Betelgeuse, also known by its Bayer designation Alpha Orionis (α Orionis, α Ori), is theninth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel (Beta Orionis) only rarely. Distinctly reddish-tinted, it is asemiregular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any first magnitude star. The star marks the upper right vertex of the Winter Triangleand center of the Winter Hexagon.
Classified as a red supergiant, Betelgeuse is one of the largest and most luminous stars known. If it were at the center of our Solar System, its surface would extend past the asteroid belt possibly to the orbit of Jupiter and beyond, wholly engulfing Mercury, Venus, Earth andMars. However, with distance estimates in the last century that have ranged anywhere from 180 to 1,300 light years from Earth, calculating its diameter, luminosity and mass have proven difficult. Betelgeuse is currently thought to lie around 640 light years away, yielding a meanabsolute magnitude of about −6.05.
In 1920, Alpha Ori was the first star (after the Sun) to have its angular diameter measured. Since then, researchers have used a number of telescopes to measure this stellar giant, each with different technical parameters, often yielding conflicting results. Current estimates of the star's diameter range from about .043 to .056 arcseconds, a moving target at best as Betelgeuse appears to change shape periodically. Because of limb darkening, variability, and angular diameters that vary with wavelength, the star remains a perplexing mystery. To complicate matters further, Betelgeuse has a complex, asymmetric envelope caused by colossal mass loss involving huge plumes of gas being expelled from its surface. There is even evidence of stellar companions orbiting within this gaseous envelope, possibly contributing to the star's eccentric behavior.
Astronomers believe Betelgeuse is only 10 million years old, but has evolved rapidly because of its high mass. It is thought to be a runaway star from the Orion OB1 Association, which also includes the late type O and B stars in Orion's belt—Alnitak, Alnilam and Mintaka. Currently in a late stage of stellar evolution, Betelgeuse is expected to explode as a type II supernova, possibly within the next million years.
The 1970s saw several notable advances in interferometry from the Berkeley Space Sciences Laboratory working in the infrared and Antoine Labeyrie in the visible, when researchers began to combine images from multiple telescopes and later invented "fringe-tracking" technology. But it was not until the late 1980s and early 1990s, when Betelgeuse became a regular target for aperture masking interferometry that significant breakthroughs occurred in visible-light and infrared imaging. Pioneered by John E. Baldwin and other colleagues of the Cavendish Astrophysics Group, the new technique contributed some of the most accurate measurements of Betelgeuse to date while revealing a number of bright spots on the star's photosphere.[21][22][23] These were the first optical and infrared images of a stellar disk other than the Sun, first from ground-based interferometers and later from higher-resolution observations of the COAST telescope, with the "bright patches" or "hotspots" potentially corroborating a theory put forth by Schwarzschild decades earlier of massive convectioncells dominating the stellar surface.[24][25]
In 1995, the Hubble Space Telescope's Faint Object Camera captured an ultraviolet image of comparable resolution—the first conventional-telescope image (or "direct-image" in NASA terminology) of the disk of another star. The image was taken at ultraviolet wavelengths since ground-based instruments cannot produce images in the ultraviolet with the same precision as Hubble. Like earlier images, this ultraviolet image also contained a bright patch, indicating a hotter region of about 2,000K, in this case on the southwestern portion of the star's surface.[26]Subsequent utraviolet spectra taken with the Goddard High Resolution Spectrograph suggested that the hot spot was one of Betelgeuse's poles of rotation. This would give the rotational axis an inclination of about 20° to the direction of Earth, and a position angle from celestial North of about 55°.[27]
[edit]Recent studies
The first decade of the 21st century has witnessed major advances on multiple fronts, the most central of which have been the imaging of the star's photosphere at different wavelengths and the study of α Ori's complex circumstellar shells. At the dawn of the millennium, Betelgeuse was measured in the mid-infrared using the Infrared Spatial Interferometer (ISI) producing a limb darkenedestimate of 55.2 ± 0.5 milliarcseconds (mas)—a figure entirely consistent with Michelson's findings eighty years earlier.[17][28] At the time of its publication, the estimated parallax from the Hipparcosmission was 7.63 ± 1.64 mas, yielding an estimated radius for Betelgeuse of 3.6 AU. However, numerous interferometric studies in the near-infrared have appeared since from the Paranal Observatory in Chile arguing for much tighter diameters. Nevertheless, on June 9, 2009, NobelLaureate Charles Townes announced that the star had shrunk 15% since 1993 at an increasing rate. He presented evidence that UC Berkeley's ISI atop Mt. Wilson Observatory had observed 15 consecutive years of stellar contraction. Despite the apparent diminution of Betelgeuse's size, Townes and his colleague, Edward Wishnow, pointed out that the star's visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers (AAVSO), had shown no significant dimming over that same time frame.[29][30] This finding of a diminishing radius coupled with a relatively constant flux puts into question some of the fundamental theories of stellar structure.
Enveloping this whole discussion have been numerous inquiries into the abstruse dynamics of Betelgeuse's extended atmosphere. For decades astronomers have understood that red giants dominate mass return to the Galaxy creating opaque outer shells, yet the actual mechanics of such stellar mass loss have remained a mystery.[31] With recent advances in interferometric methodologies, astronomers may be close to resolving this conundrum. In July 2009, images released by the European Southern Observatory, taken by the ground-based Very Large Telescope Interferometer (VLTI), showed vast plumes of gas being ejected into the surrounding atmosphere with distances approximating 30 AU.[10][32] Comparable to the distance between the Sun and Neptune, this mass ejection is but one of multiple dynamics occurring in the surrounding atmosphere. Astronomers have identified at least 6 different shells surrounding Betelgeuse. As the century unfolds, solving the mystery of mass loss in the late stages of a star's evolution may reveal those factors which precipitate the explosive deaths of these stellar giants.[29]
[edit]Visibility
Betelgeuse is easy to spot in the night sky, as it appears in close proximity to the famous belt of Orion and has a distinctive orange-red color to the naked eye. In the Northern Hemisphere, beginning in January of each year, it can be seen rising in the east just after sunset. By mid-March, the star is due south in the evening sky and visible to virtually every inhabited region of the globe, with only a few obscure research stations in Antarctica at latitudes south of 82° unable to see it. In large cities in the Southern Hemisphere (e.g., Sydney, Buenos Aires, and Cape Town) the star rises almost 49° above the horizon. Once May arrives, the red giant can be glimpsed but briefly on the western horizon just after the Sun sets.
The apparent magnitude of α Ori is listed in SIMBAD at 0.42, making it on average the ninth brightest star in the celestial sphere—just ahead of Achernar. Because Betelgeuse is a variable star whose brightness ranges between 0.2 and 1.2, there are periods when it will surpass Procyonto become the eighth brightest star. As Rigel, with a nominal apparent magnitude of 0.12, has been reported to fluctuate slightly in brightness, by 0.03 to 0.3 magnitudes,[33] it is also possible for Betelgeuse to occasionally outshine Rigel and become the seventh brightest star. At its faintest, it will fall behind Deneb as the 19th brightest star and compete with Mimosa for the 20th position.
Betelgeuse has a color index (B–V) of 1.85—a figure which points to the advanced "redness" of this celestial object. The photosphere has an extended atmospherewhich displays strong lines of emission rather thanabsorption, a phenomenon which occurs when a star is surrounded by a thick gaseous envelope. This extended gaseous atmosphere has been observed moving both away from and towards Betelgeuse, depending apparently on radial velocity fluctuations in the photosphere. Only about 13% of the star's radiant energy is emitted in the form of visible light, with most of its radiation occurring in the infrared. If our eyes were sensitive to radiation at all wavelengths, Betelgeuse would appear as the brightest star in the sky.[15]
[edit]Parallax
Since the first successful parallax measurement was conducted in 1838 by Friedrich Bessel, astronomers have been puzzled by Betelgeuse's distance, the uncertainty of which has made reliable estimates of many stellar parameters difficult. An accurate distance and angular diameter will reveal a star's radius and effective temperature, leading to a clear understanding of bolometricluminosity; luminosity combined with an understanding of isotopic abundances provides an estimate of the stellar age and mass.[4] In 1920, when the first interferometric studies were performed on the star's diameter, the assumedparallax was 0.180 arcseconds. That equated to a distance of 56 parsecs (pc) or roughly 180 light years (ly) and produced not only an inaccurate radius for the star, but every other stellar characteristic. Since then, there has been an ongoing inquiry as to the actual distance of this mysterious star with proposed distances as high as 400 parsecs or about 1,300 light years.[4]
Before the publication of the Hipparcos Catalogue (1997), there were two respected publications with up-to-date parallax data on Betelgeuse. The first was the Yale University Observatory (1991) with a published parallax of π = 9.8 ± 4.7 mas, yielding a distance of roughly 102 pc or 330 ly.[35] The second was the Hipparcos Input Catalogue (1993) with a trigonometric parallax of π = 5 ± 4 mas, a distance of 200 pc or 650 ly—almost twice the Yale estimate.[36] With such uncertainty, researchers were adopting a wide range of distance estimates, a phenomenon which fueled much debate—not only in terms of the star's distance, but also its impact on other stellar parameters.[4]
The long-awaited results from the Hipparcos mission were finally released in 1997. Instead of resolving the issue, a new parallax figure was published of π = 7.63 ± 1.64 mas, which equated to a distance of 131 pc or roughly 430 ly.[37] Because stars like Betelgeuse vary in brightness, they raise specific problems in quantifying their distance.[38] As a result, the large cosmic error in the Hipparcos solution could well be of stellar origin, relating possibly to movements of the photocenter, of order 3.4 mas, in the Hipparcos photometric Hp band.[4][39]
Recent advances in radio astronomy appear to have prevailed in this debate. New high spatial resolution, multi-wavelength radio positions of Betelgeuse conducted by Graham Harper and colleagues using NRAO's Very Large Array (VLA) have produced a more precise estimate, which combined with the recent Hipparcos data furnished a new astrometric solution: π = 5.07 ± 1.10mas, a tighter error factor yielding a distance of 197 ± 45 pc or 643 ± 146 ly.[4]
The next computational breakthrough will likely come from the European Space Agency's upcoming Gaia mission when it undertakes a detailed analysis of physical properties for each star observed, revealing luminosity, temperature, gravity and composition. Gaia will achieve this by repeatedly measuring the positions of all objects down to magnitude 20, and those brighter than magnitude 15, to an accuracy of 24 microarcseconds—akin to measuring the diameter of a human hair from 1000 km away. On-board detection equipment will ensure that variable stars like Betelgeuse will all be detected to this faint limit, thus addressing most of the limitations of the earlier Hipparcos mission. The nearest stars, in fact, will have their distances measured to within an unprecedented 0.001% error factor. Even stars near the Galactic centre, some 30,000 light-years away, will have their distances measured to within a factor of 20%.[40]
[edit]Variability
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As a pulsating variable star with sub-classification "SRC", researchers have offered different hypotheses to explain α Ori's volatile choreography—a phenomenon which causes an absolute magnitude oscillation from −5.27 and −6.27.[41] Our current understanding of stellar structure suggests that the outer layers of this supergiant gradually expand and contract, causing the surface area (photosphere) to alternately increase and decrease, and the temperature to rise and fall—thus eliciting the measured cadence in the star's brightness between its dimmest magnitude of 1.2, seen as early as 1927, and its brightest of 0.2, seen in 1933 and 1942. A red supergiant like Betelgeuse will pulsate this way because its stellar atmosphere is inherently unstable. As the star contracts, it absorbs more and more of the energy that passes through it, causing the atmosphere to heat up and expand. Conversely, as the star expands, its atmosphere becomes less dense allowing the energy to escape and the atmosphere to cool, thus initiating a new contraction phase.[14] Calculating the star's pulsations and modeling its periodicity have been difficult, as it appears there are several cycles interlaced. As discussed in papers by Stebbins and Sanford in the 1930s, there are short-term variations of around 150 to 300 days that modulate a regular cyclic variation with a period of roughly 5.7 years.[42][43]
In fact, the supergiant consistently displays irregularphotometric, polarimetric and spectroscopic variations, which points to complex activity on the star's surface and in its extended atmosphere.[21] In marked contrast to most giant stars that are typically long period variables with reasonably regular periods, red giants are generally semiregularor irregular with pulsating characteristics. In a landmark paper published in 1975, Martin Schwarzschild attributed these brightess fluctuations to the changing granulation pattern formed by a few giant convection cells covering the surface of these stars.[25][44] For the Sun, these convection cells, otherwise known as solar granules, represent the foremost mode of heat transfer—hence those convective elements which dominate the brightness variations in the solar photosphere.[25] The typical diameter for a solar granule is about 2,000 km (yielding a surface area roughly the size of India), with an average depth of 700 km. With a surface of roughly 6 trillion km2, there are about 2 million of such granules lying on the Sun's photosphere, which because of their number produce a relatively constant flux. Beneath these granules, it is conjectured that there are 5 to 10 thousand supergranules, the average diameter of which is 30,000 km with a depth of about 10,000 km.[45] By contrast, Schwardschild argues that stars like Betelgeuse may have only a dozen monster granules with diameters of 180 million km or more dominating the surface of the star with depths of about 60 million km, which, because of the very low temperatures and extremely low density found in red giant envelopes, result in convective inefficiency. Consequently, if only a third of these convective cells are visible to us at any one time, the time variations in their observable light may well be reflected in the brightness variations of the integrated light of the star.[25]
Schwarzschild's hypothesis of gigantic convection cells dominating the surface of red giants and supergiants seems to have stuck with the astronomical community. When the Hubble Space Telescope captured its first direct image of Betelgeuse in 1995 revealing a mysterious hot spot, astronomers attributed it to convection.[46] Two years later, astronomers observed intricate asymmetries in the brightness distribution of the star revealing at least three bright spots, the magnitude of which was "consistent with convective surface hotspots".[22] Then in the year 2000, another team of astronomers led by Alex Lobel of the Harvard–Smithsonian Center for Astrophysics (CfA) noted that Betelgeuse exhibits raging storms of hot and cold gas in its turbulent atmosphere. The team surmised that huge areas of the star's photosphere vigorously bulge out in different directions at times, ejecting long plumes of warm gas into the cold dust envelope. Another explanation that was also given was the occurrence of shock waves caused by warm gas traversing cooler regions of the star.[43][47] The team investigated the atmosphere of Betelgeuse over a period of five years between 1998 and 2003 with theSpace Telescope Imaging Spectrograph aboard Hubble. They found that the bubbling action of the chromosphere tosses gas out one side of the star, while it falls inward at the other side, similar to the slow-motion churning of a lava lamp.
[edit]Angular size
A third challenge that has confronted astronomers has been measuring the star's angular diameter. On December 13, 1920, Betelgeuse became the first star outside the Solar System to ever have its diameter measured.[17] Although interferometry was still in its infancy, the experiment proved a success and Betelgeuse was found to have a uniform disk of .047 arcseconds. The astronomers' insights on limb darkening were noteworthy; in addition to a measurement error of 10%, the team concluded that the stellar disk was likely 17% larger due to the diminishing intensity of light around the edges—hence an angular diameter of about .055".[17][30] Since then, there have been other studies conducted, which have produced angles that range from .042 to .069 arcseconds.[28][48][49] Combining that data with historical distance estimates of 180 to 815 ly yields a projected diameter of the stellar disk of anywhere from 2.4 to 17.8 AU, hence a radius of 1.2 to 8.9 AU respectively.[note 1] Using the Solar System as a yardstick, the orbit of Mars is about 1.5 AU, Ceres in the asteroid belt 2.7 AU,Jupiter 5.5 AU—consequently a photosphere which, depending on Betelgeuse's actual distance from Earth, could well extend beyond the Jovian orbit but not quite as far as Saturn at 9.5 AU.
The precise diameter has been hard to define for several reasons:
- The rhythmic expansion and contraction of the photosphere, as theory suggests, means the diameter is never constant;
- There is no definable "edge" to the star as limb darkening causes the optical emissions to vary in color and decrease the farther one extends out from the center;
- Betelgeuse is surrounded by a circumstellar envelope composed of matter being ejected from the star—matter which both absorbs and emits light—making it difficult to define the edge of the photosphere;[29]
- Measurements can be taken at varying wavelengths within the electromagnetic spectrum, with each wavelength revealing something different. Studies have shown that angular diameters are considerably larger at visible wavelengths, decrease to a minimum in the near-infrared, only to increase again in the mid-infrared.[50][51] The difference in reported diameters can be as much as 30–35%, yet because each wavelength measures something different, comparing one finding with another is problematic;[29]
- Atmospheric twinkling limits the resolution obtainable from ground-based telescopes since turbulence degrades angular resolution.[21]
To overcome these challenges, researchers have employed various solutions. The concept of astronomical interferometry was first conceived by Hippolyte Fizeau in 1868.[52] He proposed the observation of stars through two apertures to obtain interferences that would furnish information on the star's spatial intensity distribution. Since then, the science of interferometry has evolved considerably where multiple-aperture interferometers are now used consisting of numerous images superimposed on each other. These "speckled" images are then synthesized using Fourier analysis—a method used for a wide array of astronomical objects including the study of binary stars, quasars, asteroids and galactic nuclei.[53] The emergence of adaptive optics since 1990 has revolutionized high angular resolution astronomy,[54] while space observatories like Hipparcos, Hubble and Spitzer have produced other significant breakthroughs.[20][55] Recently another instrument, the Astronomical Multi-BEam Recombiner (AMBER), is providing new insights. As part of the VLTI, AMBER is capable of combining the beams of three telescopes simultaneously, allowing researchers to achieve milliarcsecond spatial resolution. Also by combining three baselines instead of two, which is customary with conventional interferometry, AMBER enables astronomers to compute the closure phase—an important element in astronomical imaging.[56][57]
The current debate revolves around which wavelength—the visible, near-infrared (NIR) or mid-infrared (MIR)—produces the most accurate angular measurement.[note 1] The most widely adopted solution, it appears, is the one performed with the ISI in the mid-infrared by astronomers from the Space Sciences Laboratory at U.C. Berkeley. In the epoch year 2000, the group, under the leadership of John Weiner, published a paper showing Betelgeuse with a uniform disk of 54.7 ± 0.3 mas, ignoring any possible contribution from hotspots, which are less noticeable in the mid-infrared.[28] The paper also included a theoretical allowance for limb darkening yielding a diameter of 55.2 ± 0.5 mas—a figure which equates to a radius of roughly 5.5 AU (1,180 R☉), assuming a distance of 197.0 ± 45 pc.[note 2] Nevertheless, given the angular error factor of ± 0.5 mas combined with a parallax error of ± 45 pc found in Harper's numbers, the photosphere's radius could actually be as small as 4.2 AU or as large as 6.9 AU.[58]
Across the Atlantic, another team of astronomers led by Guy Perrin of the Observatoire de Paris produced a document in 2004 arguing that the near-infrared figure of 43.33 ± 0.04 mas was a more accurate photospheric measurement.[50] "A consistent scenario to explain the observations of this star from the visible to the mid-infrared can be set-up", Perrin reports. "The star is seen through a thick, warm extended atmosphere that scatters light at short wavelengths thus slightly increasing its diameter. The scatter becomes negligible above 1.3 μm. The upper atmosphere being almost transparent in K and L—the diameter is minimum at these wavelengths where the classical photosphere can be directly seen. In the mid-infrared, the thermal emission of the warm atmosphere increases the apparent diameter of the star." The argument has yet to receive widespread support among astronomers.[29]
More recent studies done in the near-infrared with the IOTA and VLTI have brought strong support to Perrin's analysis yielding diameters that range from 42.57 to 44.28 mas with minimal error factors less than than 0.04 mas.[59][60] Central to this discussion, however, is a second paper published by the Berkeley team in 2009, this time led by Charles Townes, reporting that the radius of Betelgeuse had actually shrunk from 1993 to 2009 by 15%, with the 2008 angular measurement equal to 47.0 mas, not too far from Perrin's estimate.[30][61] Unlike most papers heretofore published, this study encompassed a 15 year horizon at one specific wavelength. Earlier studies have typically lasted one to two years by comparison and have explored multiple wavelengths, often yielding vastly different results. The diminution in angular separation equates to a range of values between 56.0 ± 0.1 mas seen in 1993 to 47.0 ± 0.1 mas seen in 2008—a contraction of almost 0.9AU in 15 years or roughly 1,000 km per hour.[note 3] What is not fully known is whether this observation is evidence of a rhythmic expansion and contraction of the star as astronomers have theorized, and if so, what the periodic cycle might be, although Townes suggests that if a cycle does exist, it is probably a few decades long.[30] Other possible explanations are photospheric protrusions due to convection or a star that is not spherical but rather asymmetric causing but the appearance of expansion and contraction as the star rotates on its axis.[62]Consequently, until a full cycle of data has been gathered, we will not know whether the 1993 figure of 56.0 mas represents a maximum extension of the star or its mean, or whether the 2008 figure of 47.0 in fact represents a minimum. It will probably take another 15 years or longer (2025) before we know with any certainty, meaning that the Jovian orbit of 5.5 AU will probably serve as the star's "average" radius for some time.[63][64]
Once considered as having the largest angular diameter of any star in the sky after the Sun, in 1997 Betelgeuse lost that distinction when a group of astronomers measured R Doradus with a diameter of 57.0 ± 0.5 mas. Betelgeuse is now considered to be in third place, although R Doradus, being much closer to Earth at about 200 ly, has an actual diameter roughly one-third that of Betelgeuse.[65]
[edit]Properties
Betelgeuse's spectral class is listed as M2Iab in the SIMBAD astronomical database, signifying that it is a red Class M star.[2] The "ab" suffix is derived from the Yerkes spectral classification system, and indicates that it is an intermediate luminous supergiant, less bright than other supergiants like Deneb.[66] However, given some of the recent findings, this classification may be outdated, as there is evidence Betelgeuse is actually much more luminous than Deneb and other stars in its class.[32]
Assuming average radius of 5.5 AU and a distance of 197 pc, theoretical calculations would yield a luminosity figure in excess of 180,000 Suns (L☉) at maximum.[4][64] When the star contracts as it appears to have since 1993, its luminosity would diminish to about 130,000L☉. Either way, that amount of electromagnetic energy dwarfs Deneb's output of about 50,000L☉.[note 4] But with most of the star's radiant energy occurring in the infrared and huge amounts being absorbed by circumstellar matter, the human eye simply cannot perceive the star's intrinsic brightness.
Given the many uncertainties surrounding Betelgeuse, no consensus has yet emerged regarding the star's mass. Estimates range from 5 to 30 solar masses(M☉) with most investigators showing a preference for a relatively large mass ranging from 10 to 20M☉.[67][68] One model reports a mass at the lower end of the scale at 14M☉, although a mass ranging from 18 to 20 is more commonplace.[6][67]
Typical of red supergiants, Betelgeuse is a cool star with surface temperatures in the last decade reported between 3,500 to 3,600K.[8][10][68]It is also a slow rotator, with the most recent velocity recorded at 5 km/s.[10] Depending on its photospheric radius, it could take the star anywhere from 25 to 32 years to turn on its axis—extremely slow when compared with a fast rotator like Pleione in the Pleiades star cluster, which turns on its axis once every 11.8 hours.[69]
In 2002, astronomers using sophisticated computer simulations began to speculate that Betelgeuse might exhibit magnetic activity in its extended atmosphere, a factor where even moderately strong fields could have a meaningful influence over the star's dust, wind and mass-loss properties.[70] A series of spectropolarimetric observations obtained in 2010 with the Bernard Lyot Telescope at Pic du Midi Observatoryrevealed the presence of a weak magnetic field at the surface of Betelgeuse, suggesting that the giant convective motions of supergiant stars are able to trigger the onset of a small-scale dynamo.[71]
[edit]Space motion
The kinematics of Betelgeuse are not easily explained. The age of a Class M supergiant with an initial mass of 20M☉ is roughly 10 million years.[4][72] Given its current space motion, a projection back in time would take Betelgeuse around 290 parsecs farther from the galactic plane where there is no star formation region—an implausible scenario. Although the radial velocity and proper motion for the 25 Ori subassociation have yet to be measured, α Ori's projected pathway across the heavens does not appear to intersect with it either. Also, formation close to the far younger Orion Nebula Cluster (ONC, also known as Ori OB1d) is doubtful. Very Long Baseline Arrayastrometry yields a distance to the ONC between 389 and 414 parsecs. Consequently, it is likely that Betelgeuse has not always had its current motion through space and has changed course at one time or another, possibly the result of a nearby stellar explosion.[4][73]
The most likely star-formation scenario for Betelgeuse is that it is a runaway star from the Orion OB1 Association. Originally a member of a high-mass multiple system within Ori OB1a, Betelgeuse was probably formed about 10–12 million years ago from the molecular cloudsobserved in Orion, but has evolved rapidly due to its unusually high mass.[4]
Like many of the young stars in Orion where masses greater than 10 solar can be found in abundance, Betelgeuse will use its fuel quickly and not live very long. On the Hertzsprung-Russell diagram, Betelgeuse has moved off the main sequence and has swelled and cooled to become ared supergiant. Although young, Betelgeuse has probably exhausted the hydrogen in its core—unlike its OB cousins born about the same time—causing it to contract under the force of gravity into a hotter and denser state. As a result, it has begun to fuse helium into carbon and oxygen producing enough radiation to unfurl its outer envelopes of hydrogen and helium. Its extreme luminosity is being generated by a mass so large that the star will eventually fuse higher elements through neon, magnesium, sodium, and silicon all the way to iron, at which point it will probably collapse and explode as a supernova.[6][43]
[edit]Density
As an early M-type supergiant, Betelgeuse is one of the largest, most luminous and yet one of the most ethereal stars known. A radius of 5.5 AU is roughly 1,180 times the radius of the Sun—a sphere so huge that it could contain over 2 quadrillion Earths (2.15 × 1015) or more than 1.6 billion (1.65 × 109) Suns. That is the equivalent of Betelgeuse being a giant football coliseum likeWembley Stadium in London with the Earth a tiny pearl, 1 millimeter in diameter, orbiting a Sun the size of a mango.[note 5] Sirius, by contrast, with a radius of 1.71R☉, would be roughly the size of a soccer ball. Moreover, recent observations of Betelgeuse exhibiting a 15% contraction in angular diameter would equate to a shortening of the star's radius from about 5.5 to 4.6 AU, assuming that the photosphere is a perfect sphere. A reduction of this magnitude would correspond to a diminution in photospheric volume of approximately 41% or 680 million Suns.[note 6]
Not only is the photosphere enormous, but the star is surrounded by a vast and complex circumstellar environment where light could take over three years just to escape.[74] In the outer reaches of the photosphere, the density is extremely low. In volume, Betelgeuse exceeds the Sun by a factor of about 1.6 billion Suns. Yet the actual mass of the star is believed to be no more than 18–19M☉, with certain mass loss estimates projected at 1–2M☉ since birth.[6] Consequently, the average density of this stellar mystery compared to the Sun is less than twelve parts-per-billion (1.119 × 10−8). If we compare such star matter to the density of ordinary air at sea level, the ratio is roughly 1.286 × 10−5, a density so ethereal, one would have to soar above the noctilucent clouds in the Earth's mesosphere to experience it.[note 7] Such star matter is so tenuous, in fact, that Betelgeuse has often been called a "red-hot vacuum".[14][15]
[edit]Circumstellar dynamics
In the late phase of stellar evolution, massive stars like Betelgeuse exhibit high rates of mass loss, possibly as much as 1M☉ every 10,000 years, resulting in a complex circumstellar environment that is constantly in flux.[32] All stars exhibit mass loss. Rates vary from about 10−14 to 10−4 
As a result of work done by Pierre Kervella and his team at the Paris observatory in 2009, astronomers may be close to solving this mystery. What Kervella noticed was a large plume of gas extending outward at least six times the stellar radius indicating that the star is not shedding matter evenly in all directions.[10][32] The plume's presence, in fact, implies that the spherical symmetry of its photosphere, often observed in the infrared, is not preserved in its close environment. Asymmetries on the stellar disk had been reported many times at different wavelengths. However, due to the refined capabilities of the NACO adaptive optics on the VLT, these asymmetries have come into focus. The two mechanisms that could cause such asymmetrical mass loss, Kervella noted, were large-scale convection cells or polar mass loss, possibly due to rotation.[10] Probing deeper still with the AMBER instrument on ESO's Very Large Telescope Interferometer, Keiichi Ohnaka from the Max Planck Institute in Bonn observed that the gas in the supergiant's extended atmosphere is vigorously moving up and down, creating bubbles as large as the supergiant itself, leading his team to conclude that such stellar upheaval is behind the massive plume ejection observed by Kervella.[32][76]
Evidence of circumstellar shells surrounding M supergiants was first proposed by Walter Adamsand Elizabeth MacCormack in 1935 when they observed anomalies in the spectral signature of such stars and concluded that the likely cause was an expanding gaseous envelope.[31][77] The first indication of vastly extended envelopes occurred in 1955 with the work of Armin Deutsch who noticed when studying the Rasalgethi system that spectroscopic peculiarities were mysteriously occurring in the G star companion, α2 Her. This led him to conclude that the whole system had to be enveloped by a circumstellar shell composed of matter being ejected by the main star, M supergiant α1 Her, and extending to at least 170 stellar radii.[31][78] In the mid 1970s, Andrew Bernat undertook a detailed analysis of four circumstellar shells, Betelgeuse, Antares, Rasalgethiand Mu Cephei, concluding that red stars dominate mass return to the Galaxy.[31]
In addition to the photosphere, researchers have now identified six other components of Betelgeuse's complex atmosphere. Extending outward, we find a compact molecular environment otherwise known as the MOLsphere, a gaseous envelope, a chromosphere, a dust environment and two outer shells (S1 and S2) composed of carbon monoxide (CO). There is also evidence of coronal plasma in the star's extended atmosphere, a phenomenon that heretofore was not believed to exist in late stage stars off the main sequence.[67] Some of these elements are known to be asymmetric while others overlap.[60]
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