Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It only takes a minute to sign up.
Sign up to join this community
I've read on Quora about Wolf-Rayet binary star WR-104, which is thought to be a precursor to a supernova explosion with two accompannying gamma ray bursts. It looks as if the rays will travel in our direction, but it is also said the ray is of by 30 degrees. Not sure what to think, so I thought to ask here. Will it have a real effect on Earth? The comments are pretty disturbing.
The dust is lit up in a spiral pattern and it is carried around in synchrony with its companion star with a rotation period of 220 days.
More detailed picture here
It looks as if it is facing us nearly face on. But spectroscopic observations of the star suggest it’s axis is at an angle of 30° - 40° (possibly as much as 45°) which would mean it would miss. See WR 104 Won't Kill Us After All - Universe Today.
Such a small angle wouldn’t be easy to detect by visual methods for a spinning spiral pattern (a 30 degree angle would make it only 14% narrower in one direction).
The paper is called “WR 104: Are we looking down the gun barrel of a future GRB?” I found no more papers on the topic. Any new paper would cite the spectroscopy article and there is only one article since 2008 citing it.
. papers citing: WR 104: Are We Looking Down The Gun Barrel of a Future GRB?
This is what it says (published in 2019)
Many pages within the popular press have been filled with the apparent doom that can be expected from the Wolf-Rayet star WR 104 (V* V5097 Sgr). This highly interesting massive WC + O star binary system sports a remarkable colliding-wind pinwheel nebula (Tuthill, et al., 2008). Initial estimates suggested that the system spin axis was pointing within a few degrees of the Solar System, and this raised the question concerning a future directed supernova (possibly GRB) jet intercept. However, given that WR 104 is at least 2.5 kpc distant, and that recent observations (Hill, 2009) find a much higher inclination of the system’s spin axis away from the Sun, the future threat from WR104 appears to be ill founded, if not entirely negligible.
. Selected Topics and Case Studies
See also WR 104: Not The Killer It Used To Be
This is a more recent article about WR104, but it doesn't discuss gamma ray bursts.
, SPHERE view of Wolf-Rayet 104
So, with WR104 then the older data is less clear but the more recent data suggests it is pointed well away from us. It would be surprising statistically if it were pointed towards us. Because it would have to be a tight beam to affect us so need to be precisely pointed towards us out of all the possible directions it could point. So it just about certain it is pointed away. And as with the other candidates, ours is the wrong kind of galaxy for gamma ray bursts and nothing special about this century either, no reason now rather than say 100,000 years from now.
GAMMA RAY BURSTS UNLIKELY IN CHEMICALLY EVOLVED GALAXIES LIKE OUR OWN
Gamma ray bursts seem to form in smaller galaxies than the Milky Way preferentially. The rate may have been greater in the past.
Also they seem to form in metal poor galaxies (ones with few elements apart from hydrogen or helium Metallicity - Wikipedia) and the local ones are normally in dwarf galaxies. For details see this paper which says
“We find that only galaxies with present stellar masses below < $10^{10}$M☉ and low metallicity reproduce the observed GRB rate.”
See also this list of Gamma Ray Burst Host Galaxies.
Our galaxy has about 100 billion times the mass of the sun, so would have a figure of 11 in the log (M*/M☉) column - at the time of the article they said they were all in dwarf galaxies - and there is currently only one example of a nearby GRB in such a large galaxy, GRB 080207.
For all these reasons they seem unlikely to occur often in the Milky Way.
The reason why they occur only in metal poor galaxies might be because to form a gamma ray burst the equator has to spin very fast. Then as it collapses to a black hole, the poles collapse first which they think may be what triggers the gamma ray burst. In a metal rich galaxy the winds blowing away from the equator of a rapidly spinning star slow it down because they carry more momentum away being heavier elements on average.
For details see Astrobiological Effects of Gamma-Ray Bursts in the Milky Way Galaxy
Conclusion of that study was
“The results in this work, and others found in the literature imply that GRBs may be uncommon in the Milky Way and may not pose a significant danger to the propensity of planets to host life in the Galaxy.”
Extract from my
which also looks at the other two candidates also pointed away from us. That's no surprise as you expect that, given the narrow size of a beam needed to reach us from so far away. Would be unlikely even one of three candidates would be pointed our way.
Should we fear Wolf-Rayet WR-104?
Not really.
What you should fear more is getting hit by a bolt of lightning from a storm that is 477 miles (768 kilometers) away. This actually happened fairly recently. Should you worry about this? No, you shouldn't. The odds are so small that this is not something to worry about, just as WR-104 is not something you should worry about.
There are plenty of things that are much more worthy of worrying about in the short term and the intermediate term. Could my city get hit by an asteroid or comet? Yep, it can. We saw that somewhat recently in Chelyabinsk in Russia. An even more significant event happened a bit over one hundred years ago in Tunguska, also in Russia. And then there's the a dinosaur-killer that happened 66 or so million years ago. Even that extremely rare event is much more likely than WR-104 hitting Earth with a GRB.
Could worldwide economic collapse, environmental collapse, political collapse, or social collapse occur in the semi-near future? Many experts claim that these are getting close to being inevitable. Those are something worthy of worrying about. A GRB from WR-104 is not worthy of worrying about.
With basis on the limited information available, we can try to get some crude estimates for an adverse outcome. From the Wikipedia article:
Recent studies suggest these effects pose a "highly unlikely" danger to life on Earth, with which, as stated by Australian astronomer Peter Tuthill, the Wolf–Rayet star would have to undergo an extraordinary string of successive events:
- The Wolf–Rayet star would have to generate a gamma-ray burst (GRB), however, these events are mostly associated with galaxies with a low metallicity and haven't yet been observed in our Milky Way Galaxy. Some astronomers believe it unlikely that WR 104 will generate a GRB; Tuthill tentatively estimates the probability for any kind of GRB event is around the level of one percent, but cautions more research is needed to be confident.
- The rotational axis of the Wolf–Rayet star would have to be pointed in the direction of our planet. The star's axis is estimated to be close to the axis of the binary orbit of WR 104. Observations of the spiral plume are consistent with an orbital pole angle of anywhere from 0 to 16 degrees relative to the Earth, but a spectrographic observation suggest a significantly larger and therefore less dangerous angle of 30° - 40° (possibly as much as 45°). Estimates of the "opening angle" jet's arc currently range from 2 to 20 degrees. (Note: The "opening angle" is the total angular span of the jet, not the angular span from the axis to one side. Earth would therefore only be in the intersecting path if the actual angle of the star's axis relative to Earth is less than half the opening angle.)
- The jet would have to reach far enough in order to damage life on Earth. The narrower the jet appears, the farther it will reach, but the less likely it is to hit Earth.
According to the text, our ignorance on the probability of a gamma-ray burst on WR 104 severely imperilling Earth's biosphere (let's call it pKill) depends on the following factors:
The main mechanism proposed, for how GBRs threaten our biosphere is ozone layer depletion, caused by the generation of large amounts of nitrogen oxides in the gamma-ray irradiated atmosphere. With nitrogen oxides shredding the ozone layer, UV rays from the Sun would reach Earth surface unfiltered, making it hostile to life until the nitrogen oxides rain down and the ozone layer reforms, what may take more than a decade. Something like that was even proposed as a possible cause for the Late Ordovincian mass extinction, about 443 million years ago. I searched the literature for figures on how much energy should be delivered to Earth by a GBR to ensure significant ozone depletion. I've found the paper (1) given in references that points to a value around 100kJ/m².
With this figure for the kJ/m² required to deplete the ozone layer, we can start to guess random values for rotational axis angle and jet opening angle for our montecarlo simulation. We could just guess values in a uniform distribution spanning the permited range of values, but I think we can do a bit better than that.
Regarding the rotational axis angle, from the text in wikipedia it appears the evidence is a bit conflicting, with probability distribution spanning the range from 0 to 45 degrees, with a maximum around 30-40 degrees. So I'm approximating it by a triangular distribution with peak at 35 degrees.
The estimates for the opening angle are placed in the range from 2 to 20 degrees, with no clue in the text about the shape of the probability distribution in this interval. But the proposed mechanism for a GBR in the literature involves the collapse of a fast spinning star, causing the poles to collapse first. From the physics of this situation, it's reasonable to guess a peak close to the larger opening angle, as slow spinning stars surely outnumber the fast spinning ones. So we end with the following probability landscape, for the relevant angles:
Given all those assumptions, I did the programming on the simulation in Pharo, as shown bellow:
| triangular generator capArea jetArc rotationalAxisAngle burstEnergy distanceLy ly2Meters pBurst results kills pCriticalHit simulationRuns |
generator := Random new.
"Defines random sampler from given triangular distribution."
triangular := [ :lowA :highB :peakC |
| ratio u |
ratio := peakC - lowA / (highB - lowA).
[
u := generator next.
u < ratio
ifTrue: [
lowA + (u * (highB - lowA) * (peakC - lowA)) sqrt ]
ifFalse: [
highB
- (1 - u * (highB - lowA) * (highB - peakC)) sqrt ] ] ].
"Calculates area of a spherical cap.
https://en.wikipedia.org/wiki/Spherical_cap"
capArea := [ :radius :angle |
2 * Float pi * radius squared * (1 - angle degreeCos) ].
jetArc := triangular value: 2 value: 20 value: 20.
rotationalAxisAngle := triangular value: 0 value: 45 value: 35.
simulationRuns := 1e7.
burstEnergy := 5e44.
distanceLy := 8400.
ly2Meters := 9.46e15.
pBurst := 0.01.
results := (1 to: simulationRuns) collect: [ :each |
| axisJetAngle |
axisJetAngle := jetArc value / 2.
rotationalAxisAngle value <= axisJetAngle
ifTrue: [
(burstEnergy
/
(2
*
(capArea
value: distanceLy * ly2Meters
value: axisJetAngle))) rounded ]
ifFalse: [ 0 ] ].
kills := results select: [ :each | each >= 1e5 ].
pCriticalHit := kills size / results size.
Transcript
clear;
show: 'Total runs: ' , simulationRuns asString;
cr;
show: 'Missing bursts: '
, (results select: [ :each | each = 0 ]) size asString;
cr;
show: 'Grazing hits: '
,
(results select: [ :each | each > 0 and: each < 1e5 ]) size
asString;
cr;
show: 'Critical hits: '
, (results select: [ :each | each >= 1e5 ]) size asString;
cr;
show: 'pCriticalHit: '
, (pCriticalHit round: 5) asFloat asString;
cr;
show:
'Overall probability (pBurst * pCriticalHit): '
, (pBurst * pCriticalHit round: 5) asString.
Running this code on the Pharo's Playground, I got the following results:
Total runs: 10000000
Missing bursts: 9659649
Grazing hits: 0
Critical hits: 340351
pCriticalHit: 0.03404
Overall probability (pBurst * pCriticalHit): 0.00034
So, this attempt at a simulated estimate for the probability of a GBR from WR 104 harming Earth's biosphere gave us a result of about 0.03%. Just reminding, as the probability distributions I used are just educated guesses in face of the uncertain data we have available, this end result also is also just a crude estimate and not to be taken too seriously.
References:
(1). Thomas, Brian C., et al. “Gamma‐Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects”. The Astrophysical Journal, vol. 634, no 1, novembro de 2005, p. 509–33. DOI.org (Crossref), https://doi.org/10.1086/496914.
It is safe to say that humans cannot be so naive as to think an extinction-level event (ELE) will not occur. It is possible, very possible. Will it happen in our lifetime? Probably not, but it is possible.
However, in such an unlikely ELE, we would never know (government cover-up to prevent mass hysteria and whatnot) or never should care. People die every day unexpectedly, we might get into our car and drive off for the last time. We might get hit by lightning. It happens.
That said, here is a bit of doom and gloom. I've noticed in all of the calculations I've seen, and I will be honest, I have not seen all of them, people have not taken into account the sheer mass of such a beam from WR-104.
Again, all the calculations I've seen are calculating the center of the beam as if it is a thin laser. The gamma "ray" from WR-104 can easily consume planets (hence the label, "Planet killer"), multiple planets larger than Jupiter in one fell swoop.
Maybe the center of the beam would miss, but the edge of it might clip Earth.
On a lighter note, though WR-104 is a curious phenomenon that people shouldn't worry themselves over, people should be more concerned about the more destructive force that is closer to home, the human kind.
We simply do not have the technology, currently, to save humankind from an ELE like WR-104, but we certainly have the technology to save ourselves from the human kind.
