I know it might be too soon to talk about some of the details in Star Wars: The Force Awakens, so you might want to wait on this blog post. This is your warning. You still have time to leave.

While you consider your decision, here is a random picture.

Img 8329

OK. You’re still here. I guess that means we can talk about Star Wars. Specifically, the physics of the awesome Starkiller Base.

Starkiller Base Assumptions

Yes, everyone knows the physics of Starkiller Base isn’t perfect. It doesn’t have to be. That doesn’t mean we can’t have a discussion about the science of a star-sucking planet killer. Before we get to the questions, though, let me start with my basic assumptions about some of the parameters of the First Order’s weapon of mass destruction. I’ve seen the movie just once (so far), but here’s what I can guess.

  • The Starkiller gets its power by sucking out a star. I am going to say the base draws all of the star’s mass into the weapon, leaving nothing behind. Clearly scholars could debate the mechanics of a star suck for decades without really deciding how it works.
  • This mass-sucking process is quick. Let’s say it takes 10 hours. Really, it doesn’t matter if it is just one hour or 48 hours. Either way, that’s really fast in terms of astronomical processes.
  • The Starkiller fires something at other planets. I have no idea what it is, but it can reach another star system and destroy planets.

This is what I’ll start with. Now I will also use two physics principles (other than saying mass is conserved). First, I will assume that momentum is conserved during the star sucking. This means that if the center of mass of the star-planet system is stationary, the center of mass will be in the same location after suckage.

Second, angular momentum will be conserved. If there are no external torques on the planet-star system, then the total angular momentum should be the same both before and after the planet absorbs the mass of the star. If I assume the star is much more massive than the planet, then just about all the angular momentum before the weapon charges is due to the orbital motion of the planet. However, there also is a component of angular momentum due to the rotation of the planet. If the rotation of the planet is the same direction as the orbit, then the total angular momentum can be written as:

La te xi t 1

I will use this expression to answer one of the following questions.

What will happen to the solar system when the star gets sucked?

Since the center of mass of the Starkiller Base plus the star would remain the same, as the base accumulated mass and the star lost mass the base would move to the center of mass. Assuming a star like our sun and a planet like Earth, the center of mass starts inside the sun. This means that at the end of the suck, there would be a planet where the star used to be.

What would this do to the rest of the planets in the star system? Nothing significant, actually. The primary interaction for the orbit of a planet is the gravitational interaction with the star. Now that the star is replaced with a planet with approximately the same mass as the star, nothing really changes.

OK, technically the planet-planet gravitational interaction would change since one of the planets moved. However, this is so small a force compared to the pull from the star that you could ignore it. In the long term it might cause a slight shift in orbits–but who cares, the star just disappeared. That’s a much bigger problem.

What would happen to the Starkiller Base as it increased in mass?

Let’s start with some numbers. Assuming a planet like Earth and a star like our sun, we have the following two masses:

  • Mass of star = 2 x 1030 kg
  • Mass of Starkiller Base = 6 x 1024 kg

The mass of the star is 300,000 times greater than the base. If all this mass is now inside the planet, then the gravitational field on the surface of the base would increase. Again, assuming an Earth-like planet the starting surface gravitational field would be 9.8 N/kg. If the radius of the base is the same as the Earth, increasing the mass by a factor of 300,000 would put the surface field at about 3 million N/kg. No one would be able to move. The would all be squashed on the surface of the planet–which also would be crushed.

If the Starkiller base maintained its size, it wouldn’t be quite small enough to turn into a black hole–but it’s close. If the radius decreased to 3 x 103 m, it would be a black hole.

But wait! There’s more. Remember what I said about angular momentum? As the Starkiller moves from its orbit to the new center of mass, it will no longer be moving around in a circle. Instead it will only be spinning. To conserve angular momentum, the planet-base must increase its rotational angular velocity. But by how much? Let’s say that the Earth is the Starkiller base so that I can use known mass and orbital data. Before the star-sucking takes place, I will assume all of the angular momentum is in the orbit (neglecting the rotation of both the sun and Earth). After sucking, it’s all angular momentum due to rotation.

La te xi t 1

Putting in values for our Earth-sun, I get a final planet rotation rate of 43.7 revolutions per second. Earth, of course, has a rotation rate of one revolution per day. This rotation rate is fast enough to fling everyone off the planet–or is it? What about the increased gravitational force from the extra mass? Would that be enough to keep a person on the surface of the planet?

Let’s assume a human is standing on the equator of this spinning planet. In the accelerating reference frame of the human, there are two forces: the downward gravitational force and the fake force pushing away from the center of rotation (centrifugal force). The centrifugal force depends on the mass of the human, the angular velocity and the radius of rotation. For the gravitational force, it depends also on the radius of the planet, but also the mass of the planet. Setting these two forces equal to each other I can solve for the mass of the planet needed to hold people down.

Spring 2016 Sketches key

In order for the human to stay on the surface, the mass of the planet would have to be greater than 2.9 x 1035 kg–which is quite a bit smaller than the mass of the star. These guys are going to fly off the planet. They’re doomed.

What happens when the Starkiller Base leaves the solar system?

I don’t know how the Starkiller Base moves, but it must have something like a hyperdrive to get it to the next star. But when it jumps out of the system, what happens to the planets left behind? Obviously, they will be literally “in the dark,” but they will also be without a star to exert gravitational forces. Just for fun, I made a quick model that shows two planets with a star that goes away.

Without a gravitational force to make planets follow a circular orbit, they would move off in a straight line. OK, technically the two planets would still interact, but this is a very small effect. But really, you could say Starkiller Base kills a bunch of planets–the ones it targets and the ones it leaves behind.

More Starkiller Base Questions

If you want something to calculate on your own, here are some suggestions:

  • Suppose you build a super awesome Starkiller Base that can shoot at stuff in another star system. How hard would it be to aim this weapon? Estimate the angular accuracy you would need to hit just one planet in a star system one lightyear away. Make a comparison to shooting something with a gun.
  • If momentum is conserved during the firing of this Starkiller, what is the recoil velocity of the planet? You will need to estimate the speed of the stuff fired from the weapon (I assume it is all the mass of the absorbed star).
  • Pretend you are a writer for Star Wars. Come up with a plausible method for Starkiller Base to shoot vast distances in just a few hours.
  • Would the process of a Starkiller Base sucking up a star create gravitational waves? Would they be detectable at Earth’s position? Is this the rumored gravitational wave detected by LIGO?

25 Comments

  1. Symphonic Metal Citadel

    No, it’s only mentioned in the movie that it’s a hyperspace weapon, i.e. the beam itself travels at hyperspeed (while remaining visible somehow).

    However, it is also supposed to have a hyperdrive. It’s just not mentioned or used in the film. http://scifi.stackexchange.com

  2. Symphonic Metal Citadel

    The author wrote, “Yes, everyone knows the physics of Starkiller Base isn’t perfect. It doesn’t have to be,” with a link to his article titled “The Physics in Star Wars Isn’t Always Right, and That’s OK.”

    And yet you decided to criticize the article on these grounds. Why? Why can’t you relax and let us have fun talking about science? (Albeit with errors, as other comments have pointed out.)

  3. The Starkiller Bases is also fitted with a hyperspace engine. This was mentioned briefly in the movie. It gathers energy from a star, then uses FTL travel to move into position to fire.

  4. came here to say this. as the energy was pulled in from the star, the mass of the planet would be increasing. .

  5. Im just some guy, but consider this:

    What if instead of assuming the base sucks the mass of a star we make the assumption that the base is actually collecting and concentrating the light of that star, like a giant “light capacitor”? The visual effect in the movie does look like its sucking mass but stick with me.

    Light does not have mass. Hypothetically if light were able to be concentrated it would have no “recoil” regardless of how much quantity was released. By the way light has been successfully trapped in scientific tests as far back as the 1990’s making it the perfect reach for science fiction: http://www.technologyreview.co

    In the most simple of terms under my assumption the Starkiller base is a giant magnifying glass, with the added ability to store light over time and release it all at once.

  6. According to this — http://nerdist.com/the-physics… — a star has plenty of energy to take many planets.

  7. RolandeDeschaine

    It’s still an issue of mass and inertia, anything that big takes forever to change direction.

    In the beginning scenes of Episode III the battle over Courasant shows some large capital ships performing extreme maneuvers but no high G rolls. There are some scenes in Episode V during the escape from Hoth where Star Destroyers are forced to make steep dives but mostly capital ships have to much mass to fly like a fighter.

    So with the example of the Falcon evading a Tie fighter they are bit closer in size and maneuverability but a Tie (with a good pilot behind the controls) will still outmaneuver the Falcon on any given day. Unless you put a crazy person behind the stick of the Falcon (Han and Rey) who are willing to try things that might kill them. Han out flew Ties in Episode V (asteroid field) because he was willing to push his limits, Rey was able to out fly the Ties because she also was willing to push her limits and knew the playing field better than the Tie pilots.

    Wow! I think I’ve expended way to much time thinking about SW piloting tactics I gotta get a life…

  8. The mass sucked off the star is stored in hyperspace, so it doesn’t contribute to the planet’s mass. Duh. Problem solved. [mic drop]

  9. alex alexiou

    More upvotes here please. We got a true scholar :o)

    Reading your post I couldn’t help wondering how come we never saw Stardestroyers performing high g roll maneuveres.

  10. alex alexiou

    Wait a sec… “Now that the star is replaced with a planet with approximately the same mass as the star, nothing really changes”.

    If I got the argument right, the conclusion is wrong because the starkiller can’t have possibly been in the same location with the sun (before the suckin). The solar system’s barycentre currently lays inside the sun’s photosphere. If the solar mass transfered to a new body (planet, Starkiller, black hole), say, 1 AU away, then the barycenter would move too, 1AU to the direction of this body. In our solar system, if the starkiller was orbiting where Earth is, the new barycenter would be (didn’t do the calculation) somewhere between the Moon and Earth. And this changes everything in the solar system, since Earth is much closer to Jupiter and the other giants. Imagine sitting in a carousel and suddenly the damn thing starts rotating around a new centre 20 inches towards the perimeter. The carousel would wobble and stress, presumably ejecting you along with your horse.

    Then, there’s the question of angular momentum (a.m.). Somehow, the sun has transfered the greatest part of his a.m. to the planets. Currently the sun’s a.m. is something like 3% of the total in the solar system, with Jupiter alone claiming more than 60% (I think). The new body would have it’s own a.m. before sucking the sun and it beats the hell out of me figuring out what would happen to this momentum if a solar mass was added to it. If the a.m. of this object was similar to the sun’s, I guess there would be no changes. But if it were quite different, then the new body would somehow have to shed it to the planets like the sun once did.

    So, if I ‘m right, Jupiter would not only have to instantly follow a new orbit but it would also change it’s spin rate. The tidal forces implied here are, well, enormous. Can you, somehow, model this?

  11. What? You say the mass is much more than necessary to keep them on the planet, and conclude that they’ll fly off? Did I misread that?

  12. Also, since the mass is kind of plasma/fluid state to get “sucked” in, wouldn’t it just keep flowing & rotating inside the planet? Preserving the angular momentum implies it forms a rigid body with the planet, which it still need to spew those mass out as a weapon soon so unlikely

  13. That was what I assumed from watching it. Siphoning the photosphere, you’d be able to fire for years before you took enough mass to kill the star.

  14. I assumed it simply siphoned a small portion of the photosphere, enough to make it go dark for a couple days. As for the travel time, in the movie they said the beams are fired into hyperspace so they don’t have to move the base to use it. Not very techy on SW, but I imagine going to hyperspace involves opening some kind of window to pass through. If they have the ability to fire plasma beams into hyperspace, that window can probably be used to aim and split the beam as needed.

  15. Right. They described it like a deathstar that fires the beam through hyperspace, so they can destroy any planet without having to move the base there.

  16. My assumption while watching the movie was that they only skimmed the photosphere, without which a star produces no light until a new layer heats up. It would take an obnoxious number of shots to kill a star that way, it would be able to fire from the same star continuously for years, if not decades.

  17. That was not at all what I got from the movie. It looked to me it was pulling in a portion of the photosphere, enough that it stopped producing light for a couple days. Once it fired the plasma jet into hyperspace as they described in the movie, they had to wait for the sun to rebuild its outer layer before they could fire again. I don’t remember them ever saying that the sun would be gone, just that it would go dark. In that manner, the Starkiller could be fired many thousands of times, maybe even millions, before the sun lost enough mass to be unable to rebuild its outer layer and stopped fusing.

  18. That’s not how the Force works!

  19. There’s a whopping amount of thermonuclear energy being transported into the base, too, that will fry everybody well before the process is complete.

  20. DarthFluttershy

    Or a “thermal resonator” perhaps? But I believe relativistic mass increases regardless of what form the energy is in… you just don’t usually notice because non massive energy is rarely concentrated in any significant amount in a fixed space. Then again, they have gravitational manipulation technology in Star Wars, so it really doesn’t matter.

  21. DarthFluttershy

    I was going to say this. It was a pretty egregious error here. Granted, the aspect of sucking up a star’s mass would still be pretty catastrophic without sci-fi tech to stop it, but it would not make you spin that fast with that much mass.

  22. RolandeDeschaine

    and this as well http://starwars.wikia.com/wiki

  23. RolandeDeschaine

    Tons of the Extended Universe books make mention of spaceflight physics and hardware (Rogue Squadron materials specifically).

    The boardgames and RPGs give even more detail and resources that have collected all of the data into one place like Wookipedia give you a great idea of what is in there http://starwars.wikia.com/wiki

  24. Don’t worry – he didn’t say energy, he said MASS. Which is actually worse because its proven wrong by actually watching the movie.

  25. Thank you for that… really. Was anything ever published that detailed it the way you described, or was it multiple sources, and you kind of condensed it?
    Do the boardgames, or tabletop RPGs go into detail? Thanks again.

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