ESO Images Cosmic Collision

Image credit: ESO
Stars like our Sun are members of galaxies, and most galaxies are themselves members of clusters of galaxies. In these, they move around among each other in a mostly slow and graceful ballet. But every now and then, two or more of the members may get too close for comfort – the movements become hectic, sometimes indeed dramatic, as when galaxies end up colliding.

ESO shows an example of such a cosmic tango. This is the superb triple system NGC 6769-71, located in the southern Pavo constellation (the Peacock) at a distance of 190 million light-years.

This composite image was obtained on April 1, 2004, the day of the Fifth Anniversary of ESO’s Very Large Telescope (VLT). It was taken in the imaging mode of the VIsible Multi-Object Spectrograph (VIMOS) on Melipal, one of the four 8.2-m Unit Telescopes of the VLT at the Paranal Observatory (Chile). The two upper galaxies, NGC 6769 (upper right) and NGC 6770 (upper left), are of equal brightness and size, while NGC 6771 (below) is about half as bright and slightly smaller. All three galaxies possess a central bulge of similar brightness. They consist of elderly, reddish stars and that of NGC 6771 is remarkable for its “boxy” shape, a rare occurrence among galaxies.

Gravitational interaction in a small galaxy group
NGC 6769 is a spiral galaxy with very tightly wound spiral arms, while NGC 6770 has two major spiral arms, one of which is rather straight and points towards the outer disc of NGC 6769. NGC 6770 is also peculiar in that it presents two comparatively straight dark lanes and a fainter arc that curves towards the third galaxy, NGC 6771 (below). It is also obvious from this new VLT photo that stars and gas have been stripped off NGC 6769 and NGC 6770, starting to form a common envelope around them, in the shape of a Devil’s Mask. There is also a weak hint of a tenuous bridge between NGC 6769 and NGC 6771. All of these features testify to strong gravitational interaction between the three galaxies. The warped appearance of the dust lane in NGC 6771 might also be interpreted as more evidence of interactions.

Moreover, NGC 6769 and NGC 6770 are receding from us at a similar velocity of about 3800 km/s – a redshift just over 0.01 – while that of NGC 6771 is slightly larger, 4200 km/s.

A stellar baby-boom
As dramatic and destructive as this may seem, such an event is also an enrichment, a true baby-star boom. As the Phoenix reborn from its ashes, a cosmic catastrophe like this one normally results in the formation of many new stars. This is obvious from the blueish nature of the spiral arms in NGC 6769 and NGC 6770 and the presence of many sites of star forming regions.

Similarly, the spiral arms of the well-known Whirlpool galaxy (Messier 51) may have been produced by a close encounter with a second galaxy that is now located at the end of one of the spiral arms; the same may be true for the beautiful southern galaxy NGC 1232 depicted in another VLT photo (PR Photo 37d/98).

Nearer to us, a stream of hydrogen gas, similar to the one seen in ESO PR Photo 12/04, connects our Galaxy with the LMC, a relict of dramatic events in the history of our home Galaxy. And the stormy time is not yet over: now the Andromeda Galaxy, another of the Milky Way neighbours in the Local Group of Galaxies, is approaching us. Still at a distance of over 2 million light-years, calculations predict that it will collide with our galaxy in about 6,000 million years!

Original Source: ESO News Release

Another Gathering of Planets

Image credit: NASA
It’s happening again: the Moon and a bunch of planets are gathering in the evening sky.

Unlike last month, when five bright planets (including Mercury) were visible, this time there are only four: Venus, Mars, Saturn and Jupiter. Four is plenty, though. Using only your eyes and, if you have one, a small telescope, you’ll be able to see some wondrous things.

The show begins on Thursday, April 22nd. Step outside after nightfall and look west. The first thing you’ll notice is piercing-bright Venus and, not far below it, the delicate crescent Moon. These are the two brightest objects in the night sky, pleasingly close together. Mars is there, too, albeit not much brighter than an ordinary star. You can find it just above Venus, at one vertex of a Moon-Mars-Venus isosceles triangle.

Point a telescope at Venus and ? it looks just like the Moon! Well, almost. Because it lies between Earth and the Sun, Venus has phases just as our Moon does. At the moment Venus is a fat crescent. It’s colored gray-white, very Moon-like, but unlike the Moon, Venus is featureless. Thick uniform clouds hide the planet’s surface; the most powerful telescopes on Earth can’t penetrate them.

The crescent Moon is more fun to look at through a telescope. Low-slanting rays from the sun cast long shadows from lunar mountains. You can see impact craters, valleys and rilles ? all cast into sharp relief.

Can you also see a ghostly glow across the Moon’s dark terrain? For millennia the glow was a mystery, until Leonardo da Vinci figured it out in the 16th century. It is sunlight reflected from Earth onto the Moon. Modern astronomers call the glow Earthshine, and it’s one of the loveliest sights in the heavens–no telescope required.

The triangle shifts on Friday, April 23rd, as the Moon moves past Venus to a spot right beside Mars. This is the best night to find Mars, dim and red, using the Moon as a guidepost. Seen through a telescope Mars is not very impressive, not like it was in August 2003 when the planet made a historic close approach to Earth.

On Saturday, April 24th, the Moon glides away from Mars and toward Saturn, which looks like a bright yellow star. With the Moon beside Saturn to mark its location, you can’t miss it. Point your telescope at Saturn: Even a small ‘scope will show the planet’s lovely rings and it’s biggest moon Titan.

The NASA-ESA Cassini spacecraft is en route to Saturn now, due to arrive in July. Cassini will orbit for four years, studying Saturn’s rings, weather and magnetic field. Cassini will also drop a probe named Huygens through the thick orange clouds of Titan to discover what lies beneath.

Titan is one of the most mysterious worlds in the solar system. It has a nitrogen atmosphere denser than Earth’s and clouds laced with organic compounds. Some researchers believe there might be puddles, lakes or even oceans of liquid hydrocarbons sloshing around on the surface. These are places where organic molecules might get together for the first stirrings of simple life.

Through a backyard telescope Titan looks like an 8th magnitude star, an unremarkable pinprick. In fact, Titan is bigger than Mercury and Pluto. If it orbited the sun it would surely be considered a planet. What do the clouds of Titan hide? It’s something to think about while you’re peering through the eyepiece.

Finally on Thursday, April 29th, the Moon glides by Jupiter. You’ve probably noticed Jupiter before: it hangs almost directly overhead at sunset and outshines everything in the sky except Venus and the Moon. The Moon and Jupiter side by side are a pleasing sight.

Look at Jupiter through a telescope and you’ll see the planet’s rust-colored cloud belts and its four largest moons: Io, Europa, Callisto, and Ganymede. You might also see the Great Red Spot–a hurricane twice as wide as Earth and at least 100 years old. On April 29th it will be crossing Jupiter’s middle (as seen from Earth) at 09:12 p.m. PDT or 04:12 UT on April 30th.

Four planets, six moons, Earthshine, lunar mountains, the phases of Venus, a planet-sized hurricane and Saturn’s rings: Mark your calendar and see them all before April is done.

Original Source: NASA Science Story

Chandra Reveals a Supernova’s Power

Image credit: Chandra
The NASA Chandra X-ray Observatory image of SNR 0540-69.3 clearly shows two aspects of the enormous power released when a massive star explodes. An implosion crushed material into an extremely dense (10 miles in diameter) neutron star, triggering an explosion that sent a shock wave rumbling through space at speeds in excess of 5 million mph.

The image reveals a central intense white blaze of high-energy particles about 3 light years across created by the rapidly rotating neutron star, or pulsar. Surrounding the white blaze is a shell of hot gas 40 light years in diameter that marks the outward progress of the supernova shock wave.

Whirling around 20 times a second, the pulsar is generating power at a rate equivalent to 30,000 Suns. This pulsar is remarkably similar to the famous Crab Nebula pulsar, although they are seen at vastly different distances, 160,000 light years versus 6,000 light years. Both SNR 0540-69.3 and the Crab pulsar rotating rapidly, and are about a thousand years old. Both pulsars are pumping out enormous amounts of X-radiation and high-energy particles, and both are immersed in magnetized clouds of high-energy particles that are a few light years in diameter. Both clouds are luminous X-ray sources, and in both cases the high-energy clouds are surrounded by a filamentary web of cool gas that shows up at optical wavelengths.

However, the extensive outer shell of 50 million degree Celsius gas in SNR 0540-69.3 has no counterpart in the Crab Nebula. This difference is thought to be due to environmental factors. The massive star that exploded to create SNR 0540-69.3 was evidently in a region where there was an appreciable amount of gas. The supernova shock wave swept up and heated the surrounding gas and created the extensive hot X-ray shell. A similar shock wave presumably exists around the Crab Nebula, but the amount of available gas is apparently too small to produce a detectable amount of X-radiation.

Original Source: Chandra News Release

Christian Huygen’s 375th Birthday

On 14 April 1629, 375 years ago today, the Dutch scientist Christiaan Huygens was born. ESA?s probe on board the NASA/ESA Cassini-Huygens mission to the Saturnian system is named after him, the lens-maker who discovered Titan in 1655.

Christian Huygens came from a wealthy and well-connected Dutch family, who were traditionally in diplomatic service to the House of Orange. As a young boy he already showed promise in mathematics and drawing.

Descartes used to correspond with Huygens’s grandfather and, impressed with the boy’s early efforts at geometry, he was a great influence on Huygens. In 1645 he went to the University of Leiden to study mathematics and law and two years later he attended the College of Breda.

Shortly after Galileo first used a telescope for astronomical purposes, many other scientists decided to use this new instrument to perform their own studies. Many realised immediately that the improvement of the quality of the telescope could mean the chance to make history in astronomy.

Huygens applied himself to the manufacture of telescopes, together with his brother Constantijn, and soon after developed a theory of the telescope. Huygens discovered the law of refraction to derive the focal distances of lenses. He also realised how to optimise his telescopes by using a new way of grinding and polishing the lenses.

In 1655, he pointed one of his new telescopes, of far better quality than that used by Galileo, towards Saturn with the intention of studying its rings. But he was very surprised to see that, besides the rings, the planet had also a large moon. This is now known as Titan. In 1659 he discovered the true shape of the rings of Saturn.

Another Dutchman, Hans Lippershey, an eyeglass maker, had first offered the invention of the telescope to the Dutch government for military use. The government did not proceed with the idea. From Lippershey, Galileo picked up the idea of building a telescope for astronomical research. Huygens, by his own efforts and too late for Lippershey, demonstrated how important the telescope was.

With his interest in the measurement of time, he then discovered the pendulum could be a regulator of clocks. Huygens became one of the founding members of the French Academy of Sciences in 1666. He stayed in Paris from 1666-81 with only occasional visits to Holland and in 1673 he famously published his work Horologium Oscillatorium.

In 1689 Huygens went to London and met Sir Isaac Newton. He had always considered himself as an outstanding genius, so much so that he refused to collaborate with Newton in finding a better and more elegant mathematical solution for a pendulum clock.

The two great scientists also had other reasons for arguing. Newton was a firm upholder of the corpuscular theory of light. On the contrary Huygens formulated a wave theory of light. Newton?s reputation at the time caused scientists to favour the Englishman’s theory. It took more than a century to give the right emphasis to the theory of the Dutch scientist.

In the field of mathematics, Huygens could not challenge Newton, because he had not developed calculus. However, he encouraged the German mathematician Gottfried Leibnitz to publish on this subject. Newton had already developed calculus independently but not yet published. This led to a dispute between Newton and Leibnitz over this important mathematical discovery.

Technicians join Huygens to Cassini
He died in 1695. Although scientific results obtained by Huygens were second only to those obtained by Newton, the Dutch scientist was not really recognised in his time, nor had he influenced the development of science as he could have done, because he preferred solitary contemplation to team efforts.

The NASA/ESA Cassini-Huygens mission to Saturn and Titan is now giving back the honour to the Dutch scientist. Over 300 years after Huygens?s discovery of Titan, the largest moon is shortly to be visited by a probe from Earth. In a few years, we will know much more about Titan?s atmosphere, its surface and possibly mystery of the origin of life.

Original Source: ESA News Release

Spitzer Reveals Hidden Massive Stars

Image credit: NASA/JPL
Hidden behind a curtain of dusty darkness lurks one of the most violent pockets of star birth in our galaxy. Called DR21, this stellar nursery is so draped in cosmic dust that it appears invisible to the human eye.

By seeing in the infrared, NASA’s Spitzer Space Telescope has pulled this veil aside, revealing a fireworks-like display of massive stars. The biggest of these stars is estimated to be 100,000 times as bright as our own Sun.

The new image is available online at and

“We’ve never seen anything like this before,” said Dr. William Reach, an investigator for the latest observations and an astronomer at the Spitzer Science Center, located at the California Institute of Technology, Pasadena, Calif. “The massive stars are ripping the cloud of gas and dust around them to shreds.” The principal investigator is Dr. Anthony Marston, a former Spitzer astronomer now at the European Space Research and Technology Centre, the Netherlands.

Located about 10,000 light-years away in the Cygnus constellation of our Milky Way galaxy, DR21 is a turbulent nest of giant newborn stars. The region is buried in so much space dust that no visible light escapes it. Previous images taken with radio and near-infrared bands of light reveal a powerful jet emanating from a huge, nebulous cloud. But these views are just the tip of the iceberg.

Spitzer’s highly sensitive infrared detectors were able to see past the obscuring dust to the stars behind. The new false-color image spans a vast expanse of space, with DR21 at the top center. Within DR21, a dense knot of massive stars can be seen surrounded by a wispy cloud of gas and dust. Red filaments containing organic compounds called polycyclic aromatic hydrocarbons stretch horizontally and vertically across this cloud. A green jet of gas shoots downward past the bulge of stars and represents fast-moving, hot gas being ejected from the region’s biggest star.

Below DR21 are distinct pockets of star formation, never captured in full detail before. The large swirling cloud to the lower left is thought to be a stellar nursery like DR21’s, but with smaller stars. A bubble possibly formed by a past generation of stars is visible within the lower rim of this cloud.

The new view testifies to the ability of massive newborn stars to destroy the cloud that blankets them. Astronomers plan to use these observations to determine precisely how such an energetic event occurs.

Launched on August 25, 2003, from Cape Canaveral, Florida, the Spitzer Space Telescope is the fourth of NASA?s Great Observatories, a program that also includes the Hubble Space Telescope, Compton Gamma Ray Observatory and Chandra X-ray Observatory.

JPL manages the Spitzer Space Telescope mission for NASA’s Office of Space Science, Washington. Science operations are conducted at the Spitzer Science Center. JPL is a division of Caltech. Spitzer’s infrared array camera, used to capture the new image of DR21, was built by NASA Goddard Space Flight Center, Greenbelt, Md. The development of the camera was led by Dr. Giovanni Fazio of Smithsonian Astrophysical Observatory, Cambridge, Mass.

Additional information about the Spitzer Space Telescope is available at

Original Source: NASA/JPL News Release

Outer Planets Could Warm Up as Sun Dies

Image credit: NASA
We are doomed. One day the Earth will be a burnt cinder orbiting a swollen red star.

This is the ultimate fate of any planet living close to a main sequence star like our sun. Main sequence stars run on hydrogen, and when this fuel runs out, they switch over to helium and become a red giant. While the sun’s transition into a red giant is sad news for Earth, the icy planets in the most distant regions of our solar system will bask in the sun’s warmth for the first time.

The sun has been slowly but steadily growing brighter and hotter over the course of its lifetime. When the sun becomes a red giant in about 4 billion years, our familiar yellow sun will turn a vivid red, as it mainly emits the lower frequency energy of infrared and visible red light. It will grow thousands of times brighter and yet have a cooler surface temperature, and its atmosphere will expand, slowly engulfing Mercury, Venus and possibly even the Earth.

While the sun’s atmosphere is predicted to reach Earth’s orbit of 1 AU, red giants tend to lose a lot of mass, and this wave of expelled gases could push Earth just out of range. But whether the Earth is consumed or merely singed, all life on Earth will have passed into oblivion.

Yet the conditions that make life possible could appear elsewhere in the solar system, according to a paper published in the journal Astrobiology by S. Alan Stern, Director of the Southwest Research Institute’s Department of Space Studies in Boulder, Colorado. He says that planets located 10 to 50 AU will be in the red giant sun’s habitable zone. The habitable zone of a solar system is the region where water can remain in a liquid state.

The habitable zone will shift gradually through the 10 to 50 AU region as the sun grows brighter and brighter, evolving through its red giant phase. Saturn, Uranus, Neptune and Pluto all lie within 10 to 50 AU, as do their icy moons and the Kuiper Belt Objects. But not all these worlds will have an equal chance at life.

The prospects for habitability on the gaseous planets Saturn, Neptune and Uranus may not be affected all that much by the red giant transition. Astronomers have discovered gaseous planets orbiting very close to their parent star in other solar systems, and these “hot Jupiters” seem to hold onto their gaseous atmospheres despite their proximity to the intense radiation. Life as we know it is not likely to appear on gaseous planets.

Stern thinks Neptune’s moon Triton, Pluto and its moon Charon, and the Kuiper Belt Objects will have the best chances for life. These bodies are rich in organic chemicals, and the heat of the red giant sun will melt their icy surfaces into oceans.

“When the sun is a red giant, the ice worlds of our solar system will melt and become ocean oases for tens to several hundreds of millions of years,” says Stern. “Our solar system will then harbor not one world with surface oceans, as it does now, but hundreds, for all of the icy moons of the giant planets, and the icy dwarf planets of the Kuiper Belt will also bear oceans then. Because temperature on Pluto will not be very different then, than Miami Beach’s temperature now, I like to call these worlds ‘warm Plutos,’ in analogy to the plethora of hot Jupiters found orbiting sun-like stars in recent years.”

The influence of the sun is not the whole story, however – the characteristics of a planetary body go a long way toward determining habitability. Such characteristics include a planet’s internal activity, the reflectivity, or “albedo” of a planet, and the thickness and composition of the atmosphere. Even if a planet has all the elements that favor habitability, life will not necessarily appear.

“We don’t know what is needed to start life,” says Don Brownlee, an astronomer with the University of Washington in Seattle and co-author of the book, “The Life and Death of Planet Earth.” Brownlee says that if warm wet interiors and organic materials are all that’s needed, then Pluto, Triton, and the Kuiper Belt Objects could harbor life.

“As a word of caution, however, the interiors of asteroids that produced the carbonaceous chondrite meteorites were warm and wet for perhaps millions of years in the early history of the solar system,” says Brownlee. “These bodies are extremely rich in both water and organic materials, and yet there is no compelling evidence that any asteroidal meteorite ever had living things in it.”

A planetary body’s orbit also will affect its chances for life. Pluto, for instance, doesn’t have a nice, regular orbit like the Earth. The orbit of Pluto is comparatively eccentric, varying in distance from the sun. From January 1979 through February 1999, Pluto was closer to the sun than Neptune, and in a hundred years, it’ll be almost twice as far out as Neptune. This type of orbit will cause Pluto to undergo extreme heating alternating with extreme cooling.

Triton’s orbit, too, is peculiar. Triton is the only large moon to orbit backwards, or “retrograde.” Triton may have this unusual orbit because it formed as a Kuiper Belt Object and then was captured by Neptune’s gravity. It’s an unstable alliance, since the retrograde orbit creates tidal interactions with Neptune. Scientists predict that someday Triton will either crash into Neptune, or break up into tiny pieces and form a ring around the planet.

“The timescale for the tidal decay of Triton’s orbit is uncertain, so it could be around, or it might have already crashed by the time the sun goes red giant,” says Stern. “If Triton is around, it’ll probably end up looking like the same kind of organic-rich ocean world as Pluto.”

The sun will burn as a red giant for about 250 million years, but is that enough time for life to get a foothold? During most of the red giant lifetime, the sun will be only 30 times brighter than its current state. Toward the end of the red giant phase the sun will grow more than 1,000 times brighter, and occasionally release pulses of energy reaching 6,000 times current brightness. But this period of intense brightness will last for a few million years, or tens of millions of years at most.

The brevity of the red giant’s brightest phases suggests to Brownlee that Pluto doesn’t hold much promise for life. Because of Pluto’s average orbit of 40 AU, the sun would have to be 1,600 times brighter for Pluto to get the same solar radiation we currently get on Earth.

“The sun will reach this brightness, but only for a very brief period of time – only a million years or so,” says Brownlee. “The surface and atmosphere of Pluto will be ‘improved’ from our point of view, but it won’t be a nice place for any significant period of time”.

After the red giant phase, the sun will become fainter, and will shrink to the size of the Earth, becoming a white dwarf. The distant planets that basked in the light of the red giant will become frozen ice worlds once again.

So if life is to appear in a red giant system, it will need a quick start. Life on Earth is thought to have originated 3.8 billion years ago, some 800 million years after our planet was born. But that is probably because the planets in the inner solar system experienced 800 million years of heavy asteroid bombardment. Even if life had gotten started immediately, the early rain of asteroids would’ve wiped the Earth clean of that life.

Brownlee says a new era of bombardment could begin for the outer planets, because the red giant sun could disturb the vast number of comets in the Kuiper Belt.

“When the red giant sun is 1,000 times brighter, it loses almost half of its mass to space,” says Brownlee. “This causes orbiting bodies to move outward. Gas loss and other effects might destabilize the Kuiper Belt and create another period of interesting bombardment.”

But Stern says that planets made habitable by a red giant sun won’t be bombarded as often as the early Earth was, because the ancient asteroid belt had much more material than the Kuiper Belt has today.

In addition, the outer planets won’t experience the same ultraviolet (UV) levels that Earth has had to endure, since red giants have very low UV radiation. The higher intensity UV of a main sequence star can be damaging to the delicate proteins and RNA strands needed for life’s origin. Life on Earth could only originate underwater, in depths protected from this light intensity. Life on Earth is therefore inextricably linked to liquid water. But who knows what sort of life might originate on planets that have no need for UV shielding?

Stern thinks we should look for evidence of life on Pluto-like worlds orbiting around red giants today. We currently know of 100 million solar-type stars in the Milky Way galaxy that burn as red giants, and Stern says that all of these systems could have habitable planets within 10 to 50 AU. “It would be a good test of the time required to create life on warm, water-rich worlds,” he says.

“The idea of organic-rich distant bodies getting baked by a red giant star is an intriguing one, and could provide very interesting if short-lived habitats for life,” adds Brownlee. “But I am glad that our sun has a good margin of time left.”

What’s Next
While much of what we know about the outer solar system is based on distant measurements made from Earth-based telescopes, on January 2, 2004, scientists caught a close-up glimpse of a Kuiper Belt Object. The Stardust spacecraft passed within 136 kilometers of comet Wild2, an enormous snowball that spent most of its 4.6 billion-year lifetime orbiting in the Kuiper Belt. Wild2 now orbits mostly inside the orbit of Jupiter. Brownlee, who is the Principle Investigator for the Stardust mission, says that the Stardust images show fantastic surface details of a body shaped both by its ancient and recent history. Stardust images show gas and dust jets shooting off the comet, as Wild2 rapidly disintegrates in the strong solar heat of the inner solar system.

To learn more about the outer solar system, we’ll need to send a spacecraft out there to investigate. In 2001, NASA selected the New Horizons mission for just such a purpose.

Stern, who is the Principal Investigator for the New Horizons mission, reports that the spacecraft assembly is scheduled to begin this summer. The spacecraft is due to launch in January 2006, and arrive at Pluto the summer of 2015.

The New Horizons mission will allow scientists to study the geology of Pluto and Charon, map their surfaces, and take their temperatures. Pluto’s atmosphere also will be studied in detail. In addition, the spacecraft will visit the icy bodies in the Kuiper Belt in order to make similar measurements.

Original Source: Astrobiology Magazine

Milky Way is a Dangerous, Turbulent Place

Image credit: ESO
Home is the place we know best. But not so in the Milky Way – the galaxy in which we live. Our knowledge of our nearest stellar neighbours has long been seriously incomplete and – worse – skewed by prejudice concerning their behaviour. Stars were generally selected for observation because they were thought to be “interesting” in some sense, not because they were typical. This has resulted in a biased view of the evolution of our Galaxy.

The Milky Way started out just after the Big Bang as one or more diffuse blobs of gas of almost pure hydrogen and helium. With time, it assembled into the flattened spiral galaxy which we inhabit today. Meanwhile, generation after generation of stars were formed, including our Sun some 4,700 million years ago.

But how did all this really happen? Was it a rapid process? Was it violent or calm? When were all the heavier elements formed? How did the Milky Way change its composition and shape with time? Answers to these and many other questions are ‘hot’ topics for the astronomers who study the birth and evolution of the Milky Way and other galaxies.

Now the rich results of a 15 year-long marathon survey by a Danish-Swiss-Swedish research team [2] are providing some of the answers.

1,001 nights at the telescopes
The team spent more than 1,000 observing nights over 15 years at the Danish 1.5-m telescope of the European Southern Observatory at La Silla (Chile) and at the Swiss 1-m telescope of the Observatoire de Haute-Provence (France). Additional observations were made at the Harvard-Smithsonian Center for Astrophysics in the USA. A total of more than 14,000 solar-like stars (so-called F- and G-type stars) were observed at an average of four times each – a total of no less than 63,000 individual spectroscopic observations!

This now complete census of neighbourhood stars provides distances, ages, chemical analysis, space velocities and orbits in the general rotation of the Milky Way. It also identifies those stars (about 1/3 of them all) which the astronomers found to be double or multiple.

This very complete data set for the stars in the solar neighbourhood will provide food for thought by astronomers for years to come.

A dream come true
These observations provide the long-sought missing pieces of the puzzle to get a clear overview of the solar neighbourhood. They effectively mark the conclusion of a project started more than twenty years ago..

In fact, this work marks the fulfilment of an old dream by Danish astronomer Bengt Str?mgren (1908-1987), who pioneered the study of the history of the Milky Way through systematic studies of its stars. Already in the 1950’s he designed a special system of colour measurements to determine the chemical composition and ages of many stars very efficiently. And the Danish 50-cm and 1.5-m telescopes at the ESO La Silla Observatory (Chile) were constructed to make such projects possible.

Another Danish astronomer, Erik Heyn Olsen made the first step in the 1980’s by measuring the flux (light intensity) in several wavebands (in the “Str?mgren photometric system”) of 30,000 A, F and G stars over the whole sky to a fixed brightness limit. Next, ESA’s Hipparcos satellite determined precise distances and velocities in the plane of the sky for these and many other stars.

The missing link was the motions along the line of sight (the so-called radial velocities). They were then measured by the present team from the Doppler shift of spectral lines of the stars (the same technique that is used to detect planets around other stars), using the specialized CORAVEL instrument.

Stellar orbits in the Milky Way
With the velocity information completed, the astronomers can now compute how the stars have wandered around in the Galaxy in the past, and where they will go in the future, cf. PR Video Clip 04/04.

Birgitta Nordstr?m, leader of the team, explains: “For the first time we have a complete set of observed stars that is a fair representation of the stellar population in the Milky Way disc in general. It is large enough for a proper statistical analysis and also has complete velocity and binary star information. We have just started the analysis of this dataset ourselves, but we know that our colleagues worldwide will rush to join in the interpretation of this treasure trove of information.”

The team’s initial analysis indicates that objects like molecular clouds, spiral arms, black holes, or maybe a central bar in the Galaxy, have stirred up the motion of the stars throughout the entire history of the Milky Way disc.

This in turn reveals that the evolution of the Milky Way was far more complex and chaotic than traditional, simplified models have long so far assumed. Supernova explosions, galaxy collisions, and infall of huge gas clouds have made the Milky Way a very lively place indeed!

Original Source: ESO News Release

Andromeda’s Carnage

Image credit: RAS
An international team of astronomers has used the UK’s 2.5-m Isaac Newton Telescope on La Palma in the Canary Islands to map the Andromeda Galaxy (otherwise known as M31) and a large area of sky all around it. Their work over the last few years has created the most detailed image of a large spiral galaxy that currently exists. Dr Mike Irwin of the University of Cambridge, one of the team leaders, reports on some of the latest findings on Wednesday 31 March, when he will tell the RAS National Astronomy Meeting at the Open University about the first clear evidence that M31 is pulling one of its bright satellite galaxies apart, and the discovery of 14 previously unknown globular clusters orbiting far from the centre of M31 which could have been left behind when Andromeda devoured their parent galaxies.

Located around 2.5 million light years away, the Andromeda Galaxy is the most distant object visible to the naked eye, and is considered to be the sister galaxy of our own Milky Way. By studying this galactic neighbour, astronomers hope to understand more about the formation and evolution of many of the billions of spiral galaxies in the universe, including the Milky Way.

For their survey, the team have taken 150 individual images with a sensitive electronic CCD camera, which reveal millions of individual stars. It extends over an area 100 times greater than all earlier studies combined. The reason for scanning such a large area is that. around bright galaxies. there is a tenuous “halo” of stars which are leftovers from the formation of the galaxy billions of years ago. Studying this “fossil” information reveals evidence for how the halo, and hence the rest of the galaxy, has built up over cosmic history.

Traditionally, galaxy halos were thought to be relatively smooth and devoid of substructure. In fact the new survey shows that Andromeda’s halo is the exact opposite: it has a wealth of structure, indicating that it has ripped apart smaller galaxies that came too close and that the halo is built up from their remains. “Given that the disk of Andromeda appears so pristine, we were shocked to discover that its halo shows so much evidence for a history of interactions with other galaxies,” says Mike Irwin.

At this year’s National Astronomy Meeting, the Andromeda team report the discovery of a large stream of stars which appears to have been pulled out of one of Andromeda’s well-known satellite galaxies, NGC205. The visible part of the apparent stream extends nearly 50,000 light years from the main body of this small elliptical galaxy and was previously unknown despite the fact that NGC 205 has been well-studied.

“This is the first clear indication that one of Andromeda’s companion galaxies is being ripped apart as we watch,” commented team member Alan McConnachie, a doctoral student at Cambridge.

The 14 globular clusters the team has found orbiting far out from M31 may be evidence of Andromeda’s past cannibalism. Globular clusters are ancient systems of hundreds of thousands of stars, which are seen around many galaxies, and provide many clues to their evolutionary history. “Since the most distant of these globular clusters is some 250,000 light years from the centre of M31, our work shows that M31’s halo extends far beyond the edge of the bright part of the galaxy disk,” said Avon Huxor of the University of Hertfordshire.

“Both these discoveries will greatly aid in understanding the evolution of these nearby galaxies and should shed light on how our own Galaxy became what it is today,” commented Nial Tanvir, another team member from the University of Hertfordshire.

Original Source: RAS News Release

Milky Way’s Centre Measured

Image credit: NRAO
Thirty years after astronomers discovered the mysterious object at the exact center of our Milky Way Galaxy, an international team of scientists has finally succeeded in directly measuring the size of that object, which surrounds a black hole nearly four million times more massive than the Sun. This is the closest telescopic approach to a black hole so far and puts a major frontier of astrophysics within reach of future observations. The scientists used the National Science Foundation’s Very Long Baseline Array (VLBA) radio telescope to make the breakthrough.

“This is a big step forward,” said Geoffrey Bower, of the University of California-Berkeley. “This is something that people have wanted to do for 30 years,” since the Galactic center object, called Sagittarius A* (pronounced “A-star”), was discovered in 1974. The astronomers reported their research in the April 1 edition of Science Express.

“Now we have a size for the object, but the mystery about its exact nature still remains,” Bower added. The next step, he explained, is to learn its shape, “so we can tell if it is jets, a thin disk, or a spherical cloud.”

The Milky Way’s center, 26,000 light-years from Earth, is obscured by dust, so visible-light telescopes cannot study the object. While radio waves from the Galaxy’s central region can penetrate the dust, they are scattered by turbulent charged plasma in the space along the line of sight to Earth. This scattering had frustrated earlier attempts to measure the size of the central object, just as fog blurs the glare of distant lighthouses.

“After 30 years, radio telescopes finally have lifted the fog and we can see what is going on,” said Heino Falcke, of the Westerbork Radio Observatory in the Netherlands, another member of the research team.

The bright, radio-emitting object would fit neatly just inside the path of the Earth’s orbit around the Sun, the astronomers said. The black hole itself, they calculate, is about 14 million miles across, and would fit easily inside the orbit of Mercury. Black holes are concentrations of matter so dense that not even light can escape their powerful gravity.

The new VLBA observations provided astronomers their best look yet at a black hole system. “We are much closer to seeing the effects of a black hole on its environment here than anywhere else,” Bower said.

The Milky Way’s central black hole, like its more-massive cousins in more-active galactic nuclei, is believed to be drawing in material from its surroundings, and in the process powering the emission of the radio waves. While the new VLBA observations have not provided a final answer on the nature of this process, they have helped rule out some theories, Bower said. Based on the latest work, he explained, the top remaining theories for the nature of the radio- emitting object are jets of subatomic particles, similar to those seen in radio galaxies; and some theories involving matter being accelerated near the edge of the black hole.

As the astronomers studied Sagittarius A* at higher and higher radio frequencies, the apparent size of the object became smaller. This fact, too, Bower said, helped rule out some ideas of the object’s nature. The decrease in observed size with increasing frequency, or shorter wavelength, also gives the astronomers a tantalizing target.

“We think we can eventually observe at short enough wavelengths that we will see a cutoff when we reach the size of the black hole itself,” Bower said. In addition, he said, “in future observations, we hope to see a ‘shadow’ cast by a gravitational lensing effect of the very strong gravity of the black hole.”

In 2000, Falcke and his colleagues proposed such an observation on theoretical grounds, and it now seems feasible. “Imaging the shadow of the black hole’s event horizon is now within our reach, if we work hard enough in the coming years,” Falcke added.

Another conclusion the scientists reached is that “the total mass of the black hole is very concentrated,” according to Bower. The new VLBA observations provide, he said, the “most precise localization of the mass of a supermassive black hole ever.” The precision of these observations allows the scientists to say that a mass of at least 40,000 Suns has to reside in a space corresponding to the size of the Earth’s orbit. However, that figure represents only a lower limit on the mass. Most likely, the scientists believe, all the black hole’s mass — equal to four million Suns — is concentrated well inside the area engulfed by the radio-emitting object.

To make their measurement, the astronomers had to go to painstaking lengths to circumvent the scattering effect of the plasma “fog” between Sagittarius A* and Earth. “We had to push our technique really hard,” Bower said.

Bower likened the task to “trying to see your yellow rubber duckie through the frosted glass of the shower stall.” By making many observations, only keeping the highest-quality data, and mathematically removing the scattering effect of the plasma, the scientists succeeded in making the first-ever measurement of Sagittarius A*’s size.

In addition to Bower and Falcke, the research team includes Robin Herrnstein of Columbia University, Jun-Hui Zhao of the Harvard-Smithsonian Center for Astrophysics, Miller Goss of the National Radio Astronomy Observatory, and Donald Backer of the University of California-Berkeley. Falcke also is an adjunct professor at the University of Nijmegen and a visiting scientist at the Max-Planck Institute for Radioastronomy in Bonn, Germany.

Sagittarius A* was discovered in February of 1974 by Bruce Balick, now at the University of Washington, and Robert Brown, now director of the National Astronomy and Ionospheric Center at Cornell University. It has been shown conclusively to be the center of the Milky Way, around which the rest of the Galaxy rotates. In 1999, Mark Reid of the Harvard-Smithsonian Center for Astrophysics and his colleagues used VLBA observations of Sagittarius A* to detect the Earth’s motion in orbit around the Galaxy’s center and determined that our Solar System takes 226 million years to make one circuit around the Galaxy.

In March 2004, 55 astronomers gathered at the National Radio Astronomy Observatory facility in Green Bank, West Virginia, for a scientific conference celebrating the discovery of Sagittarius A* at Green Bank 30 years ago. At this conference, the scientists unveiled a commemorative plaque on one of the discovery telescopes.

The Very Long Baseline Array, part of the National Radio Astronomy Observatory, is a continent-wide radio-telescope system, with 10, 240-ton dish antennas ranging from Hawaii to the Caribbean. It provides the greatest resolving power, or ability to see fine detail, of any telescope in astronomy, on Earth or in space.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Astronomers Discover Mini-Galaxies

Image credit: Steve Phillipps
A new survey made with the Anglo-Australian Telescope (AAT) has revealed dozens of previously unsuspected miniature galaxies in the nearby Fornax galaxy cluster. They belong to a class of galaxies dubbed “ultra-compact dwarfs” (UCDs), which was unknown before the same team of astronomers discovered 6 of them in the Fornax cluster in 2000. Now they say the UCDs outnumber the “conventional” elliptical and spiral galaxies in the central region of the Fornax cluster and they have found some in the Virgo galaxy cluster too. It is possible that at least some are left-over examples of the primordial “building blocks” that formed large galaxies by merging together. It is likely that they are very common but have been overlooked because they resemble nearby stars at first sight. These results will be presented to the RAS National Astronomy Meeting at the Open University on Thursday 1 April by Dr Steven Phillips of Bristol University.

UCDs were discovered by chance when Dr Phillipps and his colleagues undertook a large survey of all the moderately bright objects they could see in the direction of the Fornax cluster. Only because they used a spectrograph (the Two Degree Field, or 2dF, system on the AAT) were they able to measure redshifts, which told them that 6 objects looking like local stars in our Galaxy were in fact in the Fornax cluster about 60 million light years away. Follow-up observations with the Hubble space Telescope and the European Southern Obervatory’s Very Large Telescope (VLT) revealed just how strange they are. Although their masses are similar to those of previously known dwarf galaxies, they are amazingly small – only about 120 light years across. Tens of millions of stars are squashed into what is a tiny volume by galaxy standards.

Favouring the idea that UCDs are the nuclei of galaxies that were originally larger and have been stripped of their outer stars, the team predicted that they would find them in other dense clusters where the stripping or ‘threshing’ process could go on. They also calculated how many more they would expect to find if they searched for fainter ones.

When they put their predictions to the test, 3 nights of observations uncovered a further 46 UCDs in Fornax – even more than the team had expected – and in just 4 hours they found 8 in the Virgo cluster, also around 60 million light years away. “These results indicate that UCDs are indeed common,” says Steve Phillipps, “and part of the standard population of galaxies we can expect in rich galaxy clusters. Given that we found so many, it is even possible that a proportion of them are the remnants of a population of primordial galaxies, remnants of the original building blocks of the large galaxies we find at the centres of clusters.”

Original Source: RAS News Release