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Looking back some 13 billion years, NASA’s
Hubble Space Telescope offers a glimpse of

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the early universe, revealing countless galaxies
in a tiny area of sky.

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Galaxies are the visible foundation of the
universe; each one a collection of stars,

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planets, gas, dust, and dark matter held together
by gravity.

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Hubble’s observations give us insight into
how galaxies form, grow, and evolve through

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time.

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Hubble’s namesake, astronomer Edwin Hubble,
pioneered the study of galaxies based simply

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on their appearance.

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He divided galaxies into three basic forms:
ellipticals,

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spirals,

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and irregulars.

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Labeled the “Tuning Fork” diagram, Edwin
Hubble’s basic arrangement is still in use

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today.

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Elliptical galaxies are nearly spherical to
egg-shaped groups of old stars that lack the

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gas and dust needed to form new stars.

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Rotation doesn’t play a big part in their
shape.

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The movements of their stars, often in long
oval orbits, determines an elliptical’s

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shape.

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Elliptical galaxies are often near the center
of galaxy clusters, suggesting they may form

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when galaxies merge.

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The best known galaxies are spirals.

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The center of a spiral galaxy has a large,
roughly spherical swarm of stars, called a

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“bulge.”

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This bulge looks similar to an elliptical
galaxy, but spirals differ from ellipticals

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because they rotate.

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Rotation gives spiral galaxies the flat disk
that holds their spiral-shaped arms.

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Unlike ellipticals, spirals have a mix of
young and old stars.

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Star formation in spirals is similar to a
traffic jam on the interstate.

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Like cars on the highway, slower moving matter
in the spiral’s disk creates a bottleneck,

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concentrating star-forming gas and dust along
the inner part of their spiral arms.

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This traffic jam of matter can get so dense
that it gravitationally collapses, creating

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new stars.

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Spiral galaxies are subdivided into “Unbarred”

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and “Barred,” and organized by the size
of their central bulge and how tightly their

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arms are wound.

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Bars form in spiral galaxies when star orbits
become unstable and stretched out.

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As their orbits lengthen, they create a bar.

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The bar grows as gravity captures more nearby
stars.

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Irregular galaxies don’t fit into one of
the other categories.

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They are shapeless and have no symmetry or
ordered structure.

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Irregulars may hold old and young stars and
often have knots of gas and dust forming new

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stars.

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Astronomers have expanded Edwin Hubble’s
basic tuning fork diagram to include galaxies

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that fall between his three categories.

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Intermediate spiral galaxies sit between unbarred
and barred spirals, and have a small bar.

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Lenticular galaxies sit between elliptical
galaxies and spirals.

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They have a central bulge of stars and a flattened
disk, but no spiral arms.

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Like ellipticals, lenticular galaxies don’t
have much gas and dust, and also have mainly

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old stars.

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When viewed “edge-on,” their shape resembles
a lens, which is why they’re called “lenticular.”

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Edwin Hubble’s “tuning fork” was a first
step in understanding galaxies and how they

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evolve.

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Observations by the Hubble Space Telescope
will continue to improve our understanding

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of galaxies and their role in the evolution
of the universe.