"Long before it's in the papers"
November 09, 2012

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Comet collisions every 6 seconds by distant star?

Nov. 9, 2012
Courtesy of UCLA
and World Science staff

Eve­ry six sec­onds on av­er­age, comets col­lide near an un­usu­al star in the con­stella­t­ion Ce­tus, as­tro­no­mers say.

The sci­en­tists say the find­ings ex­plain why a tre­men­dous quanti­ty of the gas car­bon mon­ox­ide is float­ing around the star, 49 CE­TI, which is vis­i­ble to the un­aided eye. Such gas is char­ac­ter­is­tic of very young stars, but not old­er ones like this one, where it has usu­ally al­ready drift­ed away.

Ac­cord­ing to the new stu­dy, the un­usu­al gas disk around 49 CE­TI is due to comets con­stantly hit­ting each oth­er. A belt of comets sur­rounds the star, si­m­i­lar to one around our sun known as the Kuiper Belt, but far richer in comets.

“We now be­lieve that 49 CE­TI is 40 mil­lion years old, and the mys­tery is how in the world can there be this much gas around an oth­erwise or­di­nary star that is this old,” said Ben­ja­min Zuck­er­man, a co-author of the re­search, with the Uni­vers­ity of Cal­i­for­nia Los An­ge­les. “This is the old­est star we know of with so much gas.”

In find­ings pub­lished in The As­t­ro­phys­i­cal Jour­nal, Zuck­er­man and co-author In­seok Song of the Uni­vers­ity of Geor­gia pro­pose the mys­te­ri­ous gas comes from a very mas­sive disk-shaped re­gion around 49 CE­TI that is si­m­i­lar to our Kuiper Belt, which lies be­yond the or­bit of Nep­tune.

The to­tal mass, or weight, of the ob­jects mak­ing up the Kuiper Belt—in­clud­ing the dwarf plan­et Plu­to—is about one-tenth the mass of the Earth. But when Earth was young, as­tro­no­mers say, the Kuiper Belt likely had a mass that was ap­prox­i­mately 40 times larg­er than the Earth’s. By con­trast, the Kuiper Belt an­a­logue that or­bits around 49 CE­TI now has an es­ti­mat­ed mass of 400 Earths — 4,000 times the cur­rent mass of the Kuiper Belt.

“Hun­dreds of tril­lions of comets or­bit around 49 CE­TI and one oth­er star whose age is about 30 mil­lion years. Im­ag­ine so many tril­lions of comets, each the size of the UCLA cam­pus — ap­prox­i­mately one mile [wide] — or­biting around 49 CE­TI and bash­ing in­to one anoth­er,” Zuck­er­man said. “These young comets likely con­tain more car­bon mon­ox­ide than typ­i­cal comets in our so­lar sys­tem. When they col­lide, the car­bon mon­ox­ide es­capes as a gas. The gas seen around these two stars is the re­sult of the in­cred­i­ble num­ber of col­li­sions among these comets.”

49 CE­TI, which lies in the direction of the Southern con­stel­la­tion Ce­tus—named after a sea mon­ster of Greek myth—is about 200 mil­lion light-years away. A light-year is the dis­tance light trav­els in a year.

“We cal­cu­late that comets col­lide around these two stars about every six sec­onds,” Zuck­er­man said. “I was ab­so­lutely amazed when we cal­cu­lated this rap­id rate. I would not have dreamt it in a mil­lion years. We think these col­li­sions have been oc­cur­ring for 10 mil­lion years or so.” The gas or­biting 49 CE­TI was first found in 1995 by Zuck­er­man and two col­leagues, us­ing a ra­di­o tel­e­scope in the Si­er­ra Ne­vada moun­tains of south­ern Spain.

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Every six seconds on average, comets collide near an unusual star in the constellation Cetus, astronomers say. The scientists say the findings explain why a tremendous quantity of the gas carbon monoxide is floating around the star, 49 CETI, which is visible to the unaided eye. Such gas is characteristic of very young stars, but not older ones like this one, where it has usually already dissipated. According to the new study, the unusual gas disk around 49 CETI is due to comets constantly hitting each other. A belt of comets surrounds the star, similar to one around our sun known as the Kuiper Belt, but far richer in comets. “We now believe that 49 CETI is 40 million years old, and the mystery is how in the world can there be this much gas around an otherwise ordinary star that is this old,” said Benjamin Zuckerman, a co-author of the research, with the University of California Los Angeles. “This is the oldest star we know of with so much gas.” In findings published in the Astrophysical Journal, Zuckerman and co-author Inseok Song of the University of Georgia propose the mysterious gas comes from a very massive disk-shaped region around 49 CETI that is similar to our Kuiper Belt, which lies beyond the orbit of Neptune. The total mass, or weight, of the objects making up the Kuiper Belt—including the dwarf planet Pluto—is about one-tenth the mass of the Earth. But when Earth was young, astronomers say, the Kuiper Belt likely had a mass that was approximately 40 times larger than the Earth’s. By contrast, the Kuiper Belt analogue that orbits around 49 CETI now has an estimated mass of 400 Earths — 4,000 times the current mass of the Kuiper Belt. “Hundreds of trillions of comets orbit around 49 CETI and one other star whose age is about 30 million years. Imagine so many trillions of comets, each the size of the UCLA campus — approximately one mile [wide] — orbiting around 49 CETI and bashing into one another,” Zuckerman said. “These young comets likely contain more carbon monoxide than typical comets in our solar system. When they collide, the carbon monoxide escapes as a gas. The gas seen around these two stars is the result of the incredible number of collisions among these comets.” 49 CETI is about 200 million light-years away. A light-year is the distance light travels in a year. “We calculate that comets collide around these two stars about every six seconds,” he said. “I was absolutely amazed when we calculated this rapid rate. I would not have dreamt it in a million years. We think these collisions have been occurring for 10 million years or so.” The gas orbiting 49 CETI was first found in 1995 by Zuckerman and two colleagues, using a radio telescope in the Sierra Nevada mountains of southern Spain.