Hubble's Exciting Universe: Unveiling Galaxy Evolution
The Hubble Space Telescope opened up a new, far frontier in probing the universe. From the early 1990s to the present, it has allowed astronomers to leapfrog across the universe to unveil an undiscovered country where galaxies evolved from smaller and more primitive assemblages of stars.
Today, we know that galaxies are the basic building block of the universe: island cities of stars. A century ago Edwin Hubble first discovered that galaxies were actually other so-called "island universes," distributed far beyond our Milky Way in all directions.
Edwin Hubble cataloged galaxy shapes: spiral, elliptical, and irregular, and pondered if there was an evolutionary link. Did galaxies morph from one shape into another over time? He also described them as the "markers of space," because their apparent recession velocities were evidence the universe was expanding in all directions.
Sixty years after Edwin Hubble's discovery, his namesake, the Hubble Space Telescope, clearly revealed galaxies as time capsules that chronicle the evolution of the universe—from the birth of the first stars to the buildup of immense galaxy clusters (groupings of hundreds or thousands of galaxies bound together by gravity). These findings are the first hints of the origin of the Sun and Earth in the vast and wondrous chronology of the cosmos.
Before the Hubble telescope was launched, there was plenty of room for conjecture and theoretical modeling about how galaxies must have evolved if the universe was born in the big bang. Ground-based observations were not able to establish which of several competing theories best described how galaxies formed in the early universe.
However, early Hubble observations, called the Medium Deep Survey, taken even before its optical repair in 1993, showed that galaxies at a then-record-breaking distance of 9 billion light-years were more compact than those seen today. Hubble also found bizarre fragmentary objects considered the ancestors of Earth’s Milky Way galaxy.
Hubble Deep Fields
In 1995, astronomers decided to use Hubble to conduct a bold and daring experiment to probe into the universe as far as possible. For 10 consecutive days, Hubble stared at one tiny, seemingly empty patch of sky for 1 million seconds. The gamble of using precious telescope time paid off. Hubble captured the feeble glow of myriad, never-before-seen galaxies. Many of the galaxies are so far away it has taken billions of years for their light to reach us. Therefore, the view is like looking down a "time corridor," where galaxies can be seen as they looked billions of years ago.
The resulting, landmark image is called the Hubble Deep Field . At the time, the image won the gold medal for being the farthest peek into the universe ever made. Its stunning success encouraged astronomers to pursue a series of Hubble deep-field surveys. The succeeding surveys uncovered more galaxies at greater distance from Earth, thanks to new cameras installed on Hubble during astronaut servicing missions. The cameras increased the telescope's power to look even deeper into the universe.
These probing Hubble observations have opened up the early universe—the undiscovered country—in a way no other telescope has done or could have done over the past 30 years. Astronomers carried out a Herculean task of assembling a decade's worth of 2,963 Hubble deep-field exposures that required up to 800 orbits of Hubble time. The study culminated in the Extreme Deep Field, which included observations in near-infrared-light, to gaze back to within a few hundred million years of the big bang.
The combination of the deep surveys is the Hubble Legacy Field, containing a quarter-million galaxies. The image mosaic combines nearly 7,500 separate Hubble exposures, representing 16 years' worth of observations. It contains about 30 times as many galaxies as in the Hubble Ultra Deep Field in 2002. The wavelength range stretches from ultraviolet to near-infrared light, capturing all the features of galaxy assembly over time.
Collectively, these surveys allow astronomers to plot, for the first time, the star formation history of the universe, to count the number of galaxies at different epochs, to measure the growth of chemical abundances, and to study galaxy shapes over time and the rates of galaxy mergers.
This makes Hubble the "Incredible Time Machine" for pushing back through 97 percent of the universe’s history , to just 400 million years after the big bang. For the first time in human history, Hubble observations allow humans to approach the "cosmic sunrise" when the first galaxies formed after the big bang. This is an epoch that, as late as 2010, astronomers never thought would be achievable with Hubble.