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Fig 16.19.

(Continued) (e) Curvature in the paths of three stars. [Andrea Ghez, UCLA; (a)-(d) Ghez,A. M. et al., Astrophys. J., 509, 678, 1998, Figs. 3-6; (e) Ghez,A. M. et al. Reprinted by permission from Nature, 407, 307, Fig. 1, Copyright (2000) Macmillan Publishers Ltd.]

Fig 16.19.

(Continued) (e) Curvature in the paths of three stars. [Andrea Ghez, UCLA; (a)-(d) Ghez,A. M. et al., Astrophys. J., 509, 678, 1998, Figs. 3-6; (e) Ghez,A. M. et al. Reprinted by permission from Nature, 407, 307, Fig. 1, Copyright (2000) Macmillan Publishers Ltd.]

conclusively show that this mass is contained within RS, but alternative explanations, such as a very rich cluster, don't seem very likely. These masses are consistent with previous determinations using other techniques, but the new measurements place much tighter constraints on the confinement of the matter, and the coincidence with SgrA*.

By following these stars for another two years (through 1999), the group found three stars whose paths had a measurable curvature about the SgrA position (Fig. 16.19e). These stars had projected positions on the sky —0.005 pc from SgrA*. From this curvature, they could directly measure the acceleration in the circular orbit. It turns out to be numerically close to the value of the Earth's acceleration in its motion about the Sun. The shortest orbital period is 20 years, so we have the prospect of being able to watch something make a complete revolution about the galactic center in our lifetimes. Acceleration vectors don't allow for a better mass estimate, but if you use the mass estimate from the proper motions, the volume the mass is constrained to is decreased by an order of magnitude. This greatly strengthens the case for that mass to be included within its Schwarzschild radius.

There is also indirect evidence for explosive activity in the galactic center region. For example, there is an armlike feature in our galaxy, some 3 to 4 kpc from the galactic center, called the "3 kpc arm", which appears to be expanding at about 50 km/s. Speculation is that this expansion is due to some explosion in the relatively recent past (see Problem 16.14). We see other similar features closer in, suggesting that this activity has taken place on a continuing basis. We will see in Chapter 19 that the activity in our galactic center is small compared with that in many galaxies. However, it gives us our best opportunity to study such activity "close up". For this reason, study of the galactic center is a very active field.

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