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View Full Version : [science] I have a question for people that know about explosions and astronomy.



Jason California
06-12-2011, 03:45 PM
Say there was a series of explosions just outside of our solar system. How big would these need to be in order for them to be clearly visible in the night sky? Is there a resource someone can point me in for this type of information?

MayorMitch100
06-12-2011, 03:48 PM
I'm not an astronomer, but I think they'd have to be pretty damn big.

Jason California
06-12-2011, 03:55 PM
I'm not an astronomer, but I think they'd have to be pretty damn big.

Yeah, I figure as much. I am imagining a space fight that is probably more monumental than any space battle you have ever seen in a movie before. I am talking city class ships here. I am trying to figure out if part of my scenes works.

Ben
06-12-2011, 04:03 PM
With the naked eye? In a city with light pollution?

Ben
06-12-2011, 04:03 PM
Would we see the explosion if Uranus blew up? I doubt it.

Jason California
06-12-2011, 04:08 PM
With the naked eye? In a city with light pollution?


I am trying to figure out what the variables would be. THe more I think about it the more I doubt it, but the scenes works better with it, and since it is fiction I may not care.

EmarAndZeb
06-12-2011, 04:08 PM
I don't know about being visible in the night sky, but something the size of a city blowing up would certainly be detectable with instruments, if we had anything pointed that-a-way.

http://www.projectrho.com/rocket/spacewardetect.php


Wargames like GDW's STAR CRUISER describe interplanetary combat as being like hide and go seek with bazookas. Stealthy ships are tiny needles hidden in the huge haystack of deep space. The first ship that detects its opponent wins by vaporizing said opponent with a nuclear warhead. Turning on active sensors is tantamount to suicide. It is like one of the bazooka-packing seekers clicking on a flashlight: all your enemies instantly see and shoot you before you get a good look. You'd best have all your sensors and weapons far from your ship on expendable remote drones.

Well, that turns out not to be the case.

The "bazooka" part is accurate, but not the "hiding" part. If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor.

The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit.

This is with current off-the-shelf technology. Presumably future technology would be better.


EDIT: Keep in mind, the Kuiper belt extends (http://en.wikipedia.org/wiki/File:Outersolarsystem_objectpositions_labels_comp. png) about 1/1000 of a light year (http://www.google.com/search?hl=en&q=1+light+year+in+au) out from the Sun, so if you're really all that concerned about accuracy, people on Earth wouldn't actually see your big explosion for about 8 or 9 hours.

Edited again because I am dumb at reading numbers. Fuck it has been so long since I was in a physics class.

R0cketFr0g
06-12-2011, 04:21 PM
Would we see the explosion if Uranus blew up? I doubt it.

What if there were Klingons around Uranus?

Jason California
06-12-2011, 04:25 PM
I don't know about being visible in the night sky, but something the size of a city blowing up would certainly be detectable with instruments, if we had anything pointed that-a-way.

http://www.projectrho.com/rocket/spacewardetect.php




EDIT: Keep in mind, the Kuiper belt extends (http://en.wikipedia.org/wiki/File:Outersolarsystem_objectpositions_labels_comp. png) about 1/100 of a light year (http://www.google.com/search?hl=en&q=1+light+year+in+au) out from the Sun, so if you're really all that concerned about accuracy, people on Earth wouldn't actually see your big explosion for a few days

Edited again because I am dumb at reading numbers.


Not being seen for days is OK. I guess I need to find out what I would need for a single explosion that would momentarily be the brightest star in the sky.

EmarAndZeb
06-12-2011, 04:35 PM
Not being seen for days is OK. I guess I need to find out what I would need for a single explosion that would momentarily be the brightest star in the sky.

It's actually more like 8 - 20 hours, depending on which side of the sun we/the explosion are relative to one another. I am terrible at conversions.

Small objects can still be seen if they pump out a ridiculous amount of energy (see Quasars). I think it would have to do with how bright the explosion is, more than how big the ship blowing up is. We can't actually see the discs of stars outside our solar system, just the light they give off (hence they're called "point-like sources"). If your ships explode "hot" enough, they'd probably show up in the night sky. Are these alien ships powered by antimatter or captured blackholes or something crazy like that?

Jason California
06-12-2011, 04:40 PM
It's actually more like 8 - 20 hours, depending on which side of the sun we/the explosion are relative to one another. I am terrible at conversions.

Small objects can still be seen if they pump out a ridiculous amount of energy (see Quasars). I think it would have to do with how bright the explosion is, more than how big the ship blowing up is. We can't actually see the discs of stars outside our solar system, just the light they give off (hence they're called "point-like sources"). If your ships explode "hot" enough, they'd probably show up in the night sky. Are these alien ships powered by antimatter or captured blackholes or something crazy like that?

There is something huge out there that has been hiding for thousands of years. It is activated in this battle and gets blowed up. that will be my single explosion.

Ben
06-12-2011, 05:12 PM
I am trying to figure out what the variables would be. THe more I think about it the more I doubt it, but the scenes works better with it, and since it is fiction I may not care.A little hyperbole would probably be okay.

dasNdanger
06-12-2011, 05:19 PM
Look at it this way - Jupiter is in our solar system, and it's no bigger than an average star speck to the naked eye. If Jupiter was even further away - outside our solar system - and it blew up, do you think it would look impressive from earth?


das

EmarAndZeb
06-12-2011, 05:20 PM
Since I'm too math-dumb to do proper calculations, here's some other resources:

Comet Shoemaker-Levy 9, as seen through a telescope when it struck Jupiter:
http://www.youtube.com/watch?v=DgOTcIfU75Y

And the "Boom Table" which lists Shoemaker-Levy a ways down, along with a whole bunch of other explosions/energy outputs for comparison.

http://www.projectrho.com/rocket/spacegunconvent.php#boom

It looks like once you're into the hundreds-of-teratons to petaton range, you're talking about dinosaur killing asteroids and boiling the oceans off the planet and shit; if that's a reasonable range of power for this crazy-big ship when it explodes... who's to say that a larger percentage of that explosion didn't get released as light? It was totally visible in the night sky on earth.

Jason California
06-12-2011, 05:23 PM
The type of energy involved in this explosion is tremendous. We are talking about God Shops blowing up. I am sure it will be OK.

BriRedfern
06-12-2011, 05:26 PM
There is something huge out there that has been hiding for thousands of years. It is activated in this battle and gets blowed up. that will be my single explosion.

Perhaps whatever the thing is that goes kablowy, it is powered by something (such as antimatter or a cage singularity or the like) that would not make the size of the object the most important variable in the size of the final explosion. Perhaps the thing is as big as a city, but a power source the size of a nickel causes the explosion. And it is huge. Supernova huge.

Anything around the thing should obviously be obliterated if the event is viewable from Earth... Maybe a plutotoid (or whatever they are called now) should be destroyed or knocked out of orbit as a result.

GelfXIII
06-12-2011, 07:21 PM
the composition of the explosion would be of material importance in determining it's visibility, as well as its duration and decay rate. Even a small amount of mass achieving a nuclear combustion would create a large amount of visible light and also non-visible radio waves which would be easily seen on many radio telescopes around the world. A non-fission based explosion would have to be enormous to be noticed in the sky, or else very very close, like in LEO, certainly no farther than a lagrange point.

Dreaded Anomaly
06-12-2011, 10:38 PM
Astronomers have a lot of conventions that they use to describe an object's brightness and visibility. I'm taking my cue from http://en.wikipedia.org/wiki/Luminosity in order to do a sample calculation.

Using the equation given on the page for finding the apparent magnitude of a star from a given distance:
http://upload.wikimedia.org/math/3/5/a/35ac972142825954b3c601d32871bfcb.png

I'm first going to rearrange the equation to give us the luminosity (L) once we specify the apparent magnitude (M) and distance (d):
L_star = L_sun*(d_star/d_sun)^2*10^(0.4*(M_sun-M_star))

The sun quantities are known: M_sun = -26.73, d_sun = 1 AU = 1.58×10^-5 lightyears (ly), L_sun = 3.846×10^26 watts (W).

Let's take your example of an explosion just outside the solar system that is the brightest star in the sky. We can say the distance d_star = 1 ly (given the Oort Cloud is almost 1 ly from the sun), and the brightest star in the sky is Sirius, which has an apparent magnitude M_star = -1.47. (See http://en.wikipedia.org/wiki/Apparent_magnitude for a table of these values.)

Plugging all of this into the equation: L_star = 1.2×10^26 W.

To figure out the energy, we have to know how long the explosion lasts. To give us a standard for comparison, the most powerful recorded detonation of a thermonuclear weapon was 50 megatons of TNT (http://en.wikipedia.org/wiki/Nuclear_bomb), or roughly 2×10^17 joules (J). Watts are joules/second, so if I'm lazy and just say the explosion lasts for 1 second, the energy is E = 1.2×10^26 J. This is roughly 600 million thermonuclear detonations.

The order of magnitude of that number can be reduced by lowering the apparent magnitude of the explosion, the time that it lasts, or its distance from us. This is also a fairly simplistic calculation, as the formula used is only for luminosity in the visible spectrum. As Gelf points out, a real nuclear explosion would generate energy in a much larger spectrum, including radio.

Bri suggests antimatter as another explosive source. Antimatter's energy per unit mass is 9×10^16 J/kg (http://en.wikipedia.org/wiki/Antimatter#Fuel), so we can use that to figure out how much antimatter we would need to produce the above explosion. The result is 1.34×10^9 kg of antimatter, which is around the mass of the Great Pyramid of Giza (http://en.wikipedia.org/wiki/Orders_of_magnitude_%28mass%29) or a pretty small asteroid.

GelfXIII
06-13-2011, 06:03 AM
Yeah, that's what I meant to say.

BriRedfern
06-13-2011, 06:08 AM
That is fucking awesome DA. Only on the Benids Board would the question get that thorough an answer. :lol:

greg donovan
06-13-2011, 06:15 AM
now i wanna read what you are up to jason.

Dreaded Anomaly
06-13-2011, 10:03 AM
that is fucking awesome da. Only on the benids board would the question get that thorough an answer. :lol:

:grin:

Jason California
06-13-2011, 10:08 AM
now i wanna read what you are up to jason.


It part of some ideas I have been playing with for a long time. If it ever gets to the point where I am comfortable sharing you will be on the list.