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We see in other spiral galaxies that dust, gas, H II regions
(regions of ionized hydrogen), and star-forming regions are all characteristic
of the disks of spiral galaxies. The Milky Way has these in abundance.
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The location of hot young
OB star associations suggest partial spiral structure, at least in the vicinity of the Sun.
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Neutral hydrogen and giant molecular clouds have been traced by radio telescope observations and
suggest an approximate spiral pattern on larger scales.
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However, the spiral patterns suggested by these observations indicate a more complex structure for the Milky
Way than the simple two-arm spiral structure of classical spiral galaxies (what astronomers sometimes
call "grand design" galaxies). There may be more than two arms, and there is some evidence that the
arms might have some discontinuities and "spurs" sticking off them.
