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Close encounter of the galactic kind

Galaxies NGC 3169 (left) and NGC 3166

Galaxies NGC 3169 (left) and NGC 3166 (right) are close enough together to feel each other's distorting gravitational influence. The tug-of-war has warped the spiral shape of NGC 3169, and fragmented the dust lanes in NGC 3166.

  • Galaxies NGC 3169 and 3166 are 70 million light-years from Earth
  • They’re close enough together to be warped by each other’s gravity

THE GALAXIES IN THIS COSMIC PAIRING, captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile, display some curious features, demonstrating that each member of the duo is close enough to feel the distorting gravitational influence of the other.

The gravitational tug-of-war has warped the spiral shape of one galaxy, NGC 3169 (on the left), and fragmented the dust lanes in its companion, NGC 3166.

Meanwhile, a third, smaller galaxy to the lower right, NGC 3165, has a front-row seat to the gravitational twisting and pulling of its bigger neighbours.

This galactic grouping—located about 70 million light-years away in the direction of the constellation Sextans (The Sextant)—was discovered by the English astronomer William Herschel in 1783.

Modern astronomers have gauged the distance between NGC 3169 (left) and NGC 3166 (right) as a mere 50,000 light-years. That’s only about half the width of our Milky Way galaxy.

In such tight quarters, gravity can start to play havoc with galactic structure.

Mostly ‘armless

Spiral galaxies like NGC 3169 and NGC 3166 tend to have orderly swirls of stars and dust pinwheeling about their glowing centres. Close encounters with other big galaxies can jumble this configuration, often serving as a prelude to the merging of the galaxies into one larger galaxy.

So far, the interactions of NGC 3169 and NGC 3166 have just lent a bit of character. NGC 3169’s arms, shining bright with big, young, blue stars, have been teased apart, and lots of luminous gas has been drawn out from the main body.

In NGC 3166’s case, the dust lanes that also usually outline spiral arms are in disarray. The lack of blue colour indicates that NGC 3166 is not forming many new stars.

Galaxy NGC 3169 with supernova

Galaxy NGC 3169 with supernova SN 2003cg marked.

Spotting a supernova

NGC 3169 has another distinction—the faint yellow dot beaming through a veil of dark dust just to the left of and close to the galaxy’s centre. This flash is the leftover of a supernova detected in 2003 and known accordingly as SN 2003cg. (Note that this supernova does not still shine today—the image was taken back in 2003.)

A supernova of this variety, classified as a Type Ia, is thought to occur when a dense, hot star called a white dwarf—a remnant of medium-sized stars like our Sun—gravitationally sucks gas away from a nearby companion star.

This added fuel will eventually cause the whole star to explode in a runaway nuclear fusion reaction.

The new image presented here of a remarkable galactic dynamic duo is based on data selected by Igor Chekalin for ESO’s Hidden Treasures 2010 astrophotography competition. Chekalin won the first overall prize and this image received the second highest ranking of the nearly 100 contest entries.

Adapted from information issued by ESO / Igor Chekalin.

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Going out with a bang

NGC 3582

Giant loops of gas ejected by dying stars in the star formation region NGC 3582, bear a striking resemblance to solar prominences.

GIANT LOOPS OF GAS bearing a striking resemblance to solar prominences are seen in this image of the nebula NGC 3582.

The loops are thought to have been ejected by dying stars, although new stars are also being born within this stellar nursery.

These energetic youngsters emit intense ultraviolet radiation that makes the gas in the nebula glow, producing the fiery display shown here.

NGC 3582 is part of a large star-forming region in the Milky Way, called RCW 57, close to the central plane of the Milky Way.

The famous astronomers John Herschel first spotted this complex region of glowing gas and dark dust clouds in 1834, during his stay in South Africa.

Some of the stars forming in regions like NGC 3582 are much more massive than the Sun. These monster stars emit energy at prodigious rates and have very short lives that end in the stellar explosions called supernovae.

The material ejected from these explosions creates bubbles in the surrounding gas and dust. This is the probable cause of the loops visible in this picture.

Here’s a short video that takes you on a sweeping journey into NGC 3582:

The image was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at European Southern Observatory’s (ESO) La Silla Observatory in Chile.

It is a false-colour image made up of separate exposures taken through multiple filters. From the Wide Field Imager, data taken through a red filter are coloured in green and red, and data taken through a filter that isolates the red glow characteristic of hydrogen are also shown in red. Additional infrared data from the Digitised Sky Survey are shown in blue.

The image was processed by ESO using the data identified by amateur astronomer Joe DePasquale, from the United States, who participated in ESO’s Hidden Treasures 2010 astrophotography competition. The competition was organised by ESO in October-November 2010, for everyone who enjoys making beautiful images of the night sky using astronomical data obtained using professional telescopes.

ESO’s Hidden Treasures 2010 competition gave amateur astronomers the opportunity to search through ESO’s vast archives of astronomical data, hoping to find a well-hidden gem that needed polishing by the entrants.

More information: Hidden Treasures

Adapted from information issued by ESO, Digitised Sky Survey 2 and Joe DePasquale.

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Hubble bursts dark energy bubble

Galaxy NGC 5584

Galaxy NGC 5584 was one of eight galaxies astronomers studied to measure the universe's expansion rate. Two special kinds of stars—Type Ia supernovae and Cepheid variable stars—were used as "cosmic yardsticks", due to their predictable brightnesses.

  • Our universe seems to be expanding faster and faster with time
  • ‘Dark energy’ proposed as an explanation, but its nature remains a mystery
  • Hubble observations have ruled out one dark energy hypothesis

ASTRONOMERS USING NASA’s Hubble Space Telescope have ruled out one explanation for the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy.

The universe appears to be expanding at an increasing rate. Some think this is because it is filled with a ‘dark energy’ that works in the opposite way to gravity.

An alternative to that hypothesis is that an enormous ‘bubble’ of relatively empty space eight billion light-years wide surrounds our galactic neighbourhood.

If we lived near the centre of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion.

This hypothesis has now been invalidated because astronomers have refined their understanding of the universe’s present expansion rate.

“We are using the new camera on Hubble like a policeman’s radar gun to catch the universe speeding,” said Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, and leader of the science team. “It looks more like it’s dark energy that’s pressing the [accelerator] pedal.”

Portion of NGC 5584 with Cepheid locations marked

A portion of galaxy NGC 5584 with the location of Cepheid variable stars marked.

The observations helped determine a figure for the universe’s current expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble’s previous best measurement in 2009.

Cosmic yardsticks

Riess’ team first had to determine accurate distances to galaxies near and far from Earth, and then compare those distances with the speed at which the galaxies are apparently receding because of the expansion of space.

They used those two values to calculate the Hubble constant, the number that relates the speed at which a galaxy appears to recede to its distance from the Milky Way.

Because we cannot physically measure the distances to galaxies, astronomers have to find stars or other objects that serve as reliable cosmic yardsticks. These are objects with known intrinsic brightness—brightness that hasn’t been dimmed by distance, an atmosphere or interstellar dust. Their distances, therefore, can be inferred by comparing their intrinsic brightness with their apparent brightness as seen from Earth.

To calculate long distances, Riess’ team chose a special class of exploding star called Type Ia supernovae. These stellar blasts all have similar luminosity and are brilliant enough to be seen far across the universe.

By cross-correlating the apparent brightness of Type Ia supernovae with pulsating Cepheid stars (another class of stars whose intrinsic brightness is known), the team could accurately gauge the distances to Type Ia supernovae in far-flung galaxies.

WFC3

Hubble's latest camera, the Wide Field Camera 3, was instrumental in the research.

Bubble is burst

By using the sharpness of Hubble’s new Wide Field Camera 3 (WFC3) to study more stars in visible and near-infrared light, the team eliminated systematic errors introduced by comparing measurements from different telescopes.

“WFC3 is the best camera ever flown on Hubble for making these measurements, improving the precision of prior measurements in a small fraction of the time it previously took,” said Lucas Macri, a collaborator on the Supernova Ho for the Equation of State (SHOES) Team from Texas A&M in College Station.

Knowing the precise value of the universe’s expansion rate further restricts the range of dark energy’s strength and helps astronomers tighten up their estimates of other cosmic properties, including the universe’s shape and its roster of neutrinos, or ghostly particles, that filled the early cosmos.

“Thomas Edison once said ‘every wrong attempt discarded is a step forward,’ and this principle still governs how scientists approach the mysteries of the cosmos,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

“By falsifying the bubble hypothesis of the accelerating expansion, NASA missions like Hubble bring us closer to the ultimate goal of understanding this remarkable property of our universe.”

Adapted from information issued by STScI. NGC 5584 image credit: NASA / ESA / A. Riess (STScI/JHU), L. Macri (Texas A&M University) / Hubble Heritage Team (STScI/AURA). NGC 5584 illustrations credit: NASA / ESA / L. Frattare (STScI) / Z. Levay (STScI).

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3D view of exploding star

Artist’s impression of the material surrounding SN1987A

This artist’s impression of the material around a recently exploded star, known as Supernova 1987A (or SN 1987A), is based on observations which have for the first time revealed a three dimensional view of the distribution of the expelled material. This image shows the different elements present in SN 1987A: two outer rings, one inner ring and the deformed, innermost expelled material.

Astronomers using European Southern Observatory’s (ESO) Very Large Telescope (VLT) have for the first time obtained a three-dimensional view of the distribution of the innermost material expelled by a recently exploded star.

The original blast was not only powerful, according to the new results. It was also more concentrated in one particular direction.

This is a strong indication that the supernova must have been very turbulent, supporting the most recent computer models.

Unlike the Sun, which will die rather quietly, massive stars arriving at the end of their brief life explode as supernovae, hurling out a vast quantity of material.

An animation showing a 3D view of the supernova remnant.

An animation showing a 3D view of the supernova remnant.

In this class, Supernova 1987A (SN 1987A) in the nearby Large Magellanic Cloud galaxy occupies a very special place. Seen in 1987, it was the first supernova for 383 years bright enough to be seen in the sky with just the naked eye.

Because of its relative closeness, it has been possible for astronomers to study the explosion of a massive star and its aftermath in more detail than ever before.

SN 1987A has been a bonanza for astrophysicists. It provided several notable observational ‘firsts’: the detection of neutrinos from the collapsing inner stellar core triggering the explosion; the identification on archival photographic plates of the star before it exploded; the signs of a lopsided explosion; the direct observation of the radioactive elements produced during the blast; observation of the formation of dust in the supernova, as well as the detection of the gas surrounding the star.

A lopsided blast

New observations making use of a unique instrument, SINFONI, on the VLT have provided even deeper knowledge of this amazing event, as astronomers have now been able to make the first-ever 3D reconstruction of the central parts of the exploding material.

This view shows that the explosion was stronger and faster in some directions than others, leading to an irregular shape with some parts stretching out further into space.

Time sequence of Hubble images of SN1987A

A time sequence of Hubble Space Telescope images, taken in the 9 years from 1994 to 2003, showing the collision of the expanding supernova blast with a ring of dense material flung off by the star 20,000 years before it exploded.

The first material to be ejected from the explosion travelled at an incredible 100 million km per hour, which is about a tenth of the speed of light or around 100,000 times faster than a passenger jet.

Even at this breakneck speed it has taken the blast 10 years to reach a previously existing ring of gas and dust puffed out much earlier from the dying star. The images also demonstrate that another blast wave is travelling ten times more slowly and is being heated by radioactive elements created in the explosion.

“We have established the velocity distribution of the inner ejecta of Supernova 1987A,” says lead author Karina Kjær. “Just how a supernova explodes is not very well understood, but the way the star exploded is imprinted on this inner material. We can see that this material was not ejected symmetrically in all directions, but rather seems to have had a preferred direction. Besides, this direction is different to what was expected from the position of the ring.”

Such asymmetric behaviour was predicted by some of the most recent computer models of supernovae, which found that large-scale instabilities occur during the explosion. The new observations are thus the first direct confirmation of such models.

Adapted from information issued by ESO / L. Calçada.

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Telescope sees shocking blue “bullet”

Supernova remnant cloud N49 in the Large Magellanic Cloud galaxy.

Supernova remnant cloud N49 in the Large Magellanic Cloud galaxy. The blue blob in the lower right corners is being ejected at a speed of 8 million km per hour.

  • Supernova remnant in a neighbouring galaxy
  • X-ray data sees a “bullet-shaped” object shooting out
  • “Bullet” is travelling at 8 million km per hour

This beautiful composite image shows N49, the aftermath of a supernova explosion in the Large Magellanic Cloud (one of the Milky Way’s neighbour galaxies).

A new, long observation from NASA’s Chandra X-ray Observatory, shown as blue colours, reveals evidence for a “bullet-shaped” object being blown out of a debris field left over from an exploded star.

In order to detect this bullet—which can be seen as the blue “blob” in the lower right hand corner of the image—a team of researchers led by Sangwook Park of Penn State University used Chandra to observe N49 for over 30 hours. The bullet is rich in silicon, sulphur and neon.

The detection of this bullet shows that the explosion that destroyed the star was highly asymmetric.

The bullet is travelling at a high speed of about 8 million kilometres per hour away from a bright point source in the upper left part of N49. This bright source may be a so-called soft gamma ray repeater (SGR), a source that emits bursts of gamma rays and X-rays.

Artist's impression of the Chandra X-ray Observatory

Artist's impression of the Chandra X-ray Observatory.

Supernova shockwave

A leading explanation for SGRs is that they are neutron stars with extremely powerful magnetic fields. Since neutron stars are often created in supernova explosions, an association between SGRs and supernova remnants is not unexpected. This case is strengthened by the apparent alignment between the bullet’s path and the bright X-ray source.

However, the new Chandra data also shows that the bright source is more obscured by gas than expected if it really lies inside the supernova remnant. In other words, it is possible that the bright X-ray source actually lies beyond the remnant and is projected along the line of sight.

Another possible bullet is located on the opposite side of the remnant, but it is harder to see in the image because it overlaps with the bright emission—described below—from the shock-cloud interaction.

Optical data from the Hubble Space Telescope (yellow and purple colouring) shows bright filaments where the shock wave generated by the supernova is crashing into the densest regions of nearby clouds of cool, molecular gas.

Using the new Chandra data, the age of N49—as it appears in the image—is thought to be about 5,000 years, and the energy of the explosion is estimated to have been about twice that of an average supernova. These preliminary results suggest that the original explosion was caused by the collapse of a massive star.

Adapted from information issued by Chandra X-ray Centre.

Exploding star could be new type

The image on the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right.

The image on the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right.

  • Possible new class of supernovae
  • Could explain abundance of calcium in our Galaxy

In the past decade, robotic telescopes have turned astronomers’ attention to scores of strange exploding stars, one-offs that may or may not point to new and unusual physics.

But supernova (SN) 2005E, discovered five years ago by the University of California, Berkeley’s Katzman Automatic Imaging Telescope (KAIT), is one of eight known “calcium-rich supernovae” that seem to stand out as horses of a different colour.

“With the sheer numbers of supernovae we’re detecting, we’re discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes,” said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy.

“But SN 2005E was a different kind of ‘bang.’ It and the other calcium-rich supernovae may be a true suborder, not just one of a kind.”

And then there were three

Filippenko is co-author of a paper in the journal Nature describing SN 2005E and arguing that it is distinct from the two main classes of supernovae.

The Type Ia supernovae are thought to be old, white dwarf stars that accumulate matter from a companion until they undergo a thermonuclear explosion that blows them apart entirely.

The Katzman Automatic Imaging Telescope

The supernova was spotted by the Katzman Automatic Imaging Telescope

Type Ib/c or Type II supernovae are thought to be hot, massive and short-lived stars that explode and leave behind black holes or neutron stars.

In the case of SN 2005E, the team of astronomers thinks the original star was a low-mass white dwarf stealing helium from a companion star until the temperature and pressure ignited a thermonuclear explosion – a massive fusion bomb – that blew off at least the outer layers of the star and perhaps blew the entire star to smithereens.

The researchers calculate that about half of the mass thrown out was calcium, which means that a couple of such supernova every 100 years would be enough to produce the high abundance of calcium detected in galaxies like our own Milky Way, and the calcium present in all life on Earth.

More supernova confusion

Interestingly, a team of researchers from Hiroshima University in Japan argue in the same issue of Nature that SN 2005E’s original, or progenitor, star was massive – between 8 and 12 solar masses – and that it underwent a core-collapse similar to a Type II supernova.

“It’s a confusing, muddy situation now,” said Filippenko. “But we hope that, by finding more examples of this subclass and of other unusual supernovae and observing them in greater detail, we will find new variations on the theme and get a better understanding of the physics that’s actually going on.”

To make things even muddier, Filippenko and former UC Berkeley post-doctoral fellow Dovi Poznanski, currently at Lawrence Berkeley National Laboratory and also co-author on the Nature paper, reported last November another supernova, SN 2002bj, that they believe explodes by a similar mechanism: ignition of a helium layer on a white dwarf.

“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf [stealing] helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.”

Adapted from information issued by University of California – Berkeley / Sloan Digital Sky Survey / Lick Observatory.

Dead stars get the chills

Image of Cassiopeia A and an artist's impression of the neutron star

Background: An image of the Cassiopeia A supernova explosion remnant taken by the Chandra X-ray Observatory. Inset: An artist's impression of the neutron star that lives at the heart of Cassiopeia A.

Observations of how the youngest-known neutron star has cooled over the past decade are giving astronomers new insights into the interior of these super-dense dead stars.

Dr Wynn Ho presented the findings at the Royal Astronomical Society (RAS) National Astronomy Meeting in Glasgow last week.

Neutron stars are composed mostly of neutrons crushed together by gravity, compressed to over a million million times the density of lead. They are the dense cores of massive stars that have run out of nuclear fuel and collapsed in supernova explosions.

The Cassiopeia A supernova explosion, likely to have taken place around the year 1680, would have heated the neutron star to temperatures of billions of degrees, from which it has cooled down to a temperature of about two million degrees Celsius.

Dr Ho, of the University of Southampton, and Dr Craig Heinke, of the University of Alberta in Canada, measured the temperature of the neutron star in the Cassiopeia A supernova remnant nebula using data obtained by NASA’s Chandra X-ray Observatory between 2000 and 2009.

An artist's impression of a neutron star

An artist's impression of a neutron star

“This is the first time that astronomers have been able to watch a young neutron star cool steadily over time. Chandra has given us a snapshot of the temperature roughly every two years for the past decade and we have seen the temperature drop during that time by about 3%,” said Dr Ho.

Neutron stars’ cooling cores

Young neutron stars cool through the emission of high-energy neutrinos—particles similar to photons but which do not interact much with normal matter and therefore are very difficult to detect.

Since most of the neutrinos are produced deep inside the star, scientists can use the observed temperature changes to probe what’s going on in the neutron star’s core.

Initially, the core of the neutron star cools much more rapidly than the outer layers. After a few hundred years, equilibrium is reached and the whole interior cools at a uniform rate.

At approximately 330 years old, the Cassiopeia A neutron star is near this cross-over age. If the cooling is only due to neutrino emission, there should be a steady decline in temperature.

However, although Dr Ho and Dr Heinke observed an overall steady trend over the 10-year period, there was a larger change around 2006 that suggests other processes may be active.

“The neutron star may not yet have relaxed into the steady cooling phase, or we could be seeing other processes going on,” said Dr Ho. “We don’t know whether the interior of a neutron star contains more exotic particles, such as quarks, or other states of matter, such as superfluids and superconductors.”

“We hope that with more observations, we will be able to explain what is happening in the interior in much more detail,” said Dr Ho.

Adapted from information issued by NASA / CXC / Southampton / W. Ho et al / NASA / CXC / M.Weiss / MIT / UMass Amherst / M.D. Stage et al.