Hubble finds jets and explosions in NGC 7793

Credit: ESA /Hubble & NASA, Acknowledgement: D. Calzetti (University of Massachusetts) and the LEGUS Team

This new image from the NASA/ESA Hubble Space Telescope shows NGC 7793, a spiral galaxy in the constellation of Sculptor some 13 million light-years away from Earth.

NGC 7793 is one of the brightest galaxies in the Sculptor Group, one of the closest groups of galaxies to the Local Group, the group of galaxies containing our galaxy, the Milky Way and the Magellanic Clouds.

The image shows NGC 7793's spiral arms and small central bulge.

Unlike some other spirals, NGC 7793 doesn't have a very pronounced spiral structure, and its shape is further muddled by the mottled pattern of dark dust that stretches across the frame.

The occasional burst of bright pink can be seen in the galaxy, highlighting stellar nurseries containing newly-forming baby stars.

Although it may look serene and beautiful from our perspective, this galaxy is actually a very dramatic and violent place.

Astronomers have discovered a powerful micro-quasar within NGC 7793, a system containing a black hole actively feeding on material from a companion star.

A micro-quasar is an object that has some of the properties of quasars in miniature. While many full-sized quasars are known at the cores of other galaxies, it is unusual to find a quasar in a galaxy's disk rather than at its center.

Micro-quasars are almost like scale models, they allow astronomers to study quasars in detail. As material falls inwards towards this black hole, it creates a swirling disk around it.

Some of the infalling gas is propelled violently outwards at extremely high speeds, creating jets streaking out into space in opposite directions.

In the case of NGC 7793, these jets are incredibly powerful, and are in the process of creating an expanding bubble of hot gas some 1,000 light-years across.

Critical mass not needed for Type Ia supernovae explosions

A global collaboration of astronomers searching for clues about dark energy, the mysterious force that is speeding up the expansion of the Universe, have uncovered new evidence about the nature of supernovae, finding many are lighter than scientists had expected.

The findings, from an international team from the Nearby Supernova Factory project, overturn previous understanding of white dwarf stars and raise new questions about how these stars explode.

"White dwarfs are dead stars, the corpses of stars that were once like our Sun.'

Richard Scalzo

'They won't explode on their own - they need another star to help blow them up," said ANU astronomer Dr Richard Scalzo, who led the latest research.

"We now know it's much easier to blow them up than we used to think."

A supernova is a star that explodes and shines much more brilliantly as it reaches the end of its life.

By studying "nearby" Type Ia (1a) supernovae - within a billion light years from earth - astronomers can then compare them with older and fainter supernovae even further out in space, allowing them to measure distances in the Universe.

Dr Scalzo said most of the supernovae his team studied had blown up well before dinosaurs walked on Earth.

He said astronomers had previously believed white dwarfs needed to be around 1.4 times the mass of the Sun before they could explode.

Using the University of Hawaii's 2.2-metre telescope, his team studied 19 Type Ia supernovae.

By carefully watching how quickly the supernovae faded away after their brightest point, and comparing to calculations made by computer, the team could then "weigh" each explosion to figure out the white dwarf's mass.

They were surprised to find that as many as half were well below the previously-assumed tipping point for an explosion.

That meant the life the dying stars led, and the cause of their violent deaths, also had to be totally different from what scientists once thought.

Brian Schmidt

Dr. Scalzo said the ultimate aim of the research was to better understand dark energy, for which the 2011 Nobel Prize in Physics was awarded to ANU professor Brian Schmidt, Adam Riess (Johns Hopkins University), and Saul Perlmutter (Lawrence Berkeley National Laboratory - LBNL).

"Brian Schmidt used type Ia supernovae to discover that dark energy exists," he said.

"We're now trying to understand what it is. This new information about how white dwarfs explode is a huge step forward towards that goal."

Cosmologist Greg Aldering, who leads the international Nearby Supernova Factory project in Berkeley, said: "This is a significant advance in furthering Type Ia supernovae as cosmological probes for the study of dark energy."

Dr Scalzo was previously based in the Nearby Supernova Factory headquarters at Lawrence Berkeley National Laboratory in California, and is a member of the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO).

More Information: 'Type Ia supernova bolometric light curves and ejected mass estimates from the Nearby Supernova Factory' arXiv:1402.6842 [astro-ph.CO]

ESA Venus Express: Planet-sized space weather explosions

Giant perturbations called hot flow anomalies in the solar wind near Venus can pull the upper layers of its atmosphere, the ionosphere, up and away from the surface of the planet. 

Credit: NASA

Researchers recently discovered that a common space weather phenomenon on the outskirts of Earth's magnetic bubble, the magnetosphere, has much larger repercussions for Venus.

The giant explosions, called hot flow anomalies, can be so large at Venus that they're bigger than the entire planet and they can happen multiple times a day.

"Not only are they gigantic," said Glyn Collinson, a space scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

"But as Venus doesn't have a magnetic field to protect itself, the hot flow anomalies happen right on top of the planet. They could swallow the planet whole."

Collinson is the first author of a paper on these results that appeared online in the Journal of Geophysical Research in February 2014.

The work is based on observations from the European Space Agency's Venus Express.

The results show just how large and how frequent this kind of space weather is at Venus.

Earth is protected from the constant streaming solar wind of radiation by its magnetosphere. Venus, however, has no such luck.

A barren, inhospitable planet, with an atmosphere so dense that spacecraft landing there are crushed within hours, Venus has no magnetic protection.

Scientists like to compare the two: What happened differently at Earth to make it into the life-supporting planet it is today? What would Earth be like without its magnetic field?

At Earth, hot flow anomalies do not make it inside the magnetosphere, but they release so much energy just outside that the solar wind is deflected, and can be forced to move back toward the sun.

Without a magnetosphere, what happens at Venus is very different.

Venus's only protection from the solar wind is the charged outer layer of its atmosphere called the ionosphere.

A sensitive pressure balance exists between the ionosphere and the solar wind, a balance easily disrupted by the giant energy rush of a hot flow anomaly.

The hot flow anomalies may create dramatic, planet-scale disruptions, possibly sucking the ionosphere up and away from the surface of the planet.

More Information: 'Ionospheric photoelectrons at Venus: Initial observations by ASPERA-4 ELS': Journal of Geophysical Research in February 2014 dx.doi.org/10.1016/j.pss.2007.12.008

Supernova Legacy Survey: Powerful ancient explosions explain new class of supernovae

A small portion of one of the fields from the Supernova Legacy Survey showing SNLS-06D4eu and its host galaxy (arrow). 

The supernova and its host galaxy are so far away that both are a tiny point of light that cannot be clearly differentiated in this image. 

The large, bright objects with spikes are stars in our own galaxy. 

Every other point of light is a distant galaxy. 

Credit: UCSB

Astronomers affiliated with the Supernova Legacy Survey (SNLS) have discovered two of the brightest and most distant supernovae ever recorded, 10 billion light-years away and a hundred times more luminous than a normal supernova. Their findings appear in the Dec. 20 issue of the Astrophysical Journal.

These newly discovered supernovae are especially puzzling because the mechanism that powers most of them—the collapse of a giant star to a black hole or normal neutron star—cannot explain their extreme luminosity.

Discovered in 2006 and 2007, the supernovae were so unusual that astronomers initially could not figure out what they were or even determine their distances from Earth.

"At first, we had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one," said lead author D. Andrew Howell, a staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT) and adjunct faculty at UC Santa Barbara.

"I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible."

One of the newly discovered supernovae, named SNLS-06D4eu, is the most distant and possibly the most luminous member of an emerging class of explosions called superluminous supernovae.

These new discoveries belong to a special subclass of superluminous supernovae that have no hydrogen.

The new study finds that the supernovae are likely powered by the creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second.

Magnetars have the mass of the sun packed into a star the size of a city and have magnetic fields a hundred trillion times that of the Earth.

While a handful of these superluminous supernovae have been seen since they were first announced in 2009, and the creation of a magnetar had been postulated as a possible energy source, the work of Howell and his colleagues is the first to match detailed observations to models of what such an explosion might look like.

Co-author Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab created models of the supernova that explained the data as the explosion of a star only a few times the size of the sun and rich in carbon and oxygen.

The star likely was initially much bigger but apparently shed its outer layers long before exploding, leaving only a smallish, naked core.

More information: dx.doi.org/10.1088/0004-637X/779/2/98

Everything You Wanted to Know: Physics of Nuclear Explosions

Barack Obama may have instructed the Pentagon to prepare for massive cuts to the US nuclear arsenal, but a book published in Brazil has sparked fresh worries about nuclear proliferation.

Unbelievably, the book, by physicist Dalton Barroso, is called The Physics of Nuclear Explosions (translation from the Portuguese "A Física dos Explosivos Nucleares") and explains some of the physics required to engineer both fission and fusion bombs, from the dynamics of detonation to the plasma physics of the core.

The publication of is said by Brazilian media to have alarmed the International Atomic Energy Agency and the Pentagon because it may mean Brazil has a fresh interest in developing such nuclear weapons - despite being a signatory to the nuclear nonproliferation treaty.

Barroso has responded to his critics.

He told the Federation of American Scientists - whose excellent Secrecy News blog has the full story - that the book's information is deducible from public domain information in any case and represents no novel threat. It's actually a subset of his PhD thesis from the Military Institute of Engineering in Rio de Janiero.

Perspective is indeed called for here. A similar furore blew up in March 2008 over the release by the whistleblowing website Wikileaks of a Manhattan Project fission bomb design from 1947. Much of the information had been in the public domain since 2002 if anyone had cared to seek it out.

And as Wikileaks points out (scroll down its page), the British government cared so little about the "leak" it wouldn't field an official to deal with its own proposed take-down of the info. The reason? It was an Easter bank holiday weekend.