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Gravitational lensing weighs 70 galaxies

...the largest ever single collection of gravitational lens galaxies has helped settle a long-standing debate over the relationship between mass and luminosity in galaxies, as well as infer the presence of dark matter...

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Organic cemeteries could dominate ancient Mars

...two complementary studies support the popular theory that the Red Planet once hosted vast lakes, flowing rivers and a variety of other wet environments that had the potential to support life...

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XMM detects nova everyone else missed

...it was one of the brightest nova events of the decade and clearly visible to the naked eye, yet no one but the XMM-Newton space telescope was there for the party...

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STS-120 day 2 highlights

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

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STS-118: Highlights

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

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Stellar vampires make the Universe dusty
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 25, 2008

For the first time, astronomers have witnessed the expulsion of a shell of dusty gas around a freshly erupted nova and tracked its evolution for over 200 days, providing a new and invaluable way of estimating the distances to nearby novae.

Nova Scorpii 2007a, which was discovered on 4 February 2007 in the constellation Scorpius, provided the focus for the Very Large Telescope Interferometer (VLTI) study. Novae are understood to occur in a double star system once a dense white dwarf star has over-fed on material from a companion red dwarf star, causing the white dwarf to explode in a bright display of fireworks often visible to the naked eye.

Observations were carried out at the Very Large Array on the Paranal Mountain in Chile. Image: ESO.

A high resolution survey was carried out by astronomers using the VLTI over five months following the explosion, who witnessed and tracked the formation and evolution of dust around the object, the first time such measurements have been made. The first observations were secured 23 days after the nova’s discovery and showed that the stellar source was extremely compact, less than 1 milli-arcsecond in diameter, comparable to viewing one grain of sand from about 100 kilometres away. A few days later the source measured 13 milli-arcseconds.

"It is most likely that the latter size corresponds to the diameter of the dust shell in expansion, while the size previously measured was an upper limit of the erupting source," explains lead author Olivier Chesneau.

The ring of dusty debris was present for more than 200 days, during which it expanded at a rate close to 2 million kilometres per hour. The erupting source was screened from view, becoming more than 10,000 times dimmer at visual wavelengths over those 200 days.

"This is the first time that the dust shell of a nova is spatially
resolved and its evolution traced starting from the onset of its
formation up to the point that it becomes too diluted to be seen," says co-author Dipankar Banerjee from the Physical Research Laboratory in India.

The astronomers speculate that the mass of dust ejected from the star could be equivalent to over 30 times the mass of Earth, an impressive feat when considering that the star itself was less than the radius of the Earth, with the dust component comprising less than one percent the mass of the star. However, factors such as the stability of the dust, and the duration of dust formation will effect this value, and “the error bar is large” says Chesneau, “because it is difficult to know exactly when the dust production stops”.

The measurement of the angular expansion rate away from the source enabled the astronomers to derive an accurate distance to the object, in this case about 5500 light-years, providing a new and promising technique for obtaining distances of nearby novae. The observations also provided new details on the feeding mechanisms of these stellar vampires.

Chesneau reveals that the next step will be to study the emission lines of the erupting star, and to look for other novae events to learn more about their first moments. “Each line form carries a lot of information about the local density and temperature,” he says. “As interferometrists, our speciality is to spatially resolve the first moments; the goal is to see the line forming regions as early as possible.”

For Nova Scorpii 2007a, further analysis can be performed with other telescopes - such as Hubble - since the ejecta can now be more easily spatially resolved.

An animation of the evolution of the dust shell can be found on the ESO website.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
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3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
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