ASTR100 (Spring 2008) Introduction to Astronomy The Milky WayWhat does our galaxy look like?PowerPoint PresentationSlide 4Slide 5Slide 6Slide 7How do stars orbit in our galaxy?Slide 9Slide 10Slide 11Thought QuestionSlide 13Slide 14How is gas recycled in our galaxy?Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Summary of Galactic RecyclingSlide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Where do stars tend to form in our galaxy?Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42What clues to our galaxy’s history do halo stars hold?Slide 44How did our galaxy form?Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51What lies in the center of our galaxy?Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58ASTR100 (Spring 2008) Introduction to AstronomyThe Milky WayProf. D.C. RichardsonSections 0101-0106What does our galaxy look like?The Milky Way galaxy appears in our sky as a faint band of light.Dusty gas clouds obscure our view because they absorb visible light.This is the interstellar medium that makes new star systems.All-sky ViewWe see our galaxy edge-on from the inside.Primary features: disk, bulge, halo, globular clusters.If we could view the Milky Way from above the disk, we would see its spiral arms.How do stars orbit in our galaxy?Stars in the disk all orbit in the same direction with a little up-and-down motion.Orbits of stars in the bulge and halo have random orientations.Thought QuestionWhy do orbits of bulge stars bob up and down?A.They’re stuck to the interstellar medium.B.Gravity of disk stars pulls toward disk.C.Halo stars knock them back into disk.Thought QuestionWhy do orbits of bulge stars bob up and down?A.They’re stuck to the interstellar medium.B.Gravity of disk stars pulls toward disk.C.Halo stars knock them back into disk.Sun’s orbital motion (radius and speed) tells us mass within Sun’s orbit: 1.0 x 1011 MSunHow is gas recycled in our galaxy?Star-gas-star Cycle Gas from old stars recycled into new star systems.High-mass stars have strong stellar winds that blow bubbles of hot gas.10 lyLow-mass stars return gas to interstellar space through stellar winds and planetary nebulae. 1 lySupernovae generate shock waves (revealed by X-rays from hot gas) as they burst into the interstellar medium.20 lySupernova remnants cool and begin to emit visible light as they expand.New elements made by supernovae mix into interstellar medium.130 lyMultiple supernovae can create huge hot bubbles that blow out of disk.Gas clouds cooling in the halo then rain back down on disk.Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons.Molecular clouds form next, after gas cools enough to allow atoms to combine into molecules.Molecular clouds in OrionComposition:• Mostly H2• About 28% He• About 1% CO• Many other x molecules.Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle.Radiation from newly formed stars is eroding these star-forming clouds.Summary of Galactic RecyclingStars make new elements by fusion.Dying stars expel gas and new elements, producing hot bubbles (~106 K).Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K).Further cooling permits molecules to form, making molecular clouds (~30 K).Gravity forms new stars (and planets) in molecular clouds.Gas CoolsThought QuestionWhere will the gas be in 1 trillion years?A.Blown out of the galaxy.B.Still recycling just like now.C.Locked into white dwarfs and low-mass stars.Thought QuestionWhere will the gas be in 1 trillion years?A.Blown out of the galaxy.B.Still recycling just like now.C.Locked into white dwarfs and low-mass stars.We observe the star-gas-star cycle operating in Milky Way’s disk using many different wavelengths of light.RadioRadioIRIRVis.X-ray-rayInfrared light reveals stars whose visible light is blocked by gas clouds.InfraredVisibleX-rays are observed from hot gas above and below the Milky Way’s disk.X-rays21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk.Radio (21cm)Radio waves from carbon monoxide (CO) show locations of molecular clouds.Radio (CO)Long-wavelength infrared emission shows where young stars are heating dust grains. IR(dust)Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds.Where do stars tend to form in our galaxy?Ionization nebulae are found around short-lived high-mass stars, signifying active star formation.Reflection nebulae scatter the light from stars.Why do reflection nebulae look bluer than the nearby stars?For the same reason that our sky is blue!What kinds of nebulae do you see?Disk: Ionization nebulae, blue stars  star formation.Halo: No ionization nebulae, no blue stars  no star formation.Much of the star formation in the disk happens in spiral arms.Whirlpool GalaxyIonization nebulaeBlue starsGas cloudsSpiral arms are waves of star formation.1. Gas clouds get squeezed as they move into spiral arms.2. Squeezing of clouds triggers star formation.3. Young stars flow out of spiral arms.What clues to our galaxy’s history do halo stars hold?Halo Stars: 0.02-0.2% heavy elements (O, Fe, …), only old stars.Disk Stars: 2% heavy elements, stars of all ages.Halo stars formed first, then stopped.Disk stars formed later, kept forming.How did our galaxy form?Our galaxy probably formed from a giant gas cloud.Halo stars formed 1st as gravity caused cloud to contract.Remaining gas settled into spinning disk.Stars continuously form in disk as galaxy grows older.Stars continuously form in disk as galaxy grows older.Warning: This model is oversimplified!Detailed studies suggest halo stars formed in clumps that later merged.What lies in the center of our galaxy?Infrared light from centerRadio emission from centerRadio emission from centerSwirling gas near centerSwirling gas near centerOrbiting stars near centerStars appear to be orbiting something massive but invisible… a black hole?Orbits of stars indicate a mass of about 4 million MSun.X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall