WOW - Whats 20 times brighter than all the 100 billion stars comprising our Milky Way

_Tombstone_

And 570 times brighter than the Sun and 200 times more powerful than an already very powerful Supernova?


It's what the smart folk are calling a superluminous supernova, detected by scopes that look for flashes down in Chile last year. It's 3.8 Billion light years away but they are at a loss as to what type of Star could cause it or how it's fueled.

It'd take our Sun 90 Billion years to equal this things emissions.

Them are some big numbers! What Warp would the Enterprise have to be doing to outrun that?


This is an artist's impression of the record-breakingly powerful, superluminous supernova ASASSN-15lh as it would appear from an exoplanet located about 10,000 light years away in the host galaxy of the supernova.




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ps200306

I don't know if that artist's impression is based on realistic calculations of the brightness as seen from 10,000 ly.

But if it is, it occurred to me that our nearest star likely to go supernova is Betelgeuse in Orion.

At 642 ly distance, using the inverse square rule for luminosity would make it brighter by a factor of:



Betelgeuse would only be a "normal" supernova, but the lower distance would more than make up for the factor 200 lower brightness.

So we'd probably see it blazing even in the middle of the day!

Rubecula

When Betelgeuse goes supernova is it close enough to cause us problems?

ps200306

Rubecula wrote: »

When Betelgeuse goes supernova is it close enough to cause us problems?

Let's take the statement from Tombstone's post that the new supernova is "570 [billion] times brighter than the Sun and 200 times more powerful than an already very powerful Supernova". That means your very powerful supernova is 2.5 billion times as bright as the Sun. We can sanity check this. Look up Wikipedia on light curves of supernovae. Betelgeuse would be a core collapse supernova of type II, with a typical absolute magnitude of -17. Google the Sun's absolute magnitude and you get 4.83. Magnitude is a logarithmic scale whereby five magnitudes represents a brightness ratio of 100. So we can get the ratio of the supernova to Sun brightness like this:



That makes our core collapse supernova about half a billion times brighter than the Sun, which is in the ballpark of our "very powerful supernova" at 2.5 billion. It would make our very powerful supernova about 1.75 magnitudes brighter than a typical type II.

Ok, let's use the luminosity-distance inverse square relation again, to see how much dimmer something gets when we move it from the distance of our Sun to the distance of Betelgeuse. This time we convert from light years to miles before dividing by the distance to the Sun:



So for the sake of round figures we suppose that our Betelgeuse supernova is between the typical type II and the "very powerful" one, at 1.6 billion times the brightness of the sun. Dividing by our luminosity distance factor, we see that it would have an apparent brightness about a millionth that of the Sun. That may not sound like much, but it's half the apparent brightness of the moon, and concentrated in a tiny point appearing much smaller than the moon, so it would be blazingly bright even in broad daylight, and would cast strong shadows at night time. It would be a thousand times brighter than the brightest Venus.

On the other hand, think about the amounts of energy involved. A millionth of the incident solar power is about a milliwatt per square metre of the Earth's surface, compared to the Sun's kilowatt. But we also have to bear in mind that a supernova would much hotter than the Sun. This means much more of the radiation it gives off is at short wavelengths, according to the Planck curve. That means more potentially harmful X-rays and gamma rays.

But the distance to Betelgeuse would still keep us safe. Scientists reckon the nearest safe distance to a type II supernova is around 50 to 100 ly. At that sort of distance we'd get somewhere between a tenth of a watt and several watts per square metre which -- in the form of gamma rays -- would ionise molecules in our atmosphere, creating radicals that would deplete the ozone layer.

On the other hand, if the solar nebula from which our Sun formed in the first place hadn't itself been enriched by the ejecta from many supernovas, there'd be no heavy elements from which to make either the Earth or us. So we wouldn't be here to worry about it. Supernovae -- can't live with 'em, can't live without them. :pac:

Rubecula

Yeah but will it smart a bit?

_Tombstone_

Rubecula wrote: »

Yeah but will it smart a bit?

And what speed to outrun it lol?

Come on PS!

ps200306

Rubecula wrote: »

Yeah but will it smart a bit?

Nah, it'll just give you a nice gamma ray glow.

_Tombstone_ wrote: »

And what speed to outrun it lol?

I think the short answer is that you can't outrun a supernova explosion. The reason is that if you are close enough then you are going to be fried by hard radiation, and that travels at the speed of light, which can't be exceeded.

If you somehow had a magic radiation shield, and were only worried about outrunning the material shockwave from a supernova, then there may not be any definitive answer. The speed of the expanding shockwave is measured by Doppler shift of the ejecta, and has been clocked at 5,000 to 30,000 km/sec, about 10% of light speed. The spherical symmetry of a supernova produces lots of turbulence in the outflow so it slows down rapidly as the energy of the initial shock wave gets converted into heat and macro-turbulence. That said, the Crab Nebula is still expanding at 1,500 km/sec a thousand years after the supernova explosion that created it.

Very powerful supernovae like the one in the OP may be a different class of beast. Gamma ray bursts (GRB) are the most powerful events in the universe and at least some of them seem to be associated with supernovae. They only come from far distant galaxies, implying they occurred in an earlier epoch of the universe. These supernovae may be associated with very large stars with low metallicity and high rotation rates. Some mechanism seems to funnel the supernova explosion asymmetrically into narrow jets. We already know about several types of astrophysical jets, the result of material falling into black holes of either stellar mass in the case of X-Ray binary stars, or supermassive black holes at the centres of active galaxies, like this one photographed by the HST:




Although there is a wide variety of types, some GRBs may be associated with a special type of core collapse supernova. Called a hypernova, or collapsar, the idea is that a rapidly rotating black hole forms inside the star. Normal supernovae will generate a spherical shock from a rebound of the collapsed core, which compresses and blows away the outer layers of the star. But the high rotation speed of a collapsar causes material near the core to spin down into an accretion disk around the core.

In essence, we have an object that produces the sort of astrophysical jets we know about, but it's inside a star that has not yet been disrupted. Now the jets form, but they have to punch their way through the outer layers of the star. In a modern star which is contaminated with all the gunk from previous supernova explosions that went into the cloud that formed it, the higher metallicity would cause the energy of the jets to be converted to heat and turbulence and a more symmetrical explosion. (The heavier elements are much more efficient at down-scattering gamma rays and converting them to heat).

But in a collapsar, the jets punch all the way to the surface and smash out through it at 99.995% of the speed of light. Good luck trying to outrun that!

ps200306

It just occurred to me -- there is a sneaky theoretical way to outrun a supernova, even the light speed radiation, without violating the laws of physics. If you travel away from it at high speed the radiation from the supernova gets red shifted. At sufficiently high speeds, the emitted gamma rays get red shifted into harmless optical frequencies as observed by you.

The red shift, denoted z, is related to the ratio of the emitted and observed frequencies:



The frequency of light is directly proportional to its energy. So the above formula works if we replace the frequencies with energies, since gamma rays are more usually specified in energy terms than frequencies or wavelengths. There is no precise definition of where the gamma ray spectrum starts, but it is usually taken to mean energies characteristic of nuclear processes, which are usually between 0.1 and 10 MeV (million electron volts, a unit of energy).

Core collapse supernovae can occur because of the failure of electron degeneracy pressure -- the repulsive electric force between electrons -- which is the only thing holding up the star's weight when it runs out of fuel. Electrons and protons combine to form neutrons, releasing a burst of neutrinos. The combination of shockwave, neutrino burst, and neutron rich environment causes the synthesis of vast quantities of heavy elements in the outer layers of the star as they are blown away in a matter of minutes. One of the most common is the radioactive isotope nickel-56 which decays with a half life of 6 days to cobalt-56, which in turn decays with a half life of 77 days to iron-56. The decays release about 0.8 and 1.2 MeV respectively. So we could use 1 MeV as a typical energy for a 'normal' supernova.

The blue end of the visible spectrum is the most energetic, at about 3 eV. So we're going to need a red shift factor of more than 300,000 to make even the softest supernova gamma rays look like harmless blue light! This is paltry, though, compared to the energy of a gamma ray burst. Those gamma rays can have energies of 10 TeV (trillion electron volts)!

One point of information concerns the speed with which the supernova shock front is moving. When we calculate the speed we need for our desired red shift, don't we need to take that into account as well? The short answer is no. For a 'normal' supernova, the shock front speed of < 0.1c (10% of light speed) would be considered non-relativistic. For our GRB/hypernova, the shock front speed is already taken into account, since the measured energies are boosted by their speed towards us, an effect known as relativistic beaming or Doppler beaming. That makes our calculation simpler.

First we must introduce the Lorentz factor, which is just a term that crops up all over equations to do with Special Relativity. It is denoted by the Greek letter gamma, but this coincidence is nothing to do with gamma rays.



Then we can relate the red shift to the velocity using this formula:



The approximation of 2 x gamma is good for speeds above 0.9c (90% light speed). We can then convert back from gamma to a fraction of light speed like this:



So lets run the numbers. To outrun a Betelgeuse style supernova by red shifting its soft gamma ray glow to blue light, we need a red shift of about 300,000, thus a gamma (Lorentz) factor of 150,000. For the GRB energies of 10 TeV, just multiply those numbers by a factor of ten million! :eek:

We find that for our Betelgeuse supernova we need a velocity of 0.99999999998c. For the GRB/hypernova ... well, let's just say that Microsoft Excel wasn't able to calculate to enough decimal places, but a rough approximation suggests that there'd be about 25 nines after the decimal point.

Practically speaking, you'd run into a few problems. First, how would you even know the supernova had occurred? The first inkling might be when you got fried by gamma rays! If you were lucky, the surface of the star would start to brighten a few minutes before the full onslaught. Then, to achieve the necessary speeds you'd have to be supernova powered yourself ... and the acceleration would crush you to a dot. But apart from that you'd be grand

The good news is that if you managed to get to the speed needed to outrun the Betelgeuse class supernova, it would only take you about six hours in your own frame of reference to travel to the 100 light year safe distance. Then -- if you wanted to avoid overshooting and leaving the galaxy altogether -- you'd have to decelerate and get crushed to a dot for a second time. :pac:

Maximus Alexander

ps200306 wrote: »

<Hitchhicker's Guide to Outrunning a Supernova>

Is there anything to be said for just building a lead lined bunker?

mickmackey1

ps200306 wrote: »

how would you even know the supernova had occurred? The first inkling might be when you got fried by gamma rays! If you were lucky, the surface of the star would start to brighten a few minutes before the full onslaught.

We've never seen a supernova that has been predicted beforehand, and I've a sneaky suspicion the star would be showing warning signs for maybe centuries, along the lines of a giant volcano rumbling and belching ash before finally blowing its top.

ps200306

mickmackey1 wrote: »

We've never seen a supernova that has been predicted beforehand, and I've a sneaky suspicion the star would be showing warning signs for maybe centuries, along the lines of a giant volcano rumbling and belching ash before finally blowing its top.

The signs of impending doom would indeed go on for millennia. A highly evolved star would be belching lumps of its surface into space -- as Betelgeuse is doing -- due to the very low surface gravity, perhaps a thirtieth of the earth's and a thousandth of our Sun's. The material might build up into a planetary nebula (nothing to do with planets) around the star, although this could actually be a contra-indication that a supernova was likely to happen as the heavier stars don't last long enough to build a nebula. Betelgeuse doesn't have a prominent one, but lobes of material surrounding it have been discovered recently, ejected in just the last couple of tens of thousands of years. There's also a shockwave where different episodic mass ejections run into each other.

But the strong stellar winds would only be symptomatic of what was going on hundreds of millions of kilometres down in the core. The giant size of the star has been produced in the first place by the high temperatures generated as the star burns through successive generations of heavier and heavier fuel. When the fuel is finally exhausted the end comes astonishingly quickly. This is the phase I'm talking about.

There will be no immediate sign at the surface that anything has changed, but the entire structure of the core -- perhaps several solar masses worth, which have been burning for millions of years -- will have suddenly disintegrated in literally a matter of milliseconds and gone into free fall toward the centre. A quarter of a second later it gets there, and a rebound shockwave propagates outward. The star is so huge that even a blast traveling at the speed of light could take half an hour to reach the surface. But the bulk of the star is opaque to radiation, so things don't go at that speed.

The first thing that emerges might be a burst of neutrinos. They're able to travel through a light year of lead with only an average chance of interacting with anything, so the star will be almost transparent to them. Next comes the shockwave, which will have taken several hours to emerge, compressing the layers of the star outside the core on the way out. Once this shockwave reaches the photosphere, all hell breaks loose. This is the layer that is transparent to light, the visible surface of the star.

Not only is the surface now heading your way at 10% of light speed, but it is filthy, like a gazillion Chernobyls all blowing their tops together. The compression has caused exotic nuclear reactions that build up elements heavier than iron, and lots of them are intensely radioactive. They are glowing in gamma rays that will eventually be lethal to most things within a hundred light years. This is where you want to be getting out of the way pronto!