Hubble Explores the Formation and Evolution of Star Clusters in the Large Magellanic Cloud

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Globular Cluster NGC 1466

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Artist’s Impression of Blue Stragglers

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The formation of blue stragglers

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Animation of Blue Stragglers

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Just as people of the same age can vary greatly in appearance and shape, so do collections of stars or stellar aggregates. New observations from the NASA/ESA Hubble Space Telescope suggest that chronological age alone does not tell the complete story when it comes to the evolution of star clusters.

Previous research on the formation and evolution of star clusters has suggested that these systems tend to be compact and dense when they form, before expanding with time to become clusters of both small and large sizes. New Hubble observations in the Large Magellanic Cloud (LMC) galaxy have increased our understanding of how the size of star clusters in the LMC changes with time[1].

Star clusters are aggregates of many (up to one million) stars. They are active systems in which the mutual gravitational interactions among the stars change their structure over time (known to astronomers as "dynamical evolution"). Because of such interactions, heavy stars tend to progressively sink towards the central region of a star cluster, while low-mass stars can escape from the system. This causes a progressive contraction of the cluster core over different timescales and means that star clusters with the same chronological age can vary greatly in appearance and shape because of their different “dynamical ages”.

Located nearly 160 000 light-years from Earth, the LMC is a satellite galaxy of the Milky Way which hosts star clusters covering a wide range of ages. This differs from our own Milky Way galaxy which primarily contains older star clusters. The distribution of sizes as a function of age observed for star clusters in the LMC is very puzzling, as the young clusters are all compact, while the oldest systems have both small and large sizes.

All star clusters, including those in the LMC, have been found to host a special type of re-invigorated stars called blue stragglers [2]. Under certain circumstances, stars receive extra fuel that bulks them up and substantially brightens them. This can happen if one star pulls matter off a neighbour, or if they collide.

As a result of dynamical aging, heavier stars sink towards the centre of a cluster as the cluster ages, in a process similar to sedimentation, called “central segregation”. Blue stragglers are bright, making them relatively easy to observe, and they have high masses, which means that they are affected by central segregation and can be used to estimate the dynamical age of a star cluster [3].

Francesco Ferraro of the University of Bologna in Italy and his team used the Hubble Space Telescope to observe blue stragglers in five (coeval) old LMC star clusters with different sizes and succeeded in ranking them in terms of their dynamical age.

“We demonstrated that different structures of star clusters are due to different levels of dynamical ageing: they are in different physical shape despite the fact that they were born at the same cosmic time. This is the first time that the effect of dynamical ageing has been measured in the LMC clusters” says Ferraro.

“These findings present intriguing areas for further research, since they reveal a novel and valuable way of reading the observed patterns of LMC star clusters, providing new hints about the cluster formation history in the LMC galaxy,” adds co-author Barbara Lanzoni.

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Notes

[1] The observations were achieved from a set of long exposures acquired with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) for five old star clusters in the Large Magellanic Cloud galaxy, secured under proposal 14164 (PI: Sarajedini).

[2] Blue stragglers are so called because of their blue colour, and the fact that their evolution lags behind that of their neighbours.

[3] Blue stragglers combine being relatively bright and having high mass by the standards of globular cluster stars, but they are not the only stars within these clusters that are either bright or massive.

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More information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

The team’s paper will appear in Nature Astronomy.

The international team of astronomers in this study consists of F. Ferraro (University of Bologna, Italy and INAF, Italy), B. Lanzoni (University of Bologna, Italy and INAF, Italy), E. Dalessandro (INAF, Italy), M. Cadelano (University of Bologna, Italy and INAF, Italy), S. Raso (University of Bologna, Italy and INAF, Italy), A. Mucciarelli (University of Bologna, Italy and INAF, Italy), G. Beccari (European Southern Observatory, Germany), C. Pallanca (University of Bologna, Italy and INAF, Italy). More information about the paper can be found here.

Image credit: ESA/Hubble & NASA

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Links

• Images of Hubble
• Hubblesite release
• Science paper

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Contacts

Francesco R. Ferraro
Department of Physics and Astronomy
University of Bologna
Tel: +39 051 2095774
Email: francesco.ferraro3@unibo.it

Bethany Downer
ESA/Hubble, Public Information Officer
Garching, Germany
Email: Bethany.Downer@partner.eso.org

Source: ESA/Hubble/News

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Hubble Helps Solve Mystery of 'Born Again' Stars

Birth of a Blue Straggler Star 
Credit: NASA, ESA, and A. Feild (STScI). Credit: NASA, ESA, and A. Feild (STScI). Release images

About this image:   [Left] - A normal star in a binary system gravitationally pulls in matter from an aging companion star that has swelled to a bloated red giant that has expanded to a few hundred times its original size.

[Right] - After a couple hundred million years, the red giant star has burned out and collapsed to the white dwarf that shines intensely in ultraviolet wavelengths. The companion star has bulked up on the hydrogen siphoned off of the red giant star to become much hotter, brighter, and bluer than it was previously.

For the past 60 years, astronomers have been puzzled by an unusual type of star that looks hotter and bluer than it should for its age. It has been dubbed a "blue straggler" because it seems to lag behind the evolution of neighboring stars. Blue stragglers dwell inside ancient star clusters that should have stopped making youthful and short-lived blue stars billions of years ago. The most popular explanation among several competing theories is that an aging star spills material onto a smaller companion star. The small star bulks up on mass to become hotter and bluer, while the aging companion burns out and collapses to a white dwarf — a burned out cinder. To test this theory, astronomers using the Hubble Space Telescope conducted a survey of the open star cluster NGC 188 that has 21 blue stragglers. Of those they found that seven had white dwarf companions, by identifying their ultraviolet glow that is detectable by Hubble. This confirms the binary star theory for their origin.

For more images and information about this study, visit: http://mcdonaldobservatory.org/news/releases/20151207 .

Source: HubbleSite

NASA's Hubble Finds Rare Blue Straggler Stars in the Milky Way's Hub

Blue Straggler Stars in the Galactic Bulge

Image Type: Astronomical/Illustration

Credit: NASA, ESA, W. Clarkson (Indiana University and UCLA),
and K. Sahu (STScI)

View this image

Artist's View of a Blue Straggler Star

Artwork Credit: NASA, ESA, and G. Bacon (STScI)

Science Credit: NASA, ESA,
W. Clark (Indiana University and UCLA),
and K. Sahu (STScI)

This is an artist's concept of a close binary pair of stars that are merging to form a blue-straggler-class star. Blue stragglers are so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed. The merger stirs up hydrogen fuel and causes the resulting more massive star to undergo nuclear fusion at a faster rate, causing it to burn hotter and bluer. Probing the star-filled, ancient hub of our Milky Way, the Hubble Space Telescope has found blue stragglers for the first time within our galaxy's bulge.

Probing the star-filled, ancient hub of our Milky Way, NASA's Hubble Space Telescope has found a rare class of oddball stars called blue stragglers, the first time such objects have been detected within our galaxy's bulge.

The size and nature of the blue straggler population detected in the bulge will allow astronomers to better understand if the bulge is exclusively old stars, or a mixture of both young and old stars. In addition, the discovery provides a new test case for formation models of the blue stragglers themselves.

Blue stragglers — so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed — were first found inside ancient globular star clusters half a century ago. They have been detected in many globular and open clusters, as well as among the stars in the solar neighborhood. But they have never been seen inside the core of our galaxy until Hubble was trained on the region.

Hubble astronomers found blue straggler stars in an extensive set of Hubble exposures of the Milky Way's crowded hub. Blue stragglers are much hotter — and hence bluer — than they should be for the aging neighborhood in which they live. Now that blue stragglers have at last been found within the bulge, the size and characteristics of this population will allow astronomers to better understand the still-controversial processes of star formation within the bulge.

The results, to be published in The Astrophysical Journal, are being reported by lead author Will Clarkson of Indiana University and the University of California, Los Angeles, at the American Astronomical Society meeting in Boston, Mass.

These results support the idea that the Milky Way's central bulge stopped making stars billions of years ago. It is now home to aging Sun-like stars and cooler red dwarfs. Giant blue stars that once lived there exploded as supernovae billions of years ago. If our galaxy were the size of a dinner plate, the central bulge would be roughly the size of a grapefruit placed in the middle of the plate.

This discovery is a spin-off from a seven-day-long survey conducted in 2006 called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). Hubble peered at and obtained variability information for 180,000 stars in the crowded central bulge of our galaxy, 26,000 light-years away. The survey was intended to find hot Jupiter-class planets that orbit very close to their stars. But the SWEEPS team also uncovered 42 oddball blue stars among the bulge population with brightness and temperatures typical for stars much younger than ordinary bulge stars.

Blue stragglers have long been suspected to be living in the bulge. Until now, it has never been proven because younger stars in the disk of our galaxy lie along the line-of-sight to the core, confusing and contaminating the view.

But Hubble's view is so sharp that astronomers could distinguish the motion of the core population from foreground stars in the Milky Way. Bulge stars orbit the galactic nucleus at a different speed than foreground stars. Plotting their motion required returning to the SWEEPS target region with Hubble two years after the first-epoch observations were made.

Hence, the blue stragglers were identified as moving along with the other stars in the bulge. It's like looking into a deep, clear pond where the fish at the bottom of the pond are swimming at a faster rate than the fish closer to the surface.

"The size of the field of view on the sky is roughly that of the thickness of a human fingernail held at arm's length, and within this region, Hubble sees about a quarter million stars towards the bulge," Clarkson says. "Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field."

From the 42 candidate blue stragglers, the investigators estimate 18 to 37 of them are likely to be genuine blue stragglers, with the remainder consisting of a mixture of foreground objects and at most a small population of genuinely young bulge stars.

It's not clear how blue stragglers form, or if there is more than one mechanism at work. A common idea is that blue stragglers emerge from binary pairs. As the more massive star evolves and expands, the less massive star accretes material from the companion. This stirs up hydrogen fuel and causes the accreting star to undergo nuclear fusion at a faster rate. It burns hotter and bluer.

The seven-day observation allowed the fraction of blue straggler candidates presently in close binaries to be estimated by virtue of their changing light-curve. This is caused by the change of shape induced in one star due to the tidal gravitational pull of its companion. "The SWEEPS program was designed to detect transiting planets through small light variations. Therefore the program could easily detect the variability of binary pairs, which was crucial in confirming these are indeed blue stragglers," says Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md., the principal investigator of the SWEEPS program.

The observations clearly indicate that if there is a young star population in the bulge, it is very small, and it was not detected in the SWEEPS program. "Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood," Clarkson says. "While the consensus is that the bulge largely stopped forming stars long ago, many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge."

CONTACT

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Will Clarkson
Indiana University, Bloomington, Ind., and
University of California, Los Angeles, Calif.
812-855-6911
wiclarks@indiana.edu

Kailash Sahu
Space Telescope Science Institute, Baltimore, Md.
410-338-4930
ksahu@stsci.edu

WIYN 3.5-m telescope rounds up “Blue Stragglers”

Figure 1: NGC188 is an open star cluster in the constellation Cepheus

Astronomers have reported new observations of a remarkable binary star population in a well-known star cluster. In a press release (embargoed until Dec 23, 1PM EST), Nature features two interesting studies of a class of atypical stars known as “blue stragglers”.

One paper was written by Robert Mathieu and Aaron Geller, University of Wisconsin-Madison, who used the WIYN telescope at Kitt Peak National Observatory for their observations. Mathieu and Geller found that the blue stragglers in NGC188 (Caldwell 1), an open cluster in our galaxy, have a binary fraction of 76%, which is three times the frequency for normal stars of this type.

They conclude that possibly all of these stars originate in multiple star systems, and that several formation mechanisms could be operating. Geller, a PhD candidate at the University of Wisconsin-Madison, has been granted long term observing status for this project, which is part of the WIYN Open Cluster Study.

Kitt Peak National Observatory is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. The founding members of the WIYN Observatory partnership are the University of Wisconsin-Madison, Indiana University, Yale University, and NOAO.