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Q:
Why is the Milky Way galaxy said to have 5 arms
coming out from the center, yet the diagrams give 6 "arm
names"? How does this work? |
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A: There are a lot of
different variations on the names and number of arms in the
Milky Way. Spiral galaxies (like the Milky Way) have major
arms, but they may also have smaller, minor arms, side arms
(like the Orion arms of the Milky way) and/or arm fragments.
[Also, there are many stars between the arms.] For the Milky
way, some astronomers consider there to be four major arms
(Sagittarius, Centaurus, Cygnus, and Perseus Arms), plus a
side arm (the Orion arm, where we are) - but other
astronomers think that Orion is a fragment of the Perseus
arm. Some astronomers divide the Centaurus Arm into the
Scutum arm and the Crux arm, and some lump Carina arm into
the Sagittarius arm (leaving its name out altogether). Plus,
some astronomers think that the Milky way might be a barred
spiral galaxy and not a regular spiral galaxy at all
(changing the map radically). Basically, looking at a
diagram of the Mlky way today is a bit like looking at an
ancient map of the world - they're both inaccurate because
they're based on very limited data. Perhaps, in the future,
one of your students will clear this matter up for all of
us! |
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