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VLT captures young stars in detail

...details of young stars have been captured in unprecedented detail by the VLT, which could hope to end years of debate on the behaviour of matter in young stellar systems...

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Re-writing the cratering history of the Moon two separate reports planetary scientists have presented new insights into the cratering history of the Moon, and used small craters to help date the ages of geological features on Mars...

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The hunt is on for

...observations of the Earth by Venus Express, and supercomputer simulations of dusty discs around Sunlike stars may provide new clues in the quest to detect Earth-sized exoplanets...

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Video archive

STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

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Dwarf leaders shepherd galaxy gas

Posted: October 15, 2008

Dwarf galaxies that formed rapidly during the reionisation and global heating epoch of the Universe a billion years after the big bang, allowed other dwarfs to form by shepherding gas that would otherwise have blown away, according to a new theory.

According to simulations performed by astronomers at the University of Zurich, the gravitational force of some dwarf galaxies that were able to form rapidly enough, kept back enough gas to then allow later (and smaller) dwarf galaxies to form. This explanation fits well with what is observed, and could even apply to the Milky Way’s Magellanic Clouds.

The Small and Large Magellanic Cloud. The Magellanic Clouds fit the new theory regarding the formation and behaviour of dwarf galaxies. Image: NASA/ESA/A. Nota (STScI/ESA).

There has always been a problem with global heating and reionisation as an entirely satisfactory explanation for why more dwarf galaxies didn’t form. The author of the study (to be published in The Astrophysical Journal Letters), Professor George Lake points out that, “It doesn't explain why most of the dwarf galaxies have stars that form much later than this.” But the shepherding theory provides an answer. By preventing all the gas from escaping, the smaller dwarfs can capture it at a later date, allowing a later generation of stars to form. Evidence for this seems to come from the fact that the smaller dwarf galaxies group around a ‘leader’ dwarf.

Lake, and co-author Dr Elena D’Onghia (currently visiting the University of Harvard) applied their theory to the Magellanic Clouds — prominent dwarf galaxies of the Milky Way visible in the Southern Hemisphere. They say that the two dwarf galaxies were the largest members of a group of dwarfs that had entered the Milky Way’s dark halo in its recent past. Furthermore, seven of the eleven brightest satellite galaxies of the Milky Way were also part of this group. Computer simulations at the University of Zurich show that dwarf galaxies have a tendency to form in these groups and then be captured by a large galaxy later on. This group is then disrupted by gravitational tidal forces from the large galaxy, which spreads the small population around. This fits the satellite galaxy population we observe for the Milky Way today.

Lake and D’Onghia’s work seems to be backed up by others. New measurements by astronomers at Harvard University indicate that the Magellanic Clouds are moving faster than previously thought, and may have entered the Milky Way recently. "The scenario proposed by D'Onghia and Lake fits in well with these observational determinations and may account for many facets of the satellite population of the Milky Way", says Professor Lars Hernquist.