Sky & Telescope - April 2019 - 21

p BARRED SPIRAL NGC 1300 in Eridanus is the quintessential barred
spiral, its arms attached to a long central bar instead of spiraling all the
way to its core. Star-forming regions dot the arms, and a much smaller
spiral (some 3,300 light-years wide) appears in the nucleus.

BA RRED SPIR A L: N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A); CELESTIA L
SHEEP: ESA / HUBBLE, N ASA , S. SM A RT T (Q UEEN'S UNIV ERSIT Y BELFAST )

t CELESTIAL SHEEP NGC 3521 in Leo is a flocculent spiral, so called
for the woolly look of its arms. Flocculent spirals are three times as common as the iconic grand-design spirals, such as the Whirlpool.

not accrete any more, and ceases to be a spiral. This process
may explain why lenticular galaxies proliferate in galaxy clusters. The nearest galaxy cluster, Virgo, may eventually snatch
the Milky Way and Andromeda, but long before then the two
giant spirals will probably have collided, exhausting their gas
in a colossal starburst as they unite into a single giant, elliptical galaxy. Elmegreen calculates that most spiral galaxies will
plunge into clusters during the next 16 billion years.
The Sun will then be a mere white dwarf. Perhaps 20 billion
years from now astronomers will see spiral galaxies only from
afar and marvel at the ancient era when the universe swarmed
with such elegant galaxies. For however and whenever it ends,
the Spiraliferous Period is certainly splendiferous.
tain their shapes until they meet another danger: falling into
galaxy clusters, which have hot gas that strips away galactic
gas. "There are lots of ways to kill a spiral," he says. The
galaxy can metamorphose into a so-called jellyfish, in which
starbirth persists in the torn-out gas streamers that trail
behind the galaxy. Eventually the galaxy uses up its gas, can-

¢ KEN CROSWELL was born in a spiral galaxy and earned his
PhD at Harvard University for studying it. His book about our
galaxy, The Alchemy of the Heavens: Searching for Meaning in
the Milky Way, features interviews with the astronomers who
discovered the Milky Way's spiral shape.
sk yandtele scope.com * A PR I L 2 019

21



http://www.skyandtelescope.com

Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019

Contents
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Sky & Telescope - April 2019 - 1
Sky & Telescope - April 2019 - Contents
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