about 100 million lyrs and is about 40 million lys deep). Here is a series of stills on the same size scale depicting the simulation of the formation of this galaxy supercluster (left panels), for redshifts of 2, 1, and 0 (top to bottom). Red represents high density, blue very low density, with orange, yellow, and green in between. The right panel shows the same for a different set of assumptions about the matter/energy content in the universe.
• Two more MPEG movies simulating the formation of galaxy clusters (1, 2); notice especially in the second how small structures collapse/form, and merge with other small structures to form larger structures, and so on.
• here is an imaginary fly-through of the dark matter "foam" at the present epoch, around which galaxies and galaxy clusters and superclusters formed and are now located.
• The development of large scale structure depends upon the actual mixture of matter and energy in the universe, as these comparisons at redshifts of 3, 1, and 0 (now) show. Four simulations of structure at each of the redshifts shows the density of matter (brighter = denser) for a slice of space with dimensions of 1 billion light years on a side. This serves as a consistency check on our observations that measure the matter/energy content as well as on ourmodels that predict how structure developes as a function of redshift. The model appearing in the lower left represents today's best case.

• the expanding raisin bread analogy to the expanding universe, and here is an "expanding neighborhood" analogy - everybody moves away from everybody else.
• This is a more detailed animation demonstrating the concept of how we observe galaxies in an expanding universe. The following describes the problem of defining a distance in an expanding universe. Two galaxies are 4 billion light years apart when the universe is only 1 billion years old (at the start of the animation). The first galaxy (on the left) emits a pulse of light. For example, this might be the first huge burst of star formation. Because space is expanding, the second galaxy (on the right) does not receive that pulse until the universe is 14 billion years old (the present time)! By this time the two galaxies are separated by about 28 billion light years. The pulse of light has been travelling for 13 billion years, so the view the astronomers residing in the second galaxy receive is an image of the first galaxy going through its initial big burst of star formation when the universe was just 1 billion years old and the two galaxies were separated by 4 billion light years. Astronomers residing presently in the galaxy on the right would observe the galaxy on the left to have a redshift of about z = 5.9 (courtesy: Richard Powell).
  

• The predicted products of nucleosynthesis during the first few 100 seconds or so after the initial expansion of the universe. The relative abundances of these light elements depend on the density of ordinary matter (protons, neutrons, electrons) at that time and thus on the value of the mean matter density today (increasing along the horizontal axis). These two plots (1, 2) show how protons and neutrons were converted into the lightest elements as a function of time during which the universe expands and the temperature drops. This one shows the same as the first with horizontal bars indicating measurements of these light elements. The vertical grey bar shows a common solution to for the observed abundances of all of the light elements, indicating the expected relative mean matter density in the universe.
• The cosmic background radiation (CBR) blackbody spectrum, from COBE (1989-1990)
• WMAP's all-sky view of the 1 part in 10,000 and smaller fluctuations in temperature of the CBR, representing very small inhomogenieties in the matter around which structure will form from gravitational instabilities - as shown here. 
• Looking back over the early history of the universe
• Simplified 2-D representations of possible 3-D geometries of the universe, depending upon the matter-energy density, and predicted vs. observed fluctuations in the CBR depending upon the geometry of the universe
• A "standard candle": Type Ia supernova - 10 billion solar luminosities at maximum light - going off just 50 million lyrs away in the central regions of a spiral galaxy within the Virgo Cluster (most of the galaxy is not shown here).

• WWW links to sites  tutoring in cosmology and structure evolution.