I have stood under Orion The Hunter on clear evenings willing its star Betelgeuse to explode. “C’mon, blow up!” In late 2019, Betelgeuse experienced an unprecedented dimming event dropping 1.6 magnitude to 1/3 its max brightness. Astronomers wondered – was this dimming precursor to supernova? How cosmically wonderful it would be to witness the moment Betelgeuse explodes. The star ripping apart in a blaze of light scattering the seeds of planets, moons, and possibly life throughout the Universe. Creative cataclysm.
Only about ten supernova have been seen with the naked eye in all recorded history. Now we can revisit ancient astronomical records with telescopes to discover supernova remnants like the brilliant SN 1006 (witnessed in 1006AD) whose explosion created one of the brightest objects ever seen in the sky. Unfortunately, latest research suggests we all might be waiting another 100,000 years for Betelgeuse to pop. However, studying this recent dimming event gleaned new information about Betelgeuse which may help us better understand stars in a pre-supernova state.
Balancing on the Shoulder of a Giant
Betelgeuse, AKA Alpha Orionis, is a Red Supergiant star. Its name is derived from the Arabic word “ bat al-jawz’ ” which translates to “the giant’s shoulder” as the star forms the left shoulder of Orion (possibly referred to by Replicant “Roy” to in his “tears in the rain” monologue from the film Bladerunner). For their entire lives, stars are in a struggle to balance against the inward crushing force of their own gravity using the outward force of their own energy – a state known as hydrostatic equilibrium. Any disruption of this balance causes changes in the star – some dramatic others cataclysmic.
Gravity is both the beginning and end of a star. Gravity draws the raw material to create a star, hydrogen gas from the interstellar void, and crushes it together. Compressed and heated, the hydrogen ignites nuclear fusion in star’s core (our Sun’s core comprises 1/4 of its radius) which radiates energy back outward against the star’s outer layers. As long as a star has a supply of hydrogen, it can support its own weight and achieves a balanced sphere. Once out of fuel, gravity will also bring the star’s life to a crushing end. Stars in the hydrogen core burning phase of their lives are considered in the “Main Sequence” – a term from the Hertzsprung-Russel (HR) Diagram (below).
The HR Diagram helps us determine the phase of a particular star’s life based on their magnitude (brightness – Y Axis), and their colour or spectral type (assigned by a letter category – X Axis.) All stars burning hydrogen will fall on the “S” shaped central “Main Sequence” on this diagram. Think of it like a star’s prime adult years. Our own Sun falls on the Main Sequence as a class G “Yellow Dwarf” star. It is still burning hydrogen and will be for a few billion years more (don’t let anybody sell you Sun insurance). However, once the hydrogen fuel in a star’s core is exhausted, equilibrium becomes unbalanced. Energy outflow slows and gravity begins to crush the star’s core. This is where a star gets a second life – albeit in a new form.
The crushing of the core increases the core’s temperature. The increased temperature radiates outward to supplies of hydrogen that remained in layers outside the core that were previously too cold to achieve fusion. This shell of hydrogen outside the core now ignites, but burning this outer shallower layer causes the star to swell. As the outer layers expand outward, the star’s surface is now further from the core and spread over a larger area causing it to cool and turn red in colour (stars are redder when cooler, bluer when hotter). The star “evolves” into a Red Giant (or Red SUPERgiant for very massive bright stars). If the star is massive enough, rising temperatures in the core will also ignite helium which accumulated as a byproduct of burning hydrogen. Once the “ash”, the helium now becomes a secondary “emergency” supply of fuel in the losing battle against gravity.
As a Red Giant, the star’s position on the HR Diagram moves to one of the “giants” families that grow as branching stems off the “S” of the Main Sequence. It is now brighter (higher on the Y axis) and redder (farther right on the X axis). This is how we might distinguish say a class M Red Dwarf from a Class M Red Giant. A red dwarf and red giant may occupy the same position on the X axis colour-wise, but the giant will be much brighter and therefore higher on the Y axis in the one of the giant branches. Red Supergaints like Betelgeuse are in the final stages of their lives. That doesn’t necessarily mean they’ve lived a long life – just that they are in the final stages of their own life. Betelgeuse has lived a fraction of our Sun’s lifetime despite being much more massive. One would think larger star = more fuel to burn but larger stars are burning the candle at both ends. To maintain balance against their enormous mass they burn through hydrogen much faster than their lower mass counterparts.
A Breathing Star
But Betelgeuse has not achieved a new stable equilibrium yet. The star is pulsating in brightness and is therefore classified as a “variable star.” Understanding the cause or “mode” of the variability allows researchers to determine several key physical characteristics of Betelgeuse – the focus of the most recent publication by Dr. Meredith Joyce from Australian National University, Dr. Shing-Chi Leung of CalTech, and Dr. Chiaki Kobayashi associate professor at University of Hertfordshire.
A Star’s variability is either extrinsic or intrinsic. Extrinsic variability is a change in brightness due to an external source. Eclipsing binary stars is a common extrinsic variability as one star blocks the light from a companion star. Intrinsic variability is caused by something within the star itself. The recent research on Betelgeuse, inspired by the star’s dramatic dimming in late 2019, sought to determine if the event was intrinsic or extrinsic in nature. Updated observations of Betelgeuse’s regular pulses allowed researchers to confirm that the primary cause of variability is something called the “Kappa Mechanism” which destabilizes the star’s equilibrium essentially causing Betelgeuse to “breath” as it swells and shrinks in size and brightness.
So what’s the Kappa Mechanism? Stars are made of ionized gas. When you superheat hydrogen gas within a star, electrons are torn from the hydrogen atoms – the process of ionization – which turns the star into an churning soup of free flying electrons called plasma. Plasma makes up 99% of the visible Universe (us non-ionized plasma stuff are actually the rarity of space).
However, ionization is not uniform through an entire star and exists in several layers of varying partial ionization. A key characteristic of partially ionized hydrogen is that when compressed the hydrogen becomes more opaque compared to surrounding layers. These layers of opaque partially ionized hydrogen can insulate and trap energy as the energy tries to move from the core to the surface. In Main Sequence stars, that trapped energy wants to push those opaque layers upward, but there is too much of the star’s dense mass above to budge. Eventually the trapped energy finds other routes to the surface, or the uneven ionization is evened out in the churning of the star.
However, as a star expands into a Red Giant, these opaque layers of partial ionization rise closer to the star’s surface where they can more freely move. With more freedom of movement, when enough energy is trapped below the opaque insulating gas, the layer is forced upward and pushes against the star’s surface causing the star to swell further. As the layer expands, it becomes less compressed, less opaque, and more transparent to energy allowing the trapped energy to escape through the surface into space. Having lost energy, the layer loses momentum and falls back toward the star where it once again becomes compressed and opaque under the star’s surface. Think of it like the steam valve on a kettle. Enough steam builds up, the valve is pushed up to open, the steam is released, then the valve falls and closes. With each pulse, the star changes in radius and brightness. The function of this opaque partially ionized gas in causing the pulsation is the Kappa Mechanism. Here’s how the cycle works:
A) Evolution into Red Giant triggers Kappa Mechanism
- The star expands into a Red Giant as the core burns through its hydrogen fuel
- The expansion causes layers of partial ionization to rise to shallower depths within the star
- As the surface of the star expands outward, it is carried by momentum past the gravity equilibrium of the star where it slows and falls back inward.
B) Kappa Mechanism Cycle
- The infalling star compresses layers of partially ionized hydrogen near the star’s surface. The compression turns these layers more opaque trapping energy beneath
- The star continues to contract until the opacity of the partially ionized layers reaches a maximum. Eventually, enough energy is trapped beneath the opaque layers to overcome the momentum of the collapse.
This is where the star is the most compact in radius and becomes the hottest and brightest in the cycle
- The compressed opaque layer now stops falling and begins to reverse direction toward the surface pushed outward against gravity by the trapped energy beneath.
- The expanding opaque layer pushes against the star’s surface causing the star to swell. The expanding opaque layers become less compressed and more transparent releasing trapped energy. When most expanded, the star will be the coolest and dimmest in the cycle
- The outer layers of the star eventually loose momentum and fall back inward beginning the cycle over again at Step 4)
Imagine hovering near the surface of a Red Supergiant millions of times the volume of the Sun and watching its outer layers expand and contract. The star’s surface can move up to one kilometre per second! A behemoth taking one giant breath each year.
The researchers used computer models to confirm the Kappa Mechanism is responsible for a 416 day cycle or period in Betelguese’s brightness. However, the virtual model couldn’t reproduce a second 185 day period and longer 2365 day period the research team physically observed in the star itself. It’s possible that the Kappa Mechanism is interacting with other intrinsic characteristics of the star to produce another mode in the star’s variability. The researchers therefore conclude that Betelgeuse is a “Double Mode Variable Star.”
The shorter 185 day period is classified as an “overtone” in the star’s pulsations. The word “tone” is apt because the ripples through the star are essentially soundwaves in the churning plasma. The 2365 day period is referred to as an LSP or Long-Secondary Period. The origin of these other two periods is not entirely clear. The researchers encourage that more sophisticated computer models be developed in the future to further probe the star’s other periods.
There is a very narrow region on the HR Diagram where variable stars exist known as the “instability strip.” It is possible that as some stars age, they evolve through this strip until coming to a new point of equilibrium on the other side where the mode of pulsations is diminished or the pulses are amplified until the star blows its outer layers completely off.
As Betelgeuse is still pulsing, the researchers determine that the star is likely early in the helium burning phase of its transition to a Red Supergiant and could likely go on burning for another 100,000 years until gravity wins entirely and the star collapses into a supernova.
A Trail of Stellar Dominoes
Betelgeuse’s pulses allow researchers to derive other information about the star’s general characteristics such as the star’s radius. We know that the pulses travel through the star which takes a certain amount of time indicated by the pulse period. The researchers can calculate generally what speed the pulses travel (the speed of “sound” given Betelgeuse’s density) and use the timing of the period to determine what distance they’ve moved through the star. Using these calculations, Betelgeuse has been updated to 764 solar radii (764 times the Sun’s radius) about 66% of previous estimates.
Betelgeuse’s radius has been notoriously difficult to calculate because unlike our own Sun, one of the most perfectly spherical objects in the solar system, the photosphere or surface of Betelgeuse is quite “fuzzy”. Red Giants are more like star “clouds” than spheres. Betelgeuse’s surface also features bulges extending hundreds of millions of kilometers as it billows in its Red Giantyness. While the star’s new radius is smaller than originally thought, its surface would still reach past Mars and into the asteroid belt if placed at the centre of our solar system.
Like astronomy dominoes, each statistic we update about Betelgeuse provides key insights to others. With an updated radius, we can recalculate our distance to Betelgeuse based on how “wide” it appears in our sky. With a smaller radius, Betelgeuse must be closer than once thought putting the Red Supergiant at about 530 light years. While 25% closer than older calculations, the star is still too distant to kill us if it blows up. Good to know.
Finally, researchers weighed our neighbouring giant. We have a general sense of the rate Betelgeuse loses mass to space – presently about one solar mass every million years or so – blown off into the Cosmos. Experimenting by simulating with different “progenitor” or start masses when Betelgeuse was a young main sequence star, the simulation runs forward in time until the star exhibits Kappa Mechanism pulsations. Betelgeuse tips the scale at 16.5-19 solar masses (the mass of our Sun) with a progenitor mass of 18-21. These simulations also provide evidence that Betelgeuse is likely only 7-11 million years old. Imagine that – Betelgeuse is a THOUSANDTH the age of our own star and is about to explode. Stars like Betelgeuse are a fleeting spark in Cosmic time.
A Light Through the Dark
With all the new information about Betelgeuse, we still have a mystery. What caused the dimming event in late 2019? If Betelgeuse still has millennia before its magnificent death what happened? Two possible answers: The combination of multiple modes of variability in Betelgeuse aligned to enhance the dimming of the usual variability. Like dropping multiple stones into a pond, sometimes the waves can merge to create larger waves, or actually cancel eachother out. We may have witnessed that kind of event. Or, another likely cause, a massive cloud of dust moved between us and Betelgeuse temporarily blocking some of the star’s light – an extrinsic rather than intrinsic dimming event.
While our Sun has likely seen many stellar explosions during its eons-long journey around the Milky Way, a supernova is astonishing for our own limited human lifespans. The explosion of Betelgeuse would be bright enough to cast shadows at night. It would even be visible during the day. The explosion would slowly fade in the coming months. After a year, the Shoulder of Orion would disappear from the naked eye. I likely won’t be around for that, but somebody will. We may think ourselves rather impermanent, but so too is the sky itself – stars fading into the mists of space and time like “tears in the rain.”
Feature Image: Computer simulation of Betelgeuse in Space Engine Pro by Author
More to Explore:
[2006.09837] Standing on the shoulders of giants: New mass and distance estimates for Betelgeuse through combined evolutionary, asteroseismic, and hydrodynamical simulations with MESA (arxiv.org) (Original Research Paper – Open Access)
Radial Stellar Pulsations – Astro Princeton
Stellar Pulsation and Variable Stars – University of Iowa
Pulsating Variable Stars (csiro.au) – Australia Telescope National Facility