Edwin Hubble (1889-1953)

Hubble did several important things in Astronomy and Cosmology:

• Using Cepheids, measured the distance to the Andromeda galaxy and found that it was 2 million light years away, thus settling the debate.
• Developed a classification scheme for galaxies, which still has his name.
• Measured redshifts and distances to galaxies, and discovered the Hubble law:

v = H₀ l.

Here v is the recession velocity of a galaxy, and l is the distance to it. H₀ is called the Hubble constant, it is the same for all galaxies. It is normally measured in

$${{\rm\,km}/{\rm\,s}\over{\rm\,Mpc}}.$$

But since both, km and Mpc, are measures of distance, the Hubble constant can also be measured just in inverse seconds:

$$1{\rm\,Mpc} = 3.081\times10^{19}{\rm\,km},$$

and

$${{\rm\,km}/{\rm\,s}\over{\rm\,Mpc}} = 3.24\times10^{-20} {1\over{\rm\,s}}.$$