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Physics & Astronomy Department

Astr111 Photography Projects, Fall 2004

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M 74, Veronica Richards

M 74

A spiral galaxy is a type of galaxy in the”Hubble Sequence” which is characterized by the following physical properties:

•  A considerable total angular momentum

•  Composed of a central bulge surrounded by a disk

•  The bulge resembles an elliptic galaxy, containing many old, "Population II" stars, and usually a super massive black hole at its center.

•  The disk is a flat, rotating assembly consisting of interstellar matter young "Population I" stars and open star clusters.

Spiral Galaxies (Sa-d) have a central bulge and an outlying disk containing spiral arms. The arms are centered on the bulge, and vary from tightly wound (Sa) to very loose (Sc and Sd). The latter also have less pronounced central bulges. M74 is remarkably symmetric in appearance according to “Wikipedia”, the free encyclopedia. This is probably caused by the global phenomenon of density waves sweeping around M74's gaseous disk, probably induced by gravitational interaction with neighboring galaxies. When gas clouds orbiting within the disk encounter such density waves, they are accelerated into the spiral shaped wave crest, and then slowed down, so that they converge toward the spiral arm, and enhance the density wave. Moreover, collisions and mergers of neighboring clouds occur, which are thought to induce the observed starbirth activity along the spiral arms. M74 is a very large galaxy, but is also quite faint, which makes viewing this galaxy difficult unless a dark sky, unpolluted by light sources is available.

M74 is an island universe of about 100 billion stars, 30 million light-years away toward the constellation Pisces. Classified as an Sc galaxy, the grand design of M74's graceful spiral arms traced by bright blue star clusters and dark cosmic dust lanes, is similar in many respects to our own home galaxy, the Milky Way.

The spiral arms are regions where stars are being formed. Here we find the hottest, youngest and brightest stars. Stars are formed from dust, and gasses that are compressed together. This compression creates new stars, this can be seen in the spiral arms, and is why the arms are the brightest regions of the galaxy. In between the arms one can see dust lanes. This is very commonly found in galaxies. The dust lanes are made of the “left over” dust. Looking at the image one can see the spiral arms, which are classified Sc, meaning that the spiral arms are somewhat in between loosely wound and tightly wound. In the image one can also see the galaxy which is actually made up of many, many stars. However one can also see that there are other “bright” spots around the galaxy. These are stars as well; however these stars are in our own galaxy. We are actually looking through our own galaxy out onto another galaxy. The stars in our own galaxy we can see quite definitely, however the stars in M74 we can only see as a whole, the galaxy it's self, the individual stars are to far away to be seen individually.