Cerulean blue and lovely, clothed in a thin veil of clouds, Earth twirls a path through a Universe immense beyond our most unbridled imagining. Each night, a star-dusted ever widening expanse reveals itself overhead and every evening, since before recorded history, people across the globe look up and wonder about its extent, purpose and functioning. The ancients saw patterns of gods and mythical creatures and assumed that humankind dwelt at the center of everything that is. Grudgingly over the eons that passed, the truth became much more evident- Earth was but one of nine planets circling an unremarkable star that was, itself, just one of over four hundred million suns tracing a path around the center of one out of ten billion other galaxies.
Most galaxies are great stellar gatherings bound together by threads of gravity that have been shaped into gigantic, glowing disks. In the night skies of our planet, the star-studded plane of the disk forms an ethereal river of light that gives our galaxy its name- the Milky Way. So huge is our home Galaxy, that if it were reduced to the size of the United States, the Earth would be far smaller than the smallest dust fragment- barely visible in an electron microscope!
On the grandest scale, the Universe is filled with structures that resemble the shape of enormous soap bubbles. Occasionally, where the edges of one bubble meets another, herds of galaxies can be found, held together by mutual gravitational attraction. The Milky Way presides over one of these collections along with another spiral galaxy, two million light years distant, that can be seen with the naked eye from relatively dark locations toward the constellation of Andromeda. Over thirty other smaller galaxies can be found nearby and combined, they, our home galaxy and the spiral in Andromeda form, what astronomers call, the Local Group. Our group is, in turn, surrounded by other galactic aggregations.
One nearby bunch, consisting of nine galaxies, is dominated by the subject of this photograph. Known as the Northern Pinwheel or Messier 101, this enormous spiral collection of stars, gas and dust is located about 27 million light years from Earth and spans over 170 thousand light years from one side to the other!
Spiral galaxies, like the Northern Pinwheel, share many aspects in common with the Milky Way- the most prominent being a conspicuous set of expansive, glowing spiral arms extending from a compact central region.
Spiral galaxies represent about three quarters of the galactic population observed throughout the Universe. They have long been the subject of intense scrutiny because they present science with a tantalizing riddle- the spiral arms that represent their most striking characteristic should not exist in such great numbers.
On first impression, their spiral shape appears to be the result of the galaxy's natural spin- like a swirl of cream stirred into a cup of coffee. However, the speed of stars that orbit around the galactic center vary according to their distance from the galaxy's middle- the outer stars move much slower than those close in. The inner stars and gas that surround and accompany them, can make half a dozen orbits before the outer stars and material can circle once. This is known a differential rotation and it can form a spiral pattern in just a few turns but, over time, the pattern should smear just as well-stirred coffee quickly turns uniformly lighter brown.
Therefore, the spiral arm structure seen in galaxies should be transitory on the cosmic scale. Instead, they persist.
The mechanism that created the spiral arms seen in this galaxy, and others like our Milky Way, is known as a gravitational density wave. By definition, a wave is the movement of a disturbance through a medium- it is not the movement of the material itself. For example, as a boat moves through water, it disturbs nearby water molecules which, in turn, bump into molecules next to themselves and so on. This forms a wave pattern within the water but very little water actually moves with it.
Similarly, a galactic density wave is a spiral shaped disturbance that moves through the galaxy's disk. The stars, gas and dust within a galaxy orbit the central region at various speeds but all of this material travels about the center faster than the gravity density wave. Periodically, the material catches up with and falls into the slower moving gravity wave, much as a knot of cars might form around a large, slow moving truck on the highway. Over time, the stars, with mass and momentum of their own, pass through the wave and resume their original speed but the gas that accompanies the stars tends to accumulate, compress, collapse under their own weight and ignite as new, hot stars- this, in effect, lights up the spiral structure like a Christmas tree!
Thus, material that constitutes a galaxy's spiral arms are comprised of entirely different material than the arms of the same galaxy millions of years later.