Spiral Galaxies

Spiral
Galaxies

Galaxies come in a variety of shapes, with the shapes depending in a way not yet completely understood on the evolution of the galaxies. More than half of all observed galaxies are spiral galaxies.
Examples of Spiral Galaxies
The figure below right shows a nice spiral galaxy, M100, which is in the Virgo cluster Another beautiful example of a spiral galaxy is M83. Presumably our own galaxy would resemble these galaxies in appearance if we could view it from the outside. The below left image shows a class Sc spiral galaxy M101 (NGC 5457; also called the Pinwheel Galaxy), which lies at a distance of about 7 Mpc or 22 million light years (Ref).

The image shown adjacent left is of the spiral galaxy ESO 269-57, which is in the southern constellation Centaurus (Ref). The galaxy is of type is Sa(r), with a recessional velocity of just over 3100 km/sec, implying by the Hubble Law a distance of about 155 million light-years. Its angular size is over 4 arcmin, corresponding to to a diameter of about 200,000 light-years.
This spiral has a complex structure, with an inner ring-like set of tightly wound arms surrounded by outer arms that may be split into branches. It has prominent blue star-forming regions in the spiral arms.
Properties of Spiral Galaxies
The range of masses for spiral galaxies is ~ 10⁹ - 10¹² solar masses, with the typical mass being ~ 10¹¹ solar masses. The typical range of luminosities corresponds to absolute blue magnitude -16 to -23, and the typical diameter of the visible disk is 5-100 kpc.

The Spiral Structure
The spiral structure is associated with active star-forming regions. As already noted in conjunction with the Milky Way, This is in fact why the spiral arms of these galaxies are so prominent: because they are regions of active star formation, there are many hot young blue and blue-white stars there, making the spiral arms extremely visible. The adjacent image shows a star-forming region in the spiral arms of the galaxy M31 (Andromeda) called NGC 206 (Ref). Notice the diffuse blue glow associated with many young stars.
In constast the nucleus of a spiral galaxy is typically much more red in color, often resembling elliptical galaxies; this indicates the presence of many old stars in the nucleus of spiral galaxies. Here is a movie sequence of a galaxy morphing its perceived shape from spiral to almost elliptical as the wavelength is varied.
The image shown below illustrates rather clearly the spiral structure for several spiral galaxies classified according to their Hubble class. These images were taken in UV light with the Ultraviolet Imaging Telescope aboard the Space Shuttle Columbia. Because the spiral arms are dominated by hot young stars, they stand out particularly well when photographed at UV wavelengths, since only hot young stars can produce much UV (Ref).

In these images we can see clearly the progression to more tightly wound spirals as we move from Sc through Sb classifications.
Gas and Dust
Spiral galaxies are rich in gas and dust, which is often visible as lanes of dust when viewed from the "top" or "bottom", and as layers of dust when viewed from the side. In our own Milky Way the center of the galaxy is invisible from our vantage point because the interstellar dust between us and the center (in Sagittarius) is so thick.

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