Spiral Galaxies (5) ...
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.
In these images we can see clearly the progression to more tightly wound spirals
as we move from Sc through Sb classifications.
Barred Spirals
Perhaps 20 percent of observed spiral galaxies show a barred structure in the center.
These are classified by the Hubble sequence designations SBa, SBb, or SBc,
corresponding to increasing prominence of the bar.
The adjacent image shows a barred spiral, NGC 1365, which is 60 million light years away in the southern
constellation Fornax.
It is classified as an SBb spiral, and is approximately 200,000 light years in
diameter -- twice the diameter of the visible part of our own galaxy.
Development of Bars
Calculations suggest that bars develop in spiral
galaxies when the central region of the galaxy develops rotational instabilities.
It is possible that
all spiral galaxies pass through a period in which they develop such
instabilities and bars.
There is evidence for some
bar structure in the center of the Milky Way.
