Posted on by Fraser Cain

More Evidence that Meteors Caused Mass Extinctions

Image credit: NASA

A team of researchers from Louisiana State University have uncovered a connection between a meteor strike and a mass extinction that happened 380 million years ago called the middle Devonian event. It happened at a time when small plants, wingless insects and spiders inhabited the land, and everything else lived in the sea – 40% of all life disappeared from the fossil records. They found evidence of the strike by measuring the magnetic signature of layers of rock. When a large asteroid hits the Earth, it distributes a layer of dust around the entire planet – if a strata of rock has the same magnetic signature in different parts of planet, it’s evidence of a strike.

It’s the stuff of science fiction movies. Bruce Willis, by a mighty effort, saving the world from extinction by a huge meteor.

But Bruce Willis won’t do it, and in our current state of readiness, neither will anyone else. That is why LSU geophysicist Brooks Ellwood is plumbing the geologic record, trying to correlate known mass extinctions to meteor strikes.

“When we think about the human race and life in general, what do we worry about? We worry about nuclear holocaust and major glaciation. Then we worry about the giant chunks of rock that fly past Earth all the time,” Ellwood said.

“We can’t see them till they’re here, we can’t stop one, so the question is, how often do they hit the Earth and cause major mass extinctions? Are extinctions often caused by impacts? If so, we want to be sure we are prepared.”

Ellwood and four other researchers have just published an article in the journal Science in which they tie an early mass extinction to a meteor strike. This extinction happened 380 million years ago in what is called the middle Devonian. It was a time when only small plants, wingless insects and spiders inhabited the land and everything else lived in the sea. About 40 percent of all species disappeared from the fossil record at this time.

The extinction has been known to geologists for a long time but this is the first time it has been tied to a meteor strike. This is also the oldest known impact that has been tied to a mass extinction.

Ellwood is quick to point out that because the extinction and the meteor strike happened at the same time does not prove the impact caused the extinction — but it certainly suggests it.

One of the great difficulties in determining whether an extinction happened on a global scale, or was a local event caused by a volcano or some other terrestrial force, is identifying the same strata of rock at different locations on the globe. Finding a layer of earth in Colorado, for example, and finding that same layer in Australia is no simple task.

“The same layer of earth is exposed to different conditions in different parts of the world,” Ellwood said. “Weathering, upheavals, volcanos, earthquakes and flooding all confuse the geologic record, making it incomplete and open to interpretation.”

The layers can also be extremely thin, he said, showing a picture of the location of his latest research. The layer he was looking at — near the top of a barren plateau in the Anti Atlas desert near Rissani in Morocco — was about the thickness of a felt-tipped marker and only distinguishable from the soil around it by its reddish color.

What is unique about Ellwood’s work, however, is the means he uses to identify the different layers in the geologic record: induced magnetism.

“Everything is magnetic,” he said. “If I put your finger in a magnetic coil and turn it on, your finger will be magnetized.” Ellwood uses this phenomenon to take “magnetic signatures” of geologic samples. The magnetic signature of a layer of earth will be the same anywhere in the world, making it relatively easy to identify strata, if they can be found. These signatures also make it easy to identify meteor strikes. “The magnetic pattern associated with an impact layer is often distinctive, making it easier to find in a thick sequence of strata,” he said.

Working with LSU graduate students Steve Benoist and Chris Wheeler; structural geologist Ahmed El Hassani of the University of Rabat, Morocco; and Devonian biostratigrapher Rex Crick of the University of Texas at Arlington, Ellwood was able to find high concentrations of shocked quartz, microscopic spherules and microcrysts in this layer, sure signs of a meteor impact. Benoist is a paleontologist and Wheeler is an isotope geochemist; both have since moved on.

The past 550 million years are divided up by geologists into about 90 “stages.” Each stage is distinguished from another by a change in the fossil record. To date, only four of these stages show strong evidence of a meteor strike, Ellwood’s discovery being the latest, as well as the oldest. The most recent, best known extinction is the K-T boundary at which the dinosaurs died out, about 65 million years ago. There have been five major mass extinctions and many smaller ones since then.

“We know that meteors have struck the Earth hundreds of times,” Ellwood said. “If I had to guess, I would say that once every 5 million years a meteor big enough to cause a mass extinction hits the Earth.

“We could protect ourselves if we wanted. We went to the moon, we can figure out how to destroy or deflect a meteor. All it takes is the political will — and an awareness of the threat.”

The work of Ellwood and his team, published in the prestigious journal Science, is a step in that direction.

Original Source: LSU News Release

Posted on by Fraser Cain

Where’s Spirit Now?

Just a few extra notes for today:

NASA has made a Mars Explorer tracking page available on its website. Every few minutes or so, a series of images are updated that show how far along the spacecraft has gotten on its journey; the view of Mars; view of Earth, etc. Pretty cool. Check it out here.. I guess they’ll do another for “Opportunity” when it launches.

A big congratulations to Abbotsford and Calgary for winning Canada’s light pollution abatement awards. Both cities have made huge advances towards reducing obnoxious glare in the night sky (and reduced their pollution and energy use). Nice going.

Finally, there’s a free lecture at 7:00 pm PDT tonight at NASA/JPL about the Space Infrared Telescope Facility. If you’re interested in the “last of the great observatories”, you can stop by and listen in. Here’s more information. They’ll also be webcasting the presentation live on the Internet.

Posted on by Fraser Cain

Investigators Find Another Potential Shuttle Problem

Shuttle investigators have found another potential problem that could damage future space shuttles during launch. When searching through radar data of the Columbia, the investigation board discovered that an 18 kilogram bolt connecting the shuttles boosters to its fuel tank flew off . There’s no evidence that it actually hit the shuttle, but its potential damage to future missions could be catastrophic. Fragments of the bolt are supposed to be caught in a special cylinder to prevent exactly this kind of problem, but it appears that the capture device needs improvements.

Posted on by Fraser Cain

Ariane 5 Launches Two Satellites

Image credit: Arianespace

An Ariane 5 rocket successfully launched two geostationary communications satellites Wednesday evening after an hour’s delays because of poor weather. The Ariane 5G lifted off at 2238 GMT (6:38 pm EDT) and the two satellites separated 35 minutes later. The first satellite is the Australian commercial/military Optus and Defence C1 communications satellite which will provide coverage for the Asia Pacific region. The other is the Japanese BSAT-2c which will provide satellite television services throughout Japan.

Kourou, French Guiana, June 11, 2003 – Arianespace today orbited two geostationary communications satellites: Optus and Defence C1 for the Australian operator Optus and the Australian Department of Defence, and BSAT-2c for the Broadcasting Satellite System Corporation (B-SAT) of Japan under terms of a turnkey contract with Orbital Sciences Corporation of the United States.

Twelfth successful launch
With its 12th successful mission, the Ariane 5 Generic launcher confirmed its technical and operational maturity.

This latest success comes two months after the previous Ariane 5 flight — which also orbited a dual-satellite payload, and less than 10 days after Starsem’s successful Soyuz commercial mission with the European Space Agency’s Mars Express spacecraft.

Several days prior to launch, a ministerial-level ESA Council meeting authorized the Ariane 5 support plan and approved construction of a Soyuz launch pad at the Guiana Space Center, Europe’s Spaceport. These decisions give Arianespace the means to operate a full range of launch vehicles that respond to all client requirements.

Prestigious customers: Australia, Japan and the United States
The choice of Ariane by major space telecom manufacturers and operators in the United States, Japan and Australia clearly reflects international recognition of Arianespace’s top-flight launch service.

Optus and Defence C1 is the second Australian satellite to be launched by Ariane. In September 1987, Ariane orbited the Aussat K3 satellite, while Singtel — the parent company of operator Optus — had its ST-1 spacecraft launch by Ariane in 1998.

BSAT-2c is the 19 satellite launched by Ariane for Japan, and the fifth for telecom operator B-SAT — following BSAT-1a on Flight 95, BSAT-1b on Flight 108, and BSAT-2a and BSAT-2b on Flights 140 and 142. BSAT-2C is the fifth satellite built by Orbital Sciences Corporation to be launched by Arianespace using an Ariane 5 since March 2001.

Flight 161 at a glance
Flight 161 was carried out by an Ariane 5 Generic launcher from Europe’s Spaceport in Kourou, French Guiana. Liftoff was on Wednesday, June 11, 2003 at 7:38 p.m. local time in Kourou (22H38 GMT, 6:38 p.m. in Washington, D.C., 12:38 am in Paris on June 12, and at 7:38 am in Tokyo and 8:38 am in Sydney on June 12).

Provisional parameters at injection of the storable propellant upper stage were:
Perigee: 590 km for a target of 590 km (?3 km)
Apogee: 35,798 km for a target of 35,826 km (?160 km)
Inclination: 7.00 degrees for a target of 6.99 degrees (?0.06?)

Optus and Defence C1: Mitsubishi Electric Corporation of Japan is the prime contractor, and is responsible for all communications systems. Space Systems Loral of the United States designed, assembled and integrated the bus and satellite system.

Weighing about 4,725 kg at liftoff, it will be positioned at 156 degrees East. Equipped with 24 Ku-band transponders, it will provide commercial communications services for Australia, New Zealand, Southeast Asia and Hawaii. It also carries 4 X-band transponders, 4 Ku-band transponders and 6 UHF channels to provide dedicated links for the Australian Department of Defence.

Built by Orbital Sciences Corporation in Dulles, Virginia using the Star-1 platform, BSAT-2c weighed 1,275 kg at liftoff. It will be positioned at 110 degrees East. Equipped with 4 Ku-band transponders, it will provide direct TV broadcast services throughout Japan over its design life of 10 years. Over 16 million households receive programs broadcast by the B-SAT satellite.

Original Source: Arianespace News Release

Posted on by Fraser Cain

New Evidence for Cold Dark Matter

Image credit: Chandra

A new image taken by the Chandra X-ray observatory is helping astronomers to understand the composition of dark matter in the Universe. Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas – and a mass of dark matter equal to a hundred trillion Suns. The X-ray data shows that the density of the dark matter increases smoothly all the way to the centre of the galaxy, which matches predictions of the “cold dark matter” model. This model gets its name from the assumption that the dark matter particles were moving slowly when galaxies first formed, and interact with normal matter only through gravity.

Astronomers have used NASA’s Chandra X-ray Observatory to make the most detailed probe yet of the distribution of dark matter in a massive cluster of galaxies. Their results indicate that about 80 percent of the matter in the universe consists of cold dark matter – mysterious subatomic particles left over from the dense early universe.

Chandra observed a cluster of galaxies called Abell 2029 located about a billion light years from Earth. The cluster is composed of thousands of galaxies enveloped in a gigantic cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. The X-ray data show that the density of dark matter increases smoothly all the way into the central galaxy of the cluster. This discovery agrees with the predictions of cold dark matter models, and is contrary to other dark matter models that predict a leveling off of the amount of dark matter in the center of the cluster.

“I was really surprised at how well we could measure the dark matter so deep into the core of a rich cluster,” said Aaron Lewis of the University of California, Irvine, lead author of a paper describing the results in a recent issue of The Astrophysical Journal. “We still have very little idea as to the exact nature of these particles, but our results show that they must behave like cold dark matter.”

Cold dark matter gets its name from the assumption that the dark matter particles were moving slowly when galaxies and galaxy clusters began to form. Dark matter particles interact with each other and “normal” matter only through gravity.

The astronomers’ success in placing such tight constraints on the dark matter distribution was partly due to Chandra’s ability to make a high resolution intensity and temperature map, and partly due to their choice of a target. The cluster and central galaxy are unusually regular, with little or no sign of disturbance.

The hot gas in a cluster is held in the cluster primarily by the gravity of the dark matter, so the distribution of the hot gas is determined by that of the dark matter. By precisely measuring the distribution of X-rays from the hot gas, the astronomers were able to make the best measurement yet of the distribution of dark matter in the inner region of a galaxy cluster.

“While Abell 2029 might be boring for the average person to look at,” said David Buote, a coauthor of the paper, “it is a pure delight for astrophysicists to study, because it allows for a very straightforward and accurate comparison of theory and observation.”

As a case in point, earlier observations of the Hydra A galaxy cluster by Larry David of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. and colleagues found a similar result but the evidence of explosive activity in the central galaxy made it difficult to draw definite conclusions about the nature of the dark matter. The dark matter profile deduced for Abell 2029 provides evidence that the Hydra results are reliable and is an important independent confirmation of cold dark matter predictions.

John Stocke of the University of Colorado, Boulder was also involved in this research. Chandra observed Abell 2029 with the ACIS detector for 5.6 hours on April 12, 2000. NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science, NASA Headquarters, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Original Source: Chandra News Release

Posted on by Fraser Cain

Shuttle May Fly After Extreme Inspections

NASA is indicating that a new regime of inspections will be required for its fleet of space shuttle to start flying again. In the case of Columbia, it’s believed that a hairline crack only a few centimeters long was created when a chunk of foam from its fuel tank smashed into its wing. NASA administrator Sean O’Keefe said that it’s going to take an “extremely meticulous” inspection system to catch this kind of problem for future flights. When launches do resume, they will be daytime flights only, so cameras can monitor the shuttle’s flight to orbit. The shuttle may return to flight in December or some time in 2004.

Posted on by Fraser Cain

Russian Cargo Vessel Arrives at Station

Image credit: NASA

A Russian Progress 11 cargo ship arrived at the International Space Station today, after spending three days catching up to it in orbit. The Progress reached the station at 1115 GMT (7:15am EDT) and docked automatically to the Pirs Docking Compartment on the Zvezda Service Module. On board the ship is more than two tonnes of food, fuel, water, supplies and scientific gear. This makes a total of three ships docked to the space station, including another Progress ship and the Soyuz that delivered the current crew.

An unmanned Russian resupply craft successfully docked to the International Space Station this morning, delivering more than two tons of food, fuel, water, supplies and scientific gear to the Expedition 7 crew aboard the complex.

The Progress 11 vehicle automatically linked up to the Pirs Docking Compartment attached to the Zvezda Service Module over Central Asia at 6:15 a.m. Central time (1115 GMT) three days after its launch from the Baikonur Cosmodrome in Kazakhstan. As the Progress neared Pirs for docking, Expedition 7 Commander Yuri Malenchenko and NASA ISS Science Officer Ed Lu were in Zvezda, monitoring its approach. At the time of contact and capture, the ISS was flying over Central Asia at an altitude of 240 statute miles. After conducting leak checks to insure a tight seal between the Progress and the ISS, Malenchenko and Lu will open the hatch to the ship and begin to unload its cargo.

Stowed in the Progress are replacement parts for environmental systems in both the U.S. and Russian segments of the Station, office supplies, two tanks of potable water, and some clothing items for the two crewmembers. Also aboard the Progress are two experiment kits for European Space Agency cosmonaut Pedro Duque, who will launch in October on the Soyuz TMA-3 vehicle with the Expedition 8 crew for about a week?s worth of scientific research on the ISS under a contract between ESA and the Russian Aviation and Space Agency. Duque will return to Earth with Malenchenko and Lu in the Soyuz TMA-2 vehicle currently docked to the Station.

The arrival of the new Progress puts three Russian vehicles at the ISS. Docked to the aft port of Zvezda is the Progress 10 resupply craft that arrived at the Station on February 4, and docked to the Zarya Module is the Soyuz TMA-2 vehicle that brought Malenchenko and Lu to the ISS on April 28.

Original Source: NASA News Release

Posted on by Fraser Cain

Neutron Star’s Magnetism Measured for the First Time

Image credit: ESA

Using the space-based XMM-Newton X-Ray observatory astronomers with the European Space Agency have made the first direct measurement of a neutron star’s magnetic field. A neutron star is a very dense object with the mass of a large star packed into a radius of only 20-30 km, and they were predicted to have very strong magnetic fields which acted like a brake, slowing down their rotation. But after observing a neutron star called 1E1207.4-5209 for over 72 hours with the XMM, the astronomers discovered that it was 30 times weaker than they were predicting. What causes these objects to slow down is once again a mystery.

Using the superior sensitivity of ESA’s X-ray observatory, XMM-Newton, a team of European astronomers has made the first direct measurement of a neutron star’s magnetic field.

The results provide deep insights into the extreme physics of neutron stars and reveal a new mystery yet to be solved about the end of this star?s life.

A neutron star is very dense celestial object that usually has something like the mass of our Sun packed into a tiny sphere only 20?30 km across. It is the product of a stellar explosion, known as a supernova, in which most of the star is blasted into space, but its collapsed heart remains in the form of a super-dense, hot ball of neutrons that spins at a incredible rate.

Despite being a familiar class of object, individual neutron stars themselves remain mysterious. Neutron stars are extremely hot when they are born, but cool down very rapidly. Therefore, only few of them emit highly energetic radiation, such as X-rays. This is why they are traditionally studied via their radio emissions, which are less energetic than X-rays and which usually appear to pulse on and off. Therefore, the few neutron stars which are hot enough to emit X-rays can be seen by X-ray telescopes, such as ESA?s XMM-Newton.

One such neutron star is 1E1207.4-5209. Using the longest ever XMM-Newton observation of a galactic source (72 hours), Professor Giovanni Bignami of the Centre d’Etude Spatiale des Rayonnements (CESR) and his team have directly measured the strength of its magnetic field. This makes it the first ever isolated neutron star where this could be achieved.

All previous values of neutron star magnetic fields could only be estimated indirectly. This is done by theoretical assumptions based on models that describe the gravitational collapse of massive stars, like those which lead to the formation of neutron stars. A second indirect method is to estimate the magnetic field by studying how the neutron star?s rotation slows down, using radio astronomy data.

In the case of 1E1207.4-5209, this direct measurement using XMM-Newton reveals that the neutron star?s magnetic field is 30 times weaker than predictions based on the indirect methods.

How can this be explained? Astronomers can measure the rate at which individual neutron stars decelerate. They have always assumed that ‘friction’ between its magnetic field and its surroundings was the cause. In this case, the only conclusion is that something else is pulling on the neutron star, but what? We can speculate that it may be a small disc of supernova debris surrounding the neutron star, creating an additional drag factor.

The result raises the question of whether 1E1207.4-5209 is unique among neutron stars, or it is the first of its kind. The astronomers hope to target other neutron stars with XMM-Newton to find out.

Note to editors
X-rays emitted by a neutron star like 1E1207.4-5209, have to pass through the neutron star?s magnetic field before escaping into space. En route, particles in the star?s magnetic field can steal some of the outgoing X-rays, imparting on their spectrum tell-tale marks, known as ‘cyclotron resonance absorption lines’. It is this fingerprint that allowed Prof. Bignami and his team to measure the strength of the neutron star?s magnetic field.

Original Source: ESA News Release

Posted on by Fraser Cain

Flattest Star Ever Discovered

Image credit: ESO

Astronomers with the European Southern Observatory have discovered a star which is extremely flat All rotating objects in space are flattened due to their rotation; even our Earth is 21 kilometres wider at the equator than it is pole-to-pole. But this new star, called Achernar, is 50% wider at its equator than at its poles. Obviously it’s spinning quickly, but its shape doesn’t fit into the current astrophysics models. It should be losing mass into space at the rate it’s going. Time for some new models.

To a first approximation, planets and stars are round. Think of the Earth we live on. Think of the Sun, the nearest star, and how it looks in the sky.

But if you think more about it, you realize that this is not completely true. Due to its daily rotation, the solid Earth is slightly flattened (“oblate”) – its equatorial radius is some 21 km (0.3%) larger than the polar one. Stars are enormous gaseous spheres and some of them are known to rotate quite fast, much faster than the Earth. This would obviously cause such stars to become flattened. But how flat?

Recent observations with the VLT Interferometer (VLTI) at the ESO Paranal Observatory have allowed a group of astronomers [1] to obtain by far the most detailed view of the general shape of a fast-spinning hot star, Achernar (Alpha Eridani), the brightest in the southern constellation Eridanus (The River).

They find that Achernar is much flatter than expected – its equatorial radius is more than 50% larger than the polar one! In other words, this star is shaped very much like the well-known spinning-top toy, so popular among young children.

The high degree of flattening measured for Achernar – a first in observational astrophysics – now poses an unprecedented challenge for theoretical astrophysics. The effect cannot be reproduced by common models of stellar interiors unless certain phenomena are incorporated, e.g. meridional circulation on the surface (“north-south streams”) and non-uniform rotation at different depths inside the star.

As this example shows, interferometric techniques will ultimately provide very detailed information about the shapes, surface conditions and interior structure of stars.

VLTI observations of Achernar
Test observations with the VLT Interferometer (VLTI) at the Paranal Observatory proceed well [2], and the astronomers have now begun to exploit many of these first measurements for scientific purposes.

One spectacular result, just announced, is based on a series of observations of the bright, southern star Achernar (Alpha Eridani; the name is derived from “Al Ahir al Nahr” = “The End of the River”), carried out between September 11 and November 12, 2002. The two 40-cm siderostat test telescopes that served to obtain “First Light” with the VLT Interferometer in March 2001 were also used for these observations. They were placed at selected positions on the VLT Observing Platform at the top of Paranal to provide a “cross-shaped” configuration with two “baselines” of 66 m and 140 m, respectively, at 90? angle, cf. PR Photo 15a/03.

At regular time intervals, the two small telescopes were pointed towards Achernar and the two light beams were directed to a common focus in the VINCI test instrument in the centrally located VLT Interferometric Laboratory. Due to the Earth’s rotation during the observations, it was possible to measure the angular size of the star (as seen in the sky) in different directions.

Achernar’s profile
A first attempt to measure the geometrical deformation of a rapidly rotating star was carried out in 1974 with the Narrabri Intensity Interferometer (Australia) on the bright star Altair by British astronomer Hanbury Brown. However, because of technical limitations, those observations were unable to decide between different models for this star. More recently, Gerard T. Van Belle and collaborators observed Altair with the Palomar Testbed Interferometer (PTI), measuring its apparent axial ratio as 1.140 ? 0.029 and placing some constraints upon the relationship between rotation velocity and stellar inclination.

Achernar is a star of the hot B-type, with a mass of 6 times that of the Sun. The surface temperature is about 20,000 ?C and it is located at a distance of 145 light-years.

The apparent profile of Achernar (PR Photo 15b/03), based on about 20,000 VLTI interferograms (in the K-band at wavelength 2.2 ?m) with a total integration time of over 20 hours, indicates a surprisingly high axial ratio of 1.56 ? 0.05 [3]. This is obviously a result of Achernar’s rapid rotation.

Theoretical implications of the VLTI observations
The angular size of Achernar’s elliptical profile as indicated in PR Photo 15b/03 is 0.00253 ? 0.00006 arcsec (major axis) and 0.00162 ? 0.00001 arcsec (minor axis) [4], respectively. At the indicated distance, the corresponding stellar radii are equal to 12.0 ? 0.4 and 7.7 ? 0.2 solar radii, or 8.4 and 5.4 million km, respectively. The first value is a measure of the star’s equatorial radius. The second is an upper value for the polar radius – depending on the inclination of the star’s polar axis to the line-of-sight, it may well be even smaller.

The indicated ratio between the equatorial and polar radii of Achernar constitutes an unprecedented challenge for theoretical astrophysics, in particular concerning mass loss from the surface enhanced by the rapid rotation (the centrifugal effect) and also the distribution of internal angular momentum (the rotation velocity at different depths).

The astronomers conclude that Achernar must either rotate faster (and hence, closer to the “critical” (break-up) velocity of about 300 km/sec) than what the spectral observations show (about 225 km/sec from the widening of the spectral lines) or it must violate the rigid-body rotation.

The observed flattening cannot be reproduced by the “Roche-model” that implies solid-body rotation and mass concentration at the center of the star. The failure of that model is even more evident if the so-called “gravity darkening” effect is taken into account – this is a non-uniform temperature distribution on the surface which is certainly present on Achernar under such a strong geometrical deformation.

Outlook
This new measurement provides a fine example of what is possible with the VLT Interferometer already at this stage of implementation. It bodes well for the future research projects at this facility.

With the interferometric technique, new research fields are now opening which will ultimately provide much more detailed information about the shapes, surface conditions and interior structure of stars. And in a not too distant future, it will become possible to produce interferometric images of the disks of Achernar and other stars.

Original Source: ESO News Release

Posted on by Fraser Cain

Survey Finds 1000 Variable Stars in Nearby Galaxy

Image credit: ESO

An international survey by the European Southern Observatory has uncovered more than 1000 luminous red variable stars in nearby galaxy Centaurus A (aka NGC 5128). This is the first survey that’s ever been performed on a galaxy outside our own Milky Way. These stars, known as Mira-variables, pulse in a very specific way; the longer the cycle, the brighter they are – by comparing the visual brightness to their actual brightness, they can judge distances to these stars very accurately. This allows a very accurate measurement of the distance to Centaurus A.

An international team led by ESO astronomer Marina Rejkuba [1] has discovered more than 1000 luminous red variable stars in the nearby elliptical galaxy Centaurus A (NGC 5128).

Brightness changes and periods of these stars were measured accurately and reveal that they are mostly cool long-period variable stars of the so-called “Mira-type”. The observed variability is caused by stellar pulsation.

This is the first time a detailed census of variable stars has been accomplished for a galaxy outside the Local Group of Galaxies (of which the Milky Way galaxy in which we live is a member).

It also opens an entirely new window towards the detailed study of stellar content and evolution of giant elliptical galaxies. These massive objects are presumed to play a major role in the gravitational assembly of galaxy clusters in the Universe (especially during the early phases).

This unprecedented research project is based on near-infrared observations obtained over more than three years with the ISAAC multi-mode instrument at the 8.2-m VLT ANTU telescope at the ESO Paranal Observatory.

Mira-type variable stars
Among the stars that are visible in the sky to the unaided eye, roughly one out of three hundred (0.3%) displays brightness variations and is referred to by astronomers as a “variable star”. The percentage is much higher among large, cool stars (“red giants”) – in fact, almost all luminous stars of that type are variable. Such stars are known as Mira-variables; the name comes from the most prominent member of this class, Omicron Ceti in the constellation Cetus (The Whale), also known as “Stella Mira” (The Wonderful Star). Its brightness changes with a period of 332 days and it is about 1500 times brighter at maximum (visible magnitude 2 and one of the fifty brightest stars in the sky) than at minimum (magnitude 10 and only visible in small telescopes) [2].

Stars like Omicron Ceti are nearing the end of their life. They are very large and have sizes from a few hundred to about a thousand times that of the Sun. The brightness variation is due to pulsations during which the star’s temperature and size change dramatically.

In the following evolutionary phase, Mira-variables will shed their outer layers into surrounding space and become visible as planetary nebulae with a hot and compact star (a “white dwarf”) at the middle of a nebula of gas and dust (cf. the “Dumbbell Nebula” – ESO PR Photo 38a-b/98).

Several thousand Mira-type stars are currently known in the Milky Way galaxy and a few hundred have been found in other nearby galaxies, including the Magellanic Clouds.

The peculiar galaxy Centaurus A
Centaurus A (NGC 5128) is the nearest giant galaxy, at a distance of about 13 million light-years. It is located outside the Local Group of Galaxies to which our own galaxy, the Milky Way, and its satellite galaxies, the Magellanic Clouds, belong.

Centaurus A is seen in the direction of the southern constellation Centaurus. It is of elliptical shape and is currently merging with a companion galaxy, making it one of the most spectacular objects in the sky, cf. PR Photo 14a/03. It possesses a very heavy black hole at its centre (see ESO PR 04/01) and is a source of strong radio and X-ray emission.

During the present research programme, two regions in Centaurus A were searched for stars of variable brightness; they are located in the periphery of this peculiar galaxy, cf. PR Photos 14b-d/03. An outer field (“Field 1”) coincides with a stellar shell with many blue and luminous stars produced by the on-going galaxy merger; it lies at a distance of 57,000 light-years from the centre. The inner field (“Field 2”) is more crowded and is situated at a projected distance of about 30,000 light-years from the centre.

Three years of VLT observations
Under normal circumstances, any team of professional astronomers will have access to the largest telescopes in the world for only a very limited number of consecutive nights each year. However, extensive searches for variable stars like the present require repeated observations lasting minutes-to-hours over periods of months-to-years. It is thus not feasible to perform such observations in the classical way in which the astronomers travel to the telescope each time.

Fortunately, the operational system of the VLT at the ESO Paranal Observatory (Chile) is also geared to encompass this kind of long-term programme. Between April 1999 and July 2002, the 8.2-m VLT ANTU telescope on Cerro Paranal in Chile) was operated in service mode on many occasions to obtain K-band images of the two fields in Centaurus A by means of the near-infrared ISAAC multi-mode instrument. Each field was observed over 20 times in the course of this three-year period; some of the images were obtained during exceptional seeing conditions of 0.30 arcsec. One set of complementary optical images was obtained with the FORS1 multi-mode instrument (also on VLT ANTU) in July 1999.

Each image from the ISAAC instrument covers a sky field measuring 2.5 x 2.5 arcmin2. The combined images, encompassing a total exposure of 20 hours are indeed the deepest infrared images ever made of the halo of any galaxy as distant as Centaurus A, about 13 million light-years.

Discovering one thousand Mira variables
Once the lengthy observations were completed, two further steps were needed to identify the variable stars in Centaurus A.

First, each ISAAC frame was individually processed to identify the thousands and thousands of faint point-like images (stars) visible in these fields. Next, all images were compared using a special software package (“DAOPHOT”) to measure the brightness of all these stars in the different frames, i.e., as a function of time.

While most stars in these fields as expected were found to have constant brightness, more than 1000 stars displayed variations in brightness with time; this is by far the largest number of variable stars ever discovered in a galaxy outside the Local Group of Galaxies.

The detailed analysis of this enormous dataset took more than a year. Most of the variable stars were found to be of the Mira-type and their light curves (brightness over the pulsation period) were measured, cf. PR Photo 14i/03. For each of them, values of the characterising parameters, the period (days) and brightness amplitude (magnitudes) were determined. A catalogue of the newly discovered variable stars in Centaurus A has now been made available to the astronomical community via the European research journal Astronomy & Astrophysics.

Marina Rejkuba is pleased and thankful: “We are really very fortunate to have carried out this ambitious project so successfully. It all depended critically on different factors: the repeated granting of crucial observing time by the ESO Observing Programmes Committee over different observing periods in the face of rigorous international competition, the stability and reliability of the telescope and the ISAAC instrument over a period of more than three years and, not least, the excellent quality of the service mode observations, so efficiently performed by the staff at the Paranal Observatory.”

What have we learned about Centaurus A?
The present study of variable stars in this giant elliptical galaxy is the first-ever of its kind. Although the evaluation of the very large observational data material is still not finished, it has already led to a number of very useful scientific results.
Confirmation of the presence of an intermediate-age population

Based on earlier research (optical and near-IR colour-magnitude diagrams of the stars in the fields), the present team of astronomers had previously detected the presence of intermediate-age and young stellar populations in the halo of this galaxy. The youngest stars appear to be aligned with the powerful jet produced by the massive black hole at the centre.

Some of the very luminous red variable stars now discovered confirm the presence of a population of intermediate-age stars in the halo of this galaxy. It also contributes to our understanding of how giant elliptical galaxies form.

New measurement of the distance to Centaurus A
The pulsation of Mira-type variable stars obeys a period-luminosity relation. The longer its period, the more luminous is a Mira-type star.

This fact makes it possible to use Mira-type stars as “standard candles” (objects of known intrinsic luminosity) for distance determinations. They have in fact often been used in this way to measure accurate distances to more nearby objects, e.g., to individual clusters of stars and to the center in our Milky Way galaxy, and also to galaxies in the Local Group, in particular the Magellanic Clouds.

This method works particularly well with infrared measurements and the astronomers were now able to measure the distance to Centaurus A in this new way. They found 13.7 ? 1.9 million light-years, in general agreement with and thus confirming other methods.
Study of stellar population gradients in the halo of a giant elliptical galaxy

The two fields here studied contain different populations of stars. A clear dependence on the location (a “gradient”) within the galaxy is observed, which can be due to differences in chemical composition or age, or to a combination of both.

Understanding the cause of this gradient will provide additional clues to how Centaurus A – and indeed all giant elliptical galaxies – was formed and has since evolved.

Comparison with other well-known nearby galaxies
Past searches have discovered Mira-type variable stars thoughout the Milky Way, our home galaxy, and in other nearby galaxies in the Local Group. However, there are no giant elliptical galaxies like Centaurus A in the Local Group and this is the first time it has been possible to identify this kind of stars in that type of galaxy.

The present investigation now opens a new window towards studies of the stellar constituents of such galaxies.

Original Source: ESO News Release