THE WHIRLPOOL GALAXY (M51)
AND COMPANION GALAXY
The graceful, winding arms
of the majestic spiral galaxy M51 (NGC 5194) appear
like a grand spiral staircase sweeping through space.
They are actually long lanes of stars and gas laced
This sharpest-ever image of the Whirlpool Galaxy,
taken in January 2005 with the Advanced Camera for
Surveys aboard NASA's Hubble Space Telescope, illustrates
a spiral galaxy's grand design, from its curving
spiral arms, where young stars reside, to its yellowish
central core, a home of older stars. The galaxy
is nicknamed the Whirlpool because of its swirling
The Whirlpool's most striking feature is its two
curving arms, a hallmark of so-called grand-design
spiral galaxies. Many spiral galaxies possess numerous,
loosely shaped arms which make their spiral structure
less pronounced. These arms serve an important purpose
in spiral galaxies. They are star-formation factories,
compressing hydrogen gas and creating clusters of
new stars. In the Whirlpool, the assembly line begins
with the dark clouds of gas on the inner edge, then
moves to bright pink star-forming regions, and ends
with the brilliant blue star clusters along the
Some astronomers believe that the Whirlpool's arms
are so prominent because of the effects of a close
encounter with NGC 5195, the small, yellowish galaxy
at the outermost tip of one of the Whirlpool's arms.
At first glance, the compact galaxy appears to be
tugging on the arm. Hubble's clear view, however,
shows that NGC 5195 is passing behind the Whirlpool.
The small galaxy has been gliding past the Whirlpool
for hundreds of millions of years.
As NGC 5195 drifts by, its gravitational muscle
pumps up waves within the Whirlpool's pancake-shaped
disk. The waves are like ripples in a pond generated
when a rock is thrown in the water. When the waves
pass through orbiting gas clouds within the disk,
they squeeze the gaseous material along each arm's
inner edge. The dark dusty material looks like gathering
storm clouds. These dense clouds collapse, creating
a wake of star birth, as seen in the bright pink
star-forming regions. The largest stars eventually
sweep away the dusty cocoons with a torrent of radiation,
hurricane-like stellar winds, and shock waves from
supernova blasts. Bright blue star clusters emerge
from the mayhem, illuminating the Whirlpool's arms
like city streetlights.
The Whirlpool is one of astronomy's galactic darlings.
Located 31 million light-years away in the constellation
Canes Venatici (the Hunting Dogs), the Whirlpool's
beautiful face-on view and closeness to Earth allow
astronomers to study a classic spiral galaxy's structure
and star-forming processes.
Credit: NASA, ESA, S. Beckwith (STScI), and The
Hubble Heritage Team (STScI/AURA)
Note: The Hubble Heritage Team wishes to thank
Dr. Steven Beckwith (STScI) and Dr. Robert Kennicutt
(University of Arizona) for their support in
helping us to acquire the data.