Quote:
Originally Posted by Allan_L
Tarantula not in Milky Way, but still referred to as Nebula.?
So has that "rule" been relaxed, or just ignored, or what?
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Hi Allan,
This rule never existed.
But our concept of the hierarchy of objects in the universe radically
changed in the 1920's.
Nebula can be present anywhere in the universe.
As Julian explained, there was a major philosophical change in our
comprehension of the universe when Hubble first realized that
what we now refer to as galaxies were in fact outside of the Milky Way.
Before then, most astronomers believed that the Milky Way was the
entire universe and they classified all objects that were nebulous -
including what we now know as galaxies - as nebulae. *
Just before Hubble's seminal discovery, the term "non-galactic"
actually meant "away from the galactic plane".
Post the Edwin Hubble Cepheid variable/redshift discovery, what
we today refer to as galaxies were then called "extragalactic nebulae".
It was not until after Hubble's death in 1953 did the term "galaxy"
become widespread. [Reference - Steven J. Dick, Discovery and Classification
in Astronomy: Controversy and Consensus, Cambridge University Press, page 122].
Since the LMC and SMC are relatively close, many nebulae can be readily resolved in them
with amateur instruments.
With more distant galaxies, resolving nebulae within them becomes more difficult, but they
are still called nebula.
Hope this helps.
* Footnote
Astronomer Heber Curtis argued in 1920 with Harlow Shapley that "spiral nebula" were "island universes" outside of the Milky Way.
