Exam 33: Astrophysics and Cosmology

Stars A and B are both black holes but have different masses. If the mass of star A is twice as much as the mass of star B, the average density of star A, compared to the average density of star B will be

A white dwarf star with a mass equal to that of the sun is about the size of

The sun has apparent brightness $B$ at the earth, which is $1.5 \times 10^{8} \mathrm {~km}$ away. What would be the apparent brightness of the sun at Pluto, which is $6.0 \times 10 ^ { 9 } \mathrm {~km}$ from the sun?

If a galaxy is moving away from us at $1.0 \%$ of the speed of light, how far away is it from us if $H = 20 \frac { \mathrm { km } / \mathrm { s } } { \mathrm { Mly } } ? \left( c = 3.00 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s } \right)$

The apparent brightness of a star is $1.0 \times 10 ^ { - 12 } \mathrm {~W} / \mathrm { m } ^ { 2 }$ and the peak wavelength in its light is 600 $\mathrm { nm }$ . Assuming it is the same size as our sun and that it radiates like an ideal blackbody, estimate its distance from us, in parsecs. The constant in Wien's law is $0.00290 \mathrm {~m} \cdot \mathrm { K }$ , and 1 parsec is equal to $3.09 \times 10 ^ { 16 } \mathrm {~m} . \left( R _ { \text {sun } } = 6.96 \times 108 \mathrm {~m} , \sigma = 5.67 \times 10 ^ { - 8 } \frac { \mathrm { W } } { \mathrm { m } ^ { 2 } \cdot \mathrm { K } ^ { 4 } } \right)$