Once we look out beyond our immediate neighborhood, these local, random velocities are much smaller than the recession velocities caused by the expansion of space itself. This is now understood not as a genuine Doppler effect but as being literally due to the light waves from distant objects being stretched to longer wavelengths on their journey to us(itb10)
Although galaxies are far too far away from us for their movement across the sky to be perceptible, the speed at which they are moving can be inferred from spectroscopy(itb35)
The spiral arms are the visible part of a density wave, which is moving round the galaxy in the same direction as the stars themselves, but more slowly(itb200)
The density wave produces a supersonic shock wave, like the shock wave associated with a supersonic aircraft such as Concorde, at the leading edge of the spiral arm (the outer, convex part of the spiral). But the stars and clouds of material in the galaxy are moving through the shock wave with speeds of 100-300km per second. Clouds of gas and dust pile up in a traffic jam just behind a spiral arm, along the inside bend of its curve. They are squeezed into the shock wave, this squeezing leads to the birth of giant stars(itb202)
Elliptical galaxies are galaxies where little star formation seems to be going on today(itb208)
The elliptical galaxies, once thought to be the oldest, are now seen as relative newcomers(itb208)
Elliptical galaxies are less numerous than spiral galaxies, but the biggest of them are much bigger than even the biggest spiral(itb209)
Elliptical galaxies are made up mainly of cool, red stars(itb209)
The red stars in elliptical galaxies have actually come from spiral galaxies. Ellipticals form from the merger of two spiral galaxies that collide with each other, and from events in which a spiral galaxy is absorbed by an elliptical galaxy. This is how ellipticals get to be so big(itb209)
Half of all galaxies in the universe have merged with galaxies of similar size in the past seven or eight billion years(itb211)
The spread of ages shows that our galaxy formed from an amalgamation of about a billion smaller gas clouds, each weighing up to about a million times the mass of our sun(itb211)
Galaxies formed first, then clusters, then superclusters, and so on(itb239)