Hubble Reveals Ultraviolet Galactic Ring
The appearance of a galaxy can
depend strongly on the color of the light with
which it is viewed. The Hubble Heritage image
of NGC 6782 illustrates a pronounced example of
this effect. This spiral galaxy, when seen in
visible light, exhibits tightly wound spiral arms
that give it a pinwheel shape similar to that
of many other spirals. However, when the galaxy
is viewed in ultraviolet light with NASA's Hubble
Space Telescope, its shape is startlingly different.
Ultraviolet light has a shorter
wavelength than ordinary visible light, and is
emitted from stars that
are much hotter than the Sun. At ultraviolet wavelengths,
which are rendered as blue in the Hubble
image, NGC 6782 shows a spectacular, nearly circular
bright ring surrounding its nucleus. The ring
marks the presence of many recently formed hot
stars.
Two faint, dusty spiral arms emerge
from the outer edge of the blue ring and are seen
silhouetted
against the golden light of older and fainter
stars. A scattering of blue stars at the outer
edge of
NGC 6782 in the shape of two dim spiral arms shows
that some star formation is occurring there
too. The inner ring surrounds a small central
bulge and a bar of stars, dust, and gas. This
ring is
itself part of a larger dim bar that ends in these
two outer spiral arms. Astronomers are trying
to
understand the relationship between the star formation
seen in the ultraviolet light and how the bars
may help localize the star formation into a ring.
NGC 6782 is a relatively nearby
galaxy, residing about 183 million light-years
from Earth. The light
from galaxies at much larger distances is stretched
to longer, redder wavelengths ["redshifted"],
due
to the expansion of the universe. This means that
if astronomers want to compare visible-light
images of very distant galaxies with galaxies
in our own neighborhood, they should use ultraviolet
images of the nearby ones. Astronomers find that
the distant galaxies tend to have different
structures than nearby ones, even when they use
the correct procedure of comparing visible light
in
distant galaxies with ultraviolet light from nearby
ones. Since the distant galaxies are seen as they
were billions of years ago, such observations
are evidence that galaxies evolve with time.
The Hubble image of NGC 6782 was
taken with the Wide Field Planetary Camera 2 (WFPC2)
in
June 2000 as part of an ultraviolet survey of
37 nearby galaxies. The observations were carried
out
by an international "Hubble mid-UV team"
led by Dr. Rogier Windhorst of Arizona State
University. Additional observations of NGC 6782
were made by the Hubble Heritage Team in
June 2001. The color image was produced by combining
data from both observing programs that
were taken through color filters in the WFPC2
camera that isolated ultraviolet, blue, visible,
and
infrared light.
Credits: NASA and The Hubble Heritage
Team (STScI/AURA)
Acknowledgment: R. Windhorst (ASU) |
