Question

Alpha Centauri is the star system nearest to our Sun. About how long does light from Alpha Centauri take to travel to Earth, and what does this tell us about its distance?

Hint:

Recall that a light year is the distance light travels in one year, and Alpha Centauri's distance from the Sun is usually quoted directly in light years.

Answer 1

Step 1: Light travels through empty space at a constant speed of very nearly \( 3 \times 10^8 \) metre per second, and this speed does not depend on the source or the observer.

Step 2: Because interstellar distances are extremely large, astronomers often measure distance in light years, where one light year is defined as the distance light covers in one full year while travelling at this constant speed.

Step 3: Careful measurements show that the Alpha Centauri system lies at a distance of about 4.2 light years from the Sun.

Step 4: By the very definition of a light year, this means light leaving Alpha Centauri takes about 4.2 years of travel time before it reaches Earth. So whenever we observe Alpha Centauri today, we are actually seeing light that left it about 4.2 years ago, not light showing its condition at this present moment.

So light from Alpha Centauri takes approximately 4.2 years to reach Earth.