Our Galaxy Chapter Twenty Five Guiding Questions 1 What is our Galaxy How do astronomers know where we are located within it 2 What is the shape and size of our Galaxy 3 How do we know that our Galaxy has spiral arms 4 What is most of the Galaxy made of Is it stars gas dust or something else 5 What is the nature of the spiral arms 6 What lies at the very center of our Galaxy Interstellar dust obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic disk 1 As a result the Sun s location in the Galaxy was unknown for many years This dilemma was resolved by observing parts of the Galaxy outside the disk 2 Determining the distance and direction of the globular clusters gave us the Sun s location Our Sun lies within the galactic disk some 8000 pc 26 000 ly from the center of the Galaxy Observations at nonvisible wavelengths reveal the shape of the Galaxy 3 There are about 200 billion 2 1011 stars in the Galaxy Our Galaxy has a disk about 50 kpc 160 000 ly in diameter and about 600 pc 2000 ly thick with a high concentration of interstellar dust and gas in the disk The Sun orbits around the center of the Galaxy at a speed of about 790 000 km h It takes about 220 million years to complete one orbit The galactic center is surrounded by a large distribution of stars called the central bulge This bulge is not perfectly symmetrical but may have a bar or peanut shape The disk of the Galaxy is surrounded by a spherical distribution of globular clusters and old stars called the galactic halo The spin flip transition in hydrogen emits 21 cm radio waves 4 This is the same physical principle behind magnetic resonance imaging MRI an important diagnostic tool of modern medicine These emissions easily penetrate the intervening interstellar dust Spiral arms can be traced from the positions of clouds of atomic hydrogen 5 OB associations H II regions and molecular clouds in the galactic disk outline huge spiral arms 6 The rotation of our Galaxy reveals the presence of dark matter From studies of the rotation of the Galaxy astronomers estimate that the total mass of the Galaxy is about 1012 M 7 8 Only about 10 of this mass is in the form of visible stars gas and dust The remaining 90 is in some nonvisible form called dark matter that extends beyond the edge of the luminous material in the Galaxy Our Galaxy s dark matter may be a combination of MACHOs dim star sized objects massive neutrinos and WIMPs relatively massive subatomic particles Spiral arms are caused by density waves that sweep around the Galaxy There are two leading theories of spiral structure in galaxies According to the densitywave theory spiral arms are created by density waves that sweep around the Galaxy The gravitational field of this spiral pattern compresses the interstellar clouds through which it passes thereby triggering the formation of the OB associations and H II regions that illuminate the spiral arms 9 10 According to the theory of self propagating star formation spiral arms are caused by the birth of stars over an extended region in a galaxy Differential rotation of the galaxy stretches the starforming region into an elongated arch of stars and nebulae 11 The innermost part of the Galaxy or galactic nucleus has been studied through its radio infrared and X ray emissions which are able to pass through interstellar dust 12 A strong radio source called Sagittarius A is located at the galactic center This marks the position of a supermassive black hole with a mass of about 3 7 106 M 13 Key Words central bulge of a galaxy dark matter density wave disk of a galaxy far infrared flocculent spiral galaxy galactic nucleus galaxy globular cluster grand design spiral galaxy HI halo of a galaxy high velocity star interstellar extinction Local Bubble magnetic resonance imaging MRI massive compact halo object MACHO microlensing Milky Way Galaxy near infrared rotation curve RR Lyrae variable Sagittarius A self propagating star formation spin of a particle spin flip transition spiral arm 21 cm radio emission weakly interacting massive particle WIMP winding dilemma 14
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