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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 l.jpg
Other Galaxies Sagittarius

“grand design” galaxy

flocculent” galaxy

elliptical galaxy


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Structure of the Milky Way Galaxy Sagittarius

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.


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Stellar Populations Sagittarius

What does this table suggest about the origin of the Milky Way Galaxy?


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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


Measuring the structure of the galaxy using the doppler effect l.jpg

Spiral Arm Tracers Component of the Galaxy

Sun

“Zone of Avoidance”

Measuring the Structure of the Galaxy Using the Doppler Effect

Why is there a “zone of avoidance?



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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.


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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.


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Spiral Structure of Our Galaxy Component of the 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”?


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Evidence for Spiral Structure Component of the Galaxy

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 l.jpg
X-ray Image of The Center of Our Galaxy - Sagittarius A Component of the Galaxy*

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


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Radio Image of Sagittarius A Component of the Galaxy

50 pc


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Central Region of Sagittarius A Component of the Galaxy

3 pc


Near infrared image of stars near saga evidence for the presence of a supermassive black hole l.jpg
Near Infrared Image of Stars Near SagA Component of the Galaxy*(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.


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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.


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