Explanation:
The most distant object easily visible to the unaided
eye is
M31,
the great
Andromeda Galaxy
some two and a half 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.
While even casual
skygazers
are now inspired by the knowledge that there are
many distant galaxies like M31, astronomers
seriously debated
this fundamental concept less than 90 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-Curtisdebate
of 1920, which was later resolved by
observations of M31
in favor of Andromeda,
island
universe.
