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Planetary News: Stardust (2007)Familiar Star Surprises Scientists with Brilliant Streak Across the HeavensBy Amir AlexanderAugust 15, 2007
A brilliant giant of a star, thousands of times more luminous than our Sun, is blazing its way through our galaxy like a roman candle, leaving a luminous trail behind 13 light years long. The startling discovery was made by the Galaxy Evolution Explorer (“Galex”), a space telescope launched in 2003 to survey the skies at the ultraviolet range of the spectrum. “This is exactly the kind of surprise that comes from a survey mission like the Galaxy Evolution Explorer” said Galex team member James D. Neill of Caltech. The star creating such a ruckus on the galactic plane is no stranger to astronomers, but rather one of the best studied stars in the sky. Discovered in 1596 by Fabricius, it was the first variable star ever recorded, with an eleven month cycle that sees it grow in luminosity and then fade away. For this remarkable feature it was named “Mira”, meaning “marvelous,” by the astronomer Hevelius some decades later. It ultimately became the model for an entire class of stars known as “Mira variables. But despite centuries of accumulated observations no one has ever noticed the enormous comet-like tail trailing behind the fast moving star. This is probably because Mira’s brilliant tail does not register at all in the visible light range in which most observations are made, but only in the ultraviolet range, which is Galex’s specialty. As a result, the long trail that stands out so clearly in the ultraviolet, was completely invisible to earlier observers. “I was shocked when I first saw this completely unexpected humongous tail trailing behind a well-known star” added Galex Principal Investigator, Christopher Martin of Caltech. Martin and his colleagues believe they have a basic understanding of the processes producing Mira’s enormous tail that stretches three times the distance of our Sun to the nearest star. Mira began its life as a common star, similar to our Sun and only slightly more massive. But whereas the Sun is in the midst of the “main phase” and will remain so for another 5 billion years or so, Mira is much older and has moved on to the final stages of its stellar lifespan. It has grown into a “red giant,” 400 times the radius of the Sun and millions of times its volume. Mira is now in the final phases of its life as a red giant, and now belongs to a class of stars know as “Asymptotic Branch Giant” (“AGB’s”). Such stars go through cataclysmic convulsions that send their entire external mantle shooting into interstellar space, ultimately leaving only the core of the star to drift along as a “white dwarf.” The Galex team is confident that Mira’s long and brilliant tail is nothing but the stars own mantle, which it had consistently shed over the past 30,000 years.
The comet-like shape of Mira’s trail was created because Mira is moving unusually fast – about 130 kilometers (80 miles) per second relative to our branch of the galaxy, explained Martin. Much like a bullet traveling through air, Mira’s speed is creating a bow shock in front of it in the interstellar medium. This, astronomers believe, heats up the gasses blowing off the star causing them to glow in the ultraviolet range. The gases then swirl around the stars creating the turbulent wake detected by Galex. By taking into account the speed of the star, and the enormous length of the tail behind it, Martin and his team concluded that the brilliant trail was formed over the past of 30,000 years or so. This, explained Mark Seibert of the Carnegie Institution, is particularly important for astronomers studying the evolution of stars. For although scientists have long theorized about the manner in which red giants shed their mantle, they have never had a chance to observe it up close. The discovery of Mira’s brilliant wake now gives them the opportunity to study the process as it happened over a span of 30,000 years. Mira’s tail, explained Seibert, is like “tree rings,” recording the history of the star and the process of shedding. Scientists will be given a unique glimpse of the dying convulsions of a red giant star, as it sheds its mantle and gradually shrinks into oblivion. Mira’s brilliant tail is important not only for our understanding of the dying gasps of aging stars, but also for our knowledge of the birth pangs of young ones. This is because by shedding its outer layers a red giant like Mira is seeding the interplanetary medium with essential ingredients for future generations of stars. These are the “heavy elements” (i.e. heavier than hydrogen) which form in the star’s core and find their way into its mantle. Once ejected into space, they remain there in the form of interstellar gas and dust, until they are swept into a swirling cloud that produces a new star, planetary system, and perhaps even life. By looking at Mira’s brilliant tail, and its record of how this aging red giant seeded the galaxy with the building blocks of stars, scientists are also looking into our own past and distant origins. For now Mira is the only star known to possess a brilliant comet-like tail, but Martin and his team think it is unlikely to remain that way for long. “If the primary example of a “Mira Variable Star” has a tail, many pther stars probably do as well” said Michael Shara of Columbia University. Now that astronomers know what to look for, he added, they will likely ask to point Mira in the direction of many other similar stars in the hope of detecting similar phenomena. But even if Mira’s brilliant tail turns out to be a common feature in our galaxy, the Galex team will forever retain the sense of awe and wonder at the universe, which always seems to hold some startling surprises in store for us. “It’s amazing to discover such a startlingly large and important feature of an object that has been known and studies for over 400 years” said Neill. |
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