Answer:
a) Time taken for the spaceship to reach Sirius, t' = 8.86 years = 9 years(approx)
b) The mass of the interstellar gas that collides with the spaceship, m' = 1.195 * 10^(-27) kg
Explanations:
The calculations are handwritten and attached as files below. However, the following steps were taken to arrive at the solutions
a) to calculate the time taken for the spaceship to reach Sirius
Step 1: calculate the time taken to reach Sirius from the perspective of the earth using time = distance/speed
Step 2: Use lorentz time dilation formula to calculate the equivalent time to reach Sirius from the perspective of the person in the spaceship
b) to calculate the mass of the interstellar gas
Step1: Calculate the new length of the spaceship using special relativity
Step 2 Get the new volume also by using the new length gotten
Step2: Get two equations for the original mass and the new mass using mass = density * volume
Step 3: Write the original mass in terms of the new mass and make necessary substitution.
Note: Check complete solution in the file attached.