The shell of a planetary nebula is measured by the Doppler shift of emission lines to be expanding outward at a speed of 10 km/s and its diameter is measured to be 1 ly. How long has the shell been expanding?

Question

Quizplus · Accepted Answer

The shell has been expanding for 50,000 years. This is calculated by dividing the diameter of the nebula (1 light-year) by the expansion speed (10 km/s). Since 1 light-year is the distance light travels in one year, and the speed of light is approximately 300,000 km/s, we convert the expansion speed to the same units and find the time it takes to cover that distance.

The Shell of a Planetary Nebula Is Measured by the Doppler