Download Galaxies, Cosmology and the Accelera`ng Universe

Galaxies, Cosmology and the Accelera6ng Universe Class 4: Mapping the Universe Steve Bryson h?p://stevepur.com/galaxies/ Ques6ons? Life Cycle of Stars •  Most of the 6me in the “Main Sequence” –  Burn Hydrogen to Helium •  At the end, burn Helium to Carbon, then blow off outer atmosphere Size, Color and Life6me Depend on Mass •  Heavier stars burn faster and ho?er –  10 x Sun: 32 million years, up to 1 million 6mes brighter –  Sun: 10 billion years –  0.1 x Sun: 3 trillion years, 8% as bright Sun Heavier Stars Burn Beyond Carbon •  Heavier stars are hot enough to keep burning –  Carbon -­‐> Neon, Calcium, Magnesium, Oxygen –  Neon -­‐> Oxygen and Magnesium –  Oxygen -­‐> Silicon and Phosphorous –  Silicon -­‐> Sulfur, Argon, Titanium, Chromium, Iron Later Stars Have Heavier Elements •  Elements created in stars, which blow off their gas and spread the elements to clouds that create more stars… Measure the Composi6on of Stars with Spectra Distribu6on of Star Sizes •  There are many more small, cool stars •  Discovered by coun6ng the stars near the Sun Galaxies Made of Stars, Gas and Dust What Does it Mean to Map a Galaxy? •  Where are the stars? –  Are different types or ages of stars in different places? •  What are the stars doing? •  Where are the big gas clouds? •  What is the overall shape? This is a Galaxy from the Outside This is a Galaxy From the Inside •  How do we map our Galaxy? Like Mapping a Forest From the Inside •  From a single loca6on This is a Galaxy From the Inside •  Our view is obscured by lots of gas and dust But We’re Not Restricted to Visible Light •  Longer wavelengths can get through dust be?er than visible light Different Views of the Milky Way •  Using infrared and radio waves we can see through the dust Surveying the Galaxy •  Stars –  Distance •  The first rungs of the Cosmic Distance Ladder –  Composi6on •  Spectra •  Gas and dust clouds –  Radio and infrared spectra Map of the Hydrogen Clouds •  Mapped by radio telescopes The Milky Way Galaxy Barred spiral Much like many other galaxies The Milky Way is Big •  100,000 light years (30,000 parsecs) across –  We’re about 26,000 light years from the center •  If it were the size of a football field –  The distance from the Sun to the nearest star would be just over 0.2 inches •  Contains between 100 Billion and 400 Billion stars Map of the Milky Way Galaxy The Spiral Arms •  Where the big Hydrogen clouds are –  Regions of ac6ve star birth –  Lit up by the newest, ho?est, largest stars •  Up to a million 6mes brighter than the Sun Most Stars are Not In Spiral Arms •  The spiral arms are where the big bright (and short-­‐
lived) stars are •  Most stars are dim and spread evenly throughout the Galaxy –  These were also born in spiral arms much further in the past, and have drijed out of the spiral arms •  Dim stars live a lot longer Distribu6on of Stars •  Even though hot bright stars are rare, they make more light than all the other stars combined •  In a cube 10 parsecs (32 light years) on a side –  In the spiral arms •  There are just a few hot young stars •  There are 50 – 60 low-­‐mass stars –  Between the spiral arms •  There are 50 – 60 low-­‐mass stars Side View of the Milky Way Galaxy Thick vs. Thin Disk •  The thin disk contains most of the stars –  Mostly younger stars •  Sun’s age and younger –  All the Hydrogen and dust clouds for star forma6on –  A few hundred parsecs thick •  The thick disk contains fewer older stars –  Fewer heavy elements than the Sun –  A few thousand parsecs thick •  Current thinking is that the thick disk stars were originally in the thin disk but were sca?ered •  (some recent observa6ons suggest that there is not such a big difference between the thick and thin disks: there is one disk that just has fewer and fewer stars as you move above the Galaxy, but these have not been confirmed) The Galac6c Halo •  Oldest stars in highly ellip6cal orbits –  As much as 10 billion years old –  Possibly relics of the forma6on of the Milky Way •  Size of halo not well known The Galac6c Center •  Un6l about 20 years ago was thought to be a spherical bulge •  Now we believe it is a bar The Galac6c Center •  Very ac6ve star forma6on –  Lots of bright, hot, young stars –  Average distance between stars is about 12 6mes the size of the Solar System –  The night sky there contains more than a million stars brighter than Sirius, the brightest star in our sky Infrared picture of the Galac6c Center from the Spitzer Space Telescope How Do Stars Move in the Milky Way? •  We want to understand –  How do stars near the Sun orbit the Milky Way? –  How do stars at the center of the Milky Way move? –  What do these orbits tell us about mass in the Milky Way? Mo6on from the Spectrum: the Doppler Effect •  Just like for sound waves, if the source is moving towards you it sounds like a higher frequency –  Sound: higher pitch –  Light: more blue •  Moving away: lower frequency –  Sound: lower pitch –  Light: more red Doppler Effect •  Something moving away looks redder •  Something moving closer looks bluer We See a Shij in the Spectral Lines •  The size of the shij tells you the speed of the source towards or away from you (Radial Velocity) (away) (towards) Star’s Mo6on in the Sky •  We also see a star’s mo6on across the sky (Tangen6al Velocity) –  May have to wait many years •  Combining the tangen6al and radial gives us the star’s true mo6on Orbits in the Spiral Arms •  The Sun orbits the center of the Galaxy in nearly a circle once every 240 million years –  so the Sun has completed over 18 orbits since its birth •  Stars in the thin disk all orbit at about the same speed –  This is a surprise, and indicates that most mass is in the disk, not in the center The Sun’s Orbit Stars Orbi6ng the Galac6c Center •  We can observe stars at the very center of the Galaxy The Black Hole at the Center of the Galaxy •  The stars at the center of the Galaxy are moving so fast that they must be orbi6ng something very massive –  About 4 million 6mes the mass of the Sun •  This must be a supermassive Black Hole –  Gravity is so strong that when you are too close even light cannot escape –  All that mass is concentrated in an infinitely small volume •  Or so says theory, but theory is probably wrong when the volume is very very small (10-­‐35 meters across) –  Material falling in forms an “accrea6on disk” that emits a large amount of energy The Galac6c Center is Very Ac6ve •  The central black hole blasts out large amounts of energy •  We observe very high-­‐energy gas above and below the Galac6c Center The Virgo Stream •  There is a stream of stars high above the Galac6c disk –  Indicates a past collision with a smaller Galaxy –  We’ll discuss galaxy collisions when we discuss galaxy forma6on Missing Mass •  When we compute the speed that stars should orbit based on the stars, gas and dust that we see, we get a speed very different from what we observe •  The simplest explana6on is that there is stuff in the Galaxy that we do not see –  Mass of this stuff is more than the mass of the stars, gas and dust •  Our first encounter with the Dark Ma?er problem –  Much more about that in two or three weeks Looking Out We See Many Many Galaxies Besides Spiral Galaxies •  But we see two types of other shapes –  Ellip6cal, ojen very large, very li?le gas and dust –  Irregular, typically smaller Galaxy Demographics •  About 20% of all galaxies are spiral –  But they are the brightest, so they make up the majority of visible galaxies –  About 2/3 have bars •  About 60% are ellip6cal –  Most are small •  The rest (20%) are irregular •  Most spirals and large ellip6cals have central supermassive black holes Hubble’s Classifica6on of Galaxies Colliding Galaxies •  We see some galaxies colliding –  The stars just go through each other, too far apart to collide –  The gas collides, and creates new stars –  Typical result: long streams of new stars, large gravita6onal distor6on of the galaxies Galaxies Come in Clusters Our local group Galaxy cluster Galaxy supercluster More about this when we talk about large-­‐scale structure in a couple weeks The Expanding Universe •  Most surprising of all: most galaxies have a red shij –  Indica6ng that they are moving away from us •  The more distant the galaxy the larger the red shij •  Simplest explana6on: the universe is expanding! –  We’ll see next week that we have other reasons to believe this