Whirlpools and Pinwheels on the Sky
Whirlpools and pinwheels on the sky Domingos Soares Galaxies exist in many different forms and count by, at least, the hundreds of billions, according to estimations made from the observations of the Hubble Space Telescope. Among all of them, there are no doubts that the most spectacular are the so-called \spiral galaxies". Our own galaxy, the Milky Way galaxy, is a member of that family. Spiral galaxies are so named because of the distinctive aspect they present to the observer, namely, of a spiral-shaped structure similar to a whirlpool or a pinwheel. They are also called \disk galaxies" because the galactic material | stars, gas and interstellar dust | is distributed in the shape of a \thick" disk. The stars constitute the main constituents of galaxies, at least with respect to their visual appearance. They emit the major part of the visible light of a galaxy, be it a spiral or not. The typical visual aspect of spiral galaxies is due to the structures that astronomers call \spiral arms". The most bright stars of a galaxy are those that delineate the arms. These, however, contribute little to the total mass of the galaxy, but due to their extraordinary brightness they are the most noticeable on a visual inspection. The spiral galaxy disks rotate. And this is typical in those galaxies: the galactic material spins around the disk center, which is called \galactic center". But the rotation is not like the spin of a rigid disk, like, for example, a CD, in which all the points make a full turn in the same time interval.
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