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The graceful, winding arms of the majestic spiral galaxy M51 (NGC 5194) appear like a grand spiral staircase sweeping through space. They are actually long lanes of stars and gas laced with dust.

This sharpest-ever image of the Whirlpool Galaxy, taken in January 2005 with the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope, illustrates a spiral galaxy's grand design, from its curving spiral arms, where young stars reside, to its yellowish central core, a home of older stars. The galaxy is nicknamed the Whirlpool because of its swirling structure.

The Whirlpool's most striking feature is its two curving arms, a hallmark of so-called grand-design spiral galaxies. Many spiral galaxies possess numerous, loosely shaped arms which make their spiral structure less pronounced. These arms serve an important purpose in spiral galaxies. They are star-formation factories, compressing hydrogen gas and creating clusters of new stars. In the Whirlpool, the assembly line begins with the dark clouds of gas on the inner edge, then moves to bright pink star-forming regions, and ends with the brilliant blue star clusters along the outer edge.

Some astronomers believe that the Whirlpool's arms are so prominent because of the effects of a close encounter with NGC 5195, the small, yellowish galaxy at the outermost tip of one of the Whirlpool's arms. At first glance, the compact galaxy appears to be tugging on the arm. Hubble's clear view, however, shows that NGC 5195 is passing behind the Whirlpool. The small galaxy has been gliding past the Whirlpool for hundreds of millions of years.

As NGC 5195 drifts by, its gravitational muscle pumps up waves within the Whirlpool's pancake-shaped disk. The waves are like ripples in a pond generated when a rock is thrown in the water. When the waves pass through orbiting gas clouds within the disk, they squeeze the gaseous material along each arm's inner edge. The dark dusty material looks like gathering storm clouds. These dense clouds collapse, creating a wake of star birth, as seen in the bright pink star-forming regions. The largest stars eventually sweep away the dusty cocoons with a torrent of radiation, hurricane-like stellar winds, and shock waves from supernova blasts. Bright blue star clusters emerge from the mayhem, illuminating the Whirlpool's arms like city streetlights.

The Whirlpool is one of astronomy's galactic darlings. Located 31 million light-years away in the constellation Canes Venatici (the Hunting Dogs), the Whirlpool's beautiful face-on view and closeness to Earth allow astronomers to study a classic spiral galaxy's structure and star-forming processes.

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The heart of the mammoth galaxy cluster Abell 2744, also known as Pandora's Cluster, is shown in this Hubble Space Telescope image. The cluster is so massive that its powerful gravity bends the light from galaxies far behind it, making background objects appear larger and brighter in a phenomenon called gravitational lensing. These powerful lenses allow astronomers to find many dim, distant structures that otherwise might be too faint to see.

The small white boxes, labeled "a," "b," and "c," mark multiple images from the same background galaxy, one of the farthest, faintest, and smallest galaxies ever seen. The diminutive object is estimated to be over 13 billion light-years away. Enlarged views of the multiple images are shown in the insets at right. The arrows point to the tiny galaxy far behind the cluster. Each magnified image makes the galaxy appear as much as 10 times larger and brighter than it would look without the intervening lens.

To determine the background galaxy's distance, the researchers studied the galaxy's color and measured the positions between the three images. This new detection is considered one of the most reliable distance measurements of a galaxy that existed in the early universe. The galaxy appears as a tiny blob that is only a small fraction of the size of our Milky Way galaxy. But it offers a peek back into a time when the universe was only about 500 million years old, roughly 3 percent of its current age of 13.7 billion years.

An analysis of the distant galaxy shows that it measures merely 850 light-years across, 500 times smaller than the Milky Way, and is estimated to have a mass of only 40 million suns. The galaxy's star formation rate is about one star every three years (one-third the star formation rate in the Milky Way).

The galaxy was detected as part of the Frontier Fields program, an ambitious three-year effort, begun in 2013, that teams Hubble with NASA's other Great Observatories — the Spitzer Space Telescope and the Chandra X-ray Observatory — to probe the early universe by studying large galaxy clusters. Abell 2744 resides about 3.5 billion light-years away.

The image was made by combining near-infrared observations from Hubble's Wide Field Camera 3 and visible-light exposures from the Advanced Camera for Surveys. The observations were taken in 2009, 2013, and 2014.