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# 15.14: Spiral Galaxies

Spiral galaxies are often considered the most beautiful of all galaxies. Ranging from flower petal shaped flocculent spirals to pinwheel shaped grand design spirals, these magnificent systems are found in low density regions of the universe where their fragile structure can escape the tortures of gravitational interactions with similar-massed sister systems. Spiral galaxies make up roughly 30% of all systems, and often host their own armada of significantly smaller satellite galaxies. Spirals were first described by Edwin Hubble in his ground-breaking 1936 work "The Realm of the Nebulae."

The Pinwheel Galaxy. Click here for original source URL.

Spiral galaxies have three main stellar components: (1) a flat, rotating disk of stars, gas, and dust, (2) a central flattened structure called the bulge, and (3) a spherical or nearly spherical, diffuse component of stars called the halo. Structurally, spiral galaxies consist of one or more (and typically two or more) arms that spiral away from either a central nucleus or a central bar. About two thirds of spirals have bars in the present universe, although the fraction was much smaller billions of years ago. These galaxies are sub-categorized based on the presence of a bar into S for Spiral and SB for barred spiral galaxies. These general bins are further broken up based on the the size of the galactic bulge and how tightly wound the galaxy's arms may be. A galaxy with loosely wound arms and a small bulge is a Sc galaxy. Clear spiral arms and a moderate sized bulge corresponds to an Sb type spiral. The label Sa is given to galaxies with a large nucleus and tightly wound arms. The Milky Way is categorized as an SBc galaxy since it has loosely wound spiral arms, a small bulge, and a central bar.

Spiral arms typical possess gas and dust that feed ongoing star formation (although exceptions exist). Star formation is most prominent in the arms. All nearby spirals that have been searched for a super massive black hole have shown evidence of a black hole that has a mass proportional to the size of galaxy's bulge or spheroid of stars. Most of these black holes are inactive or quiescent; they are not consuming material or shining brightly and their presence is only known the the effect they have on the orbital speeds of nearby stars. By measuring galactic rotation curves it has also been shown that spiral galaxies are typically rich in dark matter. This dark matter dominates the overall mass of the galaxy and it's mostly distributed beyond the disk in the spherical halo. Dark matter cause the orbital speeds of stars in the disk to stay high out to the visible edge of the disk. Because we live in a spiral galaxy, we know more about their detailed properties than we do about other galaxy types.

Barred Spiral Galaxy NGC 1300. Click here for original source URL.