Probing Question: What are Shooting Stars?
In the early morning darkness on April 15, 1912, as the R.M.S. Titanic was sinking in the freezing Atlantic, survivors witnessed a large number of streaking lights in the sky, which many believed to be the souls of their drowning loved ones passing to heaven.
Says Kevin Luhman, what they most likely were seeing was the peak of the Lyriad meteor shower, an annual event occuring in mid-to-late April.
Though folklore of many cultures describes shooting or falling stars as rare events, 鈥渢hey鈥檙e hardly rare or even stars,鈥� says Luhman, Penn State assistant professor of astronomy and astrophysics.
鈥淔rom the dawn of civilization people have seen these streaks of light that looked like stars, but were moving quickly across the sky,鈥� he notes. 鈥淭hese 鈥榮hooting stars鈥� are actually space rocks鈥攎eteoroids鈥攎ade visible by the heat generated when they enter the Earth鈥檚 atmosphere at high speeds.鈥� These bits of ice and debris range in size from a speck of sand to a boulder. Larger objects are called asteroids, and smaller, planetary dust, Luhman explains.
Most meteoroids are about the size of a pebble and become visible 40 to 75 miles above the earth.
The largest meteoroids, called 鈥渇ireballs鈥� or 鈥渂olides,鈥� explode into flashes so bright they can be seen during the day, says Luhman.
More common, though, are falling meteors too dim to see in daylight. 鈥淢eteors are falling all the time,鈥� he adds. 鈥淭here鈥檚 debris all throughout the solar system. Every minute somewhere around the Earth there鈥檚 some little piece of rock or ice that鈥檚 falling from space.鈥�
A dark spot in the northern hemisphere promises the best viewing of falling meteors, notes Luhman. The pre-dawn hours of any clear night are the best time because, as Earth slowly spins around its axis, the side facing into its orbit tends to encounter more space grit. 鈥淵ou鈥檙e better off using just the naked eye versus the telescope, which shows just a small patch of sky,鈥� he suggests. 鈥淚f you look at the entire sky, that gives you a better chance of spotting the meteor.鈥�
The very best viewing times, when 鈥測ou might see one or two meteors per minute,鈥� says Luhman, are the eleven or so meteor showers each year when Earth passes through a debris trail left behind by a comet鈥攁 giant ball of ice and grit also orbiting the sun. 鈥淭hey are named after the constellation in the sky out of which the meteors appear to come,鈥� Luhman explains, noting the Leonid and Perseid showers as the most famous and spectacular.
The Perseids, appearing every August and named for Perseus, occur as Earth moves through a thousand-year-old cloud of cosmic debris ejected from the comet Swift-Tuttle, last seen in 1992, and鈥攁t six miles across鈥攖he largest object known to make repeated passes near Earth.
Says Luhman, it would have been hard to miss the falling lights of the 1833 Leonid meteor storm, which were so bright and fell so fast, about 100,000 in an hour, that many feared the end of the world. The storm, widely regarded as the birth of modern meteor astronomy, marked the discovery of the Leonids, visible every November 17 and caused by debris from the comet Tempel-Tuttle.
While meteors are now well-understood, meteorites鈥攆ragments of meteoroids and asteroids that survive both the passage through our atmosphere and the ground impact鈥攁re helping scientists to learn of the solar system鈥檚 origins, says Luhman. 鈥淭hese rocks are basically leftovers or raw ingredients from when the solar system was born 4.5 billion years ago.鈥�
Some meteorites even tell us about planets. 鈥淚f a comet or asteroid hits Mars, it can throw some of the pieces of the crust into space and, after millions of years, some of that material falls down to Earth鈥檚 surface,鈥� says Luhman.
One of 34 Martian meteorites reached fame in 1996 when NASA scientists announced it showed signs of primitive life from more than 3.6 billion years ago. Closer study explained the formation as a geologic effect, all but ending the scientific controversy, explains Luhman.
Yet, shooting stars have hardly fallen out of our everyday conversations. Our movies, songs and poetry still speak highly of the bright lights as a magical sight, worthy of wishes. As the Disney company鈥檚 theme song has taught generations of children, 鈥淲hen you wish upon a star, your dreams come true.鈥�
Source: by Lisa Duchene, Penn State
