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Milky Way Galaxy

Physical Properties of the Milky Way

The Milky Way Galaxy is the galaxy in which we live. It is a large barred spiral galaxy. Some properties of our Galaxy are well determined, while others are either poorly measured or totally unknown. Our best information is about the motions of nearby stars in the Galactic plane and of the structure of the Galactic center. The total mass of the Galaxy is also reasonably well determined. The number of spiral arms and their positions are roughly known. The least understood portion of the Galaxy is its halo, which is observed primarily through its gravitational influence on the observable stars.

The relative motions of the nearby stars are derived from data gathered by the Hipparcus satellite. This information provides the velocity of the Sun relative to the local stars. Also derived from this data is the measurement of shear within the Galactic plane. This data combined with measurements of the proper motion of the massive black hole Sagittarius A* (Sgr A*) provide the best measurement of the rotational velocity Ω0 of the Galactic plane. The value given below for this rotational velocity is derived by this site's author from the formula given by Reid, Readhead, Vermeulen, Treuhaft that relates the proper motion of Sgr A* to the motions of the local stars and an assumed value for R0.[5] The value in the table is derived by setting the distance R0 to the value in the table, which comes from Eisenhauer et al.[2]

The best measurement of R0, the Sun's distance from the Galactic Center, comes from the measurements of stellar orbits of stars close to Sgr A*. These stars follow Keplerian orbits around the black hole. This single fact allows both the mass of the central black hole and the distance of the black hole from Earth to be derived from measurements of the positions and radial velocities of the orbiting stars.

The total mass of the Galaxy is derived from the gravitational influence of the Galaxy on the more distant globular clusters and the two Magellanic Clouds.

In the table given below, the unit name “mas” represents milli-arc-second.

Galactic Mass[1]

7 × 1011 Msolar

Galactic Center Distance (R0)[2]

7.62±0.32 kpc

Local Disk Rotation Speed (Ω0)

208±16 km s-1

Solar Peculiar Motion (V0)

Direction of Rotation:

5.25±0.62 km s-1

To Galactic North:

7.17±0.38 km s-1

Galactic Disk Radius[1]

15 kpc

Central Black Hole Mass[2]

( 3.61±0.32 ) × 106 Msolar

Central Black Hole Position[3]

Right Ascension (2000.0J):

17h 45m 40s

Declination (2000.0J):

−29° 00 28

Central Black Hole Proper Motion

Backer and Sramek[4]

Galactic Longitude:

−6.18±0.19 mas yr-1

Galactic Latitude:

−0.65±0.17 mas yr-1

Reid et al.[5]

Galactic Longitude:

−5.90±0.35 mas yr-1

Galactic Latitude:

+0.20±0.30 mas yr-1

[1]Dehnen, Walter, and Binney, James. “Mass Models of the Milky Way,” Monthly Notices of the Royal Astronomical Society 294 (1998): 429–438.

[2]Eisenhauer, F., et al. “Sinfoni in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month.” The Astrophysical Journal 628 (20 July 2005): 246–259.

[3]Rogers, Alan E.E., Doeleman, Sheperd, Wright, Melvyn C.H., Bower, Geoffrey C., Backer, Donald C., Padin, Stephen, Phillips, J.A., Emerson, Darrel T., Greenhill, Lincoln, Moran, James M., and Kellermann, Kenneth I. “Small-Scale Structure and Position of Sagittarius A* from VLBI at 3 Millimeter Wavelength.” The Astrophysical Journal Letters 434 (20 October 1994): L59–L62.

[4]Backer, D.C., and Sramek, R.A. “Proper Motion of the Compact, Nonthermal Radio Source in the Galactic Center, Sagittarius A*.” The Astrophysical Journal, 524 (20 October 1999): 805–815.

[5]Reid, M.J., Readhead, A.C.S., Vermeulen, R.C., Treuhaft, R.N. “The Proper Motion of Sagittarius A*. I. First VLBA Results.” The Astrophysical Journal 524 (20 October 1999): 816–823.

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