How images are made
Celestial-Terrestrial Convergence: Hubble Views Beside Landscapes
A sandstone arch and a pillar of gas, a geyser and a dying star: the forms match because some of the forces that make them do too.
Put a photograph of a canyon ridge at night next to a Hubble picture of a star-forming cloud and the resemblance can be uncanny: a warm-colored mass in the foreground, a cooler sky full of stars behind. Much of that likeness is coincidence of shape and color. Some of it is not. Erosion, outflow and the way light falls on a scene work at every scale, from a few meters of sandstone to light-years of gas, and the pairings below follow those shared processes.
A ridge under a starry sky
Picture Zion National Park in Utah after dark: a rust-red ridge of iron-rich rock lit by the Moon, with the stars drawn into curved trails over it. The red comes from iron in the stone; the deep blue overhead is Earth's atmosphere scattering light.
The Hubble image at the top of this page shows a similar arrangement in NGC 3324, a region at the northwest corner of the Carina Nebula roughly 7,200 light-years away. The "ridge" is the edge of a cloud of gas and dust light-years tall, eaten away by the radiation of young stars in the cavity above it. The "sky" is a hollow full of hot gas and stars. Here color is a code, not a pigment: light from sulfur is shown red, hydrogen green and oxygen blue, one of the filter-to-color assignments explained in Are the Colors Real?
The stars give away the difference in method. A camera on a tripod stays fixed while Earth rotates, so a long exposure turns stars into arcs. Hubble tracks its target, so every star in NGC 3324 stays a sharp point no matter how long the exposure runs.
Erosion leaves the strongest part standing
Delicate Arch in Arches National Park is a span of red sandstone about 18 meters (60 feet) tall. Water and wind removed the softer rock around it, and what is left is the part that resisted longest. Seen against the low Sun, it reads as a dark silhouette.
The Pillars of Creation in the Eagle Nebula are the same story told in gas. Intense ultraviolet light and stellar winds from hot, luminous stars have stripped away the thinner material around them, and the denser columns are what remains. They, too, stand dark against a brighter background, silhouetted by the glowing nebula behind. The process differs, radiation instead of rain, but the logic is identical: the weaker material goes first.
Looking at a galaxy from inside and outside
On a dark night in Canyonlands National Park, the Milky Way rises as a pale arch above Mesa Arch. Everything in that band, every star, dark lane and glowing patch, belongs to the galaxy Earth sits in. Looking toward the brighter end of the arch means looking toward the galaxy's center, from a position out in one of its spiral arms.
NGC 1300, about 60 million light-years away in Eridanus, offers the outside view. It is considered the prototype of the barred spirals, galaxies whose arms begin at the ends of a straight bar of stars running through the nucleus. The Milky Way is thought to be a barred spiral as well, so NGC 1300 gives a rough idea of how home might look from far away. The two pictures show structures of the same kind and the same size class; only the viewpoint has changed.
Pressure finding a way out
Castle Geyser in Yellowstone erupts when water heated and pressurized by hot rock underground forces its way out of a narrow vent. The column of water and steam can block out the Sun behind it.
NGC 6302, roughly 3,800 light-years away in Scorpius, is a planetary nebula: the cast-off outer layers of a star that was once about five times the mass of the Sun. The hot remnant at the center floods the expelled gas with ultraviolet light and makes it glow, with gas in the lobes heated to nearly 20,000 degrees Celsius. A dense ring of dust around the star hides it from view and also channels the outflow, so the material escapes in two directions and forms an hourglass. As with the geyser, the shape is set by a narrow exit for something under pressure.
A wider set of pairings
The four pairs here are a sample. A larger set of matched landscapes and Hubble images is on display in the Celestial-Terrestrial Convergence gallery. Taken together, the pairings make one point plainly: the scenes in Hubble images are as physical as a canyon or a geyser, places shaped by forces, recorded by instruments that extend human sight.
Questions readers ask
Are Hubble pictures abstract images?
They can look abstract at first, without a horizon or a familiar subject. They are photographs of real places, made with specialized cameras and filters. Once the subject is understood, a picture like NGC 3324 reads much like a landscape, with a foreground, a background and visible signs of erosion.
Why are the stars sharp in Hubble images but trailed in night landscapes?
A camera on a tripod stays still while Earth rotates, so over a long exposure each star draws an arc. Hubble keeps itself locked onto its target throughout an exposure, so the stars stay as points even in very long observations.
What shapes the Pillars of Creation?
Ultraviolet radiation and winds from nearby hot, massive stars strip away the less dense gas and dust. The columns are the denser material that resists for longer, in the same way that a sandstone arch is the harder rock left after water and wind remove softer layers.
Is the Milky Way a barred spiral like NGC 1300?
Astronomers think so. NGC 1300 is the textbook example of the type, with spiral arms that start at the ends of a central bar, and it is often used to suggest how the Milky Way might appear from outside.