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Galaxies and the Universe Galaxies ______________ galaxy • Structure • Determined by using ______ telescopes • Large spiral galaxy • About 100,000 light-years wide • Thickness at the galactic nucleus is about 10,000 light-years • _____ spiral arms of stars • Sun is 30,000 light-years from the center The Milky Way galaxy appears in our sky as a faint band of light. Dusty gas clouds obscure our view because they absorb visible light. This is the ____________ ________ that makes new star systems. All-Sky View Face-on view of the Milk Way Galaxy We see our galaxy edge-on. Primary features: disk, bulge, halo, globular clusters Galaxies Milky Way galaxy • Rotation • Around the galactic nucleus • Outermost stars move the slowest • Sun rotates around the galactic nucleus once about every 200 million years • Halo surrounds the galactic disk • Spherical • Very tenuous gas • Numerous globular clusters If we could view the Milky Way from above the disk, we would see its spiral arms. Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 1011 MSun ____________________ are found around shortlived high-mass stars, signifying active star formation. ___________ nebulae scatter the light from stars. Why do reflection nebulae look bluer than the nearby stars? For the same reason that our sky is blue! Halo: No ___________ nebulae, no blue stars no star formation Disk: ___________ nebulae, blue stars star formation Much of star formation in disk happens in ________________ Whirlpool Galaxy Our galaxy probably formed from a giant gas cloud. Halo stars formed first as gravity caused the cloud to contract. The remaining gas settled into a spinning disk. Stars continuously form in the disk as the galaxy grows older. Warning: This model is oversimplified. Stars continuously form in the disk as the galaxy grows older. Infrared light from center Radio emission from center Radio emission from center Swirling gas near center Swirling gas near center Orbiting star near center Stars appear to be ________ something massive but invisible … a black hole? Orbits of stars indicate a mass of about 4 million Msun. X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in. Hubble Deep Field • Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away. Galaxies and Cosmology • A galaxy’s age, its distance, and the age of the universe are all closely related. • The study of galaxies is thus intimately connected with _________ —the study of the structure and evolution of the universe. Galaxies Other galaxies • Existence was first proposed in mid-1700s by Immanuel Kant • Four basic types of galaxies • ______ galaxy • _____ extending from nucleus • About 30% of all galaxies • Large diameter of 20,000 to 125,000 light years • Contains both _______ and ___ stars • e.g., Milky Way halo disk bulge Spiral Galaxy Great Galaxy, a spiral galaxy, in the constellation Andromeda Galaxies Other galaxies • Four basic types of galaxies • _____________ galaxy • Stars arranged in the shape of a bar • Generally quite large • About 10% of all galaxies • ___________ galaxy • Ellipsoidal shape • About 60% of all galaxies • Most are smaller than spiral galaxies; however, they are also the largest known galaxies Barred Spiral Galaxy: Has a bar of stars across the bulge Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical) Elliptical Galaxy: All spheroidal component, virtually __________ component Galaxies Other galaxies • Four basic types of galaxies • ________ galaxy • Lacks symmetry • About 10% of all galaxies • Contains mostly _______ stars • e.g., Magellanic Clouds Irregular Galaxy: Neither spiral nor elliptical Galaxies Galactic cluster • Group of galaxies • Some contain thousands of galaxies • ____________ • Our own group of galaxies • Contains at least 28 galaxies • Supercluster • Huge swarm of galaxies • May be the largest entity in the universe Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies per group). Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies). If the center of a galaxy is unusually bright, we call it an active galactic nucleus. _________ are the most luminous examples. Active Nucleus in M87 Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures. The accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars. Radio galaxies don’t appear as quasars because dusty gas clouds block our view of the accretion disk. Energy from a Black Hole • The gravitational potential energy of matter falling into a black hole turns into kinetic energy. • Friction in the accretion disk turns kinetic energy into thermal energy (heat). • Heat produces thermal radiation (photons). • This process can convert 10–40% of E = mc2 into radiation. Jets are thought to come from the twisting of a magnetic field in the inner part of the accretion disk. Do supermassive black holes really exist? ____________ _____ at center of Milky Way stars indicate a black hole with mass of 4 million Msun. Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion Msun. Black Holes in Galaxies • Many nearby galaxies—perhaps all of them— have supermassive black holes at their centers. • These black holes seem to be ________ active galactic nuclei. • All galaxies may have passed through a quasarlike stage earlier in time. Hubble also knew that the spectral features of virtually all galaxies are __________ they’re all moving away from us. Red shifts Doppler effect • Change in the wavelength of light emitted by an object due to its motion • Movement away stretches the wavelength • Longer wavelength • Light appears redder • Movement toward “squeezes” the wavelength • Shorter wavelength • Light shifted toward the blue Red shifts Doppler effect • Amount of the Doppler shift indicates the rate of movement • Large Doppler shift indicates a high velocity • Small Doppler shift indicates a lower velocity Expanding universe • Most galaxies exhibit a red Doppler shift • Moving _____ Raisin bread analogy of an expanding universe Red shifts Expanding universe • Most galaxies exhibit a red Doppler shift • Far galaxies • Exhibit the greatest shift • Greater velocity • Discovered in 1929 by Edwin Hubble • _________ Law – the recessional speed of galaxies is proportional to their distance • Accounts for red shifts By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way. Hubble’s law: velocity = H0 distance Redshift of a galaxy tells us its _________ through Hubble’s law: distance = velocity H0 We measure galaxy distances using a chain of interdependent techniques. Cosmological Principle The universe looks about the same no matter where you are within it. • Matter is ______ distributed on very large scales in the universe • No _______ and no _____ • Not proved but consistent with all observations to date • The expansion rate appears to be the same everywhere in space. • The universe has no center and no edge (as far as we can tell). One example of something that expands but has no center or edge is the surface of a balloon. Distances between faraway galaxies change while light travels. distance? Astronomers think in terms of _________ time rather than distance. We still can’t directly observe the earliest galaxies. ______________ theory Accounts for galaxies moving away from us Universe was once confined to a "ball" that was • Supermassive • Dense • Hot Big Bang theory Big Bang marks the inception of the universe • Occurred about 15 billion years ago • All matter and space was created Matter is moving ________ Fate of the universe • Two possibilities • Universe will ____________ • Outward expansion will stop and gravitational ____________ will follow The main question • Does the universe have enough kinetic energy to escape its own gravitational pull? – What is the escape velocity of the Universe? Big Bang theory Fate of the universe • Final fate depends on the average _______ of the universe • If the density is _____ than the critical density, then the universe would ________ • Current estimates point to _____ then the critical density and predict an _______________, or open, universe Fate of universe depends on the amount of _______ _______ Lots of dark matter Critical density of matter Not enough dark matter Amount of matter is ~25% of the critical density, suggesting fate is eternal expansion Not enough dark matter But expansion appears to be speeding up! ____________? Not enough dark matter old older oldest Estimated age depends on both dark matter and dark energy. Is the expansion of the universe accelerating? The brightness of distant white-dwarf supernovae tells us how much the universe has expanded since they exploded. An accelerating universe is the best fit to supernova data. http://www.vene.ws/tesina/wormhole.jpg