Expert: Philip Stahl - 7/4/2008

Question
QUESTION: As you know IK Pegasi is a binary star system that's 150 lightyears away from earth that has the potential (i heard) of blowing up as a type 1a supernova.
I cannot find a clear answer on this subject.

My question: I'm wondering when is this thing going to blow and is earth at risk from this from that current distance?
And what is the closest safe distance to be from a type 1a super nova?

ANSWER: Hello,

First, in the context of supernovae (Type 1a or other) the levels of uncertainty are such that no one can put a definite time interval on the hypothetical explosion. Besides that, there are much more immediate threats facing us right here on Earth, and I refer to global warming (or if you prefer, climate change) which people ought to be much more obsessed over than some possible supernova 150 LY away.

Anyway, let me put this in some context, since you are having problems having it addresssed. Btw you may also wish to consult this previous question I answered some weeks ago:

http://en.allexperts.com/q/Astronomy-1360/2008/3/Gamma-Ray-Bursts-Sun.htm


Now, let's get back to this star in Pegasus. It is 150 LY distant which translates into ~ 1.5 x 10^18 meters.

The Earth meanwhile is ~ 1.5 x 10^11 m from the Sun.

Imagine a derivative gamma ray burst from a major solar flare - say- which engenders a lethal (whole body) dose of 300 centigray (300 rads) of radiation to all Earth's inhabitants.

Now, in analogous fashion, imagine a supernova which delivers one hundred billion cGy to all the inhabitants of some world at an Earth distance (that, of course, will be blown to bits) near IK Pegasi. That world is gone but the radiation from the star travels to Earth over the distance given.


Now, any radiation will be diminished according to the well known inverse square law. That is, the intensity diminishes as the inverse SQUARE of the distance.

Since the distance in this case is (10^7) times farther for Earth than to reach the hypothetical (former) planet of IK Pegasi, then it must be diluted by the factor:

I'  ~  1/ (10^7) 2  or 1/ 10^14

This means a 10^9 cGy dose of radiation (at the original distance from IK) will be diluted to:   

(10^9 cGy) (10^-14) =  10^-5 cGy

which I submit is probably much less than a terrestrial air traveller would receive in making a cross-polar trip from Chicago to Oslo, Norway.

Again, all these distant supernova examples amount to storms in teacups compared to the genuine proximate threats we face on this planet, and which I wish people would pay more attention to (though I do concede that distant exploding stars are more exotic).

As for a "safe distance" from a supernova, for myself, anything more than ten light years distant, I really wouldn't lose sleep over. I DO lose sleep thinking of a runaway greenhouse effect by 2050.




---------- FOLLOW-UP ----------

QUESTION: Thankyou for your response.
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You stated that anything more than 10 light years away you would feel is a "safe distance"

The reason I was concered was the fact that I keep hearing all over the internet that you have to be more than 26 light years away from a type II supernova and something like 200 light years away from a type 1a to be safe since type 1a supernova explosions are more powerful?

My concern was that since IK pegasi may blow up as a type 1a supernova i was just wondering if we are far away enough from it to be safe?


ANSWER: Hello,

Well, let's see now, what sort of stories and rumors circulate the net? Last I checked most of them are based on more rampant speculation than anything else.

And how do they arrive at this 26 LY figure anyway? Do they realize there are NO serious supernova candidates within a sphere of radius 26 Ly to Earth? The 200 LY figure for Type 1as I would have to see directly how exactly they work that out. It just appears a preposterous figure to me. (Btw, my reference to the radiation intensity at an Earth like distance from IK Pegasi was supposed to be one billion cGy not one hundred billion. Please make that change)

Even if it is true that being within 200 Ly won't help us, so what? There isn't a thing humans can do (short of using a faster than light ship to colonize a new world at greater distance) to escape it. Even then, the colonizing humans might find themslves in a new "sphere of harm" from another possible Type 1a supernova.

So, the writing is on the wall either way. As I said, I simply am not going to worry myself into a frenzy over some star 150 LY distant blowing its gasket anytime soon in a hypothetical supernova. If it happens, it happens. We who are fretting over it now will likely be long dead by the time the radiation reaches us - IF it does!

Again, my main concern now is global warming, which poses a real and discernible threat, also one we still have time to mitigate if we act now instead of using specious rationalizations and listening to skeptics. (As columnist Philip Stephens in the Financial Times observed, most of those climate skeptics are at an age where they won't be alive to be proven wrong anyway when the worst unfolds.)

At least intelligent and expeditious human action might avoid the runaway greenhouse effect.

So, again, here is my priority scale list of worries for possible human perdition:

1- Global warming

25- Torino scale 10 asteroid strike in next 100 years.

1000+ - IK Pegasi becoming a Type Ia supernova.

---------- FOLLOW-UP ----------

QUESTION: hi again

i was reading ( section 4 of)
http://www.tass-survey.org/richmond/answers/snrisks.txt
which is a risk assessment of supernovae
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the author concluded that:

"I suspect that a type II explosion must be within a few
parsecs of the Earth, certainly less than 10 pc (33 light years),
to pose a danger to life on Earth.  I suspect that a type Ia explosion,
due to the larger amount of high-energy radiation, could be several times
farther away.  My guess is that the X-ray and gamma-ray radiation
are the most important at large distances."
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I'm still confused on this subject because on the gamma ray section (section 4) the author says this...
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"...suggests that considerably more than 100,000 erg/(cm^2) in gamma-rays are needed to destroy the ozone layer, so it seems that a type Ia would have to be closer than 1 kpc to cause significant damage." >>>end quote
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so would you agree or disagree  that a ia SN explosion has to be within 1 kilo parsec to cause damage from gamma-radiation?  

Answer
Hello,

I think what the author is saying, is essentially similar to what I have been saying - that any supernova less than 1 kpc (= 1000 parsecs or 3200 light years) distant is nothing to fret over.

The "100,000 erg/(cm^2)" is also interesting, because Class IV (optical) solar flares unleashed in (August) 1972 had an order of magnitude more intensity than this - and ended up eroding some 60% of the then ozone layer. They also helped to bring down the 'Skylab' - because of the expansion effect (via heating) on the atmosphere, so that Skylab then found itself at much too near an altitude to the atmosphere to survive.

So, in other words, our own little star the Sun has way more capacity to inflict damage on us than some distant supernova's gamma rays.

So yes, I would definitely concur that  SN explosion has to be within 1 kiloparsec to cause damage from gamma radiation.