AST 115 1st Edition Lecture 29 Outline of Last Lecture I. Introduction to Chapter 13 – The Galaxiesa. Milky WayII. Structure of the GalaxyIII. Populations in the Galaxya. Population Ib. Population IIIV. The Sun’s location in the GalaxyV. The true nature of the “Nebulae”a. CatalogsVI. The Great Spiral Nebula in AndromedaVII. The Great Spiral Galaxy in AndromedaOutline of Current Lecture I. Spiral Structure of the Galaxy’s DiskII. Rotation of the GalaxyIII. Mass of the GalaxyIV. Dark MatterCurrent Lecture Spiral Structure of the Galaxy’s Disko At optical wavelengths, dust prevents complete view of disk. Can only see about 1 to 3 thousand LY into disk.o Optically we do see bright blue O and B spectral class stars along the spiral arms near to us (just as they are seen along spiral arms of other galaxies).o Must use radio observations of cold HI clouds in the disk to map out the spiral structure. These clouds emit “21 – cm radio waves”. The Nucleus of the Galaxyo Strong radio source in Sagittarius is called “Sagittarius A”.o It lies about 25,000 LY beyond the stars that make the pattern of the constellation Sagittarius.o The VLA = Very Large Array radio telescope system in New Mexico has resolved this into components as small as 3 AU across.o The part known as “Sagittarius A” lies at the center of the Galaxy and is believed to contain a supermassive black hole.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o The diameter of the event horizon of a black hole with 2 million solar masses would be about 12 million km = about 7.2 million miles.Dkm= 6 ×mo As matter gradually falls towards this large black hole, an accretion disk has formed and is responsible for strong radio signals.*There is a supermassive black hole at the center of our Galaxy and we call it “Sagittarius A”. Rotation of the Galaxyo Motion of the Sun with respect to distant globular star clusters using the DopplerEffect in their spectra gives a roughly circular orbit with velocity of about 230 km/sec (toward Cygnus).o The Sun’s galactic orbit has a circumference of 2 π r=2 π(26,000 LY). To travelthis distance at 230 km/sec requires about 230 million years. Mass of the Galaxyo Requires use of Newton’s revision of Kepler’s 3rd Law. (M1+ M2)=a3p2o In this situation: P = 230 million years a = 26,000 LY Gives us about 100 billion times the Sun’s mass.o If the typical mass per star is ½ solar mass, then the number of stars in the Galaxyis at least twice this: 200 billion stars.(There may be more mass than this!) Dark Matter in (and around) the Galaxyo The disk of the Galaxy contains lots of interstellar dust. But this dust is NOT the dark matter.o Dark matter = material that is not emitting any kind of radiation, but have gravity.It is visible at all wavelengths.o The presence of this dark matter has been deduced by studying the orbital motion of various objects in Galaxies.o A “galactic rotation curve” shows that the velocity of objects does NOT fan off with increasing distance from the center, but stays “flat”.o 90% of the mass of the Galaxy may be this “dark matter”. o But what is this material…? We don’t know what it is, but we can make some guesses:- Black holes? (no)- Dim red stars? (no)- Brown dwarfs? (no)- Neutrinos, etc? (no)- WIMPS = Weakly Interacting Massive ParticleS? (no)- MACHOS = Massive Compact Halo ObjectS? (no) Again, no one knows what “dark matter” is, but we do know that it does exist in
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