Imagine the Universe!
Imagine Home | You Be the Astrophysicist! |

More on Hubble's Law

photo of Edwin Hubble looking into telescope
Image Credit: The Huntington Library, San Marino, California

Looking at the skies from Mt. Palomar, Edwin Hubble in the 1920s found that all far away objects (such as other galaxies) in the universe are moving away from us, and that this motion, called their recession velocity, is greater the further they are from us. In fact, he found the relationship between a galaxy's velocity (the radial component, in a straight line) away from us (v) and its distance from us (d) approaches a fairly linear one, which is known as Hubble's Law:

v = H0 x d

where H0 is an observationally determined constant (called Hubble's constant). Finding the value of Hubble's constant is a current hot topic in astronomy, and it has many implications for our understanding of how the Universe evolved since the Big Bang.

Info Click here for more information on what Hubble's findings actually mean.

How in the world (how in the Universe, rather!) did we come to know about this phenomena? Well, figuring that out was the work of Vesto M. Slipher and Edwin Hubble.

In 1912 American born astronomer Vesto M. Slipher discovered that the light from all of the galaxies that he observed, regardless of the direction he looked, appeared to be redshifted; they are moving away.

Vesto M. Slipher
portrait of Yesto Slipher
Credit: Lowell Observatory
Edwin Hubble
portrait of Edwin Hubble
Credit: Archives, California Institute of Technology

Edwin Hubble, utilizing observational data of distant galaxies, sought to find a relationship between their distance from us and their speed. He plotted recessional velocity determined by the doppler shift of stellar spectra as a function distance and established what is now know as Hubble's Law. Hubble's law applies to the objects in the Universe on the largest scales, where the force driving their motions is the expansion of the Universe.

graph of recession velocity as function of distance 
for distant galaxies

Hubble's Law

So...., in theory, if you knew the distance to M31 and you knew Hubble's constant (H0), you could use Hubble's Law to find the velocity of M31!

Quiz Click here for a quiz on what Hubble found!
Info Click here to learn about the Doppler shift and how Hubble knew these galaxies were moving away from us.
Data Click here to go to get the data to solve this problem using Hubble's Law.
Experiment Return to the beginning and try another approach.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index