The Whirlpool Galaxy is known for its sharply defined spiral arms. Their prominence could be the result of the Whirlpool's gravitational tug-of-war with its smaller companion galaxy . (Courtesty of NASA, ESA, and The Hubble Heritage Team)
Astronomers estimate that there are 100 billion galaxies. Galaxies are mostly empty space with vast distances between each star. The largest contain trillions of stars; the smallest have just a few million. A typical galaxy is about 100,000 light-years in size.
There are three major galaxy shapes: elliptical, spiral, and irregular. The Sun is just one of hundreds of billions of stars in our Milky Way galaxy, a complex structure of rotating spiral arms of younger stars surrounding a densely packed central bulge of older stars.
One of the best known spiral galaxies is the Whirlpool Galaxy (or M51). Located in the constellation Canes Venatici, the Whirlpool Galaxy is approximately 31 million light years away from us. One light year is the distance it takes light to travel one year. As the picture above shows, the Whirlpool Galaxy was given its name because of its classic spiral shape.
Astronomers have studied this famous galaxy for years in an effort to better understand galaxy structure and formation. Images from the Hubble Space Telescope have shown another galaxy, named NGC 5195, passing behind the Whirlpool Galaxy. It's possible that the gravitational interactions between these two galaxies have contributed to the defined spiral shape of the Whirlpool Galaxy.
The Whirlpool Galaxy is easily observable in the night sky under the right conditions. Anyone with a dark sky (minimal light pollution) and a good pair of binoculars or a telescope should be able to observe the galaxy as a fuzzy patch of light.
Learn more about the Whirlpool Galaxy on Hubble Site.
Dark Energy
About 13.7 billion years ago, our Universe came into being in an event know as the Big Bang. Since that time, the Universe has been expanding and cooling. Millions of years after the birth of the Universe, as it cooled,
gravity drew matter into clumps that became the galaxies we see
and dark matter, which we don't see, but infer through the
effects of its gravity.
Over the billions of years of cosmic time, gravity should slow down the expansion
of the Universe. However, in the late twentieth century, a surprising
discovery was made: the expansion of the Universe is actually
accelerating! Since gravity always pulls things together, this suggests that some form of "dark energy" is actually pushing the Universe apart.
Learn more about dark energy, how it was discovered, and what dark energy may mean for the eventual fate of the
Universe on Hubble Site.
