Sirius A has a luminosity of 26 Lsun and a surface temperature of about 9400 K. Part A What is its radius? (Hint: See Cosmic Calculations 12.2: Radius of a Star.) Express your answer using two significant figures. ? m

Suppose a star has a luminosity of 7.0x1026 watts and an apparent brightness of 4.0x10-12 watt/m?. How far away is it? Give your answer in both kilometers and light-years.

A. Estimate the surface gravity of a neutron star with R = 10 km and M = 2M. . B. Determine the density of such a neutron star in g/cm³. C. How much would a teaspoon (5 cm³) of this neutron star weigh on Earth? This material is known as neutronium. Give your answer in pounds. D. Which would be heavier: a teaspoon of neutronium weighed on Earth, or a teaspoon of water weighed on the surface of a neutron star?

A main sequence star of mass 25 M⊙has a luminosity of approximately 80,000 L⊙. a. At what rate DOES MASS VANISH as H is fused to He in the star’s core? Note: When we say “mass vanish '' what we really mean is “gets converted into energy and leaves the star as light”. Note: approximate answer: 3.55 E14 kg/s b. At what rate is H converted into He? To do this you need to take into account that for every kg of hydrogen burned, only 0.7% gets converted into energy while the rest turns into helium. Approximate answer = 5E16 kg/s c. Assuming that only the 10% of the star’s mass in the central regions will get hot enough for fusion, calculate the main sequence lifetime of the star. Put your answer in years, and compare it to the lifetime of the Sun. It should be much, much shorter. Approximate answer: 30 million years.