ASTRONOMY FUNDAMENTALS - Chapter 7, Exercise 1 Solution
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Classifying a Galaxy Using the Hubble Sequence

PROBLEM
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A newly imaged galaxy shows a flattened disk with a clear spiral
structure and a visible bar extending from its central bulge. Classify
this galaxy as precisely as possible using the chapter's own real
Hubble sequence categories.

SOLUTION
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Work through the chapter's own real classification categories:

    Elliptical? NO - the galaxy has a flattened disk and spiral
    structure, not a smooth, featureless elliptical shape.

    Spiral? YES - it has a flattened disk with a central bulge and
    spiral arms, matching the real definition of a spiral galaxy.

    Barred or regular spiral? BARRED - the galaxy has a visible bar
    extending from its central bulge, which is the defining real
    feature separating barred spirals (SB) from regular spirals (S).

ANSWER: This galaxy is a BARRED SPIRAL galaxy (SB) - it belongs to the
lower branch of the Hubble tuning-fork diagram, not the upper (regular
spiral) branch or the elliptical branch.

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WHY THIS WORKS AS AN ANSWER

The Hubble sequence works as a series of real, observable distinguishing
features checked in order: first whether the galaxy has a flattened
disk with structure (ruling elliptical in or out), then whether that
structure includes spiral arms (ruling spiral in or out against
lenticular or irregular), and finally whether a bar is present
(distinguishing SB from S). This described galaxy actually matches the
Milky Way's own real classification exactly, as covered earlier in this
chapter - our own galaxy is itself a real barred spiral, making this
exercise a direct, concrete application of a category the chapter
already used to describe home.