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Unformatted text preview: Recap the Black Holes Recap the Black Holes Describe the conditions for a stellar black hole to be formed Describe the tidal effects and time dilation near a black hole Describe the Xray observations of a black hole Describe Hawking’s proposal on white holes Is there any experimental support for this idea ? Discuss the galactic black holes Among Galaxies Among Galaxies
(from chapter 8) From stars to galaxies The Milky Way Looking into the Remote Past Galactic Explosions and Quasars Galactic Black Holes From Stars to Galaxies From Stars to Galaxies Billions of stars form galaxies, which for the old astronomy were “the nebulae” (some nebulae are actually supernova remnants). For hundreds of years stars and nebulae were the fixed constellations while the planets were the moving cosmic objects. Determining the distance to galaxies became possible only after 1923 by measuring the distance to specific stars inside those galaxies. The observations with nonoptical telescopes uncovered a world of galaxies far more violent than the world of stars. Galaxies can have different shapes: oval, S, spiral, iregular. The Milky Way The Milky Way The Sun is a medium star in our galaxy, a medium size spiral galaxy known also as the Milky Way. We are in the Orion “arm”, slightly above the galactic plane. Harlow Shapley was the first astronomer who after 1920 used measurements on stars and on the interstellar dust (hydrogen spectrum) to map the Milky Way. Our galaxy has a galactic black hole of around 100,000 solar masses surrounded by an abundance of red giants and mini white dwarfs. They are the oldest stars in the galaxy. The outside of the arms of the galaxy contains new stars and star nurseries. C.C.Lin at MIT demonstrated that our galaxy evolved from oval to spiral shape through gravitational and electromagnetic effects. Spiral Galaxies Spiral Galaxies
Spiral Galaxies from above This is the M100 spiral galaxy, similar to our Milky Way. Spiral Galaxies from a side high density galactic plane of stars black hole Stars or groups of stars Other Galaxies Other Galaxies The nearest galaxies are known as “Magellan’s Clouds”; they are about 200,000 lightyears from us. In the Northern hemisphere the nearest observable galaxy is Andromeda (M31) at 3 million lightyears from us. The light from M31 was sent to us when Earth was populated by humanoids. To measure the distance In an expanding Universe the further a galaxy is from us the faster it moves. A fast moving light source would have the optical spectrum shifted towards large wave lengths (known as red or Doppler shift). to a close galaxy one has to identify a star from the Cepheid class, which always has the same intrinsic brightness. for galaxies which are billions of lightyears away one has to look for more powerful sources of radiation as individual stars cannot be identified. Type Ia supernovas are such powerful sources. Huge Numbers of Galaxies Huge Numbers of Galaxies
The number of galaxies observed today is of the order 1011. Many of these galaxies emitted the light that we observe today long before the Sun or Earth existed. 1500 galaxies seen by the Hubble telescope in a certain part of the Universe. Many Shapes and Sizes Many Shapes and Sizes Observations found galaxies of various shapes: elliptical, spiral and irregular. Sizes vary from dwarfs (about 1 million solar masses) to giants (up to 10 trillion solar masses). Among bright galaxies about 70% are spiral and 20% elliptical, but most dwarfs are elliptical and therefore of the total only about 2030% are spiral. Explaining Different Types Explaining Different Types Two factors determine the type of a galaxy: Its rotation around the galactic nucleus The rate of stars formation Elliptical galaxies show little rotation and high rate of stars formation. Spiral galaxies have rotation which creates a disc shaped cloud of gas, with lower stars formation rate and with a lot of gas left for future generations of stars. Intergalactic Interactions Intergalactic Interactions The shapes of galaxies are also influenced by their interaction/collision with neighbors. the Cartwheel galaxy (next slide) and the Antennae pair of galaxies. Cartwheel Galaxy (I) Cartwheel Galaxy (I) Cartwheel Galaxy (II) Cartwheel Galaxy (II) A rare and spectacular headon collision between two galaxies appears in this NASA Hubble Space Telescope truecolor image of the Cartwheel Galaxy, located 500 million lightyears away in the constellation Sculptor. The striking ringlike feature is a direct result of a smaller intruder galaxy possibly one of two objects to the right of the ring that careened through the core of the host galaxy. Like a rock tossed into a lake, the collision sent a ripple of energy into space, plowing gas and dust in front of it. Expanding at 200,000 miles per hour, this cosmic tsunami leaves in its wake a firestorm of new star creation. Hubble resolves bright blue knots that are gigantic clusters of newborn stars and immense loops and bubbles blown into space by exploding stars (supernovae) going off like a string of firecrackers. The Local Group/Cluster The Local Group/Cluster The “Magellan’s Clouds” at 170,000 lightyears and Andromeda (M31) and its companion M33 at 2.5 million lightyears are the best known neighbouring galaxies. These galaxies plus 14 others form our local group/ cluster of galaxies. The nearest of all is an elliptic dwarf only 50,000 lightyears from the center of Milky Way (hidden from us by the galactic center) Inside a cluster a lot of interaction between galaxies moving with speeds up to 1500 km/s and huge amounts of dark matter keep the cluster together. Clusters and Superclusters Clusters and Superclusters Beyond the center of the Milky Way one can see the Virgo group containing over a thousand galaxies. In center Virgo has 3 giant ellipticals, of which M87 has a mass more than 30 times the mass of our galaxy. Virgo and our local group share the same supercluster. Our supercluster is relatively modest although it comprises a total about 5000 galaxies. Superclusters typically occupy 100250 million lightyears and have various shapes. The most famous is the “great wall” of galaxies with 500x200x15 million lightyears. Between superclusters there are true empty spaces measuring hundreds of millions of lightyears. Radio Galaxies Radio Galaxies As astronomers started using radiotelescopes many galaxies appeared to be in an explosive state, with irregular shapes and with jettype emissions. The first galactic explosion was seen in 1951 by G.Smith at Cambridge. Cygnus A was a radio source at about 750 million ly. Its emission was also seen through optical telescopes. Observations using interferometry determined a structure in the radio emission, corresponding to periods of different level of activity. Quasars Quasars Rapid variations of brightness always imply a concentration of matter. The neutron stars which are a few kilometers wide emit more radio signals than a regular star. Quasars are a few lightweeks or lightmonths wide and emit much more radio waves than a regular galaxy. Most quasars are visible through optical telescopes too. The current explanation: the galactic black holes swallows large clouds. In the scenario which includes a black hole of 108 solar masses, a star approaching it would be broken into fragments by the tidal forces outside the event horizon. Those fragments would be accelerated producing the observed radiation. Seyfert Galaxies Seyfert Galaxies One of the oldest variable radiosources is 3C120. It took many years for a clear explanation of its true origin. Now we know that it is a Seyfert galaxy, a galaxy with a very bright center. About one in 100 galaxies are of Seyfert type. Although Seifert galaxies are much brighter than regular galaxies, quasars are still more than 100 times brighter than Seifert galaxies. Many astronomers believe that the Seyfert galaxies and quasars are two stages in the evolution of the same galaxy. The majority of quasars correspond to galaxies about 8 billion lightyears away and therefore very young galaxies. Galactic Black Holes (I) Galactic Black Holes (I) The use of radiointerferometers allowed the creation of very detailed radio photos. They showed the radio source Cygnus A was a huge elliptical area almost 100 times larger than the center which was the galaxy NGC 5128. Its shape was suggesting not one but several explosions separated by long intervals of time. Blandford and Rees at Cambridge have developed a double emission model with two jets emitted by a rotating mass of gas. This model agreed well with the Cygnus A observations and also with the photo of Scorpius X1, one of the known stellar black holes. Galactic Black Holes (II) Galactic Black Holes (II)
The existence of black holes in the center of galaxies is today a common belief among cosmologists. It is not clear yet if they were the Our galaxy contains a central actual seeds in the black hole of about 105 solar masses and its activity is relatively formation of galaxies or formed in later stages. small. Galactic Black Holes (III) Galactic Black Holes (III) The galactic black holes model can explain all 3 types of active galaxies. Quasars and Seifert galaxies correspond to direct observations of the galactic center (with quasars being brighter because they are younger stages with more material around the black hole) Radio galaxies correspond to cases where the center is obscured by cosmic clouds and the observations focus on the two jets perpendicular to the galactic plane. ...
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This note was uploaded on 05/03/2011 for the course NATS 1740 taught by Professor Hall during the Spring '10 term at York University.
- Spring '10