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THE MILKY WAY

THE MILKY WAY. Basic Facts about the Milky Way. The Sun is one of about 200 billion stars in the Milky Way Galaxy The Milky Way is a spiral galaxy , with a flat disk, central bulge and bar, and spherical halo The disk contains stars, gas, and dust.

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THE MILKY WAY

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  1. THE MILKY WAY

  2. Basic Facts about the Milky Way • The Sun is one of about 200 billion stars in the Milky Way Galaxy • The Milky Way is a spiral galaxy, with a flat disk, central bulge and bar, and spherical halo • The disk contains stars, gas, and dust. • The halo contains stars and globular clusters

  3. First, a little history: • The first efforts to map the structure of the Milky Way through star counts concluded that we were in the center of the Milky Way, because extinction of dust was neglected • William and Caroline Herschel's Picture of the Milky Way (18th century):

  4. William and Caroline Herschel • William H. was a musician and composer, and did astronomy as a hobby at first • He and his sister discovered comets, the planet Uranus, and demonstrated that there was light “beyond” the reddest light you can see with your eyes: infrared light

  5. The Herschels built over 400 telescopes, including one with a mirror of diameter 50 inches – and length 40 feet. c. 1788

  6. At the end of his life, William Herschel was named “King’s Astronomer” and spent his time counting stars to map out the structure of the Milky Way. He deduced that the Sun is in the center of the Milky Way – because he saw equal numbers of stars in every direction.

  7. What he didn’t know:Interstellar Space in the disk of the Milky Way is filled with Gas and Dust Dust in the gas clouds make stars look RED and FAINT.

  8. Harlow Shapley (Harvard Observatory) used RR Lyrae Stars in Globular Clusters to map the halo of the Milky Way in about 1915 He realized that the Sun is not in the center of the Milky Way X = Center of The Milky Way

  9. The modern view of the structure of the Milky Way uses IR and radio maps to sort out the structure of the disk, and map out the spiral arms through the dust.

  10. Milky Way • Rotating disk of gas, dust and stars • Bulge of stars • Central bar of gas, dust and stars • Spherical Halo of stars and globular clusters

  11. Stars rotate in the disk, with a little up and down motion

  12. In the halo, the orbits of the stars are in random planes

  13. Here's the actual structure of the spiral arms of the Milky Way: The Milky Way’s disk is ROTATING The Sun travels once around the Milky Way disk in 220 Million years

  14. If you could fly out of the Milky Way and look back on it, it might look something like this: M83

  15. Amazing Fact About the MILKY WAY #1: Stars are forming today out of the gas clouds in the disk.

  16. Amazing Fact About the MILKY WAY #1: Stars are forming today out of the gas clouds in the disk. Molecular Clouds: Mostly H2 and CO Supernovae Red Giant Mass Loss Planetary Nebulae

  17. Star formation is triggered by spiral density waves, which gives a spiral galaxy its characteristic ”spiral arms".

  18. Why are there spiral arms? Are the spiral arms in galaxies just the result of stars closer to the center orbiting faster than those further out? NO The spiral arms would have wound up and disappeared long ago

  19. Some fun numbers: The Sun became a star 4.5 billion years ago: 4.5 x 109 years The Sun orbits once around the center of the Milky Way once every 220 million years Years per orbit: 220 x 106 years or 2.20 x 108 years So how many times has the Sun orbited around the Milky Way?

  20. SPIRAL DENSITY WAVES Instead, the spiral arms are standing waves, and the gas and stars streams through (traffic accident analogy, see above) The spiral arms are rotating, but very slowly As gas streams through the spiral arm, it gets shocked, and starts to make stars So the young, bright blue stars are bunched up in the spiral arms

  21. Tucsonans are familiar withstanding waves in flooding rivers

  22. Standing (stationary) Wave in river

  23. Surf!

  24. Back to spiral arms and Spiral Density Waves • The spiral arms are a pattern, that rotates more slowly than the gas and dust in the disk • When the gas moves through the spiral arms, it is compressed and forms stars • The young, bright blue stars are therefore found in the arms. • By the time the disk moves through the spiral arm, the young stars have left the main sequence and are red.

  25. Green gas cloud passes through red arm Blue stars form, then explode as supernovae Animation from G. Rieke

  26. Brown Dust Star formation regions

  27. What starts the spiral density waves?Maybe the waves are excited by a satellite galaxy Computer model Real galaxy and its satellite

  28. A few words about the gas and dust in the disk of the Milky Way

  29. Most of the interstellar gas is hydrogen. Astronomers map the hydrogen using radio telescopes, using the 21-cm line – energy released by the spin flip of electron

  30. The Milky Way mapped in 21cm:atomic hydrogen In this picture, the sky is projected such that the plane of the Milky Way disk is running horizontally across the center Some streams of hydrogen above the disk may be bubbles blown out of the disk by supernova explosions

  31. 21-cm Map of Spiral arms in the Milky Way Yellow arrow: The Sun Blue Dot: Center of The Milky Way

  32. In the densest gas clouds, most of the gas is H2 = molecular hydrogen Ultraviolet photons from hot stars can destroy the molecular hydrogen, dividing the molecules into individual hydrogen atoms So only the densest clouds have molecular clouds

  33. Carbon Monoxide MoleculeSpectrum has lines at wavelength of 2-mm Interstellar Molecules like CO (Carbon Monoxide) emit radiation by rotation and vibration rather than electrons changing energy levels

  34. Rotation and vibration of molecules at specific rates: • Changes in rotation and vibration •  changes in energy of the molecule •  spectral lines, observed by mm telescopes

  35. Giant Molecular Clouds • Rotating in disk of the Milky Way • 100,000 – a million solar masses in molecules • 100-300 light years across • Cold: 10-20 degrees Kelvin • Densities: million molecules per cubic cm • If they pass through a spiral density wave, • they start to collapse and form stars

  36. Dust • Mostly carbon and silicon • Dust particles particles are a few micro-meters in size • Dust which is deep in molecular clouds has frozen water and CO2 Atomic collisions and ultraviolet light heats the dust, so the dust grains radiate like black bodies, in the infrared Mass in dust is only 1% of the mass in gas Interstellar dust was formed in the cool outer regions of red giants Distributed into interstellar space by planetary nebulae, stellar winds

  37. Amazing Fact About the Milky Way #2: Despite the appearance of galaxies in these pictures, the Milky Way is mostly empty space • A few analogies: • If the Sun were the size of a baseball, the density of the stars in our galaxy • would be comparable to scattering fifty baseballs across the United States, • so that there would be one star per state. • If the distance from home plate to the pitcher's mound were equal to the • distance from Earth to the Sun, the next star would be 800 miles away. • Average gas density is one atom per cubic centimeter. • On the surface of the Earth, air has about • 2x1019 molecules per cubic centimeter

  38. Amazing Fact About the Milky Way #3: About 90% of the Mass of the Milky Way is something besides stars, black holes, gas or dust We use the Doppler shift to measure the velocities of stars and gas clouds in the disk and halo of the Milky Way Then we can calculate the MASS the Milky Way must have to keep the stars and gas clouds from flying away  HUGE MASS, The mass of the Milky Way is about 1 trillion solar masses

  39. Let’s do the math

  40. The Sun goes around the center of the Milky Way Galaxy on a nearly circular orbit. Thanks to Barbara Ryden for the slides

  41. Orbital radius = 8000 parsecs = 26,000 light-years Orbital speed = 220 km/second = 490,000 miles/hour Orbital period = 220 million years

  42. Sun moves on a (nearly) circular orbit rather than a straight line because of the mass within its orbit.

  43. The mass in the Galaxy which is outside the Sun’s orbit exerts gravitational forces on the Sun, but they CANCEL OUT

  44. What is M, the mass required to keep the Sun on its orbit around the Galactic Center? This requires a little math. If you don’t want to follow, just start paying attention again at the end when I tell you the answer

  45. Newton’s Law of Gravity F = force m = mass of one object M = mass of other object r= distance between centers of objects G = “universal constant of gravitation” (G = 6.7 × 10-11 Newton meter2 / kg2)

  46. Newton: shape of orbit depends on speedof the satellite at launch. Low speed = closed orbit (circle, ellipse). High speed = open orbit (parabola, hyperbola)

  47. A satellite will have a circular orbit if its initial speed = circular speed ( vcirc) Presented without proof (life is too short). r= radius of circular orbit M= mass of object being orbited

  48. square each side: rearrange:

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