The child’s poem, “Twinkle, twinkle, little star,” describes a familiar sight in the night sky, according to U-M astronomer Richard G. Teske.
“During cold February nights, the twinkling of bright stars is especially striking; sometimes they seem to shatter into dazzling shards of blue, red, and green,” Teske says. “While the vivid display delights most star watchers with its movement and color, the twinkling interferes with delicate observations made by astronomers.”
Twinkling of stars—called “scintillation” by astronomers—is caused by moving air currents in the atmosphere. The beam of light from a star passes through many regions of moving air while on its way to an observer’s eye or telescope. Each atmospheric region distorts the light slightly for a fraction of a second.
“The total result of a great number of momentary small distortions is the twinkling that we see,” Teske explains. “Although these atmospheric motions are present all the time, they are especially noticeable on cold winter nights.”
Teske says that Michigan sky watchers can observe twinkling by watching February’s stars between 7 and 8 p.m. on clear, crisp evenings. Sirius, the brightest star in Earth’s sky, is low in the southeast, while the stars of Orion the Hunter hang higher in the south.
Sirius is closer to the horizon than Orion, Teske explains, so its light passes through more air than the light from Orion’s stars. Early in the evening, Sirius flickers more intensely than the stars of Orion. A few hours later, when Sirius is nearly directly south and its light comes through less of the atmosphere, its twinkling diminishes noticeably.
“Anyone who uses a telescope is aware that scintillation causes images of stars to wobble and waver. The effect blurs and enlarges star images on photographs captured during astronomical time exposures,” Teske notes. “If there was no atmosphere overhead, the photos would be much sharper. This is one of the reasons the Hubble Space Telescope was positioned high above the atmosphere—to achieve the extremely sharp pictures that can be taken when no air motions interfere.”
Ground-based astronomers have tried to get around the problem by putting telescopes on high mountains to get them above most of the atmosphere.
One of the world’s finest observing sites, Teske says, is nearly 14,000 feet up on the extinct volcano Mauna Kea on the island of Hawaii. There, several huge modern telescopes take advantage of tranquil skies to image the heavens with a sharpness never achieved before the launch of the Hubble Space Telescope.
“But the 14,000-foot altitude is about as high as most ordinary people can work comfortably for several hours at a time, and it is unlikely that large, manned observatories will be placed on taller peaks,” Teske says.
The future is coming into sharp focus. A telescope placed on Mauna Kea can just about distinguish George Washington’s presence on a quarter from three miles away. Astronomers are hopeful that in the near future the same telescopes, when equipped with new devices to compensate for bad images, will come close to being able to distinguish his face from the same distance.
“But even this will not surpass the performance of the newly refurbished Hubble Space Telescope,” Teske says. “It will recognize—at three miles’ distance—the ribbon that ties Washington’s pigtail.”
