74 ASTRONOMY • SEPTEMBER 2015
A once and
future Sun
Roughly 7 billion years
from now, the Sun will
have exhausted all the
hydrogen and helium in
its core and ballooned
into a red giant (for a
second time). Then, it will
puff off its outer layers to
form a planetary nebula
like the Medusa (Abell 21)
seen here. Named after
the Gorgon Medusa from
Greek mythology, which
famously had snakes
instead of hair, the nebu-
la’s serpentine tendrils are
actually the dying star’s
expelled gases. In this
image from the European
Southern Observatory’s
8.2-meter Very Large
Telescope, hydrogen
glows red and ionized
oxygen green. ESOBREAK
THROUGH