Transcript Milky Way
•Earlier in the course, I told you stellar spectra are black bodies •Why are there all these features? Spectral Lines High resolution solar spectrum •In dense inner part, light gets randomized – black body spectrum •Light passes through cooler stellar atmosphere •But some wavelengths get absorbed! +- Stellar spectra •Approximate black body spectrum – color tells you temperature •Red is cool, blue is hot •Spectral lines tell you a lot about the star •Different lines for each element (and ionization state) •Strength tells you about composition and temperature More about stars •Stars are formed from clouds of cool gas •They come in different sizes: •High mass stars tend to burn hotter (bluer) and don’t last long •Low mass stars tend to burn cooler (redder) and last a long time •The color of a collection of stars (clusters) tells you about them: •Young clusters look bluish •Old clusters look reddish Doppler Effect Revisited f0 f 1 v cos c 1 v c f f0 1 v c •In astronomy, most of the motion is towards you f0 f 1 vr c or away from you •Let vr = - v cos be the velocity away from you f c 0 f 0 •In astronomy, we work with wavelength instead of frequency •Except they actually use z, the fractional increase in wavelength 2 f0 1 z 1 vr 1 z 1 vr c 0 f c 1 z 2 1 •For non-relativistic: z vr c 0 f f0 1 v c 1 v c Orbits •When a low mass object orbits a high mass object, there is a simple relationship between the distance and the velocity: 2 GMm mv F 2 R R GM v R Distance 1 AU = Earth-Sun distance = 1.496 1011 m 1 ly = cy = 9.46 1015 m 1 pc = 3.086 1016 m 50 Mean Velocity (km/s) 45 40 Planets 35 Fit to formula 30 25 20 15 10 5 0 0 5 10 15 20 25 Mean distance (AU) 30 35 40 Mass 1 MSun = 1.989 1030 kg The Milky Way: Visible Light •Obscured by gas and dust Center of Galaxy Galactic Plane The Milky Way The central portion Baade’s Window The Milky Way – From Outside The Milky Way – From Outside The Milky Way – Edge On The Milky Way – Artist’s Conception •Our galaxy is hard to study because we are inside it •We are in the galactic plane, filled with obscuring dust •Other galaxies are hard to study because they are far away •Generalize from far away to nearby and vice versa You are here The Milky Way – Basic Structure •Galaxies and larger structures are incredibly large compared to anything we have studied up to now •We need new units to describe it Distances ly = 0.931016 m kly = 103 ly Mly = 106 ly Gly = 109 ly •The disk •The bulge •The nucleus •The halo •Globular clusters The Sun The Disk – Dimensions and Structure •The disk •The bulge •The nucleus •The halo •Globular clusters •A large, flat disk, shaped like a pancake •About 100 kly in diameter •About 3 kly thick •We are about half way out •Has prominent spiral structure 27 kly The Sun 100 kly 3 kly The Disk - Composition •Stars, mixture of young and old •Circular orbits in plane of galaxy •From 0 to about 10 Gyr •Open clusters •The interstellar medium •Hot bubbles •Atomic hydrogen clouds •Molecular clouds •Ionization nebulae •Dust •Obscures and reddens things •Causes “reflection nebulae” •The disk •The bulge •The nucleus •The halo •Globular clusters Open Clusters M35 NGC 290 NGC 2158 M6 M36 Pleiades Hot Bubbles •Gas heated by supernovae and other violent events •Gas is very thin •Gas is very hot and ionized •Temperatures up to 106 K •Can be traced out by X-rays •The disk •The bulge •The nucleus •The halo •Globular clusters Hot Bubbles - Images Atomic Hydrogen Clouds Slightly cooler regions of gas •Hydrogen atoms produce 21 cm line •Electron and proton are spinning and have magnetic interactions •When electron spin flips over, 21 cm radio emission is seen •21 cm line used to map out our disk •Can also get accurate Doppler shift •The disk •The bulge •The nucleus •The halo •Globular clusters Radio waves The 21 cm line Approximate Map of Galaxy Molecular Clouds Coldest and densest regions •Atoms join together to make molecules •Principally hydrogen (H2), but this is difficult to detect •Other molecules vibrate to produce characteristic radio waves •These regions are where new stars can form Carbon monoxide (CO) emissions from cool clouds in our galaxy Molecular Clouds – Eagle Nebula Molecular Clouds Molecular Clouds – Keyhole and Orion Molecular Clouds Molecular Clouds – Horsehead Nebula Ionization (Emission) Nebulae •Light from hot stars ionizes hydrogen •Hot thin gas •When it recombines, light is made Light •The disk •The bulge •The nucleus •The halo •Globular clusters Ionization (Emission) Nebulae Ionization (Emission) Nebulae Labeled Eagle Nebula Dust •Lots of dust in spiral galaxies •Dust absorbs and scatters short wavelength light •The color of objects is reddened if we look through thin dust clouds •Dust clouds look blue •You can peek through dust in near infrared light Dust cloud •The disk •The bulge •The nucleus •The halo •Globular clusters Dust •Dust is warmed by absorbing light from stars •It glows in the infrared •The disk •The bulge •The nucleus •The halo •Globular clusters A reflection nebula – the Pleiades •The disk •The bulge •The nucleus •The halo •Globular clusters A reflection nebula – the Merope Nebula •The disk •The bulge •The nucleus •The halo •Globular clusters Reflection Nebulae IC 349 Witch Head Nebula Spiral Arms •The disk •The bulge •The nucleus •The halo •Globular clusters Spiral Arms – Signs of Rotation •The disk •The bulge •The nucleus •The halo •Globular clusters •Bulge is flattened - rotation •Disk - rotation vs. gravity •Spiral arms - what causes them? •Simple winding? No! •Density waves? Yes! Complicated! Simple Winding – The Wrong Theory •Spiral arms wind up in one cycle •20 cycles since beginning of galaxy •Something else is going on An Analogy from Driving •Knot in traffic causes slowdown •Slowdown causes other cars to slow down •Knot in traffic moves •Different cars at different times Density Waves – The explanation •Gas is in a disk •Inevitably, some parts are more dense than others •Gravity/rotation causes dense parts to become denser •Different gas goes in/out •Dense parts form new/bright stars The Bulge •Our view of it is (mostly) blocked by dust •We can see through the dust in near infrared •The disk •The bulge •The nucleus •The halo •Globular clusters The Bulge The Sun •Flattened sphere approximately 20 kly across •Composition •Mostly older stars •Little gas and dust •Recent evidence indicates it is bar shaped, with left side closer to us than right side. •The disk •The bulge •The nucleus •The halo •Globular clusters The Bulge – Color and Appearance •Older stars give it a redder appearance The Nucleus •The disk The Sun •The bulge •The nucleus •The halo •At the center of our galaxy lies a complex region •Globular •Fast star formation clusters •Recent supernovae remnants •Hot gas •Fast motion •Density of stars is very high here •Intense radio sources can penetrate the gas and dust The Nucleus in Radio •Close in, we see streamers of gas apparently flowing in •At the heart is an intense radio source called Sagittarius A* What is that thing in the Nucleus? •Copious quantities of X-rays close in •Stars orbit this source very quickly •Kepler’s Laws tell us mass X-ray image •It appears to be a 4 million solar mass black hole The Monster in the Middle •Radio waves can’t come from black hole itself •Gas from nearby attracted by gravity •Accelerates to near light speed •Friction creates heat/X-rays/etc. •More efficient than any other power source •Black hole •4.0 million MSun •The disk •The bulge •The nucleus •The halo •Globular clusters New Discovery? •Gamma Rays coming from pair of lobes •Outburst of energy from nucleus? •Was our Galaxy formerly active? The Halo •Roughly spherical shape •At least 2 size of disk •Stars: •Old stars (about 10 Gyr) •Orbits well out of plane •Globular Clusters •Up to a million stars each •Oldest stars (up to about 13 Gyr) •Little gas or dust •The disk •The bulge •The nucleus •The halo •Globular clusters Total Mass in the Galaxy Object Mass (MSun ) •Total of a few hundred billion stars Disk Stars 60 billion •Total mass about 100 billion Sun •Stars mostly concentrated near center Disk Gas ~10 billiion 20 billion •Treat gravity as if it all comes from a Bulge Halo Stars 1 billion point source in the center Nucleus 4 million •Use Kepler’s Third If all the mass is in the center, how would law: MP2 = a3 we expect the velocity of the stars to d 2 a M change as we move away from the center? v 2 t P a A) Fastest for stars near the center B) Fastest for stars far out C) Roughly same speed for all Where is the Mass? •Measure rotational velocities using Doppler shift of 21 cm line •Plot vs. distance from center of galaxy Dark Matter •85% of mass is not concentrated in the center •It does not glow, it is dark •We do not know what it is •We know a lot of things it isn’t: •Living Stars •Gas What is the dark matter? •Dust A) Neutron Stars •Could it be B) Black Holes MACHOs? C) White Dwarfs D) Planets without stars E) Something else •The disk •The bulge •The nucleus •The halo •Globular clusters Massive Compact Halo Objects MACHOs How to catch a MACHO •All of these objects are dark •But they do have gravity! •Einstein says they can bend light •They can magnify a distant star MACHO: MACHOs Neutron Stars Black Holes White Dwarfs Planets without stars We see: MACHO’s: What the Data Tells Us •MACHO’s do exist •Substantial fraction of stars •But not the majority •There are not enough of them to account for the dark matter •They are probably mostly white dwarfs •The disk •The bulge •The nucleus •The halo •Globular clusters •Most likely dark matter is some weird new particle