Now I am become Death, the destroyer of worlds
This article was originally published in Substack
WR-104, The Pinwheel
WR-104 in Sagittarius imaged by the WM Keck twin 10-meter telescopes
Both the title and subtitle above are from the Hindu holy book, the Bhagavad-Gita and were famously uttered by Julius Robert Oppenheimer, the American physicist whose life and career were recently featured in a full-length Hollywood production. Oppenheimer spoke these words during a 1965 television interview when discussing that first nuclear test blast at the site that would later become known as Trinity.
He is less known for his contributions to Astrophysics, however. Oppenheimer collaborated in the discovery of the upper mass limit for a Neutron star, a mass limit above which a degenerate neutron core can no longer provide a barrier to collapse and will thus, collapse to become a black hole. Known as the TOV limit, short for Tolman-Oppenheimer-Volkoff limit, this upper limit is 3 solar masses and is the analogous limit for a Neutron Star as the Chandrasekhar Limit is for White Dwarfs (1.44 Solar masses).
Imagine, if you will, a star system containing a massive, intensely hot, luminous star that’s emitting a fierce and prodigious stellar wind in a binary orbit around a 20 solar-mass O4 – O5 main-sequence star. The former, a 10 solar-mass Wolf-Rayet star, is at the very end of its productive life.
Wolf-Rayet Stars
Exemplified by broad, strong emission lines of Helium, Oxygen, Nitrogen and Carbon, Wolf-Rayet stars are stars in the last productive phase of their lives for highly evolved, massive stars (> 10 Solar Masses). Most stellar spectra exhibit absorption lines (dark lines representing elements or compounds at specific “colors” or wavelengths). Wolf-Rayet stars are known for their extremely intense stellar winds and prodigious mass loss.
Discovered by their distinctive spectra by French astronomers C.J. Wolf and Georges Rayet in 1867, many Wolf-Rayet stars present without any hydrogen in their spectra and all, with broad emission lines of ionized Carbon, Nitrogen, Oxygen and Silicon.
Classic Wolf–Rayet stars have completely lost their outer hydrogen shell and are fusing helium or heavier elements in their cores. The Wolf-Rayet phase of a high-mass star’s evolution is considered to be the last stable point before the star’s spectacular demise as a Type-II, core-collapse supernova.
Add to the “mix”, that the material being ejected from WR-104 is producing a continuous spiral 200 Astronomical Units (1 AU is the earth-sun distance) wide with a period of 242 days.
If you have imagined this, then you have a clear idea of what WR-104 is. It is a potentially lethal stellar system about 8,400 light years (2.5 Kiloparsecs) distant in the direction of Sagittarius that sports a remarkable colliding-wind pinwheel nebula.
A Lethal Star System?
Width of spiral: 200 AU with a 242-day period
First discovered in 1999 by Peter Tuthill of the School of Physics, University of Sydney, NSW, Australia, WR-104 could be aptly referred to as the destroyer of worlds, most significantly, this world. All stars in this system are candidates for core collapse supernovae:
- the WC9d (Carbon rich spectra) Wolf-Rayet component is a Type Ib or Ic supernova candidate,
- its spectroscopic binary companion (resolvable only using spectroscopy) is a luminous, early B (B0.5) main sequence star for a combined mass of 10 solar masses for both stars and,
- a luminous, 20 solar-mass O4-O5 main sequence star.
The combined mass of the entire system is over 30 solar-masses.
There are three high-mass stars in the WR-104 system, all supernova candidates for the price of one: triple the jeopardy!
So why the lethality when the system is 8,400 light years distant?
.
Gamma-Ray Bursts are believed to be tightly focused beams of Gamma rays produced by a rare subclass of Type II supernovae known as Type Ic-BL (broad-lined), originating from the specific type of WR star as WR-104 (stripped-envelope — a powerfully-emitting carbon shell or envelope is sitting above the core)! The typical angular spread of those beams is in the range of 2 – 20º. Initial estimates place the alignment angle of WR-104 at < 16º (less than 16 degrees).
Typical type II supernovae aren’t thought to be Gamma-Ray Burst progenitors. However, the Wolf-Rayet component of WR-104 is a member of that rare class of star as described above! And, even though the system is 8,400 light years distant, a safe distance for a Type II supernova, the polar axis of the system, identified by the Archimedes spiral produced as the WR component loses mass, appears to be aligned with us, to our location in the galaxy!
In essence, when the WR star explodes as a supernova, the gamma rays will be tightly focused along the polar axis of the exploding star. You might say, even though it could be closely aligned with our location in the galaxy, it’s still 8,400 light years distant.
Gamma rays, the highest energy of all electromagnetic radiation (light, X-rays, infrared, etc.), are ionizing radiation and lethal to all life. Ionizing radiation strips electrons off atoms and changes their chemical properties. Ionized Oxygen, for example, is a precursor in the formation of ozone (O3), and can lead to oxidative stress and DNA damage and is lethal in high concentrations but, in its nominal form as molecular oxygen (O2), is necessary for life. The same is true for other elements, such as nitrogen or carbon.
Even from 8,400 light years, a tightly-focused, powerful beam of lionizing radiation could degrade and compromise the Earth’s Ozone layer over the long term, allowing greater exposure to solar ultraviolet (also ionizing, but less so) with a higher risk of cancer. Think of earth being hit by a gamma-ray pulse-laser from 8,400 light years, and you have a good idea of what this threat could be.
The literature published on this system since Tuthill’s discovery has been sporadic, with stories vacillating between a veritable certainty that we’re in the system’s cross-hairs to a complete miss with an off-axis orientation of 30–40 degrees. The almost perfectly-circular Archimedes spiral strongly suggests a very close alignment with our line-of-sight. When there is uncertainty, it is helpful to believe what you see with your eyes.
There were one or two stories published in non-academic blogs, one citing a presentation by Grant Hill at WM Keck observatory without any published data. Then there was speculation that maybe WR stars and supernovae weren’t the killers we all thought they were, again, without any data – spoiler alert: they still are the killers we all knew they were.
Again, believe what you see with your eyes. To wit and to quote Grant Hill:
“But if you look at WR 104 and the image of its pinwheel,” said Hill, “it really is a visceral and powerful argument that the thing is face on with an inclination of zero.”
Four years later in 2013, after the 2009 30–40 degree misalignment narrative was floated by Hill without any published study or supporting data, it turns out that…., surprise, surprise, he revised his earlier oh-so-certain pronouncement. Another study, published in 2018, explored several aspects of the WR-104 system that are still unknown. This included the opening angle of the spiral, dust formation, and the link between the central binary star and a candidate companion star detected with the Hubble Space Telescope.
And now, as recently as November, last year (2024), another study has been published with no clear, definitive reconciliation between the visual interpretation as an on-axis aligned system and data to the contrary. Again, from this study and regarding the system’s alignment with our vantage point in the galaxy:
The most surprising result here relates to the orbital inclination. WR 104 is widely believed to be a face-on binary. Not only do images of the dust spiral very much look to the eye to be face-on, modelling of them finds i ≤ 16◦. (i, the angle of inclination, is between 0 and 16 degrees).
So, even now, as of 3 months ago, the jury is still out.
The given timeline for the demise of the Wolf-Rayet component of WR-104 is 0 to a few hundred thousand years. According to current evolutionary models for evolved, high-mass stars, the Wolf-Rayet component of WR-104 is towards the end of its Helium burning phase (core temperatures in excess of 100,000,000 K). When the Helium burning phase concludes, the Carbon and Oxygen that has been building in the stellar core will become the new nuclear fuel and will require temperatures in excess of 600,000,000 K to burn.
Neon burning follows Carbon and requires core temperatures in excess of 1.2 billion degrees Kelvin. Each productive phase is shorter and shorter and yields less and less energy as the binding energy of each successive and more massive nucleus increases. The last productive phase of any high-mass star is Silicon burning, a phase that lasts 24 hours or one day! This phase ensues when the Oxygen fuel from the previous phase is depleted and the core temperature, achieved by gravitational collapse, exceeds 2.5 billion degrees Kelvin.
As far as the fate of WR-104 is concerned, we’ll just have to wait and see what happens — stay tuned!
Note: stellar types cited in this article are according to the Morgan-Keenan Spectral classification system used in the Hertzprung-Russell Diagram. As a point of reference, the sun is a G2-V (Roman numeral “5”) main sequence star.
Where is WR-104?
This stellar ticking time bomb is located in the prominent summer constellation Sagittarius, visible now in the pre-dawn hours but soon well-placed as spring progresses and summer is neigh.
A view to the south during today’s pre-dawn hours. The 3rd quarter moon is in Sagittarius in the foreground with the galactic center set in the background, WR-104 is located to the north of the moon. Image via Stellarium.
Please see my latest post in Substack here.
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