Final review: Milky Way Galaxies Active galaxies Cosmology: –The future of the universe –The beginning of the universe Test schedule (in LL203) –8 am class:
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Final review: Milky Way Galaxies Active galaxies Cosmology: –The future of the universe –The beginning of the universe Test schedule (in LL203) –8 am class: Wednesday, 4/29, 7-9 –9 am class: Tuesday, 4/28, 9:30-11:30
Galaxy interactions and spiral arms Close encounters between galaxies provide another way of forming spiral arms. –A simulation is found on the text website. Some astronomers argue that the spiral arms in the Milky Way are due to interactions with the Large Magellanic Cloud.
Quasars These objects were called quasi-stellar radio sources which was soon shortened to quasars. –Soon many starlike objects with large redshifts were discovered that emitted no radio waves. –Called “radio-quiet” quasars and comprise 90% of all known quasars. All quasars have large redshifts meaning they are very distant.
Quasar distribution There are no quasars with small redshifts. –This means there are no nearby quasars. –Nearest is about 800 million ly from Earth. Quasars were common in the distant past, but there a none in present-day universe.
Density of the universe A flat universe is a special case with a specific density. –Call this density the critical density or c. Spherical: 0 > c, Flat: 0 = c, Hyperbolic: 0 < c, Alternatively, we define the curvature of the universe by the ratio of the combined average mass density to the critical density. – c
Results of curvature measurements We find that 0 =1.0 with an uncertainty of 2%. –This says the universe is flat. ( 0 = c ) Unfortunately, m is measured to be only about 24% of the critical density. –Radiation density is insignificant. Radiation, matter and dark matter acount for 24% of the total density of the universe. What accounts for the rest? –Must be some form of energy we cannot detect gravitationally or electromagnetically. –Dark energy!
The inflationary model Theory suggests that the universe experienced a brief period of inflation shortly after the Planck time. –Planck time: First 1.35x10 -43 s of the lifetime of the universe. Before this time the laws of physics as we know them didn’t apply. During inflation the universe expanded by a factor of 10 50 in about 10 -32 s!
Nucleosynthesis At t=3 min the universe was cool enough for protons to combine to form helium. –Same process as in the center of a star. Lithium (3 protons) and beryllium (4 protons) also formed this way. At t=15 min the universe was too cool for this to happen and no further nucleosynthesis occurred until stars formed.
Population III stars Population II stars can’t be the oldest stars in the universe. –The original stars were Population III stars. –These stars had masses from 30 to 1000 M . –The death of these stars provided matter incorporated into next generation of stars.