The Milky Way Galaxy Distribution of Globular Clusters around a Point in Sagittarius Infrared Image of the Milky Way Other Galaxies “grand design” galaxy flocculent” galaxy elliptical galaxy Structure of the Milky Way Galaxy
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Other Galaxies “grand design” galaxy flocculent” galaxy elliptical galaxy
Structure of the Milky Way Galaxy Halo: globular clusters, red dwarfs, low metal abundance – old objects dominate. Disk: stars, gas, dust, O and B stars, emission nebulae, open clusters, high metal abundance – young objects dominate. Disk Bulge Halo Disk includes spiral arms, delineated by young objects such as O and B stars. Bulge: old, low metal abundance stars.
Stellar Populations What does this table suggest about the origin of the Milky Way Galaxy?
Composition of the the Disk Component and the Spherical Component of the Galaxy Disk Component Spherical Component • Includes spiral arms. • Open clusters. • Young stars. • High metal abundance. • Lots of gas and dust. • Orbits nearly circular and all revolve in the same direction with approximately the same orbital orientation • Population I • Includes the nuclear bulge. • Globular clusters. • Old stars. • Low metal abundance. • Little gas and dust. • Orbits have high eccentricities and random orientations • Population II
Spiral Arm Tracers Sun “Zone of Avoidance” Measuring the Structure of the Galaxy Using the Doppler Effect Why is there a “zone of avoidance?
Example 1The Sun moves in a nearly circular orbit around the center of the Galaxy, which is about 8.5 kpc away.How long does it take to complete 1 revolution? Example 2Using the data from Example 1, calculate the mass of the part of our galaxy that is inside of Earth’s orbit.
Example 3 How much mass is inside the orbit of a star 16 kpc from the center of the Galaxy? Most of the Galaxy’s mass is evidently not visible. More than 90% of it may be part of the “dark halo”, also known as the “galactic corona” ,but it is not yet known what kind of mass it is. Other galaxies also have a large dark matter component.
Spiral Structure of Our Galaxy The Sun is about 8.5 kpc from the center. The thickness of the disk at the location of the Sun is only about 1000 pc What causes the spiral arms? What causes the “spurs”?
Evidence for Spiral Structure Spiral arm tracers are large, luminous objects that are concentrated in the spiral arms. This includes O and B stars, HI clouds, and HII regions. O and B stars are concentrated in spiral arms because that is where most stars are formed, and they are bright and short-lived. After birth, they spend most of their lives near their birthplaces in the spiral arms. HII regions are large and bright. They are created by the ionizing UV radiation from O and B stars. What causes a galaxy to have spiral arms?
X-ray Image of The Center of Our Galaxy - Sagittarius A* The nucleus of our galaxy contains a 3 million solar mass black hole. This fact is deduced from the motion of objects near the center. The lobes of hot gas have temperatures of about 20 million kelvins. High temperatures are presumably due to supernova shock waves and stellar wind from hot stars. http://chandra.harvard.edu/photo/2003/0203long/index.html 8.4 arcminutes
Near Infrared Image of Stars Near SagA*(Evidence for the Presence of a Supermassive Black Hole) Speeds as high as 1400 km/s imply that these stars orbit a black hole with a mass of about 3 million solar masses.
Example 4 One of the stars near SagA* has an orbital speed of 1400 km/s and is about 0.0057 pc from SagA*. Calculate the mass of the central object.