Largest collection of anomalous white dwarfs observed by Hubble

Apr 23, 2009
Twenty-four unusual stars have been spotted in an ancient swarm of stars in the Milky Way. These white dwarfs, a type of dead star, are made of helium rather than the usual carbon and oxygen, and appear as faint, pale blue stars. This collection of helium-core white dwarfs was discovered by a team including Professor Adrienne Cool of San Francisco State University and Rachel Strickler of UC Santa Cruz. They suggest that thousands of stars in star cluster NGC 6397 are coupled with close companions that interfere with their normal development, resulting in these odd white dwarfs made of helium. Credit: Jay Anderson / Space Telescope Science Institute

Twenty-four unusual stars, 18 of them newly discovered, have been observed in new Hubble telescope images. The stars are white dwarfs, a common type of dead star, but they are odd because they are made of helium rather than the usual carbon and oxygen. This is the first extensive sequence of helium-core white dwarfs to be observed in a globular cluster, a dense swarm of some of the oldest stars in our galaxy.

A study, accepted for publication in the , suggests that these helium-core have had their lives cut short because of their orbital dance around a partner star.

"Helium-core white dwarfs have only about half the mass of typical white dwarfs, but they are found concentrated in the center of the cluster" said Adrienne Cool, professor of physics and astronomy at San Francisco State University, who co-authored the study with graduate student Rachel R. Strickler. "With such low masses, the helium-core white dwarfs ought to be floating all around the cluster, according to theory. The fact that we find them only in the central regions suggests that they have heavy companions -- partner that anchor them to the cluster center."

Being coupled with companions also helps explain the stars' atypical chemical make up. White dwarfs are stars that have reached the end of their lives and have run out of fuel. Most stars burn their fuel leaving behind a dense ball of carbon and oxygen, but these white dwarfs are made of . Cool suggests that a star that goes on to become a helium-core white dwarf must have a close companion so that when the star became a red giant and expanded, its outer layers spilled onto the companion. The star never had the chance to reach maturity and burn its helium into carbon and oxygen.

The study focused on star cluster NGC 6397, one of the globular clusters closest to Earth at approximately 7,200 away. Six helium-core white dwarfs have been observed before in this cluster. Cool and colleagues discovered the first three in 1998.

"This is the first time that helium-core white dwarf stars have been discovered in partnerships with other white dwarfs in a globular cluster," Cool said. "This large sample allows us to answer questions about the mass and nature of the partner stars, and the prevalence of these kinds of binaries in the globular cluster."

Binary stars play an important role in the evolution of star clusters. Their continual dance around each other provides energy to the cluster that astronomers believe can help prevent black holes from forming. From the data, Cool and her team are able to infer that one to five percent of stars in this globular cluster will end their lives as helium-core white dwarfs with companion stars, a finding that will help improve theoretical models of cluster dynamics. "It may not sound like a lot but it doesn't take many binaries to stir things up," Cool said.

One finding remains a mystery. Despite the Hubble camera's high level of sensitivity, the faintest helium-core white dwarfs appear to be missing.

"It's possible that these helium-core white dwarfs cool so slowly that they haven't had time to get very faint yet," Cool said. Another possibility is that the oldest binaries containing helium-core white dwarfs have been destroyed by interactions with other stars in the cluster.

More information: "Helium-Core in the Globular Cluster NGC 6397" will be published in the July 1, 2009 issue of the Astrophysical Journal.

Source: San Francisco State University (news : web)

Explore further: Astronomers: 'Tilt-a-worlds' could harbor life

add to favorites email to friend print save as pdf

Related Stories

Astronomers discover how white dwarf stars get their 'kicks'

Dec 04, 2007

University of British Columbia astronomer Harvey Richer and UBC graduate student Saul Davis have discovered that white dwarf stars are born with a natal kick, explaining why these smoldering embers of Sun-like stars are found ...

Hubble Sees Faintest Stars in a Globular Cluster

Aug 17, 2006

NASA's Hubble Space Telescope has uncovered what astronomers are reporting as the dimmest stars ever seen in any globular star cluster. Globular clusters are spherical concentrations of hundreds of thousands ...

Astronomers Find Origin Of Extreme-Helium Stars

Mar 20, 2006

Astronomers have determined the origin of a very unusual and rare type of star. New data indicate that extreme-helium stars, as they are called, form from the merger of two white dwarfs.

New type of pulsating white dwarf star discovered

May 01, 2008

University of Texas at Austin astronomers Michael H. Montgomery and Kurtis A. Williams, along with graduate student Steven DeGennaro, have predicted and confirmed the existence of a new type of variable star, ...

Recommended for you

Astronomers: 'Tilt-a-worlds' could harbor life

13 hours ago

A fluctuating tilt in a planet's orbit does not preclude the possibility of life, according to new research by astronomers at the University of Washington, Utah's Weber State University and NASA. In fact, ...

Pushy neighbors force stellar twins to diverge

21 hours ago

(Phys.org) —Much like an environment influences people, so too do cosmic communities affect even giant dazzling stars: Peering deep into the Milky Way galaxy's center from a high-flying observatory, Cornell ...

Image: Multiple protostars within IRAS 20324+4057

Apr 14, 2014

(Phys.org) —A bright blue tadpole appears to swim through the inky blackness of space. Known as IRAS 20324+4057 but dubbed "the Tadpole", this clump of gas and dust has given birth to a bright protostar, ...

Research group to study interstellar molecules

Apr 11, 2014

From April 2014, a new group will study interstellar molecules and use them to explore the entire star and planet formation process at the Max Planck Institute for Extraterrestrial Physics. Newly appointed ...

Astronomers suggest more accurate star formation rates

Apr 10, 2014

(Phys.org) —Astronomers have found a new way of predicting the rate at which a molecular cloud—a stellar nursery—will form new stars. Using a novel technique to reconstruct a cloud's 3-D structure, ...

User comments : 0

More news stories

Astronomers: 'Tilt-a-worlds' could harbor life

A fluctuating tilt in a planet's orbit does not preclude the possibility of life, according to new research by astronomers at the University of Washington, Utah's Weber State University and NASA. In fact, ...

NASA Cassini images may reveal birth of a Saturn moon

(Phys.org) —NASA's Cassini spacecraft has documented the formation of a small icy object within the rings of Saturn that may be a new moon, and may also provide clues to the formation of the planet's known ...

Vegetables on Mars within ten years?

The soil on Mars may be suitable for cultivating food crops – this is the prognosis of a study by plant ecologist Wieger Wamelink of Wageningen UR. This would prove highly practical if we ever decide to ...

Patent talk: Google sharpens contact lens vision

(Phys.org) —A report from Patent Bolt brings us one step closer to what Google may have in mind in developing smart contact lenses. According to the discussion Google is interested in the concept of contact ...

Tech giants look to skies to spread Internet

The shortest path to the Internet for some remote corners of the world may be through the skies. That is the message from US tech giants seeking to spread the online gospel to hard-to-reach regions.

Wireless industry makes anti-theft commitment

A trade group for wireless providers said Tuesday that the biggest mobile device manufacturers and carriers will soon put anti-theft tools on the gadgets to try to deter rampant smartphone theft.