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Astronomy Picture of the Day |
APOD: 2003 March 10 - M101: The Pinwheel Galaxy
Explanation:
Why do many galaxies appear as
spirals?
A striking example is M101,
shown above, whose relatively close distance of about 22 million light years allow it to be studied in some detail.
Recent evidence indicates that a
close gravitational interaction with a neighboring galaxy created
waves of high mass and condensed gas which
continue to orbit the galaxy center.
These waves compress existing gas and cause star formation.
One result is that
M101, also called the
Pinwheel Galaxy, has several extremely
bright star-forming regions
(called HII regions) spread across its spiral arms.
M101 is so large that its
immense gravity distorts smaller nearby galaxies.
APOD: 2003 April 13 - NGC 1365: A Nearby Barred Spiral Galaxy
Explanation:
Many spiral galaxies have bars across their centers.
Even our own
Milky Way Galaxy is
thought to have a bar, but perhaps not so
prominent as the one in NGC 1365, shown above.
The persistence and motion of the
bar imply relatively
massive spiral arms.
The placements of
bright young blue stars and dark
dust lanes also
indicate a strong rotating density wave of star formation.
NGC 1365 is a member of the
Fornax Cluster of Galaxies. Because
NGC 1365 is relatively nearby, simultaneous
measurements of its
speed and distance are possible, which help
astronomers estimate
how fast our universe is expanding.
APOD: 2005 December 22 - Andromeda Island Universe
Explanation:
The most distant object easily visible to the unaided eye is
M31,
the great
Andromeda Galaxy
some two million light-years away.
But without a telescope, even this immense spiral galaxy - spanning over
200,000 light years - appears as a
faint, nebulous cloud in the
constellation
Andromeda.
In contrast, a bright yellow nucleus, dark winding dust lanes,
gorgeous blue spiral arms and star clusters are recorded in this
stunning
telescopic
digital mosaic with a cumulative exposure of over 90 hours.
While even casual
skygazers
are now inspired by the knowledge that there are
many distant galaxies like M31, astronomers
seriously debated
this fundamental concept only 80 years ago.
Were these "spiral nebulae" simply outlying components of our own
Milky Way Galaxy or were they instead "island universes" -- distant
systems of stars comparable to the Milky Way itself?
This question was central to the famous
Shapley-Curtis
debate
of 1920, which was later resolved by
observations of M31
in favor of Andromeda,
island
universe.
Authors & editors:
Robert Nemiroff
(MTU) &
Jerry Bonnell
(USRA)
NASA Web Site Statements, Warnings, and
Disclaimers
NASA Official: Jay Norris.
Specific rights apply.
A service of:
EUD at
NASA /
GSFC
& Michigan Tech. U.