How are the stars proof that the universe is expanding?

When looking at the radiation emitted by distant stars or galaxies, scientists see emission spectra shifted towards the red end of the electromagnetic spectrum. The observed wavelengths are longer than expected. And something like causes the wavelength to stretch.

:]

When looking at the radiation emitted by distant stars or galaxies, scientists see emission spectra 'shifted' towards the red end of the electromagnetic spectrum—the observed wavelengths are longer than expected. Something causes the wavelength of the radiation to 'stretch'.

Explanation:

The average speed can be calculated as the quotient between the distance travelled and the time needed to travel that distance.

To go to the school, he travels 2.4 km in 0.6 hours, then here the average speed is:

s = (2.4km)/(0.6 hours) = 4 km/h

To return to his home, he travels 2.4km again, this time in only 0.4 hours, then here the average speed is:

s' = (2.4 km)/(0.4 hours) = 6 km/h.

Now, if we want the total average speed (of going and returning) we have that the total distance traveled is two times the distance between his home and school, and the total time is 0.6 hours plus 0.4 hours, then the average speed is:

S = (2*2.4 km)/(0.6 hours + 0.4 hours)

S = (4.8km)/(1 h) = 4.8 km/h