The Caldwell Objects

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This means that in shape NGC 4559 lies somewhere
between a barred spiral and an unbarred
one (SAB), whose inner arms form a ring that
surrounds the needle-sharp nucleus (rs) while
farther out the arms fly freely open (cd).
Recent advances in our knowledge of the
high-redshift universe are providing us with a
better understanding of the history of galaxy
morphology. For instance, the Hubble Space
Telescope has shown that the distinction between
ellipticals and spirals grows less definite with
increasing distance, becoming nearly
imperceptible in images that show how the
universe looked when it was only one-tenth its
current age. Other studies suggest that ellipticals
formed remarkably early in the universe while
spiral galaxies took much longer to form. The
origins and evolution of galaxy morphology
remain active (and contentious) fields of research.
NGC 4559 is a sizable galaxy, spanning
100,000 light-years, with a total mass of 40 billion
Suns. Together with NGC 4565 it is a member of
the Coma-Sculptor Cloud of galaxies,
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and it is receding from Earth at a speed of 820 km
per second. The galaxy is inclined 69° from face
on. In photographs NGC 4559 looks like a
spinning saw blade whisking toward the observer
at a skewed angle. Its untamed arms
appear tormented, as if they have been stretched
by three people in three different directions. The
arms are dotted with bright ΗII regions that seem
to cling to them like lint to a sweater.
In the mid-1990s the Rosat spacecraft
revealed 17 discrete X-ray sources within NGC
4559. The most luminous one, sited in the
galaxy's nucleus, is slightly extended, suggesting
either an unresolved superposition of several
compact sources or a diffuse source of emission.
Another bright source was found in an outlying
spiral arm, where it coincides with a conspicuous
group of Η II regions. Since little radio or infrared
emission emerges from this location, the X-rays
may have originated from a single, severalhundred-year-old
supernova remnant.
Small-telescope users will have no trouble
locating NGC 4559. The galaxy lies only 2° eastsoutheast
of magnitude-4.3 Gamma (γ)
Deep-Sky Companions: The Caldwell Objects