The Furor over FUORs

FU Orionis and its associated nebula. Image cedit: ESO


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.

This artist's concept shows a young stellar object and the whirling accretion disk surrounding it. NASA/JPL-Caltech

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.

Unprecedented Eruption Catches Astronomers By Surprise

Artists rendering of a symbiotic recurrent nova. Image credit: David A. Hardy & PPARC


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.

Aurora Watch! Valentine’s Day Sun-Earth Weather Alert

Image: Wikimedia Commons


Sky watchers observing from high latitude areas may be in for a treat tonight. Forecasters from the National Oceanic and Atmospheric Administration have predicted a 30-35% chance of geo-magnetic storm activity during the late hours of February 14-15. Activity could reach as far south as Michigan, Minnesota, and Wisconsin. The source of this potential light show is a solar wind stream flowing energetically from a coronal hole on the Sun. If you’re lucky, the night sky where you live could look like this for Valentine’s Day night.

The coronal holes in this image are the two dark spots at approximately 4 o'clock. Image credit: SOHO/MDI

Coronal holes are darker, colder areas of the Sun’s corona that have lower-density plasma than elsewhere in the Sun’s outer atmosphere. Coronal holes are areas where the fast-moving component of the solar wind is known to pass through as it escapes into space.

When energy from the Sun interacts with the Earth’s outer atmosphere, it excites oxygen and nitrogen molecules some 100 and 400km above the surface, respectively, emitting a green (oxygen) or red (nitrogen) glow. This in turn excites observers on the ground, who may see the auroral dance take on any of several characteristic forms.

There may just be a glow to the north (or south if you live in the southern hemisphere), just over the horizon; or you may see arcs or bands of light, sometimes with vertical rays spiking high into the night sky. During strong events you may witness the famous curtain effect, or the coronal effect where all the rays appear to converge almost directly overhead.

So if the sky is clear tonight, take your Valentine outside, find a place to snuggle up and you may be rewarded with a light show you can share and remember for years to come.

Long Anticipated Eruption of U Scorpii Has Begun

Artists rendition of the recurrent nova RS Oph Credit: David Hardy/PPARC


Today, two amateur astronomers from Florida detected a rare outburst of the recurrent nova U Scorpii, which set in motion satellite observations by the Hubble Space Telescope, Swift and Spitzer. The last outburst of U Scorpii occurred in February of 1999. Observers around the planet will now be observing this remarkable system intensely for the next few months trying to unlock the mysteries of white dwarfs, interacting binaries, accretion and the progenitors of Type IA supernovae.

One of the remarkable things about this outburst is it was predicted in advance by Dr. Bradley Schaefer, Louisiana State University, so observers of the American Association of Variable Star Observers (AAVSO) have been closely monitoring the star since last February, waiting to detect the first signs of an eruption. This morning, AAVSO observers, Barbara Harris and Shawn Dvorak sent in notification of the outburst, sending astronomers scrambling to get ‘target of opportunity observations’ from satellites and continuous coverage from ground-based observatories. Time is a critical element, since U Sco is known to reach maximum light and start to fade again in one day.

There are only ten known recurrent novae (RNe). This, coupled with the fact that eruptions may occur only once every 10-100 years, makes observations of this rare phenomenon extremely interesting to astronomers. Recurrent novae are close binary stars where matter is accreting from the secondary star onto the surface of a white dwarf primary. Eventually this material accumulates enough to ignite a thermonuclear explosion that makes the nova eruption. ‘Classical novae’ are systems where only one such eruption has occurred in recorded history. They may indeed have recurrent eruptions, but these may occur thousands or millions of years apart. RNe have recurrence times of 10-100 years.

The difference is thought to be the mass of the white dwarf. The white dwarf must be close to the Chandrasekhar limit, 1.4 times the mass of the Sun. This higher mass makes for a higher surface gravity, which allows a relatively small amount of matter to reach the ignition point for a thermonuclear runaway. White dwarfs in RNe are thought to be roughly 1.2 times solar, or greater. The rate at which mass is accreted onto the white dwarf must be relatively high also. This is the only way to get enough material accumulated onto the white dwarf in such a short time, as compared to classical novae.

Recurrent novae are of particular interest to scientists because they may represent a stage in the evolution of close binary systems on their way to becoming Type IA supernovae. As mass builds up on the white dwarf they may eventually reach the tipping point, the Chandrasekhar limit. Once a white dwarf exceeds this mass it will collapse into a Type IA supernova.

A problem with this theory is the mass that is blown off the white dwarf in the erruption. If more mass is ejected during an eruption than has accreted during the previous interval between eruptions, the white dwarf will not be gaining mass and will not collapse into a Type IA supernovae. Therefore, scientists are eager to obtain all the data they can on these eruptions to determine what is happening with the white dwarf, the mass that is ejected and the rate of accretion.

AAVSO comparison sequence chart for U Sco

Observations from amateur astronomers are requested by the AAVSO. Data from backyard telescopes will be combined with data from mountaintop observatories and space telescopes to help unravel the secrets of these rare systems. AAVSO finder charts with comparison star sequences are available at:

Kepler Goes Fishing and Reels in Two KOI

Sizes and temperatures of Kepler discoveries compared to Earth and Jupiter

By now you’ve probably heard about the first results from the Kepler mission to find extrasolar planets. Five new exoplanet discoveries were announced recently at the American Astronomical Society meeting in Washington, D.C. Four of the five new planets are larger than Jupiter, about 1.4 times its radius, and they all have orbital periods around their host stars of 3-5 days. Kepler 4b, the oddball of that bunch is about the size of Neptune.


All of them are close to their parent stars, so they have high surface temperatures, 1500-1800K or 2240-2780 degrees F. None of this is very surprising. We knew Kepler could detect extrasolar planet transits, and the first ones were liable to be fairly large and close in. These are the easiest to detect in the shortest amount of time.

KOI- Planet, star or fish?

What is even more interesting to me, is the discovery of two so-called Kepler Objects of Interest (KOI). What is so interesting about KOI?

First, they were discovered because when they disappear behind the parent star from our point of view the light from the system is dimmed dramatically. I mean a lot! Usually this phase is known as the secondary eclipse, and is much less noticeable than the primary eclipse of the star as the planet transits across the face of the star. The fact that the light was dimmed more by the disappearance of the KOI means they are blazing hot and emitting a lot of light on their own. Astronomers estimate the temperature for KOI-74b to be 12250K (21590F) and KOI-81b a blazing 13500K (23840F). The hottest known exoplanet to date is a mild 2300K (3700F) in comparison.

Both KOI are actually hotter than their host stars. KOI-74b companion star is an A1V type star with a surface temperature around 9400K. KOI-81b has a companion type B9-A0V, with a temperature of approximately 10000K.

KOI-74b and KOI-81b are not massive enough to be stars, with solar masses of 0.111 and 0.212 respectively. That’s just not massive enough to start nuclear burning in the core. Yet each object is far too hot to only be shining by heat absorbed from its companion star and then re-emitted into space. This may mean they have evolved from hot stars into their current state, and they are slowly cooling with time. The problem with that theory is, with host stars that are relatively young type A and B stars, there doesn’t appear to have been enough time for these to have evolved from massive, hot stars to the state they are in now.

So what are they? They are the first of what will undoubtedly be a host of newly discovered objects from the Kepler mission. We live in exciting times for astronomy. We are now going fishing with new flies in the tackle box, and we have no idea what other KOI we’ll pull out of the next round of data.

Surf’s Up! Astronomers Ride Stellar Waves

Astronomers peer inside stars by interpreting the waves they create


This week, first results from the Kepler mission are coming out in waves from the meeting of the American Astronomical Society (AAS) in Washington, DC. Carried along on those waves are papers on waves in stars. I’m referring to a branch of astronomy you’ll be hearing more about as Kepler and other missions begin to reveal the interior structures of stars- asteroseismology. So, what is asteroseismology?

Seismology is the study of earthquakes on Earth. But more importantly to our discussion, it is the study of seismic waves. Earthquakes produce different types of seismic waves that travel through different layers of rock, providing us with a way to image structures deep within the Earth. Essentially, large earthquakes provide us with a natural sonogram to look inside the Earth, far deeper then we can tunnel or drill. Since these waves propagate all the way from one side of the planet to the other we can look all the way to the center of the Earth. This is how we know the outer core of the Earth is liquid, and the relative dimensions and densities of the other parts of the Earth’s internal and surface structure.

Copyright Nick Strobel

Asteroseismology, also known as stellar seismology, gives us the same kind of insight into the structure of stars. By studying the oscillations in pulsating stars, astronomers can peer into the very hearts of stars, one of the most difficult places to observe in the entire universe. The reason stellar interiors can be probed from oscillations is that different oscillation modes penetrate to different depths inside the star. Combining the rate, and amplitude of pulsation with other information, such as spectra, which reveals what the composition of the star is we obtain information on the internal structure of stars.

Stellar oscillation modes are divided into three categories, based on the force that drives them: acoustic, gravity, and surface-gravity wave modes. p-mode, or acoustic waves, have pressure as their force, hence the name “p-mode”. These waves can tell us things about the structure and density of regions below the surface of a star. g-mode, or gravity waves, are confined to the interior of the star. f-mode, or surface gravity waves are also gravity waves, but occur at or near the outer layers of stars, so they give us information about the surface conditions of stars.

Helioseismology is the study of the propagation of wave oscillations in the Sun. Since the Sun is the closest star to us, it is much easier to study its pulsations in greater detail. By interpreting solar oscillations we can even detect sunspots on the far side of the Sun before they rotate into view. Many of our models of stellar interiors are based on information gained through studying the Sun’s oscillations. But the Sun is only one star at one point in its evolution, so to really understand stars we need to observe many more stars of different size, mass, composition and age.

Kepler stares at a portion of the sky taking hi-precision photometric data

That is precisely what Kepler is doing right now. The satellite is staring at a 100 square degree section of the sky between Cygnus and Lyra continuously taking data on the brightness of over 150,000 stars for the next three to five years. While Kepler’s primary mission is to discover the existence and abundance of earth-like planets around stars, all this high precision photometry will be used for other science, especially studying variable stars of all types and performing asteroseismology on stars showing solar-like oscillations.

The much-anticipated release of the first science results from the Kepler mission January 4th, included numerous papers on asteroseismology and the potential for understanding stellar structure in unprecedented detail. Astronomers are riding the new wave of information on wave propagation in stars. Surf’s up!

Further reading:

The asteroseismic potential of Kepler: first results for solar-type stars
W. J. Chaplin, T. Appourchaux, Y. Elsworth, et al

Solar-like oscillations in low-luminosity red giants: first results from Kepler
T. R. Bedding, D. Huber, D. Stello, et al

Kepler Asteroseismology Program: Introduction and First Results
Ronald L. Gilliland, T. M. Brown, J. Christensen-Dalsgaard

Eta Carinae- A Naked Eye Enigma

Credit: X-ray: NASA/CXC/GSFC/M.Corcoran et al.; Optical: NASA/STScI


Eta Carinae is a beast of a star. At more than 100 solar masses and 4 million times the luminosity of our Sun, eta Car balances dangerously on the edge of stellar stability and it’s ultimate fate: complete self-destruction as a supernova. Recently, Hubble Space Telescope observations of the central star in the eta Carinae Nebula have raised an alert on eta Car among the professional community. What they discovered was totally unexpected.

“It used to be, that if you looked at eta Car you saw a nebula and then a faint little core in the middle” said Dr. Kris Davidson, from the University of Minnesota. “Now when you look at it, it’s basically the star with a nebula. The appearance is completely different. The light from the star now accounts for more than half the total output of eta Car. I didn’t expect that to happen until the middle of this century. It’s decades ahead of schedule. We know so little about these very massive objects, that if eta Car becomes a supernova next Thursday we should not be very surprised.”

In 1843, eta Carinae underwent a spectacular eruption, making it the second brightest star in the sky behind Sirius. During this violent episode, eta Car ejected 2 to 3 solar masses of material from the star’s polar regions. This material, traveling at speeds close to 700 km/s, formed two large, bipolar lobes, now known as the Homunculus Nebula. After the great eruption, Eta Car faded, erupted again briefly fifty years later, then settled down, around 8th magnitude. Davidson picks up the story from there.

This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.
This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.

“Around 1940, Eta suddenly changed its state. The spectrum changed and the brightness started to increase. Unfortunately, all this happened at a time when almost no one was looking at it. So we don’t know exactly what happened. All we know is that by the 1950’s, the spectrum had high excitation Helium lines in it that it didn’t have before, and the whole object, the star plus the Homunculus, was gradually increasing in brightness. In the past we’ve seen three changes of state. I suspect we are seeing another one happening now.”

During this whole time eta Car has been shedding material via its ferocious stellar winds. This has resulted in an opaque cloud of dust in the immediate vicinity of the star. Normally, this much dust would block our view to the star. So how does Davidson explain this recent, sudden increase in the luminosity of eta Carinae?

“The direct brightening we see is probably the dust being cleared away, but it can’t be merely the expansion of the dust. If it’s clearing away that fast, either something is destroying the dust, or the stellar wind is not producing as much dust as it did before. Personally, I think the stellar wind is decreasing, and the star is returning to the state it was in more than three hundred years ago. In the 1670’s, it was a fourth magnitude, blue, hot star. I think it is returning to that state. Eta Carinae has just taken this long to readjust from its explosion in the 1840’s.”

After 150 years what do we really know about one of the great mysteries of stellar physics? “We don’t understand it, and don’t believe anyone who says they do,” said Davidson.  “The problem is we don’t have a real honest-to-God model, and one of the reasons for that is we don’t have a real honest-to-God explanation of what happened in 1843.”

Can amateur astronomers with modest equipment help untangle the mysteries of eta Carinae? Davidson think so, “The main thing is to make sure everyone in the southern hemisphere knows about it, and anyone with a telescope, CCD or spectrograph should have it pointed at eta Carinae every clear night.”