• SAO Astronomy News

Semester 1, 2003

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2nd June, 2003: New Galaxies - Ultra Compact

Small, condensed galaxies have been discovered in the Fornax cluster of galaxies, some 60 million light-years away. The Ultra-Compact Dwarf (UCD) galaxies are probably dwarf galaxies that are being pulled apart by the gravitational field of the cluster.

"There has always been the concern that galaxy surveys are biased against finding very diffuse galaxies, because they are so faint, and very compact galaxies, because they look just like stars", says Dr Micheal Drinkwater, the lead scientist in the research programme.

Dr Micheal Drinkwater closely examining the barred spiral NGC 1365 in the Fornax cluster. Credits: UQ Communications/European Southern Observatory (background).

These UCDs are 1,000 times smaller than the average large galaxy, housing only about 10 million stars. The UCDs were first discovered with the 3.9m Anglo-Australian Telescope (AAT) at Siding Spring Observatory, Coonabarabran. Dr Drinkwater said this challenging task was made possible with the AAT's Two Degree Field Spectrograph (2dF), which can measure 400 objects simultaneously. Amongst the 2500 objects that had previously been identified as (Galactic) stars, 7 were actually found to be members of the Fornax cluster. At this distance they were too bright to be stars, but rather had to be a new class of extremely compact `dwarf' galaxy, which had never been seen before.

The researchers then won rare time on the Hubble Space Telescope to measure how big these dwarf galaxies were, as well as time on the European Southern Observatory's Very Large Telescope in Chile to measure how fast stars are moving around inside these galaxies. These two measurements are used in combination by astronomers to "weigh" galaxies and find out how massive they are. This confirmed the UCDs to be a new type of low-mass galaxy held together by gravity.

The Fornax Cluster of galaxies with insets, 60-times magnified, of a normal dwarf galaxy (upper) and a newly-discovered "ultra-compact dwarf" (UCD) galaxy (lower). A simulation of the remains of a a normal dwarf galaxy after disruption to form a UCD is shown in the centre. Credits: Insets: Hubble Space Telescope; background Michigan Curtis Schmidt Telescope and Arna Karick (University of Melbourne).

For more information see: Web site - Dr Drinkwater * UQ Media Release * ABC News in Science

3rd May, 2003: Mercury - The Winged Messenger transits the Sun

On Wednesday, 2003 May 07, Mercury will transit the Sun for the first time since 1999. The entire event will be widely visible from the Europe, Africa and Asia as shown in the map below. Japan, Australia, and New Zealand will witness the beginning of the transit but the Sun will set before the event ends. Similarly, observers in western Africa, eastern North America and eastern South America will see the end of the event since the transit will already be in progress at sunrise from those regions.

The 1999 transit of Mercury as viewed by the ESA/NASA SOHO spacecraft.

Warning. Do not look directly at the Sun, nor look at the Sun directly through binoculars or a telescope without correct solar filters in place. Since Mercury is only 1/158 of the Sun's apparent diameter, a telescope with a magnification of 50x to 100x is recommended to watch this event. Naturally, the telescope must be suitably equipped with adequate filtration to ensure safe solar viewing. The visual and photographic requirements for observing a transit are identical to those for solar viewing. The ESO Information Page has information on how to view the transit safely.

Regions of transit visibility. see "2003 Transit of Mercury" by Fred Espenak at http://sunearth.gsfc.nasa.gov/eclipse/OH/transit03.html below

Links to Live Web Cams: Transit from SOHO * ESO Transit WebCam * Institute of Theoretical Astrophysics- Norway

For more information see: Information for Australian Observers * 2003 Transit of Mercury by Fred Espenak * ESO Information Page

1st May, 2003: RAVE on!

RAVE (RAdial Velocity Experiment) is an all-sky survey (complete to V = 16) to measure the radial velocities, metallicities and abundance ratios of 50 million stars using the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO), together with a northern counterpart, over the period 2006 - 2010. The first part of RAVE, a pilot programme to detect 100,000 stars using the existing 6 degree field (6dF) fibre spectrograph in unscheduled bright time over the period 2003 - 2005 has just begun on 11 April, 2003.

RAVE will offer the first truly representative inventory of stellar radial velocities for all major components of the Galaxy. Its completeness and homogeneity will make it an invaluable stand-alone resource, but its full potential will be realised when the radial velocities are combined with proper motions and parallaxes from other sources (USNO, Tycho, DIVA).

Swinburne Astronomy Online instructor Prof. Brad Gibson is a member of the RAVE Science Working Group. He says "RAVE is an unprecedented attempt at Galactic Archaeology - over a 300 night observing run at Australia's UK Schmidt Telescope, a consortium of 20 astronomers from 7 countries will measure the three-dimensional space velocities of more than 100,000 Milky Way stars - an order of magnitude more than have been measured throughout the course of history. The motions and chemical abundances of these stars contain the clues to how our Galaxy formed. The unique facilities at the UK Schmidt Telescope make it the only telescope on Earth capable of taking on such an ambitious undertaking."

RAVE will help current studies into the possible modes of formation of our Galaxy, in particular if the halo has been built from numerous mergers of dwarf-sized galaxies. The figure below shows a simulation of the halo of our Galaxy as if it was formed by the accretion of small galaxies. The stars of the individual galaxies are colour-coded.

The image above, made from models by Paul Harding, shows what a halo might look like which was formed by accretions at various times during the last 10 Gyr of 50 dwarf galaxies. see http://smaug.astr.cwru.edu/heather/spag.html below

For more information see: RAVE * "Spaghetti Project" Morrison et al. * astro-ph paper on RAVE by Steinmetz

25th April, 2003: Third Transitting Extrasolar Planet Discovered - Shortest Orbital Period Yet

Extrasolar planet searches usually rely on detecting the change in velocity of a parent star. Over 100 extrasolar planets have been found. Sometimes the planet passes directly in front of the stars disk but this is a rather rare occurence. However a third planet has just been discovered in which this is happening. The transit of the planet can be detected as a very faint decrease in the light of the parent star as the planet blocks some of the starlight.

The Optical Gravitational Lensing Experiment (OGLE) detected a solar-like star in the direction of the Galactic Center, OGLE-TR-3, showing slight brightness variations, indicative of an orbiting planet. The 2 per cent dip in the brightness of OGLE-TR-3, (sse figure), occurs every 28 hours 33 minutes (1.1899 days). The orbital period is the shortest known for any exoplanet and the distance between the star and the planet is correspondingly small, only 3.5 million kilometres. The temperature of the side of the planet facing the star must therefore be very high, of the order of 2000 C. Clearly, the planet must be losing its atmosphere by evaporation.

ESO UVES velocity measurements fit this period well. The spectroscopic observations firmly exclude that the brightness variations could be due to a small stellar companion. A red dwarf star would have caused velocity variations of 15 km/s and a brown dwarf star 2.5 km/s; both would have been easy to observe with UVES.

Astronomers provisionally deduce a true mass of the planet of the order of one half of that of Jupiter. The density is found to be about 250 kg/m3, only one-fifth of that of Jupiter, so the planet is quite big for this mass - a bit "blown up". It is obviously a "gas giant" planet.

Folded photometry of the star, OGLE-TR-3 showing the small decrease due to the planet transit.

For more information see: ESO Press Release 09/03 * OGLE Web Site * Extrasolar Planets Encyclopedia - Unconfirmed Planets

9th April, 2003: The Age of Large Telescope Optical Interferometry

Direct measurements of stellar disks (apart from our own Sun) has proven difficult due to their extreme small sizes. One way around this for optical telescopes is to adopt radio interferometry techniques. Telescopes with large separations or baselines can act as if they are one telescope with the diameter of the baseline. The ESO VLT has now proven its ability to do such optical interferometry and has gathered important disk diameter measurements or angular sizes for stars in the Alpha Centauri system.

This has also allowed a unique and very detailed comparison between "real nature" and current stellar theory for solar-type stars. There is clearly very good agreement, indicating that the structure and evolution of stars like our Sun are well understood.

The Alpha Centauri triple stellar system is our closest neighbour in space. It is located at a distance of 4.36 light-years, or 41 million million km, in the direction of the southern constellation Centaurus. The two main stars in the system, Alpha Centauri A and Alpha Centauri B, are rather similar to the Sun; their stellar spectral types are "G2V" and "K1V", respectively. The third star is a "red dwarf" known as Proxima. It is much cooler and smaller than the other two. It was observed in 2002 with the VLT Interferometer (VLTI), see ESO Press Release 22/02.

The VLTI measurements provided high-quality angular diameter values for both stars, 8.512 +- 0.022 milliarcsec and 6.002 +- 0.048 milliarcsec for A and B, respectively. The true radii were then found to be 854,000 km and 602,000 km, or 1.227 +- 0.005 and 0.865 +- 0.007 times the radius of the Sun, respectively.

ESO astronomer Pierre Kervalla says: "The agreement between the VLTI measurements and the theory is very satisfactory for both stars. This confirms the validity of the multi-technique approach and with the interferometric measurement of Proxima obtained with the two 8.2-m VLT ANTU and MELIPAL telescopes, we now have a rather complete view of this famous triple system".

His colleague Frederic Thevenin at the Nice Observatory adds: "Alpha Centauri is not only the nearest stellar system - thanks to these studies, it is now also the best known one!".

The relative sizes of a number of objects, including the three (known) members of Alpha Centauri triple system and some other stars for which the angular sizes have also been measured with Very Large Telescope Interferometer (VLTI) at the ESO Paranal Observatory (Chile) during the past year, cf. ESO PR 22/02. The Sun and planet Jupiter are shown for comparison.

For more information see: ESO Press Release 05/03 * ESO Press Release 22/02

25th March, 2003: From the embers ...

Science still flows from the halls of Mt Stromlo despite the devastating bushfires of January, 2003.

ANU astronomers have played a crucial role in the first ever accurate measurement of the age of our universe. An international team of astronomers has announced that the universe started in a fiery explosion, thirteen thousand, seven hundred million years ago.

"The human race lives on a tiny, insignificant planet in a remote corner of the universe," said team leader Dr Matthew Colless. "It is amazing that we should even think of trying to measure the size, shape and age of the whole universe. But what really beggars belief is that Australian astronomers, working at a burnt out observatory, should have played a pivotal role in solving this age old question".

More details are now becoming apparent about the bushfire that struck the observatory.

"It was no ordinary bushfire - it was a superheated firestorm," said Vince Ford, a research officer at the observatory, Australia's oldest and one of the largest. "If it was made of aluminium, it burned and blew away. Made of brass, it melted. Made of steel, it softened and drooped. So we're talking temperatures around 1,000C to 1,200C."

Ford said it feels strange to be at an observatory with no telescopes, and even stranger to see what was spared by the fire. "The petrol bowser is untouched," he said, bemused. "And the BBQ? Not a mark on it."

Mt Stromlo Observatory, looking towards the remnants of the Administration building. Photograph by - Glen Mackie

For more information see: ANU Media Release * ABC Space News * Mt Stromlo Observatory, ANU * ABC Space News

7th March, 2003: What a gas!

Occasionally we like to blow our own trumpet! SAO instructors are currently in the news with a remarkable result. Professors Brad Gibson (Swinburne; this semesters HET611 instructor) and Chris Flynn (Tuorla Observatory in Finland, this semesters HET605 instructor) in collaboration with Finnish and American counterparts, have made the first accurate measurement of the rate of helium in the universe.

Their results were published in the 7 March issue of international journal Science.

The astronomers estimated the cosmic production of helium using K dwarf stars (in the Hipparcos catalogue) to yield a ratio of helium to heavy (excluding hydrogen and helium) elements. Their value agrees with previous theoretical predictions.

"There are many mysteries in astronomy, but it is safe to say that we can finally put a tick mark beside one of the longer, perplexing mysteries!" ... "Australian researchers have played a fundamental role in demonstrating that we are not only the stuff of stars, as proclaimed by the late Carl Sagan, but also the importance of stars as helium 'factories'" said Professor Gibson.

This plot shows how the luminosity (y-axis) of K dwarf stars increases as the amount of Helium (denoted by Y) and heaver elements (denoted by Z) increases. The data points represent real K dwarf stars observed with the Hipparcos satellite. Three computer calculations are shown by the dark blue, light blue and green lines: they represent different amounts of Helium produced in stars relative to the amount of heavier elements. The data indicate that the amount of Helium produced has been twice the amount of all the heavier than Helium elements combined (purple line). - from C. Flynn Press Release

For more information see: Swinburne Media Release * Press Release - C. Flynn

26th February, 2003: Farewell Long Distance Traveller

Pioneer 10 is silent. After travelling 12.2 billion kilometres from Earth and after nearly 31 years, it appears that we have lost contact with our furthest spacecraft. The last signal received by Jet Propulsion Laboratory's Deep Space Network was on 22 January. The spacecraft was so distant that the signal, travelling at the speed of light took 11 hours and 20 minutes to arrive. (For comparison the light travel time from Earth to Pluto is approx. 5 hours.)

"It was a workhorse that far exceeded its warranty, and I guess you could say we got our money's worth," said Larry Lasher, Pioneer 10 project manager at NASA's Ames Research Center.

impression of Pioneer 10s encounter with Jupiter

Pioneer 10 was launched on March 2, 1972, on a 21 month mission. It became the first spacecraft to pass through the asteroid belt and the first to obtain close-up images of Jupiter. In 1983, it became the first manmade object to leave the Solar System when it passed the orbit of Pluto.

Pioneer 10 Plaque

The Pioneer 10 spacecraft, destined to be the first man-made object to escape our solar system, carries this plaque. It is designed to show scientifically educated inhabitants of some other star system-who might intercept it millions of years from now-when Pioneer was launched, from where, and by what kind of beings. The design is engraved into a gold-anodized aluminum plate, 152 by 229 millimeters (6 by 9 inches), attached to the spacecraft's antenna support struts in a position to help shield it from erosion by interstellar dust.

Text from: Great Images in NASA

For more information see: NSSDC GSFC NASA site * NASA Spacelink