Western Washington University   College of Sciences and Technology   Physics/Astronomy Dept.   Bellingham, WA USA
 
ASTRO 101

Analemma
Asteroids
Aurora
Big Bang
Black Holes
Bode Titius
Brightest Stars
Comets
Constellations
Coordinates
Cosmology
Cruithne
Dark Matter
Eclipses
Galaxies
Historical
HR Diagram
Hubble's Law
Intelligent Life
Kepler's Laws
Leap Year
Light Waves
Lunar Libration
Messier Objects
Meteors
Milky Way
Moon
Moon Phases
Planets
Precession
Rainbows
Redshift
Seasons
Stellar Evolution
Stardust
Sun & Fusion
Telescopes
Tides
Time of Day
Twilight
Zodiac
 
Hubble's Law
Hubbles Law   Hubble's Constant   Raisin Pudding   Activity Lab   Galactic Recession   Galactic Redshifts   Exam

Edwin Hubble 1889-1953
Edwin Hubble 1889-1953
The motion of galaxies in the universe, in relation to each other, appears to be a smooth recession away from each other. The relative velocity of a galaxy to any observer is proportional to the distance from that observer.

Some galaxies that are in close proximity to each other, such as Andromeda and the Milky Way, are actually moving towards one another because gravity at a close distance is a greater factor than the recession. Also, galaxies probably have some momentum of their own that they inherited from the clouds that formed them. Nevertheless, the overall effect is a general expansion of the Universe.

An observational effect of the expansion, is that the farther a galaxy is from you, the faster it appears to be receding. This creates a relationship between the recessional velocity and distance. This phenomenon is known as Hubble's Law (historical note).



Recessional Velocity = Hubble's Constant times Distance

V = Ho D


V is the observed velocity of the galaxy away from us, usually in km/sec
Ho is Hubble's Constant, in km/sec/Mpc
D is the distance to the galaxy in Mpc