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Finding “triples” number of stars in universe

Dec. 2, 2010
Courtesy of Yale University
and World Science staff

Lit­tle stars called red dwarfs are much more com­mon than pre­vi­ously thought—so much so that any es­ti­mate of the to­tal num­ber of stars ex­ist­ing must be tripled, some as­tro­no­mers have an­nounced.

The find­ings could boost the chances of life ex­ist­ing in the rel­a­tively near­by uni­verse, as red dwarfs are con­sid­ered good can­di­dates for host­ing plan­ets with po­ten­tially com­plex life.

Fil­ter­ing out the light from brighter stars, as­tro­no­mers de­tected the faint sig­na­ture of small, dim red dwarf stars in near­by el­lip­ti­cal ga­lax­ies (right), and found these are much more nu­mer­ous than in our own Milky Way (left). This find­ing sug­gests that the to­tal num­ber of stars in the uni­verse could be up to three times high­er than pre­vi­ously thought. (Cour­te­sy of Yale U.)


Be­cause red dwarfs are rather dim, as­tro­no­mers had­n’t been able to de­tect them in most ga­lax­ies oth­er than our own be­fore now, the re­search­ers ex­plained. As such, it was­n’t known how much of the to­tal stel­lar popula­t­ion of the uni­verse con­sists of red dwarfs.

Now as­tro­no­mers have used pow­er­ful in­stru­ments on the Keck Ob­serv­a­to­ry in Ha­waii to de­tect the faint sig­na­ture of red dwarfs in eight mas­sive, rel­a­tively near­by ga­lax­ies called el­lip­ti­cal ga­lax­ies. These lie about 50 mil­lion to 300 mil­lion light years away; a light year is the dis­tance light trav­els in a year.

The in­ves­ti­ga­tors found that the red dwarfs, which weigh only 10 to 20 per­cent as much as the Sun, were far more boun­ti­ful than ex­pected. “No one knew how many of these stars there were,” said Pie­ter van Dokkum, a Yale Uni­vers­ity as­tron­o­mer who led the re­search, which was pu­blished Dec. 1 in the ad­vance on­line is­sue of the jour­nal Na­ture. “D­if­fer­ent the­o­ret­i­cal mod­els pre­dicted a wide range of pos­si­bil­i­ties.”

The team dis­cov­ered that there are about 20 times more red dwarfs in el­lip­ti­cal ga­lax­ies than in the Milky Way, said Char­lie Con­roy of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., who was al­so in­volved in the re­search. “We usu­ally as­sume oth­er ga­lax­ies look like our own. But this sug­gests oth­er con­di­tions are pos­si­ble in oth­er ga­lax­ies,” Con­roy said. “So this dis­cov­ery could have a ma­jor im­pact on our un­der­stand­ing of gal­axy forma­t­ion and evo­lu­tion.”

For in­stance, Con­roy said, ga­lax­ies might con­tain less dark mat­ter—a mys­te­ri­ous sub­stance that is un­dect­ect­able ex­cept through its grav­i­ta­tion­al effects—than pre­vi­ously est­im­ated. In­stead, the abun­dant red dwarfs could con­trib­ute more mass than real­ized.

In ad­di­tion to boost­ing the to­tal num­ber of stars in the uni­verse, the dis­cov­ery al­so in­creases the num­ber of plan­ets or­bit­ing those stars, which in turn el­e­vates the num­ber of plan­ets that might har­bor life, van Dokkum said. In fact, a re­cently dis­cov­ered plan­et that as­tro­no­mers be­lieve could po­ten­tially sup­port life or­bits a red dwarf star, called Gliese 581.

“There are pos­sibly tril­lions of Earths or­bit­ing these stars,” van Dokkum said, adding that the red dwarfs they dis­cov­ered, which are typ­ic­ally more than 10 bil­lion years old, have been around long enough for com­plex life to evolve. “It’s one rea­son why peo­ple are in­ter­est­ed in this type of star.”


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Little stars called red dwarfs are much more common than previously thought—so much so that any estimate of the total number of stars must be tripled, some astronomers have announced. Moreover, the findings could boost the chances of life existing in the relatively nearby universe. Red dwarf stars are considered good candidates for hosting habitable planets. Because red dwarfs are relatively dim, astronomers hadn’t been able to detect them in most galaxies other than our own before now, the researchers explained. As such, it wasn’t known how much of the total stellar population of the universe consists of red dwarfs. Now astronomers have used powerful instruments on the Keck Observatory in Hawaii to detect the faint signature of red dwarfs in eight massive, relatively nearby galaxies called elliptical galaxies, which lie about 50 million to 300 million light years away. A light year is the distance light travels in a year. The investigators found that the red dwarfs, which weigh only 10 to 20 percent as much as the Sun, were far more bountiful than expected. “No one knew how many of these stars there were,” said Pieter van Dokkum, a Yale University astronomer who led the research, described Dec. 1 in the advance online issue of the journal Nature. “Different theoretical models predicted a wide range of possibilities.” The team discovered that there are about 20 times more red dwarfs in elliptical galaxies than in the Milky Way, said Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was also involved in the research. “We usually assume other galaxies look like our own. But this suggests other conditions are possible in other galaxies,” Conroy said. “So this discovery could have a major impact on our understanding of galaxy formation and evolution.” For instance, Conroy said, galaxies might contain less dark matter—a mysterious substance that has mass but cannot be directly observed—than previous measurements of their masses might have indicated. Instead, the abundant red dwarfs could contribute more mass than realized. In addition to boosting the total number of stars in the universe, the discovery also increases the number of planets orbiting those stars, which in turn elevates the number of planets that might harbor life, van Dokkum said. In fact, a recently discovered planet that astronomers believe could potentially support life orbits a red dwarf star, called Gliese 581. “There are possibly trillions of Earths orbiting these stars,” van Dokkum said, adding that the red dwarfs they discovered, which are typically more than 10 billion years old, have been around long enough for complex life to evolve. “It’s one reason why people are interested in this type of star.”