New Evidence that Stars Form Like Dominos

Image credit: Hubble

An international team of astronomers have gathered new evidence to support the “domino theory” of star formation; that star formation occurs in sequence in galaxies driven by the movements of gas and stars at the core. A new instrument attached to the 8m Gemini South Telescope, called CIRPASS, allowed the astronomers to measure the composition of a whole range of stars at the centre of galaxy M83. A detailed analysis of the data is now underway.

An international team of astronomers have used a unique instrument on the 8m Gemini South Telescope to determine the ages of stars across the central region of the barred spiral galaxy, M83. Preliminary results provide the first hints of a domino model of star formation where star formation occurs in a time sequence, driven by the movements of gas and stars in the central bar.

The new instrument, called CIRPASS, simultaneously produces 500 spectra, taken from across the whole region of interest, which act as a series of ‘fingerprints’. Encoded in these ‘fingerprints’ is not only all the information the team required to determine when individual groups of stars formed, but also information on their movements and chemical properties. Dr. Johan Knapen, project co-investigator, ‘The unique combination of a state-of-the-art instrument like CIRPASS with one of the most powerful telescopes available is now providing us with truly sensational observations.’

M83 is a grand design spiral galaxy undergoing an intense burst of star formation in its central bar region. Large scale images, of the visible light from the galaxy, taken with ground based telescopes, show a pronounced bar across the middle of the galaxy) seen as the diagonal white structure in figure 1 . Astronomers believe that it is the influence of this bar which leads a concentration of gas in the central regions of the galaxy from which stars are born. ‘The central region of M83 is enshrouded in dust, but by using CIRPASS, which operates in the infra-red not the visible, we are able to see through this dust and investigate the hidden physical processes at work in the galaxy,’ said Dr Ian Parry, leader of the CIRPASS instrumentation team.

Two competing theories strive to explain the burst of star formation in the centre of the galaxy, M83. One theory suggests that stars form randomly across the whole nuclear region. A second model, favoured by the observational team, proposes that star-formation is triggered by the bar structure. In this model, the rotation of gas and stars in the bar causes stars to be formed sequentially, in a domino manner.

Using a technique first demonstrated by Dr. Stuart Ryder and colleagues, the team searched for a hydrogen emission feature, the Paschen-beta line, within the galaxy’s ‘fingerprints’. The measurement of this feature indicates the presence of hot young stars. By comparing the strengths of the Paschen-beta emission with the amount of absorption from carbon-monoxide (arising in the cool atmospheres of old giant stars) the team are able determine the age of the stars in each region of the galaxy. ‘A detailed analysis of the data is underway but initial results hint at a complex sequence of star formation,’ said Dr Robert Sharp, instrument support scientist with CIRPASS.

Preliminary analysis of other emission features (due to Paschen-beta and ionized iron) revealed a potentially intriguing result. ‘Ionized iron enables us to trace past supernova explosions. The observations indicate that energy from exploding stars (supernovae) may be being passed into regions of undisturbed gas causing further massive star formation.’ said Dr. Stuart Ryder, principle investigator.

While some members of the instrument team are presenting their work at the Royal Society Science Exhibition in London, CIRPASS is back on the Gemini South Telescope in Chile, performing the next set of observations.

Original Source: Cambridge News Release

Adaptive Optics Improve Images of the Sun

Image credit: NSO

A new adaptive optics system is helping the National Solar Observatory take much more vivid images of the Sun. Earth-based telescopes are limited in resolution by atmospheric distortion, so there was no real point of building them larger than 1.5 metres across – bigger didn’t help. With the new NSO system; however, solar telescopes can now be built 4-metres and larger. This should allow solar astronomers to better understand the processes of solar magnetism and other activities.

Impressive, sharp images of the Sun can be produced with an advanced adaptive optical system that will give new life to existing telescopes and open the way for a generation of large-aperture solar telescopes. This AO system removes blurring introduced by Earth’s turbulent atmosphere and thus provides a clear vision of the smallest structure on the Sun.

The new AO76 system — Adaptive Optics, 76 subapertures — is the largest system designed for solar observations. As demonstrated recently by a team at the National Solar Observatory at Sunspot, NM, AO76 produces sharper images under worse seeing conditions for atmospheric distortion than the AO24 system employed since 1998.

“First light” with the new AO76 system was in December 2002, followed by tests starting in April 2003 with a new high-speed camera that significantly enhanced the system.

“If the first results in late 2002 with the prototype were impressive,” said Dr. Thomas Rimmele, the AO project scientist at the NSO, “I would call the performance that we are getting now truly amazing. I’m quite thrilled with the image quality delivered by this new system. I believe its fair to say that the images we are getting are the best ever produced by the Dunn Solar Telescope.” The Dunn is one of the nation’s premier solar observing facilities.
Dual-purpose program

The new high-order AO system serves two purposes. It will allow existing solar telescopes, like the 76-cm (30-inch) Dunn, to produce higher resolution images and greatly improve their scientific output under a wider range of seeing conditions. It also demonstrates the ability to scale the system up to enable a new generation of large-aperture instruments, including the proposed 4-meter Advanced Technology Solar Telescope (see below) that will see at higher resolutions than current telescopes can achieve.

High resolution observations of the Sun have become increasingly important for solving many of the outstanding problems in solar physics. Studying the physics of flux elements, or solar fine structure in general, requires spectroscopy and polarimetry of the fine structures. The exposures are typically about 1 second long and the resolution currently achieved in spectroscopic/polarimetric data typically is 1 arc-second, which is insufficient for study of fine solar structures. Further, theoretical models predict structures below the resolution limits of 0.2 arc-sec of existing solar telescopes. Observations are needed below the 0.2 arc-sec resolution limit to study the important physical processes that occur on such small scales. Only AO can provide a consistent spatial resolution of 0.1 arc-sec or better from ground based observatories.

AO technology combines computers and flexible optical components to reduce the effects of atmospheric blurring (“seeing”) on astronomical images. Sunspot’s solar AO76 system is based on the Shack-Hartmann correlating technique. In essence, this divides an incoming image into an array of subapertures viewed by a wavefront sensor camera. One subaperture is selected as a reference image. Digital signal processors (DSPs) calculate how to adjust each subaperture to match the reference image. The DSPs then command 97 actuators to reshape a thin, 7.7 cm (3-inch) deformable mirror to cancel much of the blurring. The DSP also can drive a tilt/tip mirror, mounted in front of the AO system, that removes gross image motion caused by the atmosphere.

Closing the loop for sharper images
“A major challenge for astronomers is correcting the light entering their telescopes for the effect of the Earth’s atmosphere,” explained Kit Richards, NSO’s AO lead project engineer. “Air of different temperatures mixing above the telescope makes the atmosphere like a rubber lens that reshapes itself about a hundred times each second.” This is more severe for solar astronomers observing during the day with the Sun heating Earth’s surface, but still causes the stars to twinkle at night.

Further, solar physicists want to study extended bright regions with low contrast. That makes it more challenging for an AO system to correlate the same parts of several slightly different subapertures, and to maintain the correlation from one image frame to the next as the atmosphere changes shape.

(Nighttime astronomy has used a different technique for several years. Lasers generate artificial guide stars in the atmosphere, letting astronomers measure and correct for atmospheric distortion. This is not practical with instruments that observe the Sun.)

In 1998 NSO pioneered use of a low-order AO24 system for solar observations. It has 24 apertures and compensates 1,200 times/second (1,200 Hertz [Hz]). Since August 2000, the team focused on scaling the system up to the high-order AO76 with 76 apertures and correcting twice as fast, 2,500 Hz. The breakthroughs started in late 2002.

First, the servo loop was successfully closed on the new high-order AO system during its first engineering run at the Dunn in December. In a “closed loop” servo system the output is fed back to the input and the errors are driven to 0. An “open loop” system detects the errors and makes corrections but the corrected output is not feed back to the input. The servo system doesn’t know if it is removing all the errors or not. This type of system is faster but very hard to calibrate and keep calibrated. At this point the system used a DALSA camera, which operates at 955 Hz, as the interim wavefront sensor. The optical setup was not finalized and preliminary; “bare-bone” software operated the system.

High-speed wavefront sensor
Even in this preliminary state — intended to demonstrate that the components worked together as a system– and under mediocre seeing conditions, the high-order AO system produced impressive, diffraction-limited images. Time sequences of corrected and uncorrected images show that the new AO system provides fairly consistent high-resolution imaging even as the seeing varies substantially, as is typical for daytime seeing.

Following this milestone, the team installed a new high-speed wavefront sensor camera custom developed for the AO project by Baja Technology and NSO’s Richards. It operates at 2,500 frames/second, which more than doubles the closed-loop servo bandwidth possible with the DALSA camera. Richards also implemented improved control software. In addition, the system was upgraded to drive the tip/tilt correction mirror either directly from the AO wavefront sensor or from a separate correlation/spot tracker system that operates at 3 kHz.

The new high-order AO76 was first tested in April 2003 and immediately started producing excellent images under a wider range of seeing conditions that normally would preclude high-resolution images. The new high-order AO76 was first tested in April 2003 and immediately started producing excellent images under a wider range of seeing conditions that normally would preclude high-resolution images. Striking differences with the AO on versus off are readily visible in images of active areas, granulation, and other features.

“That’s not to say that seeing does not matter anymore,” Rimmele noted. “To the contrary, seeing effects such as anisoplanatism — wavefront differences between the correlation target and the area we want to study — still are limiting factors. But in halfway decent seeing we can lock up on granulation and record excellent images.”

To make large instruments like the Advanced Technology Solar Telescope possible, the high-order AO system will have to be scaled up more than tenfold to at least 1,000 subapertures. And NSO is looking beyond that to a more complex technique, multiconjugate AO. This approach, already being developed for nighttime astronomy, builds a three-dimensional model of the turbulent region rather than treating it as a simple distorted lens.

For now, though, the project team will focus on the completion of the optical setup at the Dunn, installation of the AO bench at the Big Bear Solar Observatory followed by engineering runs, optimization of reconstruction equations and servo loop controls, and characterization of system performance at both sites. Then, the Dunn AO system is to become operational in fall of 2003. The Diffraction Limited Spectro-Polarimeter (DLSP), the main science instrument that can take advantage of the diffraction-limited image quality delivered by the high-order AO, is scheduled for its first commissioning runs in fall of 2003. NSO is developing the DLSP in collaboration with the High Altitude Observatory in Boulder.

Original Source: NSO News Release

Gamma Ray Bursts and Hypernovae Linked

Image credit: ESO

On March 29, 2003 NASA’s High Energy Transient Explorer detected a bright burst of gamma rays, and shortly after telescopes from around the world focused in on the object; now called GRB 030329 and measured to be 2.6 billion light-years away. By measuring the afterglow of the explosion, astronomers realized that it matches the spectrum of a hypernova – explosions of extremely large stars, at least 25 times larger than our own Sun. By matching the spectra, astronomers have compelling evidence that there is some connection between gamma ray bursts and the explosions of very large stars.

A very bright burst of gamma-rays was observed on March 29, 2003 by NASA’s High Energy Transient Explorer (HETE-II), in a sky region within the constellation Leo.

Within 90 min, a new, very bright light source (the “optical afterglow”) was detected in the same direction by means of a 40-inch telescope at the Siding Spring Observatory (Australia) and also in Japan. The gamma-ray burst was designated GRB 030329, according to the date.

And within 24 hours, a first, very detailed spectrum of this new object was obtained by the UVES high-dispersion spectrograph on the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile). It allowed to determine the distance as about 2,650 million light-years (redshift 0.1685).

Continued observations with the FORS1 and FORS2 multi-mode instruments on the VLT during the following month allowed an international team of astronomers [1] to document in unprecedented detail the changes in the spectrum of the optical afterglow of this gamma-ray burst. Their detailed report appears in the June 19 issue of the research journal “Nature”.

The spectra show the gradual and clear emergence of a supernova spectrum of the most energetic class known, a “hypernova”. This is caused by the explosion of a very heavy star – presumably over 25 times heavier than the Sun. The measured expansion velocity (in excess of 30,000 km/sec) and the total energy released were exceptionally high, even within the elect hypernova class.

From a comparison with more nearby hypernovae, the astronomers are able to fix with good accuracy the moment of the stellar explosion. It turns out to be within an interval of plus/minus two days of the gamma-ray burst. This unique conclusion provides compelling evidence that the two events are directly connected.

These observations therefore indicate a common physical process behind the hypernova explosion and the associated emission of strong gamma-ray radiation. The team concludes that it is likely to be due to the nearly instantaneous, non-symmetrical collapse of the inner region of a highly developed star (known as the “collapsar” model).

The March 29 gamma-ray burst will pass into the annals of astrophysics as a rare “type-defining event”, providing conclusive evidence of a direct link between cosmological gamma-ray bursts and explosions of very massive stars.

What are Gamma-Ray Bursts?
One of the currently most active fields of astrophysics is the study of the dramatic events known as “gamma-ray bursts (GRBs)”. They were first detected in the late 1960’s by sensitive instruments on-board orbiting military satellites, launched for the surveillance and detection of nuclear tests. Originating, not on the Earth, but far out in space, these short flashes of energetic gamma-rays last from less than a second to several minutes.

Despite major observational efforts, it is only within the last six years that it has become possible to pinpoint with some accuracy the sites of some of these events. With the invaluable help of comparatively accurate positional observations of the associated X-ray emission by various X-ray satellite observatories since early 1997, astronomers have until now identified about fifty short-lived sources of optical light associated with GRBs (the “optical afterglows”).

Most GRBs have been found to be situated at extremely large (“cosmological”) distances. This implies that the energy released in a few seconds during such an event is larger than that of the Sun during its entire lifetime of more than 10,000 million years. The GRBs are indeed the most powerful events since the Big Bang known in the Universe, cf. ESO PR 08/99 and ESO PR 20/00.

During the past years circumstantial evidence has mounted that GRBs signal the collapse of massive stars. This was originally based on the probable association of one unusual gamma-ray burst with a supernova (“SN 1998bw”, also discovered with ESO telescopes, cf. ESO PR 15/98). More clues have surfaced since, including the association of GRBs with regions of massive star-formation in distant galaxies, tantalizing evidence of supernova-like light-curve “bumps” in the optical afterglows of some earlier bursts, and spectral signatures from freshly synthesized elements, observed by X-ray observatories.

VLT observations of GRB 030329
On March 29, 2003 (at exactly 11:37:14.67 hrs UT) NASA’s High Energy Transient Explorer (HETE-II) detected a very bright gamma-ray burst. Following identification of the “optical afterglow” by a 40-inch telescope at the Siding Spring Observatory (Australia), the redshift of the burst [3] was determined as 0.1685 by means of a high-dispersion spectrum obtained with the UVES spectrograph at the 8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory (Chile).

The corresponding distance is about 2,650 million light-years. This is the nearest normal GRB ever detected, therefore providing the long-awaited opportunity to test the many hypotheses and models which have been proposed since the discovery of the first GRBs in the late 1960’s.

With this specific aim, the ESO-lead team of astronomers [1] now turned to two other powerful instruments at the ESO Very Large Telescope (VLT), the multi-mode FORS1 and FORS2 camera/spectrographs. Over a period of one month, until May 1, 2003, spectra of the fading object were obtained at regular rate, securing a unique set of observational data that documents the physical changes in the remote object in unsurpassed detail.

The hypernova connection
Based on a careful study of these spectra, the astronomers are now presenting their interpretation of the GRB 030329 event in a research paper appearing in the international journal “Nature” on Thursday, June 19. Under the prosaic title “A very energetic supernova associated with the gamma-ray burst of 29 March 2003”, no less than 27 authors from 17 research institutes, headed by Danish astronomer Jens Hjorth conclude that there is now irrefutable evidence of a direct connection between the GRB and the “hypernova” explosion of a very massive, highly evolved star.

This is based on the gradual “emergence” with time of a supernova-type spectrum, revealing the extremely violent explosion of a star. With velocities well in excess of 30,000 km/sec (i.e., over 10% of the velocity of light), the ejected material is moving at record speed, testifying to the enormous power of the explosion.

Hypernovae are rare events and they are probably caused by explosion of stars of the so-called “Wolf-Rayet” type [4]. These WR-stars were originally formed with a mass above 25 solar masses and consisted mostly of hydrogen. Now in their WR-phase, having stripped themselves of their outer layers, they consist almost purely of helium, oxygen and heavier elements produced by intense nuclear burning during the preceding phase of their short life.
“We have been waiting for this one for a long, long time”, says Jens Hjorth, “this GRB really gave us the missing information. From these very detailed spectra, we can now confirm that this burst and probably other long gamma-ray bursts are created through the core collapse of massive stars. Most of the other leading theories are now unlikely.”
A “type-defining event”

His colleague, ESO-astronomer Palle M?ller, is equally content: “What really got us at first was the fact that we clearly detected the supernova signatures already in the first FORS-spectrum taken only four days after the GRB was first observed – we did not expect that at all. As we were getting more and more data, we realised that the spectral evolution was almost completely identical to that of the hypernova seen in 1998. The similarity of the two then allowed us to establish a very precise timing of the present supernova event”.

The astronomers determined that the hypernova explosion (designated SN 2003dh [2]) documented in the VLT spectra and the GRB-event observed by HETE-II must have occurred at very nearly the same time. Subject to further refinement, there is at most a difference of 2 days, and there is therefore no doubt whatsoever, that the two are causally connected.

“Supernova 1998bw whetted our appetite, but it took 5 more years before we could confidently say, we found the smoking gun that nailed the association between GRBs and SNe” adds Chryssa Kouveliotou of NASA. “GRB 030329 may well turn out to be some kind of ‘missing link’ for GRBs.”

In conclusion, GRB 030329 was a rare “type-defining” event that will be recorded as a watershed in high-energy astrophysics.

What really happened on March 29 (or 2,650 million years ago)?
Here is the complete story about GRB 030329, as the astronomers now read it.

Thousands of years prior to this explosion, a very massive star, running out of hydrogen fuel, let loose much of its outer envelope, transforming itself into a bluish Wolf-Rayet star [3]. The remains of the star contained about 10 solar masses worth of helium, oxygen and heavier elements.

In the years before the explosion, the Wolf-Rayet star rapidly depleted its remaining fuel. At some moment, this suddenly triggered the hypernova/gamma-ray burst event. The core collapsed, without the outer part of the star knowing. A black hole formed inside, surrounded by a disk of accreting matter. Within a few seconds, a jet of matter was launched away from that black hole.

The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly formed radioactive nickel-56 blowing off the disk inside, shattered the star. This shattering, the hypernova, shines brightly because of the presence of nickel. Meanwhile, the jet plowed into material in the vicinity of the star, and created the gamma-ray burst which was recorded some 2,650 million years later by the astronomers on Earth. The detailed mechanism for the production of gamma rays is still a matter of debate but it is either linked to interactions between the jet and matter previously ejected from the star, or to internal collisions inside the jet itself.

This scenario represents the “collapsar” model, introduced by American astronomer Stan Woosley (University of California, Santa Cruz) in 1993 and a member of the current team, and best explains the observations of GRB 030329.

“This does not mean that the gamma-ray burst mystery is now solved”, says Woosley. “We are confident now that long bursts involve a core collapse and a hypernova, likely creating a black hole. We have convinced most skeptics. We cannot reach any conclusion yet, however, on what causes the short gamma-ray bursts, those under two seconds long.”

Original Source: ESO News Release

First Light: An Introduction to Stargazing

Interested in space and astronomy but you’ve never actually looked through a telescope? Until you’ve actually gone out and done some actual observing with your own two eyes, you won’t know what you’re missing. In this article, Fraser gives you a kick in the pants to get out there under the skies and start enjoying the heavens above. You don’t need any special equipment or advanced university degrees, just some enthusiasm, a little time and the ability to look up.

I know there are a lot of subscribers interested in space and astronomy, but I’m wondering how many of you have actually taken a look through a telescope and seen some of the objects I talk about with your own eyes.

One of my fondest memories was when I was 13 years old, and set up my 4″ telescope at my Dad’s birthday party. I was in a darkish corner of our property and would sneak away a few partygoers to show them Saturn. Fortunately the rings were at their greatest angle, and people looking through the eyepiece couldn’t believe their eyes. Looking at pictures taken by Hubble is one thing, but when you’re actually looking through the eyepiece at Saturn, it’s an incredible experience.

Stargazing has since played a big part of my life: I organized a star party, hit on my future wife by pointing out constellations, and started a space-related website, but I’m still amazed at the number of people who’ve never actually gone out there and gotten to know their sky.

With all the new observatories and space news, I think that people are starting to think that astronomy is one of those sciences reserved for people with the expensive instruments, but that couldn’t be further from the truth. It’s one of the few sciences that amateurs still make valuable contributions, and it costs absolutely nothing to get started – you just need your eyes, and a little knowledge.

Find your community
The first thing you need to do is make a commitment to get involved in astronomy. It’s not as easy as just turning on your television; you’ve got to get organized; make some phone calls; set aside some time to explore.

I’ll bet you didn’t know, but there’s an astronomical society lurking in almost every population centre on the planet. We’ve got dozens just here in Canada, and there are literally thousands in the US. The members of the society will usually meet on a regular basis and will have observing nights where they all get together and point their telescopes at different objects. This is a great way to quickly see what the night sky has to offer.

Do a search on Google with the search terms: yourtown astronomical society. For example, I would do a search for: Vancouver astronomical society. If nothing turns up for your specific location, broaden the search a bit. Eventually you should come up with something. Find the contact information for the society and drop them an email or give them a phone call. Trust me, they’ll be happy to give you more information and have you join them for an evening.

Next, see if there’s an observatory in your region. Although most of the largest telescopes are fully booked up for years in advance, some of the smaller ones have open nights where people can come down, ask to see stuff and they’ll move the scope around. Often these open nights are run by the local astronomical society. Once again, contact the society and find out if they can recommend an observatory to check out. Or, you can do a search on Google (search for: yourtown observatory) and contact them directly.

Learn your constellations
Whether you actually contact a society or just decide to go solo is up to you, but your first step is to learn some of your constellations. Maybe you already know the Big Dipper or Orion’s belt, but there are 88 constellations in Northern and Southern hemispheres. It’s pretty cool to be able to ask a person what their sign is, and then point it out in sky.

Learning your constellations is also the first step to finding some of the more interesting stuff to look at in the night sky. They’re like your guides. For example, our nearest galaxy, Andromeda (aka M31) is easily visible in binoculars or a telescope. It’s just a little up from the middle of the constellation Andromeda, which is just above Aries. I can spot M31 in a second whenever a look up in the sky (at the right time of year). Once you start to learn your constellations, they all start to fit together like a puzzle. And the great thing is the knowledge never goes away, even if it’s been a few years since you’ve done any observing.

There are many great resources for learning your constellations. One option is to do a search, once again on Google, for the term: astronomy sky charts. Some of these are fairly detailed, however, and make it hard to just learn the basic constellations.

The book that taught me, and I can’t recommend it highly enough is Nightwatch, by Terrence Dickinson. The book breaks the night sky into seasons and then has single pages for each chunk of sky with clearly defined stars and constellations – similar to one of those road maps that sit open on the car seat next to you. The book also has fabulous information on starting equipment, etc. (Order Nightwatch from Amazon.com – $20.97)

Another handy tool is Astronomy magazine. The middle of each issue is a star chart for the current month. The advantage of using a magazine like Astronomy is that it also has the current positions of the planets. (Click here to get a subscription to Astronomy for 32% off the newsstand price)

Finally, you can use a software product like Starry Nights, which lets you define your location and time to produce a custom star chart that includes the locations of the planets. (Click here for more information on Starry Nights)

Once you’ve got your sky chart together, I suggest you also get a flashlight with a red-light filter. You can usually pick them up at camping stores or army surplus. This way you can look at your charts without ruining your night vision.

Now, hit the road! If you live in an area with reasonably dark skies, you can just turn out the lights in your house and head into your back yard. If you live in a city, you’ll have to get a little ways out. Even a dark park or dimly lit suburb will be a vast improvement over the downtown core. City lights cause two problems: the streetlights will send a glare up into your night sky, dimming your visibility; and the lights will ruin your night vision directly.

Give yourself a couple of hours, and by the end of it you’ll be familiar with most of the constellations in the sky. You’ll probably also see a few meteors and even some satellites. Quality family entertainment if you ask me.

Improving your stargazing experience
Astronomy is one of those hobbies that you can enjoy for free, but you can really improve your experience with some basic equipment.

Binoculars
Chances are you’ve already got one of the most useful pieces of stargazing equipment already in your home: binoculars. Anywhere you look in the night sky is significantly improved by a simple pair of astronomical binoculars, from the Moon to star clusters. In fact, some stuff looks better in binoculars than a more powerful telescope.

Binoculars generally have two measurements: magnification and field of view. For example, a common kind is 7×35. This means it has a 7x magnification and 35mm field of view. For astronomy, power isn’t necessarily a good thing. Some go as high as 20x or even 30x, but this usually creates a very small field of view. And since you’re holding the binoculars with your hands, it can get very shaky.

It’s much better to go with a lower power set of binoculars with a large field of view: 8×50 is a perfect combination of power and field of view.

Obviously it’s important to have good quality optics, but that’s one of those things that you should experience with first to get a sense of the equipment you already have. If it’s too high-power, or you can’t focus the image to get really crisp stars, you might want to consider upgrading your gear.

It’s also really useful to have a tripod adapter hole on the bottom of your binoculars. This will let you screw them onto the top of a tripod and then let other people come and take a look through the eyepieces to share your view.

Here are some links to Binoculars.com for some good astronomical binoculars:

Celestron 7×50 Enduro. Straightforward pair of binoculars with good magnification and field of view. $57.40 USD

Bausch & Lomb 10×50 Legacy. Higher magnification with 50mm field of view. $111.00 USD.

Canon 15×50 IS. Pretty much the best binoculars you can get. Higher resolution but image stabilization keeps the image from shaking. $899.00 USD

Telescope
If you’re thinking of buying a telescope, then you’ve really got the bug. However, don’t just run down and purchase a telescope from a department or toy store. These usually have low quality optics, a jiggly mount and generally stink for astronomy – those “in the know” call them “Christmas trash scopes”.

For the same price or a little more you can purchase a real telescope with quality optics and mount and have a much better experience with the night sky.

There are many different kinds of telescopes, and explaining the differences of how to select a good telescope can fill a book so I won’t go into the details here. Remember your contacts at your local astronomical society? Let them know your budget and objectives and they can probably recommend a good telescope. They might even know someone who’s selling one used. Of course, these folks are going to be astronomy fans, so they might have bigger ideals than what you’re looking for.

There are two main kinds of telescopes: refractors and reflectors.

Refractors work through a series of lenses which focus light into the telescope’s eyepiece (think of your traditional ship captain’s spyglass) and typically have a main lens between 70mm and 100mm. These can be solid telescopes, but the optics can make them more expensive than reflectors.

One example refractors would be the Meade EXT-70AT ($298.00 USD). A small portable refractor with with a computer-controlled mount. Put the telescope on a flat surface, align it with the sky and then it can automatically pick out targets in the sky. These automated telescopes can take some of the fun out of stargazing, but it definitely speeds things up.

Reflectors use a big mirror to reflect and focus incoming light to the telescope’s eyepiece. They’re usually shorter and fatter than a refractor, starting at 4 inches and going up from there. I started, and still use a 4″ telescope, which is perfectly fine to see the major planets and all kinds of astronomical objects.

An example reflector is a Celestron 4.5″ Firstscope ($149.00). No computer on this telescope, so you’ll get a chance to learn the location of sky objects on your own.

Bigger telescopes gather more light, so they can display fainter objects, but they come with a higher price. My recommendation is to start small, get some experience before considering a higher-end telescope.

Probably the best starting telescope is something like a 6″ Dobsonian reflector. Unlike most telescopes you’ve seen, the Dobsonians have their mount down at the base and then point up. They’re solid, inexpensive, and easy to use. Some of the largest, most powerful amateur-built telescopes are Dobsonians.

Here’s a link to a Swift Instruments 6″ Dobsonian telescope ($382.95 USD).

An a link to a much larger Meade Starfinder 16″ Dobsonian ($1,386.00 USD).

Now Get Out There!
Enough reading, start sky watching. Early Summer is a great time to get involved in astronomy (and a terrible time to watch TV) – warm summer nights and stargazing go hand in hand. Do a little research, grab some supplies, gather the friends and family, and get out under the stars. And please, email me your summer experiences. Trust me, you’ll get some memories you’ll never forget.

Glimpse Into a Star Factory

Image credit: ESO

A new series of photographs taken by the European Southern Observatory show a rare look into the very early stages of heavy star formation. This time in a star’s life is usually obscured from sight because of thick clouds of gas and dust, but in star cluster NGC 3603, the stellar wind from hot stars are blasting away the obscuring material. Inside this cluster, astronomers are finding massive protostars which are only 100,000 years old. This is a valuable discovery because it helps astronomers understand how the early stages of heavy star formation begins – is it through gravity pulling together gas and dust, or something more violent, like smaller stars colliding together.

Based on a vast observational effort with different telescopes and instruments, ESO-astronomer Dieter N?rnberger has obtained a first glimpse of the very first stages in the formation of heavy stars.

These critical phases of stellar evolution are normally hidden from the view, because massive protostars are deeply embedded in their native clouds of dust and gas, impenetrable barriers to observations at all but the longest wavelengths. In particular, no visual or infrared observations have yet “caught” nascent heavy stars in the act and little is therefore known so far about the related processes.

Profiting from the cloud-ripping effect of strong stellar winds from adjacent, hot stars in a young stellar cluster at the center of the NGC 3603 complex, several objects located near a giant molecular cloud were found to be bona-fide massive protostars, only about 100,000 years old and still growing.

Three of these objects, designated IRS 9A-C, could be studied in more detail. They are very luminous (IRS 9A is about 100,000 times intrinsically brighter than the Sun), massive (more than 10 times the mass of the Sun) and hot (about 20,000 degrees). They are surrounded by relative cold dust (about 0?C), probably partly arranged in disks around these very young objects.

Two possible scenarios for the formation of massive stars are currently proposed, by accretion of large amounts of circumstellar material or by collision (coalescence) of protostars of intermediate masses. The new observations favour accretion, i.e. the same process that is active during the formation of stars of smaller masses.

How do massive stars form?
This question is easy to pose, but so far very difficult to answer. In fact, the processes that lead to the formation of heavy stars [1] is currently one the most contested areas in stellar astrophysics.

While many details related to the formation and early evolution of low-mass stars like the Sun are now well understood, the basic scenario that leads to the formation of high-mass stars still remains a mystery. It is not even known whether the same characterizing observational criteria used to identify and distinguish the individual stages of young low-mass stars (mainly colours measured at near- and mid-infrared wavelengths) can also be used in the case of massive stars.

Two possible scenarios for the formation of massive stars are currently being studied. In the first, such stars form by accretion of large amounts of circumstellar material; the infall onto the nascent star varies with time. Another possibility is formation by collision (coalescence) of protostars of intermediate masses, increasing the stellar mass in “jumps”.

Both scenarios impose strong limitations on the final mass of the young star. On one side, the accretion process must somehow overcome the outward radiation pressure that builds up, following the ignition of the first nuclear processes (e.g., deuterium/hydrogen burning) in the star’s interior, once the temperature has risen above the critical value near 10 million degrees.

On the other hand, growth by collisions can only be effective in a dense star cluster environment in which a reasonably high probability for close encounters and collisions of stars is guaranteed.

Which of these two possibilties is then the more likely one?

Massive stars are born in seclusion
There are three good reasons that we know so little about the earliest phases of high-mass stars:

First, the formation sites of such stars are in general much more distant (many thousands of light-years) than the sites of low-mass star formation. This means that it is much more difficult to observe details in those areas (lack of angular resolution).

Next, in all stages, also the earliest ones (astronomers here refer to “protostars”), high-mass stars evolve much faster than low-mass stars. It is therefore more difficult to “catch” massive stars in the critical phases of early formation.

And, what is even worse, due to this rapid development, young high-mass protostars are usually very deeply embedded in their natal clouds and therefore not detectable at optical wavelengths during the (short) phase before nuclear reactions start in their interior. There is simply not enough time for the cloud to disperse – when the curtain finally lifts, allowing a view of the new star, it is already past those earliest stages.

Is there a way around these problems? “Yes”, says Dieter N?rnberger of ESO-Santiago, “you just have to look in the right place and remember Bob Dylan…!”. This is what he did.
“The answer, my friend, is blowing by the wind…”

Imagine that it would be possible to blow away most of the obscuring gas and dust around those high-mass protostars! Even the strongest desire of the astronomers cannot do it, but there are fortunately others who are better at it!

Some high-mass stars form in the neighbourhood of clusters of hot stars, i.e., next to their elder brethren. Such already evolved hot stars are a rich source of energetic photons and produce powerful stellar winds of elementary particles (like the “solar wind” but many times stronger) which impact on the surrounding interstellar gas and dust clouds. This process may lead to partial evaporation and dispersion of those clouds, thereby “lifting the curtain” and letting us look directly at young stars in that region, also comparatively massive ones at a relatively early evolutionary stage.

The NGC 3603 region
Such premises are available within the NGC 3603 stellar cluster and star-forming region that is located at a distance of about 22,000 light-years in the Carina spiral arm of the Milky Way galaxy.

NGC 3603 is one of the most luminous, optically visible “HII-regions” (i.e. regions of ionized hydrogen – pronounced “eitch-two”) in our galaxy. At its centre is a massive cluster of young, hot and massive stars (of the “OB-type”) – this is the highest density of evolved (but still relatively young) high-mass stars known in the Milky Way, cf. ESO PR 16/99.

These hot stars have a significant impact on the surrounding gas and dust. They deliver a huge amount of energetic photons that ionize the interstellar gas in this area. Moreover, fast stellar winds with speeds up to several hundreds of km/sec impact on, compress and/or disperse adjacent dense clouds, referred to by astronomers as “molecular clumps” because of their content of complex molecules, many of these “organic” (with carbon atoms).

IRS 9: a “hidden” association of nascent massive stars
One of these molecular clumps, designated “NGC 3603 MM 2” is located about 8.5 light-years south of the NGC 3603 cluster, cf. PR Photo 16a/03. Located on the cluster-facing side of this clump are some highly obscured objects, known collectively as “NGC 3603 IRS 9”. The present, very detailed investigation has allowed to characterise them as an association of extremely young, high-mass stellar objects.

They represent the only currently known examples of high-mass counterparts to low-mass protostars which are detected at infrared wavelengths. It took quite an effort [2] to unravel their properties with a powerful arsenal of state-of-the-art instruments working at different wavelengths, from the infrared to the millimeter spectral region.

Multi-spectral observations of IRS 9
To begin with, near-infrared imaging was performed with the ISAAC multi-mode instrument at the 8.2-m VLT ANTU telescope, cf. PR Photo 16b/03. This allowed to distinguish between stars which are bona-fide cluster members and others which happen to be seen in this direction (“field stars”). It was possible to measure the extent of the NGC 3603 cluster which was found to be about about 18 light-years, or 2.5 times larger than assumed before. These observations also served to show that the spatial distributions of low- and high-mass cluster stars are different, the latter being more concentrated towards the centre of the cluster core.

Millimeter observations were made by means of the Swedish-ESO Submillimeter Telescpe (SEST) at the La Silla Observatory. Large-scale mapping of the distribution of the CS-molecule showed the structure and motions of the dense gas in the giant molecular cloud, from which the young stars in NGC 3603 originate. A total of 13 molecular clumps were detected and their sizes, masses and densities were determined. These observations also showed that the intense radiation and strong stellar winds from the hot stars in the central cluster have “carved a cavity” in the molecular cloud; this comparatively empty and transparent region now measures about 8 light-years across.

Mid-infrared imaging (at wavelengths 11.9 and 18 ?m) was made of selected regions in NGC 3603 with the TIMMI 2 instrument mounted on the ESO 3.6-m telescope. This constitutes the first sub-arcsec resolution mid-IR survey of NGC 3603 and serves in particular to show the warm dust distribution in the region. The survey gives a clear indication of intense, on-going star formation processes. Many different types of objects were detected, including extremely hot Wolf-Rayet stars and protostars; altogether 36 mid-IR point sources and 42 knots of diffuse emission were identified. In the area surveyed, the protostar IRS 9A is found to be the most luminous point source at both wavelengths; two other sources, designated IRS 9B and IRS 9C in the immediate vicinity are also very bright on the TIMMI 2 images, providing further indication that this is the site of an association of protostars in its own right.

The collection of high-quality images of the IRS 9 area shown in PR Photo 16b/03 is well suited to investigate the nature and the evolutionary status of the highly obscured objects located there, IRS 9A-C. They are situated on the side of the massive molecular cloud core NGC 3603 MM 2 that faces the central cluster of young stars (PR Photo 16a/03) and were apparently only recently “liberated” from most of their natal gas and dust environment by strong stellar winds and energetic radiation from the nearby high-mass cluster stars.

The combined data lead to a clear conclusion: IRS 9A-C represent the brightest members of a sparse association of protostars, still embedded in circumstellar envelopes, but in a region of the pristine molecular cloud core, now largely “blown-free” from gas and dust. The intrinsic brightness of these nascent stars is impressive: 100,000, 1000 and 1000 times that of the Sun for IRS 9A, IRS 9B and IRS 9C, respectively.

Their brightness and infrared colours give information about the physical properties of these protostars. They are very young in astronomical terms, probably less than 100,000 years old. They are already quite massive, though, more than 10 times heavier than the Sun, and they are still growing – comparison to the currently most reliable theoretical models suggests that they accrete material from their envelopes at the relatively high rate of up to 1 Earth mass per day, i.e., the mass of the Sun in 1000 years.

The observations indicate that all three protostars are surrounded by comparatively cold dust (temperature around 250 – 270 K, or -20 ?C to 0?C). Their own temperatures are quite high, of the order of 20,000 – 22,000 degrees.

What do the massive protostars tell us?
Dieter N?rnberger is pleased: “We now have convincing arguments to consider IRS 9A-C as a kind of Rosetta Stones for our understanding of the earliest phases of the formation of massive stars. I know of no other high-mass protostellar candidates which have been revealed at such an early evolutionary stage – we must be grateful for the curtain-lifting stellar winds in that area! The new near- and mid-infrared observations are giving us a first look into this extremely interesting phase of stellar evolution.”

The observations show that criteria (e.g., infrared colours) already established for the identification of very young (or proto-) low-mass stars apparently also hold for high-mass stars. Moreover, with reliable values of their brightness (luminosity) and temperature, IRS 9A-C may serve as crucial and discerning test cases for the currently discussed models of high-mass star formation, in particular of accretion models versus coagulation models.

The present data are well consistent with the accretion models and no objects of intermediate luminosity/mass were found in the immediate neighbourhood of IRS 9A-C. Thus, for the IRS 9 association at least, the accretion scenario is favoured against the collision scenario.

Original Source: ESO News Release

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

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

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

Watch the Mars Explorer Launch Live

If you’re going to be anywhere near a computer connected to the Internet on Tuesday, why not tune into NASA television and watch the launch of the first Mars Explorer rover. NASA’s set up a fast-loading page that will have operational links when their coverage gets started, so you can tune in.

Coverage begins 1600 GMT (12:00pm EDT). Click here to see the page, and then bookmark it so you can check back tomorrow.

I’ll be watching.

Fraser Cain
Publisher
Universe Today

New Station Modules Arrive in Florida

Image credit: NASA

Two major components of the International Space Station arrived at NASA’s Kennedy Space Center in Florida this week. Node 2, built by the European Space Agency, will increase the station’s living and work space, while the Japanese Experiment Module (JEM) will enhance its research capabilities. NASA engineers will perform integration tests over the course of the summer and then the modules will be moved to the KSC Space Station Processing Facility for a future launch on the space shuttle.

After traveling thousands of miles, two major components of the International Space Station completed the first leg of a journey that will eventually end 240 miles above the Earth. NASA’s Node 2, built for the agency by the European Space Agency (ESA) in Italy, and the Pressurized Module of the Japanese Experiment Module (JEM) arrived in Florida and are
being transported to the Kennedy Space Center (KSC) this week.

“Delivery of these components, built in Europe and Japan, to KSC for integrated testing prior to flight is yet another indication of the significant global cooperation and proactive planning required for successful operation of the International Space Station program,” said Bill Gerstenmaier, NASA’s Station Program Manager. “Their arrival in the United States signifies the Space Station international partnership is continuing to move forward with the steps necessary to construct our unique research platform in space,” he said.

The arrival of Node 2, the next pressurized module to be installed on the Station, sets in motion the final steps toward completing assembly of essential U.S. components. When
installed, Node 2 will increase the living and working space inside the Space Station to approximately 18,000 cubic feet. It will also allow the addition of international laboratories
from Europe and Japan.

The Pressurized Module is the first element of the JEM, named “Kibo” (Hope), to be delivered to KSC. The JEM is Japan’s primary contribution to the Station. It will enhance the unique research capabilities of the orbiting complex by providing an additional environment for astronauts to conduct science experiments.

The JEM also includes an exposed facility (platform) for space environment experiments, a robotic manipulator system, and two logistics modules. The various JEM components will be
assembled in space over the course of three Shuttle missions.

An Airbus Beluga heavy-lift aircraft, carrying Node 2, departed May 30 from Turin, Italy, where the Italian Space Agency’s (ASI) contractor, Alenia Spazio, built it. Following post-transportation inspections, ASI will formally transfer ownership of Node 2 to ESA, which, in turn, will sign it over to NASA.

The container transport ship carrying JEM departed May 2 from Yokohama Harbor in Japan for the voyage to the United States. The National Space Development Agency of Japan (NASDA) developed the laboratory at the Tsukuba Space Center near Tokyo.

Later this summer, integrated testing will confirm module compatibility and, ultimately, lead to pre-launch processing at KSC’s Space Station Processing Facility.

Original Source: NASA News Release