They have a period-luminosity relation which allows astronomers to determine their luminosity by measuring their period. Knowing both their luminosity and their apparent brightness allows astronomers to determine their distances. Cepheids can be seen in nearby galaxies, allowing astronomers to determine the distances to the galaxies. He determined the periods of these stars, from which he inferred their luminosities and thus calculated their distance. We now know that most of the nebulae are actually external galaxies rather than objects within our own Milky Way Galaxy.
Elliptical galaxies consist of old stars and have little or no gas and dust.
Describe a theory for the formation of S0 galaxies. S0 galaxies have disks, like spiral galaxies, but do not show spiral patterns in their disks. S0 galaxies have almost no gas and dust, unlike spiral galaxies but like elliptical galaxies. S0 galaxies have a substantial spheroidal component, like many elliptical galaxies. S0 galaxies may be formed by collisions between spiral galaxies, which sweep out the gas and dust.