Which astronomer argued, in 1920, that the Milky Way was just one of many galaxies?

Which astronomers "unlocked the mystery" of spiral nebulae in the 1920's?

In the 1920's, what type of pulsating stars were observed in the spiral nebula M31 in Andromeda?

It was known, for Cepheid variables, that the time it took them to pulse was directly related to their ___

If you can measure the period of Cepheid variables, you can determine how far away they are simply by ___

In the 1920's it was determined that the "Great Andromeda Nebula" was actually ___

We still classify galaxies according to Hubble's original system, based on ___

What type of galaxy is the Milky Way?

What type of galaxy is characterized by a broad, flat, rotating disk of stars, gas and dust, and a central bulge?

What type of galaxy often has a bar of stars extending across the central part of the galaxy?

The choppy or patchy arms, that resemble tufts of cotton, in some spiral galaxies are called ___

Star forming nebulae in spiral arms are colored pink by the characteristic glow of ___

When seen edge-on, what feature can bisect a spiral galaxy "like a racing stripe right down the middle"?

What type of galaxy is puffy, with no overall structure and populated with older stars?

Current thinking is that elliptical galaxies are the products of ___

Colliding galaxies can glow blue and pink as stars are born and ___

We think most large galaxies, including our own Milky Way, grew to their current bulk by ___

What type of galaxy has a shape that's weird and distorted, essentially all due to collisions?

What type of galaxy tends to be small and are truly shapeless?

The Large and Small Magellanic Clouds are satellite galaxies of the Milky Way, visible ___

What is the biggest and most active star forming gas cloud in any nearby galaxy?