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Transcript
Galaxies Chapter 23: Our Galaxy A galaxy is a large collection of stellar and interstellar matter Isolated in space All held together by gravity The galaxy we live in the Milky Way Galaxy PHY 150 Dr. Akins Shape 3 Views of the MW The Galaxy or MW, for short Seeds, SS&B, 5th 2 Shape, cont In the late 1700’s astronomers attempted to determine the shape of the galaxy Herschels counted stars Found approximately equal numbers of stars in all directions Concluded we are at the center of the galaxy Seeds, SS&B, 5th Stars, gas, dust, neutron stars, black holes, etc Today astronomers think the MW is similar to the Andromeda Galaxy Seeds, SS&B, 5th 4 Our View of the MW Top is an artist’s conception of the MW Bottom is an optical view We see a faint band of light across the sky Remember, we are in the galaxy We can’t get a view of the entire galaxy Seeds, SS&B, 5th 5 Seeds, SS&B, 5th 6 1 Variable Stars Pulsating variable stars became important to studying the Galaxy Instability Strip Became a new yardstick RR Lyrae and Cepheid Both have characteristic light curves Pulsations are due to internal instabilities Period is the time it takes to complete a cycle of bright to faint to bright Seeds, SS&B, 5th The pulsating stage occurs when the star has moved off the main sequence The star is internally unstable Cepheid variables are high mass stars RR Lyrae are low mass stars that hit the bottom edge of the strip 7 Seeds, SS&B, 5th Cepheids RR Lyrae Pulsate in distinctive sawtooth pattern Large variation in periods From 1 to 100 days In the upper instability strip of the HR diagram All have very similar light curves Periods range from 0.5 to 1 day Lie at the bottom of the instability strip Ionized gas in their atmosphere causes the pulsations Seeds, SS&B, 5th 9 Measuring Distance with Variable Stars Seeds, SS&B, 5th Once identified as a Cepheid or RR Lyrae variable, you can tell its luminosity You can observe its apparent brightness Form its luminosity, you can determine the distance Same rule we used for stars in previous chapters All RR Lyrae have approximately the same luminosity Cepheids have luminosities related to their period Seeds, SS&B, 5th 10 Period Luminosity Relationship 8 11 ~100 x Sun Long periods → high luminosities Short periods → low luminosities Seeds, SS&B, 5th 12 2 Globular Clusters Globular Cluster Distribution Many RR Lyrae stars are found in globular clusters Using the variable stars, Shapley found Globular clusters are at great distances from the Sun The clusters’ three-dimensional distribution in space is roughly spherical, with a 30 kpc diameter Many thousands of parsecs Not centered on the Sun Seeds, SS&B, 5th 13 Seeds, SS&B, 5th Shapley’s Conclusions Center of the Galaxy Distribution of clusters maps out the extent of the Galaxy About 8 kpc from the Sun In Sagittarius Called the Galactic Center 15 MW in Non-Visible Wavelengths Distances in 1000’s of pc Gas and dust block our view of the center Also way astronomers originally thought the galaxy was much smaller Seeds, SS&B, 5th 16 MW In Other Wavelengths Visible light suffers interstellar extinction Globular cluster distribution centered around Sagittarius The center of the distribution was the center of the Galaxy Now we call it the halo Seeds, SS&B, 5th 14 Galactic nucleus is completely obscured from our view The longer the wavelength of the radiation used to “see” into the galaxy, the farther we can see Longer wavelengths, less interstellar extinction Seeds, SS&B, 5th 17 Seeds, SS&B, 5th 18 3 A Picture of the MW Components of the Galaxy Another view of the structure of the galaxy Two main components Seeds, SS&B, 5th Disk Sphere Each have different characteristics 19 Seeds, SS&B, 5th Spherical Component Halo Scattered in a roughly spherical shape around the center Includes Consists of thinly scattered stars and globular clusters Roughly spherical Contains only old stars (Pop II) No gas and dust No current star formation Stars have random orbits in three dimensions Reddish in color Halo Nuclear bulge Seeds, SS&B, 5th 21 Contains both young and old stars More old stars farther from the center Football shaped Little gas and dust, especially in outer regions Possibly some ongoing star formation in inner regions Ring of gas and dust near center Yellow-white/reddish-yellow Seeds, SS&B, 5th None for last 10 billion years 22 Disk Component Dense cloud of stars surrounding the center of the galaxy Slightly flattened Or at least very little Seeds, SS&B, 5th Nuclear Bulge 20 Contains all matter confined to the plane of rotation General characteristics 23 Highly flattened Contains both young and old stars Contains gas and dust Site of on-going star formation Gas and stars move in circular orbits in the galactic plane Overall white, blue in the spiral arms Seeds, SS&B, 5th 24 4 Disk Component, cont Diameter ~25 kpc Part of the disk component Long curves containing Also seen as 75,000 ly Spiral Arms A bit smaller than the 25 kpc Bright stars Star clusters Gas Dust Uncertainty No sharp boundaries Star density decreases Different types of stars are located in different parts Gas and dust block view Affects both thickness and diameter estimates Seeds, SS&B, 5th 25 Seeds, SS&B, 5th Star Orbits, Summary Seeds, SS&B, 5th Orbit Comparison 27 Seeds, SS&B, 5th Stellar Populations, Summary 28 21 cm Radiation Neutral hydrogen emits 21 cm wavelength radiation when its electron flips Different spin relationships have different energies When the electron moves to the lower energy state, the photon given off has λ = 21 cm Seeds, SS&B, 5th 26 29 Radio wavelength Seeds, SS&B, 5th 30 5 21 cm Map Spiral Arms Seeds, SS&B, 5th The 21-cm radiation also allows astronomers to map the spiral arms Analysis of the Doppler shifts of the signals 31 Seeds, SS&B, 5th Spiral Arms, Map Spiral Arms Looking at various objects at various wavelengths leads to Seeds, SS&B, 5th 33 Spiral Arms, final Seeds, SS&B, 5th 32 At least four major spiral arms Several short arm segments Bar instead of circular central bulge Seeds, SS&B, 5th 34 Artist’s Conception of the MW 35 Seeds, SS&B, 5th 36 6 M83 Mass Measured by studying the motion of gas clouds and stars in the Galactic disk Total mass (in solar masses) = orbital size (AU)3 / orbital period (years)2 Kepler’s Seeds, SS&B, 5th Third Law as modified by Newton Using the information for our Sun, the mass of the Galaxy is ~1011 solar masses 37 But this is only the mass that is within the orbit of the Sun Seeds, SS&B, 5th 38 Uncertainties in Mass Measurement Mass and Sun’s Orbit Knowing actual distance from center to Sun Gives a lower limit since only the mass inside the Sun’s orbit, but how to increase to account for the rest? Complex rotation of the galaxy Seeds, SS&B, 5th 39 Differential rotation Seeds, SS&B, 5th Differential Rotation 40 Mass, cont The orbital speed of any object depends on the mass inside its orbit Therefore, we need to measure the orbital speed of objects as far out to the edge of the Galaxy as possible Seeds, SS&B, 5th 41 15 kpc is as far as we have measured so far Seeds, SS&B, 5th 42 7 Rotation Curve Dark Matter Note the rotation curve does not drop as it would in Keplerian motion If all the mass of the Galaxy was within the visible part, the rotation curve should follow the dashed line However, the curve actually rises out to the ~50 kpc Seeds, SS&B, 5th This is the distance that can be reliably measured 43 Seeds, SS&B, 5th Dark Matter, cont Dark Halo Mass calculates indicate there is ~9 x 1011 solar masses of mass contained within the 50 kpc The region of a galaxy beyond the visible halo where dark matter is believed to reside Composition? MACHOs WIMPs Other Something else This is three times more than was in the visible part of the Galaxy Dark matter is the general term given to this mass we cannot see Brown and black dwarfs, white dwarfs, ordinary matter that’s too dim or small to see Particles have not been confirmed experimentally Neutrinos (with mass) Or not really there, Newton’s Laws and General Relativity have to be modified Seeds, SS&B, 5th 44 45 Not generally accepted, but being discussed Seeds, SS&B, 5th Dark Halo 46 “Seeing” Dark Matter Can only be determined by its gravitational effect on other matter Rotation curves in galaxies Gravitational Lensing Temperature distributions Seeds, SS&B, 5th 47 Seeds, SS&B, 5th 48 8 Gravitational Lensing Bullet Cluster Seeds, SS&B, 5th 49 Galactic Center Colliding galaxy clusters Best evidence of dark matter The stars, gas, and dark matter all behave differently This allows the various types of matter to be distinguished Seeds, SS&B, 5th 50 View of the Center There should be an area densely populated with stars at the center of the Galaxy The interstellar medium blocks our view of the center The center of the Galaxy is in the direction of Sagittarius Seeds, SS&B, 5th 51 Seeds, SS&B, 5th 52 53 Seeds, SS&B, 5th 54 More Views Seeds, SS&B, 5th 9 Near the Center At the Center Measurements indicate a large mass and a small size A supermassive black hole fits these requirements The object at the center of the Galaxy is call Sgr A* Bright radio sources Filaments suggest a strong magnetic field in the vicinity Also many bright X-ray sources in the region Seeds, SS&B, 5th 55 Evidence of Sgr A* Seeds, SS&B, 5th Seeds, SS&B, 5th 56 Sag A* 57 The Milky Way Galaxy Seeds, SS&B, 5th Violent High energy output Seeds, SS&B, 5th 58 The Milky Way Galaxy 59 Seeds, SS&B, 5th 60 10