SOLUTIONS HOMEWORK # 6


  1. (Chp.16. Review 11) What evidence do we have that galaxies collide and merge?

    The clearest evidence is found in optical images of interacting galaxies such as the Antennae Galaxy or the Whirpool Galaxy which show the galaxies colliding and material being affected by the gravitacional forces of each galaxy. Additional evidence is found in the bursts of star formation in galaxies that have very little gas or dust. The star formation is believed to have been triggered when to galaxies collided and the gas and dust of the galaxy show star formation has been stripped by the gravitacional field of the second galaxy.

    Also the LMC and SMC show evidence of disturbances caused by interactions with the Milky Way. Ring galaxies appear to originate when two galaxies collide head on at high speed, many of these galaxies do have nearby companions.


  1. (Chp.16. Problem 8) We have found a galaxy in which the outer stars have orbital velocities of 150 km/s. If the radius of the galaxy is 4kpc, what is the orbital period of the outer stars? (Hint: 1pc = 3.08 x 10^13 km and 1 year = 3.15 x 10^7 sec ).

    radius = 4kpc = 4 x 10^3 pc = (4 x 10 ^3)(3.08 x 10^13)km = 1.23 x 10^17 km

    period= (2 x pi x radius)/ velocity= 2 x pi x 1.23 x 10^17km / (150km/s)

    = 5.16 x 10^15 sec

    period= (5.16 x 10^15 sec)/(3.15 x 10^7 sec/yr)= 164 x10^6 yrs = 164 million yrs

  2. (Chp.17. Review 5) How does the unified model explain the two kinds of Seyfert galaxies?

    The unified model of active galactic nuclei produces the observed properties of a Type 2 when the model is viewed edge-on. The disk of gas and dust hides the hot central region of the AGN and only the narrow line region above the disk is visible. The type 1 Seyferts are seen when the model is viewed slightly inclined so that the sight line is above the disk and into the hot central cavity so that the broad line region is visible.


  1. (Chp.17. Problem 5) If the active core of a galaxy contains a black hole of 10^6 solar masses, what will the orbital period for the matter orbiting the black hole at a distance of 0.33AU)?

    M = a^3 / P^2 so, orbital period = P = (a^3 / M)^½

    where M is in solar masses, a in AU and P in years

    P= (0.33^3/ 10^6) ^ ½ yrs= (3.59 x 10^-8)^ ½ yrs= 1.9 x 10^-4yrs=1.66 hrs.



  1. (Chap.18. Review 9) Why do we conclude that the universe must have been very uniform during its first million years?

    Because the Cosmic Microwave Background (CMB) is very isotropic, i.e uniform. This suggests that the gas at the time of recombination was very uniformly distributed.