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Semester 2, 2003

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28th November, 2003: Eta Carinae: The Next Big Thing

As Semester 2, 2003 comes to a close we bring to your attention a star that may make you stay awake at night (at least until Semester 1, 2004 starts)! As Eta Carinae comes to the end of its life, a group of astronomers have an excellent view of the rumblings of stellar old-age.

Eta Carinae is the most luminous star known in our Galaxy. It is 100 times more massive than our Sun and 5 million times as luminous. This star has now entered the final stage of its life and is highly unstable. It undergoes giant outbursts from time to time; one of the most recent happened in 1841 and created the beautiful bipolar nebula known as the Homunculus Nebula. Eta Carinae is so big that, if placed at the position of our Sun, it would extend beyond the orbit of Jupiter.

The image to the left shows the mushroom-shaped clouds, known as the Homunculus Nebula, that surround the massive star Eta Carinae (Credit: NASA/ESA HST). To the right is an image obtained with the VLT NACO adaptive-optics camera that reveals the structure of the star's immediate surroundings. The central region displays a complex morphology of luminous objects.

Its outer layers are continually being blown into space by radiation pressure - the impact of photons on atoms of gas. The resulting mass loss is enormous (about 500 Earth-masses a year) and it is therefore difficult to define the border between the outer layers of the star and the surrounding stellar wind region.

A team of astronomers have used two infrared-sensitive instuments on ESO's Very Large Telescope (VLT) at the Paranal Observatory (Chile), to study the shape of the stellar wind region. The image (above, right) shows the surroundings of Eta Carinae, with a spatial resolution comparable to the size of the solar system. The image shows that the central region of the Homunculus nebula is dominated by an object that is seen as a point-like light source with many luminous "blobs" in the immediate vicinity.

In order to obtain an even sharper view, the astronomers used interferometry techniques. The astronomers thus used VINCI, the VLT INterferometer Commissioning Instrument, together with two 35-cm siderostat (outrigger) telescopes. The siderostats were placed at selected positions on the VLT Observing Platform to provide different configurations and a maximum baseline of 62 meters. The astronomers succeeded in resolving the shape of the outer layer of Eta Carinae. They were able to provide spatial information on a scale of 0.005 arcsec, that is about 11 AU (1650 million km) at the distance of Eta Carinae, corresponding to the full size of the orbit of Jupiter.

The observations brought the astronomers a surprise. They indicate that the wind around Eta Carinae is amazingly elongated: one axis is one-and-a-half times longer than the other. Moreover, the longer axis is found to be aligned with the direction in which the much larger mushroom-shaped Homunculus clouds were ejected. Spanning a scale from 10 to 20-30,000 AU, the star itself and the Homunculus Nebula are thus closely aligned in space. Apparently, this stellar wind is much stronger in the direction of the long axis than of the short axis.

From Roy van Boekel, the team leader, "the current overall picture only makes sense if the stellar wind of Eta Carinae is elongated in the direction of its poles. This is a surprising reversal of the usual situation, where stars (and planets) are flattened at the poles due to the centrifugal force."

Eta Carinae has experienced large outbursts in the past and it is certain that this unstable giant star will not settle down. At present, it is losing so much mass so rapidly that nothing will be left of it after less than 100,000 years. More likely, though, Eta Carinae will destroy itself long before that in a supernova blast that could possibly become visible in the daytime sky with the naked eye. This may happen "soon" on the astronomical time-scale, perhaps already within the next 10-20,000 years.

One of the siderostats is placed at a station and prepared for observations. The tracking mirror can be seen and the (blue) vertical tube through which the light beam is sent down into the Interferometric Tunnel. Credit: ESO

The astromomy team is composed of Roy van Boekel (ESO and the University of Amsterdam, The Netherlands), Pierre Kervella, Francesco Paresce and Markus Scholler (ESO), Wolfgang Brandner, Tom Herbst and Rainer Lenzen (MPI for Astronomy, Heidelberg, Germany), Alex de Koter and Rens Waters (University of Amsterdam, The Netherlands), John Hillier (University of Pittsburgh, USA), and Anne-Marie Lagrange (Observatoire de Grenoble, France).

For more information and original source material see: ESO Press Release * SEDS * Chandra images * Astronomy Picture of the Day * HST Treasury Program

11th November, 2003: Still Hungry!

A survey in infrared light has discovered a small galaxy being devoured by our Galaxy, the Milky Way. As well, the small galaxy is now our closest neighbour, some 42,000 light years away from the centre of our Galaxy and about 25,000 light years from the Sun. The galaxy, a dwarf, is called Canis Major after the constellation it is in. The survey enabled astronomers to see through dust near the plane of our Galaxy, and detect M-giant stars - cool red stars that shine brightly in the infrared. The M-giants were used as beacons to trace the shape and location of Canis Major.

A simulation. The Milky way (blue) swallows up a nearby neighbour (pink) Credit: Nicolas Martin & Rodrigo Ibata, Observatoire de Strasbourg

The galaxy was identified by the detection of a strong elliptical-shaped stellar over-density, close to the Galactic plane at (l=240, b=-8). A number of globular clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), surround this structure, as do a number of open clusters. The population of M-giant stars in this over-density is similar in number to that in the core of the Sagittarius dwarf galaxy, another small galaxy being devoured by our Galaxy.

This discovery suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disk. (The "thick disk" is found in many S0 and spiral galaxies, and it is detected as an excess of light beyond that expected for disk galaxies. Arguably, disks can now be thought of as having "thin" and "thick" components.) Therefore the thick disk continues to grow via such merger events, even at the present time.

For more information and original source material see: ABC Space and Astronomy News * Astro-ph e-print 0311010 * www.astronomy.com

29th October, 2003: Flares are back in vogue

One of the largest solar flares has been detected. At about 1100 GMT 28th October, 2003, satellites watched as the event occurred. The flare was significantly larger than several unleashed since last week. The outburst was classified an X17.2 flare, the third largest on record. The Earth was immediately affected by intense X-ray radiation, which ionised the upper layers of the atmosphere, causing serious disruption to radio communications. Solar flares are associated with coronal mass ejections, or CMEs, eruptions from the sun that, if headed our way, can also disrupt power grids.

Image from the SOHO satellite showing the eruption. Light from the solar disk is blocked by a disk. It is the second largest flare observed by SOHO. Credit: NASA/ESA/SOHO

Our atmosphere protects people on the Earth, but passengers and crews on commercial jets at high latitudes could receive exposure equivalent to a normal medical chest X-ray. The CME associated with this flare is larger than the Sun itself and is one of the most dramatic halo CMEs ever recorded by SOHO (Solar & Heliospheric Observatory). The material ejected by this CME is travelling towards Earth at 2145 kilometres per second, as compared to a "normal" speed for these events of 400 kilometres per second.

The high level of energetic particles have caused two SOHO instruments to take precautionary measures: CDS has stopped taking science, and is continually reading out (or "draining") their detectors to prevent damage. Other instruments are less affected, although the onslaught of particles is quite noticeable for some. The "snow" in EIT and LASCO images is making it difficult to spot new CMEs.

LASCO image from the SOHO satellite showing the high level of extraneous "snow" particles. Credit: NASA/ESA/SOHO

For more information and original source material see: SOHO Hot Shot - movies * CNN Science and Space * Spaceflight Now * Todays Space Weather - Space Environment Center - NOAA * ESA Press Release

7th October, 2003: Isaac, Albert ... and Mordehai?

Two Swinburne astronomers have published a paper that challenges the standard laws of gravity. They have produced the first simulations that depict what the universe would look like using Modified Newtonian Dynamics (MOND). MOND, developed by Mordehai Milgrom in 1983 is an alternative theory of gravity.

The simulations developed by Dr Alexander Knebe (SAO Project Supervisor) and Professor Brad Gibson (SAO HET603 instructor) show a model universe that looks very similar to that observed.

Credit: A. Knebe and B. Gibson (Swinburne University of Technology)

The current paradigm requires that 90 percent of the universe be composed of presently unknown "dark matter". Instead of resorting to dark matter as an explanation for the motions of galaxies and the stars within them, MOND proponents suggest that the laws of gravity need revision. Under MOND, any object whose acceleration drops below some small threshold value receives an extra accelerative "kick" - this kick is tiny, a 1 part in 10 billion effect. An undetectable modification on Earth today, but its effect on the scale of galaxies is profound, potentially eliminating the need to invoke dark matter.

Dr Knebe explained that MOND did not make the traditional laws of gravity redundant. "Our traditional laws are very well-tested on planetary scales, but they have not been confirmed on the larger scales of galaxies or the universe as a whole. MOND can be thought of as a subtle extension to the traditional laws, becoming relevant only in cases where the traditional laws may not hold", Knebe said.

This research has been accepted for publication in the Monthly Notices of the Royal Astronomical Society

For more information see: New research challenges the laws of gravity - Web release * Astro-ph preprint (Knebe and Gibson) * MOND - A PEDAGOGICAL REVIEW by Mordehai Milgrom

3rd October, 2003: Deep, dudes!

HST's Advanced Camera for Surveys (ACS) has taken over from where the Hubble Deep Fields finished. Over 100 orbits of HST time has been used to observe a small region of space on the outskirts of the Andromeda galaxy, M31.

Credit: NASA, ESA and T.M. Brown (STScI)

The image provides a new, deep view into the stellar halo of M31. Using this data, astronomers determine that approximately one-third of the stars in M31's halo formed only 6 to 8 billion years ago which is much younger than the 11 to 13 billion year old stars in our Galaxy. They suggests three possibilities to explain this age discrepancy: (1) collisions destroyed the young disk of M31 and dispersed many of its stars into the halo; (2) a collision destroyed a relatively massive galaxy and dispersed its stars and some of Andromeda's disk stars into the halo; and/or (3) many stars formed during the collision itself.

Five background galaxies beyond M31. Most show distortions consistent with recent interactions. The stars in these panels are located in Andromeda's halo. The object [bottom row, right] is a a globular cluster, located in M31's halo. Credit: NASA, ESA and T.M. Brown (STScI)

Looking beyond the M31 halo stars, the ACS image reveals thousands of background galaxies (down to 31st magnitude) billions of light-years away. Plans for even deeper images are well underway.

For more information see: HubbleSite * Astro-ph preprint

18th September, 2003: Farewell Galileo

The Galileo spacecraft will end its mission on September 21, 2003. Launched in 1989 aboard Space Shuttle Atlantis, Galileo has been exploring Jupiter and its moons since December 1995. The spacecraft has been purposely put on a collision course with Jupiter to eliminate any chance of an unwanted impact between the spacecraft and Jupiter's moon Europa. The long-planned impact is necessary now that the onboard propellant is nearly depleted.

Galileo will transmit it's final data as it takes a final plunge into the planet's crushing atmosphere. The Galileo science team expects the spacecraft to transmit a few hours of science data in real time leading up to impact.

It found evidence of subsurface liquid layers of salt water on Europa, Ganymede and Callisto and it examined a diversity of volcanic activity on Io. Galileo is the first spacecraft to fly by an asteroid and the first to discover a moon of an asteroid.

This image of Europa and an enlargement of the Thrace region gives visual evidence of the dramatic advance in our knowledge of Jupiter's second Galilean satellite due to the Galileo mission.

For more information see: Galileo Countdown * Galileo Homepage

5th September, 2003: SIRTF ... finally

SIRTF (Space InfraRed Telescope Facility) was launched on Monday, 25 August 2003 at 1:35:39 a.m. EDT from Cape Canaveral Air Force Station in Florida. SIRTF is a cryogenically-cooled infrared observatory capable of studying objects ranging from our Solar System to the distant reaches of the Universe. SIRTF is the final element in NASA's Great Observatories Program.

Who says optical astronomers are the height of geeky-ness? The SIRTF team recorded the launch in the infrared! Go to:

IR Movie of SIRTF launch

Artists impression of SIRTF.

SIRTF underwent two significant "descoped" designs within five years, changing from a massive observatory with development costs in excess of 2.2 billion dollars to a modest-sized (but still powerful) observatory with comparable costs of less than 0.5 billion dollars. The various versions of SIRTF were named after the NASA rocket that would be used to launch the observatory (see below).

Various designs of SIRTF.

For more information see: SIRTF at CalTech * Latest on SIRTF

27th August, 2003: Mars Attacks

With a once-in-60,000 years occurence there was little doubt what this Astronomy News item was going to be about!

Today, 27 August, 2003 Mars will pass within just 55.76 million km of Earth. The last time Mars came nearer was on around 12 September 57617 BC, when Mars was about 55.72 million km from us. However if you miss it this time, you will only have to wait another 284 years for such a close encounter.

The red planet as seen by Mars Global Surveyor in June 2001. Courtesy NASA/JPL/Malin Space Science Systems.

"Mars will be the brightest object in the sky except for the moon and its reddish colour will make it easy to find," said Nick Lomb, curator of astronomy at Sydney Observatory. "Telescopes, even small ones, will allow you to see detail on Mars, including a polar cap and other surface features."

The brilliant planet, in Aquarius, glares at magnitude -2.9. Mars is now extraordinarily large, ~25 arcseconds in apparent diameter, and shows much detail in a good telescope, especially when viewed high in the sky during good atmospheric seeing.

For more information see: ABC News in Science * Sky and Telescope * Space Daily.com

8th August, 2003: What's for Dinner?

A small, satellite galaxy has been captured in its' death-throes. A team of astronomers (including SAO instructors Prof. Duncan Forbes and Dr. Mike Beasley; HET606) have identified a galaxy being eaten by a larger galaxy in one of the early release images from the HST ACS imager.

Whilst this is an important discovery in terms of galaxy evolution it also has implications for current mass (dark matter dominated) models of the universe. These models predict many more satellite galaxies around larger galaxies than seen. Some may be destroyed in a similar way to this galaxy, but it is still difficult to match theory to observations.

"This has been called the 'missing satellite' problem" , says Prof. Forbes. "An average galaxy might have two or three satellites when the model predicts there should be 10 or 100 times more."

A HST ACS image of the satellite galaxy, with tidal tails of stars being pulled out of it by the gravitational field of the larger galaxy.

Computer simulations (by team member Dr. Kenjii Bekki, U. N.S.W.) can reproduce the morphology of the smaller galaxy in good agreement with observations.

A model of the satellite galaxy (blue) being destroyed by the larger galaxy (pink) over several billion years.

"Until now, no one has seen the expected characteristic signature of these satellite galaxies in the process of being swallowed up," said Forbes. "As it orbits around the bigger galaxy, gravity causes the little galaxy to be disrupted and it pulls stars off in two directions, giving rise to plumes or tails."

The original HST ACS image can be viewed at http://hubblesite.org/newscenter/archive/2002/11/image/e

A paper on this research has been accepted in the magazine, Science

For more information see: ABC News in Science * D. Forbes web page (images, movies)