Galaxies

Our Solar System is in the Milky Way galaxy, one of hundreds of billions of galaxies in the observable Universe.

Image at right: A picturesque spiral galaxy known as Messier 81, or M81 (courtesy of NASA/JPL-Caltech/ESA/Harvard-Smithsonian CfA)

Features

: 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, we would expect gravity to 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.

Missions

: An estimated 10,000 galaxies are revealed in humankind's deepest portrait of the visible universe ever taken by the Hubble Space Telescope. Photo credit: NASA/ESA/S. Beckwith(STScI) and The HUDF Team.

Thanks to new technology we have been able to see things very far away from us in space! Light travels about 6 trillion miles a year and we have been able to observe other galaxies that are ten billion light years away! Now that is far away.

For ten days in 1995, The Hubble Space Telescope took a series of 342 pictures of a very small sample of the sky. These pictures were constructed to create what is known as the Hubble Deep Field. Adler astronomer Grace Wolf-Chase, Ph.D. gives some more information about these deep field images. Learn more! (PDF)

Another galaxy observing mission is the GALEX mission. Since its launch in April 2003, the Galaxy Evolution Explorer (GALEX), an orbiting space telescope, has been observing galaxies over 10 billion years of history. These observations will help scientists determine how galaxies, the basic structures of our universe, evolve and change.

Learn more about GALEX on NASA's website.

Myths, Stories, and More

: Artist's concept of the Milky Way galaxy (Courtesy of NASA)

From the Latin words Via Lactea, meaning road of milk, the Milky Way has fascinated many cultures. Greek legend tells us that the hazy band of light across the night sky was made of milk spilled by the baby Heracles (Hercules in Roman mythology). The milk then sprayed over the heavens.

Earth Matters

: The Andromeda galaxy, in a new composite image from NASA's Galaxy Evolution Explorer and the Spitzer Space Telescope. (Courtesy of NASAJPL-Caltech)

The Universe is the totality of matter, energy, and space, including Earth, the Solar System, the galaxies, and the contents of the space between the galaxies. Although we cannot visit other galaxies with current technology, telescopes let us know that they exist.

The Andromeda Galaxy is the closest neighboring galaxy and has a similar large spiral shape to our Milky Way Galaxy. It is about 2.5 million light years away. If we look about 4 billion years into the future, the Milky Way Galaxy and the Andromeda Galaxy will collide and combine to make a new and even larger galaxy. Because stars in the galaxies are so spread out, they will probably pass each other without colliding, but the two galaxies also contain giant clouds of dust and gas which are the raw material for new stars. When the galaxies collide, these clouds will crash into each other and many new stars will form as the clouds are compressed.

It's possible to observe the Andromeda Galaxy from your own backyard. For tips on how to find it, visit NASA's Imagine the Universe page NASA's Imagine the Universe page

Discover more about Earth's connection to galaxies at Curious About Astronomy.