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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“grand design” 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 includes spiral arms, delineated by young objects such as O and B stars.
Bulge: old, low metal abundance stars.
What does this table suggest about the origin of the Milky Way Galaxy?
Spiral Arm Tracers Component of the Galaxy
“Zone of Avoidance”Measuring the Structure of the Galaxy Using the Doppler Effect
Why is there a “zone of avoidance?
Example 1 Component of the GalaxyThe 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 Component of the Galaxy
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.
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”?
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?
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.
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 Component of the Galaxy
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.