Question about stars going nova:

I am reading a sci fi book by Micheal McCullum, Sails of Tau Ceti, and one of his sci fi props, the setup, is to say Tau Ceti went nova. I won't bore you with plot details but wondered if anyone knows enough about novae to say whether Earth would be effected by this star, only 12 LY away. It masses about 80% of Sol, and a radius about the same ~80%, so the surface gravity is about the same as the sun. It is lighter in 'metals' than the sun and is estimated to be about 10 billion years old. Can anyone here figure out the energy given off by such a star going nova and if it would lead to any damage say, to the ozone layer or some such?

Originally posted by sonhouse
I am reading a sci fi book by Micheal McCullum, Sails of Tau Ceti, and one of his sci fi props, the setup, is to say Tau Ceti went nova. I won't bore you with plot details but wondered if anyone knows enough about novae to say whether Earth would be effected by this star, only 12 LY away. It masses about 80% of Sol, and a radius about the same ~80%, so the ...[text shortened]... ch a star going nova and if it would lead to any damage say, to the ozone layer or some such?

If it´s mass is 80% of the sun´s it can´t undergo a core collapse supernova. For a type Ia supernova it needs to be part of a binary system - Tau Ceti has only one stellar component and so cannot supernova. This also rules out a nova explosion as that is caused by a white dwarf accreting material from a giant twin, which then undergoes fusion on the surface of the dwarf. Tau Ceti is in the main sequence, so it is also in the wrong part of it´s evolution and will be for billions of years. If Tau Ceti were to go supernova then it would be very bad news for earth - a nova is less of a problem as the energy release is much smaller. There are no supernova candidates within a few hundred light years.

If this star is less massive than the Sun (and not part of any binary system, not colliding with anything etc.) it will most likely fade out and turn into a white dwarf.

Originally posted by DeepThought
If it´s mass is 80% of the sun´s it can´t undergo a core collapse supernova. For a type Ia supernova it needs to be part of a binary system - Tau Ceti has only one stellar component and so cannot supernova. This also rules out a nova explosion as that is caused by a white dwarf accreting material from a giant twin, which then undergoes fusion on the su ...[text shortened]... gy release is much smaller. There are no supernova candidates within a few hundred light years.

In the story, it was a nova not a supernova, and we are dealing with an alien race, who maybe had the technology to FORCE it to nova. Under those conditions, what effects would show up on Earth?

Originally posted by sonhouse
In the story, it was a nova not a supernova, and we are dealing with an alien race, who maybe had the technology to FORCE it to nova. Under those conditions, what effects would show up on Earth?

The only thing I can think of is if they could induce the core to collapse when it would proceed like pair-instability supernova imposter event Reveal Hidden Content, when you´ll get an event like the one at Eta Carinae in the 1860´s. The difference is Eta Carinae is a truely huge star weighing over 100 solar masses. Tau Ceti is smaller than the sun and still burning hydrogen. If you assume they have some plot device which allows them to compress the core then maybe they could induce a hydrogen flash.

Estimates for the safe distance from a type II (core collapse) supernova start at 26 light-years. The only sane model that approximates what happens in the book is a pair instability induced supernova imposter event - so probably some damage to the ozone layer, and not much else since the energy emitted will be a lot less than in a true supernova.

http://en.wikipedia.org/wiki/Near-Earth_supernova

Originally posted by DeepThought
The only thing I can think of is if they could induce the core to collapse when it would proceed like pair-instability supernova imposter event [hidden]see e.g. http://en.wikipedia.org/wiki/Eta_Carinae[/hidden], when you´ll get an event like the one at Eta Carinae in the 1860´s. The difference is Eta Carinae is a truely huge star weighing over 100 solar ...[text shortened]... ill be a lot less than in a true supernova.

http://en.wikipedia.org/wiki/Near-Earth_supernova

So a type I is a regular nova and a type II is a supernova? How much energy difference are we talking about? Thousand times? Million times?

Originally posted by sonhouse
So a type I is a regular nova and a type II is a supernova? How much energy difference are we talking about? Thousand times? Million times?

There are type I and type II supernovae. The difference in magnitude of a nova and a supernova is roughly a factor million.

Originally posted by sonhouse
So a type I is a regular nova and a type II is a supernova? How much energy difference are we talking about? Thousand times? Million times?

No, the classification of supernovas depends on the spectrum of light emitted. Type II supernovas show hydrogen absorption lines, type I supernovas do not. Type I supernovas are divided into Type Ia and types Ib and Ic, type Ia has a absorption line for silicon. Type Ib and Ic do not, Ib has helium absorption lines and Ic does not. They classify them like this because it depends on observations - which won´t change - not on theories as to why stars do this which might.

Type II supernovas are believed to happen due to core collapse of stars > 3 solar masses. Type Ib and Ic supernovas happen due to core collapse in a star which has lost some of its outer layers, which explains the absence of hydrogen and in Ic supernovas helium lines.

Type Ia supernovas are believed to be caused by a white dwarf star accreting material from a giant binary partner which causes it to gain mass until it reaches the Chandrasakar limit which means it collapses, the collapse causes it to undergo a fusion burn of carbon of a large fraction of the material in the star in a few seconds. The reaction is enough to completely destroy the star.

A nova is a white dwarf star which is not (necessarily) close to the Chandresakar limit which is accreting hydrogen. As the hydrogen builds up on the surface of the star it is occasionally explosively burned in a fusion reaction to helium. This is a hydrogen flash and different from a supernova event which is far more energetic. The star survives the hydrogen flash more or less intact and after accreting more material may supernova in the future.

The star in the science fiction novel cannot supernova as it is too young, and there is no known mechanism for a star in it´s stage of life and of its size to nova. The aliens would have to have some method to either compress the core or make it overheat. This is more like a type II supernova than a nova.

Originally posted by DeepThought
No, the classification of supernovas depends on the spectrum of light emitted. Type II supernovas show hydrogen absorption lines, type I supernovas do not. Type I supernovas are divided into Type Ia and types Ib and Ic, type Ia has a absorption line for silicon. Type Ib and Ic do not, Ib has helium absorption lines and Ic does not. They classify them ...[text shortened]... ither compress the core or make it overheat. This is more like a type II supernova than a nova.

I didn't know there were that many classifications of nova and supernova.
So it sounds like the hydrogen flash kind of nova could happen more than once if the dwarf star survives the first flash, it could accrete another layer of H2 and blow off again, unless that kind of thing takes more time than the age of the universe to redo. I think in the story, (which I haven't actually read far enough to know for sure) that aliens blew up the star to provide a push for a solar sail spacecraft, which sounds like a pretty lame plot if that is what is going on in the book, like if they have the technology to actually blow up a star like that, why would they need to do it just to run a light sail spacecraft. I guess I'll find out in a few chapters🙂

Originally posted by sonhouse
I didn't know there were that many classifications of nova and supernova.
So it sounds like the hydrogen flash kind of nova could happen more than once if the dwarf star survives the first flash, it could accrete another layer of H2 and blow off again, unless that kind of thing takes more time than the age of the universe to redo. I think in the story, (wh ...[text shortened]... need to do it just to run a light sail spacecraft. I guess I'll find out in a few chapters🙂

There are further subdivisions of supernova depending on how the light curve decays. There is only one type of nova, which is hydrogen accretion. Tau Ceti can´t do any of this stuff, it´s too small for core collapse and not a white dwarf yet, and without a partner has no way of accreting material. The novel´s bizarre if you can cause a star to do something like that then propulsion for star ships really isn´t going to be a problem.

You are right about white dwarves being able to nova recurrently, they can do it quite frequently, RS Ophiuchi novas once every 20 years or so see http://en.wikipedia.org/wiki/RS_Ophiuchi

Originally posted by DeepThought
There are further subdivisions of supernova depending on how the light curve decays. There is only one type of nova, which is hydrogen accretion. Tau Ceti can´t do any of this stuff, it´s too small for core collapse and not a white dwarf yet, and without a partner has no way of accreting material. The novel´s bizarre if you can cause a star to do some ...[text shortened]... uently, RS Ophiuchi novas once every 20 years or so see http://en.wikipedia.org/wiki/RS_Ophiuchi

That is awesome! It goes from 12.5 to 5 mag. every twenty years! And is INSIDE the extended atmosphere of a giant red star! Amazing! So if I get it right, the difference in energy is 12.5-5 or 7.5 and that would be 2.5^7.5 right? That comes out to a power ratio 965, nearly a thousand times the energy from quiescent to blasting. So that kind of thing could never happen to Tau Ceti even if super advanced aliens could dump the same amount of hydrogen on the surface of that star because the surface gravity of Tau is about the same as our sun and a white dwarf has a surface gravity thousands of times greater which would make for thousands of times more dense concentration of hydrogen on its surface. I can't see even some theoretical way to make Tau go nova then. If that star RS were at 12 LY distance then, it wouldn't do much to earth since its normal output would be probably in the Picowatt/square meter whereas a star like Sirius puts out about 0.1 milliwatt per square meter at 8 LY away (9.8 microwatts per square foot) So even if Sirius was to undergo some kind of H2 blowoff, and increased a thousand times, it would only give Earth about 100 milliwatts per square meter max it would seem so there would be nothing to fear from even mighty Sirius if it went nova. If it went SUPERnova though, we would be all dead within a matter of weeks after the flash hit the Earth though, it would seem. If it was a million times its normal energy than that would represent 100 watts per square meter, still 1/10th the output of the sun at Earth's distance. I guess the problem there would not be the actual intensity but the fact it would be coming at you in Gamma and X-ray, right? I guess the way RS nova's it must not even do much damage to its host star, the red giant.