VOA常速英语: Scientists discover 'tail' on distant star

Images captured by a NASA science satellite reveal the star Mira has left in its wake a comet-like tail that is some 13 light years long — more than 120 trillion kilometers. It's a phenomenon that's never been seen before.

The Galaxy Evolution Explorer satellite, known as Galex, was launched four years ago with the aim of studying the history of the universe, using a telescope equipped with ultraviolet detectors. Finding a gigantic tail on a fast-moving star was not something astronomers had anticipated. Chris Martin, the principal investigator on Galex, announced the discovery.

"The Galaxy Evolution Explorer has discovered that Mira has a vast, turbulent tail stretching across interstellar space," Martin told reporters. "And it's truly remarkable that this star, which has been studied for centuries, has surprised us with a completely new and unexpected phenomenon."

Mira's "tail" is composed of carbon, oxygen and other chemical elements. The discovery may help scientists understand how old, dying stars like Mira shed material that is later incorporated into new stars and planets. The astronomers calculate that the oldest parts of the tail were left behind tens of thousands of years ago, as the star grew and gravity weakened farther from its center.

"In fact, if we had had, 30,000 years ago — If Neanderthal man had had ultraviolet eyes and could look above the atmosphere, he could have seen the beginning of this tail forming," said Martin.

The age of the tail makes it something of a time capsule, says Columbia University astronomer Michael Shara, like the rings of a tree. "Because we have this tail that was generated over 30,000 years, we can look at small individual pieces of the tail and deduce what the mass loss rate was like and what the chemistry along the tail was, which gives us some hint as to the differentiation of elements in the atmosphere of Mira before it started to shed mass."

In addition to the tail, images from the Galex satellite show a curved shock wave in front of Mira as it speeds through the surrounding space at 130 kilometers per second. Astronomer Mark Siebert of the Carnegie Institution says that the bow shock, as they call it, and the random-looking tail behind the star resemble a picture of a bullet in supersonic flight. "In front of the bullet there's a leading shock, much like Mira's bow shock. And then trailing the bullet there's also a classic turbulent wake, which is very reminiscent of the structures that we see behind Mira."

Seibert says Mira is a very common type of star, so it may be that this kind of tail is actually not very rare and that more will be discovered. In any event, this one is the first.

Shara, the Columbia University astronomer, says the new and completely unexpected discovery is an example of what can happen as astronomy advances.

"Any time astronomers take a look at a new part of what we call observation space — that is, they start using a telescope which has much better angular resolution like Hubble, or much greater sensitivity in the ultraviolet like Galex, or a very large field of view like Galex — they tend to find something new, and that's exactly what's happened here," he said.

The astronomers say they are really just beginning to study this huge structure. They published their discovery in the journal Nature.