The Hubble classification scheme
for galaxies is based on their appearance.
|SHAPE||STAR CONTENT||GAS/DUST||STAR FORMATION|
disc bulge in middle, spiral or barred halo arms
|halo, mainly old stars||none in halo, lots in disc||spiral arms|
|Ellipticals||Ellipsoidal/spherical, uniform star
|old||v little||nothing new for the last 10bn y|
|Irregulars||no structure||young and old||lots of both||plenty|
Our Galaxy, the Milky Way is part of the Local Group (LG) extending for 10m ly containing about 20 galaxies, the nearest being the Large Magellanic Cloud at 2m ly
It contains the Andromeda Spiral (largest member)
Beyond this – a supercluster ( diameter = 15Mpc) of which our LG is a member.
How far away? We calculate from Doppler shift measurements on known spectral lines (either H or He, usually)
as long as we realise that the velocity as calculated is as if the star is moving directly away from us.
distance d and recession speed v are proportional in an isotropic universe, but there is controversy over its exact value. Notice the bubble in the middle of the (much simplified) graph.
The Particle data Group offer the best estimate of the Hubble constant (“per second”) at 72 km s-1 Mpc-1 by observing Type 1 supernovae whose distances are known to better than 5%. It implies that at some point in time the Universe was a point – indirectly implying a Big Bang at r=0.
Thus, 1/H approximates to the age of the Universe, currently thought to be 13.7 bn years. You might like to fiddle with the units to check this value.