"Late" Spiral Galaxies: Sc and Sd
"Late" Spirals are designated as Sc and Sd. They have extensive arms showing vigorous star formation. Their bulges are nearly non-existent. Typical gas and dust mass fraction is greater than 20-30%. All galaxies contain some old stars, but late-type Spiral galaxies have the highest ratio of new, young stars.
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M74 is an Sc galaxy, the largest among a small group of galaxies 35 million light years away. The spiral arms are dotted with enormous HII regions, which trace the gas inward all the way to the small central bulge. |
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M33 is a very late spiral, Sd, 1 million light years away. This is the third largest galaxy in our Local Group of galaxies. Only recently has a true bulge stellar population been found. The central nuclear stellar population appears to be less than 1 billion years old. The red knot to the upper left is a very well known, ENORMOUS, HII region called NGC 604. |