(i) What's the period of the cluster?
(ii) What's the mass and mean density interior to its orbit?
(iii) If the cluster has radius of ~10pc and internal velocity dispersion ~10 km/s, what's its mass density?
(i) comet nuclei in the Oort cloud, ~¼ pc from the sun.
(ii) gas in an accretion disk 1000 AU from
a 108M black hole.
(iii) stars orbiting in the galactic center cluster (distance 8.5 kpc) of radius ~30 arcsec and density
~106 M pc-3
(i) Hubble's original estimate was Ho = 530 km s-1Mpc-1.
What cosmic age does this imply?
(ii) For Ho = 72 km s-1 Mpc-1, what's the Universe's critical density in M
pc-3 (use c = 3Ho2/8G).
(iii) How much denser than the critical density is a typical galaxy (radius ~ 10 kpc, Vrot ~ 250 km/s) and a typical galaxy cluster (radius ~1 Mpc and velocity dispersion ~500 km/s).
(i) What's the total kinetic energy (KE) of a giant E galaxy with L ~ 1011 L, ~ 300 km s-1 and M/L ~ 10?
(ii) From the virial theorem, the galaxy's gravitational binding energy (BE) is equal to its KE, which was liberated when it formed. If it collapsed on its current dynamical timescale, what was the "collapse luminosity", LBE, in PLU and L?
(iii) Is this significant compared to the luminosity of the associated starburst (which would be classified as a "ULIRG" -- or Ultra-Luminous Infrared Galaxy)?
(2) Magnitudes and Mass to Light Ratios :
m - M = -5 + 5 log10 dpc
µV = 26.39 - 2.5 log(IV)
Hint: consider placing the object at a distance such that 1 pc = 1 arcsec. You will also need to use the solar absolute magnitudes given in the Lecture notes: Topic 1.3e .
(3) You outshine the stars!
Putting mass-to-light ratios into physical units can be surprising:
(i) What would its luminosity be, expressed in Watts and in L?
(ii) Compare the total energy liberated by the sun and the human star integrated over their respective lifetimes.
(iii) Compare this ratio to the ratio of typical outer-electron binding energies (driving chemistry) with typical nuclear binding energies (driving fusion).
Notice: solar type stars live so long for two reasons: (a) their fuel is indeed very energy rich; but (b) they burn it at an extremely frugal rate -- their furnaces are surprisingly feeble, per kg, well below even to your own metabolic rate.
When your supervisor next asks you whether you have "fire in the belly" for your work, you can honestly reply, "more, even, than the sun and stars!"
(4) Alien Astronomers in Virgo study the Milky Way
For example, the disk of the Milky Way has Rd = 3.5 kpc, and I(R) at the solar radius (8.5 kpc) is 15 LV, pc-2.
(5) Scaled Hubble Constant: Ho :