Astronomy 490†††††††††††††††††††††† †††††††††† Homework #5 †††††††††††††††††††††††††††††††††††††††Spring 2007



Due date:†† Monday, March 5 at start of class.

Please make a copy of your assignment to refer to during the class discussion.



Journal paper:††

†† Ghez et al. 2005, Stellar Orbits around the Galactic Center Black Hole,

†† ApJ, 620, 744



1.Read the Ghez paper carefully.Record your questions and comments and include

†††† them with the rest of your HW assignment.Try to include at least 2 questions and/or

††† comments per section.



2.Notice the orbital data reported in Table 2.


††† (i)†† Using Newtonís form of Keplerís 3rd law (i.e., the version that includes the masses

††††††††† of the black hole and star), write down an expression for the mass of the black in

††††††††† terms of the period and semi-major axis.What assumption if any do you have to

†††††††††† make about the mass of the star relative to that of the black hole?


†† (ii)Use your expression above to determine the mass (in solar masses) of the putative

†††††††† black hole from each of the 7 starís orbits given in the table.Comment on your

††††††††† results.



3.Compute the Schwarzschild radius of a black hole of the mass reported by Ghez.

†††† Express your result in km and in AU and compare it to the orbit of Mercury.



4.Write a short (<1 page), type-written essay addressing the following questions.


††††† Prior to reading this paper, how convinced or unconvinced were you that black holes

††††† existed?After reading the paper, has your view about black holes changed in any

††††† way?What arguments in this paper did you find most and least convincing regarding

††††† the possible existence of a black hole at the center of the Milky Way?


†††† Assume your reader is a fellow physics student who knows no astronomy.