Reminders!Website: http://starsarestellar.blogspot.com/ Lectures 1-15 are available for download as studyaids.Reading: You should have Chapters 1-14 read.Read Chapters 15-17 by the end of the week.Homework: Homework #4 is due tomorrow, June24th.Observing Projects: Both due Monday. They willNOT be accepted late!!!Our Home, the Milky WayToday’s Lecture:GRB Review (Chapter 14, pages 337-339)• Two types of gamma-ray bursts• Universal “central engine”The Milk Way (Chapter 15, pages 344-365) • The main parts of our Galaxy • Nebulae • What the spiral arms?GRBs: Fantastic explosions!• In 1991 new telescopes showed GRBs are isotropic onthe sky, proving they are (usually) NOT from our galaxy• Afterglows (optical) were seen from some GRBs. Thisallowed the host galaxies to be identified and distancesto be measured.• The luminosities of these things are HUGE: 1052 ergs/s.Equivalent to vaporizing a star into pure energy in amatter of 10 seconds.• Also highly beamed and relativistic (indicating jetsmoving near the speed of light)• Now we have the satellite Swift that can rotate quickly inthe sky to do detailed studies of GRBsLong and short GRBsFurther observations show that there are two classes:• Long gamma-ray bursts (> 2 sec): Found in young,star forming regions. Some are clearly massivesupernovae (hypernovae?) because spectra are seen.• Short gamma-ray bursts (< 2 sec): Found in youngand old regions. Thought to be two merging neutron starsor a neutron star plus a black hole.In both classes, the “central engine” is the same: ablack hole rapidly accreting from a massiveaccretion diskThe difference in times basically reflects the differentreservoirs of mass available to accrete in each case.Milky Way Galaxy: Our Home!• A huge, rotating, gravitationally bound system of about 400billions stars. The Sun takes 250 millions years to orbit, so itis about 18 orbits old.• A spiral galaxy: disk with spiral arms in which new starstend to form.NucleusSun1000 lyGlobularclustersbulgeHalo80,000 lySide view(not to scale)Are we at center of our Galaxy?• The gas and dust in the disk of the galaxy absorb andscatter much of the light. Also called extinction orobscuration. So we can’t “see” very far at visiblewavelengths.• Because of this, we seem to be at the center if we justcount stars in different directions.We are NOT at the center!• Harlow Shapley (1917) noticed that globular clusters seemvery concentrated in one direction.• By assuming that globular clusters orbit the center of thegalaxy, he derived the distance from the Sun to the center.The Milky Way in the Sky• Band of light stretching across the sky on a dark, moonlessnight: “Milky Way” (countless faint stars, gas, dist in plane ofgalaxy)• Very splotchy distribution.• Nebulae (clouds of gas and dust): Emission nebula: Gas is ionized by ultraviolet light from hot stars. Glows at optical wavelengths when electrons recombine with ions, or when electrons excite atoms and ions by colliding with them. Reflection nebula: Lots of dust and gas reflects light fromadjacent stars. Dark (absorption) nebula: Blocks light.SunTypes of NebulaeDiffuse gas, clouds of gas and dust between stars: theinterstellar medium (ISM). The raw material for new stars!Hα emissionUVDark nebulae generallyhave lots of dense gasand dust.Reflection nebulaRadio Observations of Our Galaxy• Because of all the dust and gas in our Galaxy, longerwavelength observations are key.• Radio observations are very important for thisreason. But where does radio come from?• Hydrogen spin-flip transition: emits a 21 cm photon!• Molecules: water (H2O), ammonia (NH3),carbonmonoxide (CO), molecular hydrogen (H2), and hundredsof other molecules are seen in the radio• These molecules weren’t initially expected (gas toolow-density for molecule formation), but we now knowthat dust grains help to allow molecules to form.“Rotation Curve” of the Milky WayFor a large part of the distance out from the center of theMilky Way Galaxy, the speed of orbiting objects is just aboutconstant. It is said that the rotation curve is “flat”Solution: Arms are gas “density waves” of compression thatrotate more slowly than the galaxy disk. Massive stars form there.Analogy: Cars moving past a slowly moving road blockbottleneck. Road block causes a moving overdensity of cars.Why don’t arms wrap up quicklyOther Galaxies• Originally called “spiral nebulae” because of their shape.• A big controversy in the early 1900s was: Are thesenebulae gas clouds in Milky Way or other “universes”?• Edwin Hubble - 1924 - observed very faint “Cepheidvariable stars” in some of them.• Cepheids are evolved supergiant stars that brightenand fade periodically as their size oscillates. Remember,these are standard candles.• If Cepheids appear faint, then they must be wayoutside the Milky Way Galaxy.• From the distance and angular size of spiral nebulae,Hubble deduced that they are HUGE stellar system,“island
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