The primary mission of the twin STEREO probes is to explore the 3-dimensional makeup of our Sun. Each craft carries a variety of instruments. One of them, the Heliospheric Imager (HI), doesn’t look directly at the Sun, but rather, explores a wide field near the Sun in order to explore the physics of coronal mass ejections (CMEs), in particular, ones aimed at the Earth. But while not focusing on solar ejections, the HI is free to make many other observations, including its first detection of an extrasolar planet.
As the Heliospheric Imager stares at the space between the Earth and Sun, it has made many novel observations. The device first opened its shutters in 2006 the instrument has observed the interaction of CMEs with the atmosphere of Venus, the stripping of a tail of a comet by a CME, atomic iron in a comet’s tail, and “the very faint optical emission associated with so-called Corotating Interaction Regions (CIRs) in interplanetary space, where fast-flowing Solar wind catches up with slower wind regions.”
The spacecraft allows for long periods of time to stare at patches of sky as the satellites precede and follow Earth in its orbit. The spacecraft is able to take pictures roughly every 40 minutes for almost 20 days in a row giving excellent coverage. As a result, the images taken have the potential to be used for detailed survey studies. Such information is useful for conducting variable star studies and a recent summary of findings from the mission reported the detection of 263 eclipsing variable stars, 122 of which were not previously classified as such.
Another type of variable star observed by the STEREO HI, was the cataclysmic sort, in particular, V 471 Tau. This red giant/white dwarf binary in the Hyades star cluster is a strong source of interest for stellar astrophysicists because the system is suspected to be a strong candidate for a type Ia supernova as the red giant dumps mass onto its high mass, white dwarf companion. The star system is extremely erratic in its light output and observations could help astronomers understand how such systems evolve.
Although planetary hunting is at the very edge of the capabilities of the HI’s limitations, eclipses caused by planet sized objects are feasible for many of the brighter stars in the field of view as dim as approximately 8th magnitude. Around one star, HD 213597, the STEREO team reported the detection of an object that seems too small to be a star based on the light curve alone. However, follow up studies will be necessary to pin down the object’s mass more accurately.
[/caption]Early into the celebration of its centennial year, observers of the American Association of Variable Star Observers (AAVSO) passed another milestone over the weekend, when an amateur astronomer from Belgium contributed the 20 millionth observation of a variable star on February 19, 2011.
Amateur astronomers have been recording changes in the brightness of stars for centuries. The world’s largest database is run by the AAVSO. Started in 1911, it is one of the oldest, continuously operating citizen science projects in the world.
“The long-term study of stellar brightness variation is critical to understanding how stars work and the impact they have on their surroundings. The noble efforts of the engaged AAVSO volunteers play an important role in astronomy and help expand human knowledge,” said Dr. Kevin Marvel, Executive Officer of the American Astronomical Society.
The AAVSO currently receives variable star brightness estimates from about 1,000 amateur astronomers per year. Some variable stars are bright enough to be seen with the unaided eye while others require high-tech equipment. The AAVSO also has a network of robotic telescopes available to members free of charge.
“Because some variable stars are unpredictable and/or change their brightness over long time scales, it is not practical for professional astronomers to watch them every night. Thus, amateurs were recruited to keep tabs on these stars on behalf of professionals,” Dr. Arne Henden, Director of the AAVSO, said.
The 20 millionth observation was made by Dr. Franz-Josef “Josch” Hambsch of Belgium. The observation was of GV Andromeda, member of a class of older, pulsating stars smaller than our Sun. “I like these stars because you can see their entire variation cycle in one night. There have not been many recent observations made of this particular star, so that is why I am monitoring it,” Hambsch said. Hambsch is also a member of the Belgian variable star organization, Vereniging Voor Sterrenkunde, Werkgroep Veranderlijke Sterren (VVS, WVS).
The process of estimating a star’s brightness can range from less than a minute to many hours per estimate, but typically takes about five minutes. At that rate, observers have invested the equivalent of about 1.67 million hours of time in collecting observations for the database. Assuming a current median salary of US$33,000, this would be the roughly equivalent to 27.5 million dollars worth of donated time if all the observations were reported today.
“The reality is these observations are invaluable. The database spans many generations and includes data that cannot be reproduced elsewhere. If an astronomer wants to know the history of a particular star, they come to the AAVSO,” Henden said.
The AAVSO’s mission is to coordinate, collect, and distribute variable star data to support scientific research and education. The AAVSO International Database is openly available to the public through their web site (www.aavso.org), where it is queried hundreds of times per day.
As the “V” in the designation of V445 Puppis indicates, this star was a variable star located in the constellation of Puppis. It was a fairly ordinary periodic variable, although with a rather complex light curve, but still showing a distinct periodicity of about fifteen and a half hours. It wasn’t especially bright, yet something seemed to tug at my memory regarding the star’s name as I scanned through articles to write on. Just over a year ago, Nancy wrote a post on V445 Puppis stating it’s a supernova just waiting to happen. A new article challenges this claim.
In December of 2000, V445 Puppis underwent an unusual nova. It was first noticed on December 30th, but archival records showed the eruption began in early November of that year and reached a peak brightness on November 29th. The system was known to be a binary star system with a shared envelope in which the primary star was a white dwarf and thus, a nova was the most readily available explanation.
However, this wasn’t a normal nova. Spectroscopic observations early the next year showed the ejecta lacked the helium emission seen in classical novae in which hydrogen piles up on a white dwarf surface until it undergoes fusion into helium. Instead, astronomers saw lines of iron, calcium, carbon, sodium, and oxygen expanding at nearly 1,000 km/sec. This fit better with a proposed type of explosion where, instead of hydrogen collecting on the dwarf’s surface, it was helium and the eruption seen was a helium flash in which it was helium that underwent fusion. Slowly the star faded, and debris from the eruption cooled to form dust. Today, the star itself is completely obscured in the visible portion of the spectrum.
The 2009 paper by Woudt, Steeghs, and Karowska that Nancy cited, suggested accretion might continue until the white dwarf passed the Chandrasekhar limit and exploded as a type Ia supernova. However, the authors of the new paper, led by V. P. Goranskij at Moscow University, say that this 2000 detonation has effectively ruled out that possibility because an explosion of that magnitude would likely destroy the envelope of the donor star. Their evidence for this is the very same structure Woudt noted in his paper (shown above).
While the structure looks to be bipolar in nature, other observations have suggested that there is an additional component along the line of sight and that the structure is more of a doughnut shape. In this case, the total amount of material lost is higher than originally anticipated and must have come from from the envelope of the companion star. Additionally, observations in wavelengths able to pierce the dust have been unable to resolve a strong stellar source which suggests that the donor star’s envelope has been largely blown away as well. Additionally, this large and rapid loss of mass from the system may have broken the gravitational bond between the two stars and allowed the giant star to be ejected from the system, which would also preclude the possibility of a supernova in the future.
The conclusion is that V445 Puppis is not a candidate for a supernova of any type in the future. It’s own premature fireworks have likely destroyed whatever chance it may have had for an even grander show in the future.
November 23rd, astronomers from the Asiago Novae and Symbiotic Stars collaboration announced recent changes in the symbiotic variable star, AX Persei, could indicate the onset of a rare eruption of this system. The last major eruption took place between 1988 and1992. In the (northern hemisphere) spring of 2009, AX Per underwent a short outburst that was the first time since 1992 this star had experienced a bright phase. Now AX Per is on the rise again. This has tempted astronomers to speculate that another major eruption could be in the making.
Symbiotic variable stars are binary systems whose members are a hot compact white dwarf in a wide orbit around a cool giant star. The orbital periods of symbiotic variables are between 100 and 2000 days. Unlike dwarf novae, compact binaries whose periods are measured in hours, where mass is transferred directly via an accretion disk around the white dwarf, siphoned directly from the surface of the secondary, in symbiotic variables the pair orbit each other far enough away that the mass exchanged between them comes from the strong stellar wind blowing off the red giant. Both stars reside within a shared cloud of gas and dust called a common envelope.
When astronomers look at the spectra of these systems they see a very complex picture. They see the spectra of a hot compact object superimposed on the spectra of a cool giant star tangled up with the spectrum of the common envelope. The term “symbiotic” was coined in 1941 to describe stars with this combined spectrum.
Typically, these systems will remain quiescent or undergo slow, irregular changes in brightness for years at a time. Only occasionally do they undergo large outbursts of several magnitudes. These outbursts are believed to be caused either by abrupt changes in the accretion flow of gas onto the primary, or by the onset of thermonuclear burning of the material piled up on the surface of the white dwarf. Whatever the cause, these major eruptions are rare and unpredictable.
AX Per underwent a short-duration flare about one year before the onset of the major 1988-1992 outburst. Now astronomers are tempted to speculate. Could the 2009 short outburst be a similar precursor type event? The present rise in brightness by AX Per might be the onset of a major outburst event similar to that in 1988-1992. The watch begins now, and professional and amateur variable star observers will be keeping a close eye on AX Per in the coming months.
Ranging from 8.5 to 13th magnitude, AX Persei is visible to anyone with an 8-inch telescope, and if it erupts to maximum it will be visible in binoculars. You can monitor this interesting star and report your observations to the American Association of Variable Star Observers (AAVSO). Charts with comparison stars of known brightness can be plotted and printed using the AAVSO’s Variable Star Chart Plotter, VSP.
Cepheid variable stars – a class of stars that vary in brightness over time – have long been used to help measure distances in our local region of the Universe. Since their discovery in 1784 by Edward Pigott, further refinements have been made about the relationship between the period of their variability and their luminosity, and Cepheids have been closely studied and monitored by professional and amateur astronomers.
But as predictable as their periodic pulsations have become, a key aspect of Cepheid variables has never been well-understood: their mass. Two different theories – stellar evolution and stellar pulsation – have given different answers as to the masses that these stars should be. What has long been needed to correct this error was a system of eclipsing binary stars that contained a Cepheid, so that the orbital calculations could yield the mass of the star to a high degree of accuracy. Such a system has finally been discovered, and the mass of the Cepheid it contains has been calculated to within 1%, effectively ending a discrepancy that has persisted since the 1960s.
The system, named OGLE-LMC-CEP0227, contains a classical Cepheid variable (as opposed to a Type II Cepheid, which is of lower mass and takes a different evolutionary track) that varies over 3.8 days. It is located in the Large Magellanic Cloud, and as the stars orbit each other over a period of 310 days, they eclipse each other from our perspective on Earth. It was detected as part of the Optical Gravitational Lensing Experiment, and you can see from the acronym soup that this yields the first part of the name, the Large Magellanic Cloud the second, and CEP stands for Cepheid.
A team of international astronomers headed by Grzegorz Pietrzynski of Universidad de Concepción, Chile and Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland measured the spectra of the system using the MIKE spectrograph at the 6.5-m Magellan Clay telescope at the Las Campanas Observatory in Chile and the HARPS spectrograph attached to the 3.6-m telescope of the European Southern Observatory at La Silla.
The team also measured the changes in brightness and slight red and blueshift of the light from the stars as they orbited each other, as well as the pulsing of the Cepheid. By taking all of these measurements, they were able to create a model of the masses of the stars that should yield the orbital mechanics of the system. In the end, the mass predicted by stellar pulsation theory agreed much more with the calculated mass than that predicted by stellar evolution theory. In other words, stellar pulsation theory FTW!!
They published their results today in a letter to Nature, and write in the conclusion of the letter: “The overestimation of Cepheid masses by stellar evolution theory may be the consequence of significant mass loss suffered by Cepheids during the pulsation phase of their lives – such loss could occur through radial motions and shocks in the atmosphere. The existence of mild internal core mixing in the main-sequence progenitor of the Cepheid, which would tend to decrease its evolutionary mass estimate, is another possible way to reconcile the evolutionary mass of Cepheids with their pulsation mass.”
Cepheid variables take their names from the star Delta Cephei (in the constellation Cepheus), which was discovered by John Goodricke to be a variable star a few months after Pigott’s discovery in 1784. There are many different types of variable stars, and if you are interested in learning more or even participating in observing and recording their variability, the American Association of Variable Star Observers has a wealth of information.
In 1937, an ordinary 16th magnitude star in the constellation Orion began to brighten steadily. Thinking it was a nova, astronomers were astounded when the star just kept getting brighter and brighter over the course of a year. Most novae burst forth suddenly and then begin to fade within weeks. But this star, now glowing at 9th magnitude, refused to fade. Adding to the puzzle, astronomers could see there was a gaseous nebula nearby shining from the reflected light of this mysterious star, now named FU Orionis. What was this new kind of star?
FU Ori has remained in this high state, around 10th magnitude ever since. Because this was a form of stellar variability never seen before and there were no other examples of this behavior, astronomers were forced to learn what they could from the only known example, or wait for another event to provide more clues.
Finally, more than 30 years later, FU Ori-like behavior appeared again in 1970 when the star now known as V1057 Cyg increased in brightness by 5.5 magnitudes over 390 days. Then in 1974, a 3rd example was discovered when V1515 Cyg rose from 17th magnitude to 12th magnitude over an interval lasting years. Astronomers began piecing the puzzle together from these clues.
FU Orionis stars, commonly called FUOrs, are pre-main sequence stars in the early stages of stellar development. They have only just formed from clouds of dust and gas in interstellar space, which occur in active star- forming regions. They are all associated with reflection nebulae, which become visible as the star brightens.
Astronomers are interested in these systems because FUOrs may provide us with clues to the early history of stars and the formation of planetary systems. At this early stage of evolution, a young stellar object (YSO) is surrounded by an accretion disk, and matter is falling onto the outer regions of the disk from the surrounding interstellar cloud. Thermal instabilities, most likely in the inner portions of the accretion disk, initiate an outburst and the young star increases its luminosity. Our Sun probably went through similar events as it was developing.
One of the major challenges in studying FU Orionis stars is the relatively small number of known examples. Although approximately 20 FU Orionis candidates have been identifed, only a handful of these stars have been observed to rise from their pre-outburst state to their eruptive state.
Now, in the last year, several new FUOrs have been discovered. In November 2009, two newly discovered objects were announced. Patrick Wils, John Greaves and the Catalina Real-time Transient Survey (CRTS) collaboration had discovered them in CRTS images.
The first of these objects appeared to coincide with the infrared source IRAS 06068-0641 in Monoceros. Discovered on Nov. 10, it had been continuously brightening from at least early 2005, when it was magnitude 14.8, to its present 12.6 magnitude. A faint cometary reflection nebula was visible to the east. A spectrum taken with the SMARTS 1.5-m telescope at Cerro Tololo, on Nov. 17, confirmed it to be a YSO. The object lies inside a dark nebula to the south of the Monocerotis R2 association, and is likely related to it.
Also inside this dark nebula, a second object, coincident with IRAS 06068-0643, had been varying between mag 15 and 20 over the past few years, much like UX-Ori-type objects with very deep fades. This second object is also associated with a variable cometary reflection nebula, extending to the north.
Light curves, spectra and images can be found here.
Then, in August 2010, two new eruptive, pre-main sequence stars were discovered in Cygnus. The first object was an outburst of the star HBC 722. The object was reported to have risen by 3.3 magnitudes from May 13 to August 16, 2010. Spectroscopy reported by Ulisse Munari on August 23rd, support this object’s classification as an FU Ori star. Munari and his team reported the object at 14.04V on Aug 21, 2010.
The second object, coincident with another infrared source, IRAS 20496+4354, was discovered by K. Itagaki of Yamagata, Japan, on August 23, 2010. The object appears very faint, approximately magnitude 20, in a Digital Sky Survey image taken in 1990. Subsequent spectroscopy and photometry of this object by Munari showed that this object also has the characteristics of an FU Ori star. Munari reported the object at 14.91V on August 26, 2010.
Both these objects are now the subjects of an AAVSO observing campaign announced October 1, 2010 in AAVSO Alert Notice 425. Dr. Colin Aspin, University of Hawai’i, has requested the help of AAVSO observers in performing long-term photometric monitoring of these two new YSOs in Cygnus. AAVSO observations will be used to help calibrate optical and near-infrared spectroscopy to be obtained during the next year.
Since these stars are newly discovered, very little is known about their behavior. Their classification as FU Ori variables is based on spectroscopy, but establishing a good optical light curve and maintaining it, over the next several years, will be crucial to understanding these stars. This kind of long-term monitoring is one of the things at which amateur astronomers excel.
So after a very slow start, discoveries of new YSOs and our understanding of the dusty disk environments around them are starting to heat up. With new tools and new examples to study we are peering into the early stages of stellar and planetary formation and finding some of our models have been pretty close to the truth. We expect to find more and similar objects as new all-sky surveys begin to cover the sky, but these objects will still be relatively rare and therefore interesting, because this period in a star’s evolution is short-lived and only takes place in the active star forming regions of galaxies.
In my article two weeks ago, I discussed how data mining large surveys through online observatories would lead to new discoveries. Sure enough, a pair of astronomers, Ivan Zolotukhin and Igor Chilingarian using data from the Virtual Observatory, has announced the discovery of a cataclysmic variable (CV).
Cataclysmic variables are often called “novae”. However, they’re not a single star. These stars are actually binary systems in which their interactions cause large increases in brightness as matter is accreted from a secondary (usually post main-sequence) star, onto a white dwarf. The accretion of matter piles up on the surface until the it reaches a critical density and undergoes a brief but intense phase of fusion increasing the brightness of the star considerably. Unlike type Ia supernovae, this explosion doesn’t meet the critical density required to cause a core collapse.
The team began by considering a list of 107 objects from the Galactic Plane Survey conducted by the Advanced Satellite for Cosmology and Astrophysics (ASCA, a Japanese satellite operating in the x-ray regime). These objects were exceptional x-ray emitters that had not yet been classified. While other astronomers have done targeted investigations of individual objects requiring new telescope time, this team attempted to determine whether any of the odd objects were CVs using readily available data from the Virtual Observatory.
Since the objects were all strong x-ray sources, they all met at least one criteria of being a CV. Another was that CV stars often are strong emitters for Hα since the eruptions often eject hot hydrogen gas. To analyze whether or not any of the objects were emitters in this regime, the astronomers cross referenced the list of objects with data from the Isaac Newton Telescope Photometric Hα Survey of the northern Galactic plane (IPHAS) using a color-color diagram. In the field of view of the IPHAS survey that overlapped with the region from the ASCA image for one of the objects, the team found an object that emitted strongly in the Hα. But in such a dense field and with such different wavelength regimes, it was difficult to identify the objects as the same one.
To assist in determining if the two interesting objects were indeed the same, or whether they just happened to lie nearby, the pair turned to data from Chandra. Since Chandra has much smaller uncertainty in the positioning (0.6 arcsecs), the pair was able to identify the object and determine that the interesting object from IPHAS was indeed the same one from the ASCA survey.
Thus, the object passed the two tests the team had devised for finding cataclysmic variables. At this point, followup observation was warranted. The astronomers used the 3.5-m Calar Alto telescope to conduct spectroscopic observations and confirmed that the star was indeed a CV. In particular, it looked to be a subclass in which the primary white dwarf star had a strong enough magnetic field to disrupt the accretion disk and the point of contact is actually over the poles of the star (this is known as a intermediate polar CV).
This discovery is an example of how discoveries are just waiting to happen with data that’s already available and sitting in archives, waiting to be explored. Much of this data is even available to the public and can be mined by anyone with the proper computer programs and know-how. Undoubtedly, as organization of these storehouses of data becomes organized in more user friendly manners, additional discoveries will be made in such a manner.
An alert was raised March 11 when Japanese amateur astronomers announced what might have been the discovery of a new 8th magnitude nova in the constellation of Cygnus. It was soon realized that this eruption was not what it appeared to be. It was actually the unexpected nova-like erruption of a known variable star, V407 Cygni. Typically varying between 12th and 14th magnitude, V407 Cyg is a rather mundane variable star. So what caused this well-behaved star to suddenly go ballistic?
V407 Cyg is a symbiotic variable. These are close, interacting binary pairs usually containing a red giant and a hotter, smaller white dwarf. They orbit a common center of gravity inside a shared nebulosity. A typical symbiotic variable consists of an M type giant transferring matter to a hot white dwarf via its stellar wind. This wind is ionized by the white dwarf, giving rise to the symbiotic nebula.
Symbiotic variables are complex systems with many sources of variability. They can vary periodically due to the binary motion, the red giant can vary due to pulsation, the stars may be obscured by circumstellar dust, or the light emitted my change due to the formation of giant star spots. The white dwarf component may glow more or less constantly as it accretes material from the red giant and heats it up at a steady rate, or the material may form an accretion disk around the white dwarf, like in dwarf novae. Mass accreted onto the white dwarf can result in flickering and quasi-periodic oscillations. If there is a sudden increase in the rate of accretion, or the material in the accretion disk reaches a point of instability and crashes down onto the surface of the white dwarf the symbiotic system may undergo a nova-like eruption.
About 20% of symbiotics consist of a Mira-type variable as the giant of the pair. These binaries reside in much dustier envelopes. V407 Cyg is one of these dusty, Mira-type symbiotics. Its typical variation of a few magnitudes is due mainly to the pulsation of the Mira component of the system. Astronomers had never before witnessed a nova-like outburst of this interacting binary. You can imagine their surprise when Japanese amateurs, searching for novae along the galactic plane, suddenly detected this mild mannered, dusty Mira, symbiotic variable glowing nearly 100 times brighter than ever before.
That was just the beginning of the story. The first new spectra taken of the system, on March 13th, was different from any ever recorded for this star or any other symbiotic Mira variable in outburst. The normal absorption spectra of the Mira star was completely overwhelmed by the blue continuum of the outbursting white dwarf. The characteristics of the emission spectra revealed two distinct types of activity. One was the relatively slow ionized wind of the Mira star. The other looked like the fast expanding ejecta of a nova outburst. In fact, the spectrum looked remarkably similar to the symbiotic recurrent novae, RS Ophiuchi.
Typical outbursts of known symbiotic binaries, and symbiotic Miras in particular, usually exhibit a very slow rise to maximum, taking months, and no real significant mass ejection. This appears to be a much more quickly evolving and violent event, more like the eruptions of the recurrent novae RS Oph and T CrB. V407 Cyg may join this rare class of symbiotic recurrent novae.
As if that weren’t enough, another twist was added to the story on March 19th, when the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope detected the star in gamma-rays, something never observed in a symbiotic system before. The gamma-rays could be caused by shock driven acceleration of the ejected material, and its capture by strong magnetic fields within the system.
Like many novae and recurrent novae outbursts, this eruption may last for weeks or months and the variation in light output could be quite complex and interesting. Because the giant secondary is losing mass, the system is likely to have a large amount of circumstellar material. The ejected shell from the nova explosion on the white dwarf will interact with this material as the shell propagates outward, and will likely produce a wide variety of variable phenomena.
V407 Cyg has our attention now, and professional and amateur astronomers will be keeping a close eye on it from now on.
From the left: Klaus Beuermann (group leader), Jens Diese (back,teacher), and the high-school students Joshua Zachmann (front), Alexander-Maria Ploch (back), Sang Paik (front). JD, JZ, and AMP are from the Max-Planck-Gymnasium, SP is from the Felix-Klein-Gymnasium.
High school students from Germany have now done what many scientists strive for: had their research work published by a science journal. The Astronomy & Astrophysics science journal published a paper co-authored by three students who observed the light variations of the faint (19th magnitude) cataclysmic variable EK Ursae Majoris (EK UMa) over two months. Led by astronomer Klaus Beuermann from the University of Göttingen, and the students’ high school physics teacher, the team made use of a remotely-controlled 1.2-meter telescope in Texas. Astronomy & Astrophysics says the team “presents an accurate, long-term ephemeris,” and that “they participated in all the steps of a real research program, from initial observations to the publication process, and the result they obtained bears scientific significance.”
The students, Joshua Zachmann, Alexander-Maria Ploch, Sang Paik and their teacher, Jens Diese, made observations, analyzed the CCD images, produced and interpreted light curves, and looked at archival satellite data. Beuermann, the astronomer they worked with said, “Although it is fun to perform one’s own remote observations with a professional telescope from the comfort of a normal school classroom, it is even more satisfying to be involved in a project that provides new and publishable results rather than to perform experiments with predictable outcomes.”
Cataclysmic variable research is a field where the contributions of small telescopes has a long tradition. Cataclysmic variables are extremely close binary systems containing a low-mass star whose material is being stripped off by the gravitational pull of a white dwarf companion. Due to the transfer of matter between the stars, these systems vary dramatically in brightness on timescales in the whole range between seconds and years. This largely unpredictable variability makes them ideal targets for school projects, particularly since professional observatories are generally unable to provide enough observation time for regular monitoring.
An accurate ephemeris is needed to keep track of the orbital motions of the two stars, but none was available because EK UMa is faint in the optical range and requires a long-term observation of the light variations. The strong magnetic field of the white dwarf turns the light of the hot matter striking the surface of the white dwarf into two “lighthouse” beams. By measuring the times of the minimum between the beams, the group was able to determine an orbital period accurate enough to keep track of the eclipse that took place in 1985, over 100 000 cycles earlier. By combining their own measurements with those made by the Einstein, ROSAT, and EUVE satellites, they estimated the orbital period over 137 000 cycles to an accuracy of a tenth of a millisecond. Surprisingly, the orbital period is extremely stable, although the period of such very close binaries is expected to vary due to the presence of third bodies and magnetic activity cycles on the companion star.
The team’s paper: (not yet available) A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris, by K. Beuermann, J. Diese, S. Paik, A. Ploch, J. Zachmann, A.D. Schwope, and F.V. Hessman.
Betelgeuse is the ninth brightest star in the sky, and the second brightest in the constellation of Orion (it’s the red one, on the opposite side of the Belt from Rigel, which is the blue one, and the brightest).
With a mass of some 20 sols (= the mass of 20 Suns), Betelgeuse is evolving rapidly, even though it’s only a few million years old. It’s now a red supergiant, burning helium in a shell, and (very likely) burning carbon in another shell (closer to the nucleus), and (possibly) oxygen, silicon, and sulfur in other nested shells (like Russian dolls).
Betelgeuse is enormous … if it were where the Sun is, all four inner planets would be inside it! Because it’s so big, and is only approx 640 light-years away, Betelgeuse appears to about 1/20 of an arcsecond in size; this made it an ideal target for optical interferometry. And so it was that in 1920 Michelson and Pease used the 100″ Mt Wilson telescope, with a 20 m interferometer attached to the front, to measure Betelgeuse’s diameter.
Since the 1920s Betelgeuse has been observed, from the ground, by many different optical interferometers, at many wavelengths. Its diameter varies somewhat, as does its brightness (Herschel is perhaps the first astronomer to describe its variability, in 1836). It also has ‘hotspots’, which are ginormous.
Betelgeuse is also shedding mass in giant plumes that stretch to over six times its diameter. Although these plumes will certainly cause it to ‘slim down’, they won’t be enough to stop its core turning to iron (when the silicon there is exhausted, if it hasn’t already done so). Not long afterwards, perhaps within the next thousand years or so, Betelgeuse will go supernova … making it the brightest and most spectacular supernova visible from Earth in perhaps a million years. Fortunately, because we are not looking directly down on its pole, when Betelgeuse does go bang, we won’t be fried by a gamma ray burst (GRB) which may occur (while a core collapse supernova can cause one kind of GRB, it is not yet known if all such supernovae produce GRBs; in any case, such a GRB is one of a pair of jets which rip through the poles of the dying star).
AAVSO has an excellent article on Betelgeuse, and COAST’s (Cambridge Optical Aperture Synthesis Telescope) webpage on its observations of Betelgeuse gives a good summary of one interferometric technique (and some great images too!).