When Galileo turned his telescope toward the Milky Way in 1612, this cloudy band that stretches across the nighttime sky was resolved into “countless stars.” Galileo’s sketches were the first published astronomical observations of our home galaxy, which we call the Milky Way galaxy, and the “countless stars” are now thought to number around three hundred billion, including our own Sun!

The true size of the Milky Way galaxy was not fully realized until the beginning of the 20th century, when astronomers developed new ways to measure the vast distances to stars. The two astronomers that pioneered ways to measure the universe were Henrietta Leavitt, who discovered that some pulsating stars could be used as cosmic yardsticks, and Edwin Hubble, who used Leavitt’s pulsating stars to prove that the Milky Way was just one galaxy in a universe filled with countless galaxies. The search for Leavitt’s pulsating stars in distant galaxies by the space telescope named for Hubble is still a cutting-edge research program at the start of the 21st century.

Our Milky Way galaxy is a spiral-shaped city of stars, so big that it would take a beam of light 100,000 years to cross from one edge to another. Like most galaxies, the Milky Way resides in a group of galaxies held together by gravity. The Milky Way shares local space with its sister spiral, the Andromeda galaxy, as well as dozens of smaller galaxies of differing sizes and shapes. (Intergalactic distances are truly unimaginable on human scales. It takes starlight from Andromeda 2.5 million years to reach us.)

This galactic collection is called the Local Group. Our Local Group of galaxies is in turn, a member of the Virgo Supercluster, which contains about 2500 large galaxies, 50,000 dwarf galaxies, and 200 trillion stars. It is named after the Virgo Cluster, the largest grouping of galaxies in the Virgo Supercluster and the supercluster’s dominant gravitational force. Sometimes the Virgo Supercluster is also referred to as the Local Supercluster. Welcome to our galactic neighborhood!

With so many galaxies bound by their mutual gravity, collisions can and do happen. Several galaxies in the Virgo Cluster are interacting, including the pair called Markarian’s Eyes. Such collisions happen over millions of years, changing the galaxy’s shape and triggering huge bursts of star formation. These newborn stars go on to enrich the galaxies with new elements and molecules that are the building blocks for future stars, planets, and perhaps life.

The universe is built out of galaxy clusters, with our own Virgo Supercluster being just one in a web of clusters that fill the observable universe. Starlight from the most distant galaxies takes billions of years to travel through space to us, which means that we see such galaxies as they appeared when the light began its journey, billions of years in the past. In this way, a telescope is a sort of time machine, looking out and looking back in time.

Modern telescopes such as NASA’s Great Observatories—namely, the Compton Gamma Ray Observatory, Chandra, Hubble, and Spitzer—have allowed us to see beyond our local galactic region. Another orbiting space telescope, the ultraviolet Galaxy Evolution Explorer (GALEX), will produce the first comprehensive map of galaxies under construction, bringing us closer to understanding how galaxies like our own Milky Way were formed. And NASA’s James Webb Space Telescope, due to launch in 2013, will be able to detect light that has been traveling for over twelve billion years, from a time when the galaxies themselves were born.

Galaxies—the building blocks of the universe—are constantly in motion, rotating, interacting, and generating new stars. The story of how galaxies form, change and evolve is one of the most important areas of research in astronomy—research that began four centuries years ago when Galileo looked up at the Milky Way.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Galileo's sketch of a sky filled with stars. Credit: Octavo Corp./Warnock Library.

The Andromeda galaxy, our nearest neighbor, is 2.5 million light years away. Credit: Copyright Robert Gendler.

GALEX traces Andromeda's spiral arms. The wisps of blue are neighborhoods that harbor hot, young, massive stars. Credit: NASA/JPL-Caltech.

This view of nearly 10,000 galaxies, called the Hubble Ultra Deep Field, is the deepest visible-light image of the cosmos. Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team.