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Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the monthNovember 3, 2008 Dr. Uwe Trittmann Today’s Topics • Galaxies – Island Universes • The Night Sky in November Galaxies – Island Universes • A historic tour of the discovery of the dwindling significance of humans in the universe: • From the center of the universe towards the edge of in an average galaxy amongst 100 billion others How do we know where we are? • “Obviously” we are living on a flat Earth at the center of the universe, as a quick look tells us: – – – – The stars, Sun, Moon and planets rotate us There is no apparent curvature of the ground The Milky Way is a band that surrounds us There are no signs for any movement of the Earth (like wind, or forces throwing us off) Science to the Rescue • How do we avoid these wrong conclusions? – The data were sound – The interpretation not Further observations are necessary to decide! • Do we have to question everything? – Yes, in principle. – The signature of genius is to ask the right question, not necessarily to answer them. Exploring our own Island Universe: The Milky Way • A galaxy is a huge collection of stars, gas, dust, neutron stars, and black holes, isolated from others and held together by gravity Our view of the Milky Way • Appears as a milky band of light across the sky • A small telescope reveals that it is composed of many stars (Galileo again!) • Our knowledge of the Milky Way comes from a combination of observation and comparison to other galaxies How do we know? Obviously a bogus picture of our milky way! • Question: How can we say anything about our Milky Way, if we cannot see it from outside? Enter the Genius • William Herschel (XVIII century) • Simple model: – Assumed all stars have the same absolute brightness – Counts stars as a function of apparent magnitude – Brighter stars closer to us; fainter stars further away – Cut off in brightness corresponds to a cut off at a certain distance. • Conclusion: there are no stars beyond a certain distance Herschel’s Findings • Stars thinned out very fast at right angles to Milky Way • In the plane of the Milky Way the thinning was slower and depended upon the direction in which he looked • Flaws: – Observations made only in visible spectrum – Did not take into account absorption by interstellar gas and dust Discovering other Island Universes • Data: Lots of nebulous spots known in the nightsky • Questions: What are they? All the same? Different things? • Need more observations! Build bigger telescopes Famous Telescopes - Herschel Herschel detected Uranus (1781) (Uranus is visible with the unaided eye) Famous Telescopes – Lord Ross • 72 inch Reflector • built during potato famine in Ireland • Largest Telescope until Mt Wilson (1917) The first nebula discovered to have spiral structure: M51 Lord Rosse, 1845 HST: M51 Spiral Galaxy M99 is a spiral, too! • Q: do we live in a spiral? • Q: Are we in the center of the spiral? • Philosophical answer: probably not! Enter: next genius • Harlow Shapley used variable stars, e.g. RR Lyrae stars, to map the distribution of globular clusters in the galaxy • Found a spherical distribution about 30 kpc (30,000 pc) across – This is the true size of the galaxy • Sun is (naturally!) not at the center – it’s about 26,000 ly out Standing on the shoulders of Giants • Shapley used methods developed by others to measure the distance to globulars • Cepheid variables show luminosity-period correlations discovered by Henrietta Leavitt • Shapley single-handedly increase the size of the universe tenfold! Cepheids • Henrietta Leavitt (1908) discovers the period-luminosity relationship for Cepheid variables • Period thus tells us luminosity, which then tells us the distance • Since Cepheids are brighter than RR Lyrae, they can be used to measure out to further distances Properties of Cepheids or: How do I know if this star is a Cepheid? • Period of pulsation: a few days • Luminosity: 200-20000 suns • Radius: 10-100 solar radii Properties of RR Lyrae Stars or: How do I know if this star is of RR Lyrae type? • Period of pulsation: less than a day • Luminosity: 100 suns • Radius: 5 solar radii Cepheids and RR Lyrae: Yard-Sticks • Normal stars undergoing a phase of instability • Cepheids are more massive and brighter than RR Lyrae • Note: all RR Lyrae have the same luminosity • Apparent brightness thus tells us the distance to them! – Recall: B L/d2 Scientists are humans - Humans err • Shapley was so convinced by his own findings that he held the opinion that our galaxy IS the universe • He even won the great debate (1920) against Heber Curtis who correctly held the opposing view • See: http://www.aip.org/history/cosmology/ideas/island.htm Hubble supersedes Shapley • Edwin Hubble identified single stars in the Andromeda nebula (“turning” it into a galaxy) • Measured the distance to Andromeda to be 1 million Ly (modern value: 2.2 mill. Ly) • Conclusion: it is 20 times more distant than the milky way’s radius Extragalacticity! Shapley’s theory falsified! New Tools – New Discoveries Hubble in prime focus of Mt Palomar. Einstein visits Mt Wilson Hubble detected the Expansion of the Universe “Proof” of Einstein’s General Relativity Theory Structure of the Galaxy Q: How many galaxies are there? • Hubble Deep Field Project – 100 hour exposures over 10 days – Covered an area of the sky about 1/100 the size of the full moon • Probably about 100 billion galaxies visible to us! • About 1,500 galaxies in this patch alone • Angular size ~ 2 minutes of arc Other Galaxies • there are ~ 100 billion galaxies in the observable Universe • measure distances to other galaxies using the periodluminosity relationship for Cepheid variables • Type I supernovae also used to measure distances – Predictable luminosity – a standard candle • Other galaxies are quite distant – Andromeda (M31), a nearby (spiral) galaxy, is 2 million light-years away and comparable in size to Milky Way • “Island universes” in their own right Q: How does our galaxy look like from the outside? • Probably like others, so observe them! Hubble Classification Scheme • Edwin Hubble (~1924) grouped galaxies into four basic types: – – – – Spiral Barred spiral Elliptical Irregular • There are sub-categories as well Spirals (S) • All have disks, bulges, and halos • Type Sa: large bulge, tightly wrapped, almost circular spiral arms • Type Sb: smaller bulge, more open spiral arms • Type Sc: smallest bulge, loose, poorly defined spiral arms Barred Spirals (SB) • Possess an elongated “bar” of stars and interstellar mater passing through the center Elliptical (E) • • • • No spiral arms or clear internal structure Essentially all halo Vary in size from “giant” to “dwarf” Further classified according to how circular they are (E0–E7) S0/SB0 • Intermediate between E7 and Sa • Ellipticals with a bulge and thin disk, but no spiral arms Q: How do we know we live in a Spiral Galaxy? • After correcting for absorption by dust, it is possible to plot location of O- and B- (hot young stars) which tend to be concentrated in the spiral arms • Radio frequency observations reveal the distribution of hydrogen (atomic) and molecular clouds • Evidence for – galactic bulge – spiral arms Rotation of the Galaxy • Stars near the center rotate faster; those near the edges rotate slower (Kepler) • The Sun revolves at about 250 km/sec around the center • Takes 200-250 million years to orbit the galaxy – a “galactic year” How do spiral arms persist? Why don’t the “curl up”? “Spiral Density Waves” • A spiral compression wave (a shock wave) moves through the Galaxy • Triggers star formation in the spiral arms • Explains why we see many young hot stars in the spiral arms Shock Waves The Mass of the Galaxy • Can be determined using Kepler’s 3rd Law – Solar System: the orbital velocities of planets determined by mass of Sun – Galaxy: orbital velocities of stars are determined by total mass of the galaxy contained within that star’s orbit • Two key results: – large mass contained in a very small volume at center of our Galaxy – Much of the mass of the Galaxy is not observed • consists neither of stars, nor of gas or dust • extends far beyond visible part of our galaxy (“dark halo”) The Missing Mass Problem • Dark Matter is dark at all wavelengths, not just visible light • The Universe as a whole consists of up to 25% of Dark Matter! Strange! • What is it? – – – – – Brown dwarfs? Black dwarfs? Black holes? Neutrinos? Other exotic subatomic particles? • Actually: Most of the universe (70%) consists of Dark Energy Even stranger! Missing Mass Problem Galaxy Masses • Rotation curves of spiral galaxies comparable to milky way • Masses vary greatly The Night Sky in November • The sun is past autumn equinox -> longer nights! • Autumn constellations are coming up: Cassiopeia, Pegasus, Perseus, Andromeda, Pisces lots of open star clusters! Moon Phases • Today (Waxing Crescent) • 11 / 5 (First Quarter Moon) • 11 / 13 (Full Moon) • 11/19 (Last Quarter Moon) • 11/ 27 (New Moon) Today at Noon Sun at meridian, i.e. exactly south 10 PM Typical observing hour, early November Uranus Neptune Star Maps Celestial North Pole – everything turns around this point Zenith – the point right above you & the middle of the map 40º 90º West The summer triangle lingers on … Due North Big Dipper points to the north pole High up – the Autumn Constellations • W of Cassiopeia • Big Square of Pegasus • Andromeda Galaxy Andromeda Galaxy • “PR” Foto • Actual look SouthEast Perseus, Auriga & Taurus with Plejades and the Double Cluster SouthWest – 2006 • Planets – Uranus – Neptune • Zodiac: – Capricorn – Aquarius SouthWest – 2007 • Planets – Uranus – Neptune • Zodiac: – Capricorn – Aquarius South – 2008 • Planets – Uranus – Neptune • Zodiac: – Capricorn – Aquarius Mark your Calendars! • Next Starry Monday: February 2, 2009, 7 pm (this is a Monday • Observing at Prairie Oaks Metro Park: – Friday, January 30, 7:00 pm "The Journey to Palomar" airs on PBS on November 10, 2008 Web pages: – http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) – http://www.otterbein.edu/dept/PHYS/ (Physics Dept.) ) Mark your Calendars II • • • • Physics Coffee is every Wednesday, 3:30 pm Open to the public, everyone welcome! Location: across the hall, Science 244 Free coffee, cookies, etc. Famous Telescopes – Mt Palomar • 5 Meter Telescope – Huge and heavy mirror • On Mt. Palomar in California Visiting Mount Palomar • November 2005 Model of the 200” Telescope Transport of the Mirror ~ 1945