Top Ten Signs Your Astronomy Professor May Be Nuts Astronomy 122 (based on Lee Carkner List) 10) The title of every lecture is: "Man, Them Stars is Hot!". 9) His so called "telescopes" are really just paper towel rolls covered in aluminum foil. This Class (Lecture 22): 8) To illustrate the vastness of the universe, he makes everybody walk to Springfield. 7) Thinks he's married to the slide remote. The Milky Way: 6) Your grade is based entirely on how many ping-pong balls you can fit in your mouth. 5) His so-called Drake Equation video is really just an old episode of Alf. Not just a Candy Bar 4) He makes everyone wear a soup pot on their head to protect the class from "Klingon mind control lasers". Next Class: 3) About 90% of all classes involve dressing monkeys up to look like Isaac Newton. 2) When you go to his office hours, he is always hiding under the desk so that the "space Normal Galaxies squirrels" can't get him. 1) The only observing advice he ever gives is, "Keep an eye out for the mothership." HW9 due on Sunday. Music: Under the Milky Way – The Church Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Outline Astronomy: • Our Milky Way The Big Picture • Where are we in the Milky Way Moving from the birth/death of stars to a better understanding of • Problems with stars – Dark Matter the Galaxy! Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Milky Way The Milky Way • Our galaxy is a collection of stars, nebulae, molecular clouds, and stellar remnants – All bound together by gravity – Connected by the stellar evolution cycle http://home.arcor-online.de/axel.mellinger/mwpan_aitoff.html Apr 8, 2008 Astronomy 122 Spring 2008 What is it? Which is a picture of the Milky Way? A A is what we see from Earth inside the Milky Way. B is what the Milky Way In the infrared “might” look like if we were far away looking back at our own galaxy from B some other galaxy . http://antwrp.gsfc.nasa.gov/apod/image/0001/milkyway_cobe_big.jpg Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Milky Way is made of all the stars in our galaxy– about 100 billion. All the stars you can Question see in the sky are in our Galaxy. The Milky Way is a) effectively the observable Universe b) a galaxy made up of stars, stellar remnants, dust, and gas. c) a tasty treat. d) the beautiful band of light seen in the night sky with an unknown origin. e) all of the stars you can see with a telescope. Enormous clouds of dust obscure our view of most of the stars in our Galaxy Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Discovery of the Milky Way! The Importance of Being Earnestly Dust • There is clearly dust in the Milky Way disk. 6400 ly How does dust effect the measurement? • The number of – Makes stars dimmer and redder stars were • There is more dust toward Galaxy center. counted in all ly 1300 • Consequence: Under-estimation in the number directions from Sun of stars in one direction the Sun by William Herschel (the guy who discovered • We appear to be at the center, Uranus) and his sister Caroline but we’re not! • They assumed that all stars have the same brightness – Bad assumption! and that space is completely transparent – Nope! Us Star • They concluded that the Sun is at the center of the Universe Sun Apr 8, 2008 Astronomy 122 Spring 2008 AprCan’t 8, 2008 see stars here Astronomy 122 Spring 2008 But, We Are in a Disk of Stars! How Do We Find the Center? • But they were correct in determining that the • If dust blocks our view, distribution of stars in the how do we find the center? Milky Way is in a thin disk • We need to look outside • The Milky Way is very of the disk! thin in comparison to its – Get around the dust diameter – imagine 3 CDs stacked • A collection of relatively bright objects, outside the disk. Few stars More stars Sun Many stars Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Globular Clusters Finding Our Place! • Extremely regular, gravitationally bound groups of stars– very dense Globular Clusters Early view evenly distributed • About 10 5 – 10 6 stars each Sun at center • HR diagram of these groups of stars show that they are very old– 10 billion years! Sun • Generally outside disk of the Galaxy Globular Clusters unevenly distributed • About 150 known orbiting our Galaxy Harlow Shapley Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Our Place The Structure of Our Galaxy • Shapley showed that we are not the center of the Galaxy in the 1920s. • Disk – All kinds of stars, many (30 kpc) – 2nd Copernican revolution! younger – Open clusters • All of the globular clusters are orbiting around a point in (8 kpc) Sagittarius– 25000 lyrs or 8000 parsecs away. – Gas and dust • Halo • That must be the center of our Galaxy. – Old, red dwarfs and Globular clusters giants – Little gas and dust – Globular clusters • Bulge Sun – Mixture of halo and disk 8 kpc Side View Galaxy Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Disk Spiral Arms? • Other disk galaxies show • The disk of our spiral arms Galaxy contains – Made of O- & B-type stars, most of its diffuse nebulae, and most of visible mass the giant molecular clouds • How do we know our – 90% of the The disk in infrared light Galaxy’s stars Galaxy has them? • It’s where “the action” occurs • It’s the problem of not – Star formation, nebulae, etc.. seeing the forest for the • Relatively thin trees – 700 parsecs thick vs. 30,000 parsecs across Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Seeing the Galaxy Hints of Spiral Arms in Hydrogen Emission • We plot the locations of nearby O- and B-type • Look for 21-cm stars in our Galaxy wavelength photons • Find the stars are – Emitted by interstellar arranged in arms hydrogen – most • Our Sun is abundant stuff! in-between – Easily pass through gas spiral arms & dust along the way • But we can’t see beyond a few thousand light years – Map the Galaxy! • What about the rest of the Galaxy? Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Visible and Radio The Galaxy’s Spiral Arms • The 21-cm radio emission shows the spiral arms (below) • We find five main arms in the Galaxy (right) 100000 ly M83 Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Galaxy’s Spiral Arms The Galactic Halo • Our Galaxy’s disk is surrounded by a spherical halo of stars & globular clusters – Red dwarfs and red giants – old stars – Only about 2% the number of stars in the disk • There is some structure to the halo – Denser towards the center – Two sets of globular clusters • Outer clusters – older, spherical distribution The disk in infrared light • Inner clusters – slightly younger (but still old), flattened spherical distribution Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Bulge The Center of Our Galaxy • The region where the disk and the halo merge – About 2,000 pc across – Contains about 10% of the Galaxy’s stars • Mix of primarily old stars, but also contains some young stars and gas & dust • Like an extension of both the disk and halo Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 The Galactic Nucleus Question • Buried in the center of the bulge Massive O-type stars are not found in the • 8,000 pc away Galactic halo. What can we conclude from this? • Incredibly dense region of stars and gas a) Massive stars never formed in the halo. b) The halo consists of only old stars. c) Massive stars can only be formed now. d) The halo was accreted from another galaxy. Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Do Galaxies Spin? Rotation of the Galaxy • Similar to the planets orbiting the Sun, the stars and gas of the Galaxy orbit the nucleus • How does the Galaxy rotate? • Like a CD? DVD? • How about disk vs. halo? • Measure Doppler shifts to Spiral galaxies really suggest it. BTW, our Galaxy find out. probably looks more like the right galaxy. Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Solid vs. Differential Rotation Rotation of the Galaxy • Stars in the disk all orbit the Galaxy in the same direction – Stay in the disk (they may drift up and down) – Orbits roughly circular • Stars in the halo and bulge orbit the Galactic nucleus randomly – No organization to the orbits – Many very elliptical orbits Same angular speed Same linear speed (degrees per year) (parsecs per year) Apr 8, 2008 Astronomy 122 Spring 2008 Apr 8, 2008 Astronomy 122 Spring 2008 Is the Solar System Moving Wow! That’s fast! Too? Yes… the whole Stop and think about it. Galaxy has • That’s traveling to Chicago differential in 1 second! rotation– us • But Milky Way is big! included • Only orbited 50 times! • Last time the Sun was here, the dinosaurs were just starting out.