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Stars that race through space at nearly the speed of light
- hirundo 5y ago> Stars That Race through Space at Nearly the Speed of Light > These hypervelocity stars move at up to 2 percent of the speed of light Either there is some sense I'm missing in which 1/50th of C is "nearly" C, or this is a click bait and switch title.
- JoeAltmaier 5y agoC is very large. Nothing goes that fast normally. Not by orders of magnitude. So 2% is huge. Our sun moves about the Milky Way at 828,000mph. 2% of the speed of light is around 13,000,000mph That's the speed where relativistic effects become important.
- taeric 5y agoStill doesn't mesh with the colloquial "nearly". :(
- deleted 5y ago[deleted]
- yesenadam 5y agoI think your sun speed should be km/h? Every website has a different figure, but they mostly seem to be around 500,000 miles/hr ~ 800,000 km/h.
- JoeAltmaier 5y agoYeah missed that, thanks. 536,000mph
- gpvos 5y agoThose two numbers are actually closer to each other than I expected.
- ganafagol 5y agoWhere they become important depends on your precision of measurenment. GPS satellites don't move at 2% the speed of light, but relativistic effects do play a role for the system to work since they need to be very precise in measuring and broadcasting time.
- tectonic 5y agoIt's odd that he didn't mention it in the writeup because his paper predicted that massive black hole binaries could accelerate stars to half the speed of light.
- pmoriarty 5y agoIf a star is moving at nearly the speed of light relative to us, doesn't that also mean that we're moving at nearly the speed of light relative to them? If that's so, wouldn't that mean that the Earth is a "hypervelocity planet" in the article's terminology.. at least relative to some other objects in the universe?
- NikolaeVarius 5y agoYes.
- JoeAltmaier 5y agoWell, every cosmic ray makes you hypervelocity too I guess. But to be meaningful you want to talk about speed relative to your local galactic frame or some such. Most of the mass around us is moving relatively slowly to one another. That makes these fast stars an anomoly. And it means their (relative) effect is going to be interesting.
- tectonic 5y agoThat's right. From their perspective, a hypervelocity galaxy is moving past them.
- simonh 5y agoThe difference is these stars experienced a massive acceleration, while we didn’t, so it’s not actually a symmetric relationship.
- savant_penguin 5y agoFrom the point of view of the star didn't we accelerate though? The twin paradox never made sense to me. I've never understood the asymmetry
- enkid 5y agoAcceleration isn't relative, velocity is. An object undergoes acceleration when a force is applied to it. Objects "know" when they are being accelerated, just like how you know you're being pulled down by gravity. The same can't be safe for velocity.
- tectonic 5y agoWe wrote about this in Orbital Index recently (https://orbitalindex.com https://orbitalindex.com) and so I did some background research: Loeb's paper [1], mentioned in the article, found that hypervelocity stars accelerated by massive black hole binaries could hypothetically reach as much as *half the speed of light*. This is not something you want to be anywhere near. Relatedly, Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely. They used China's LAMOST optical telescope and ESA's Gaia and doubled the number of known high-velocity stars. Apparently this is useful for measuring the mass of the Milky Way. [1] https://arxiv.org/abs/1411.5030 https://arxiv.org/abs/1411.5030 [2] http://english.cas.cn/newsroom/research_news/phys/202012/t20201228_260875.shtml http://english.cas.cn/newsroom/research_news/phys/202012/t20... [3] https://iopscience.iop.org/article/10.3847/1538-4365/abc16e https://iopscience.iop.org/article/10.3847/1538-4365/abc16e P.S. HN needs a better way to make links.
- jessriedel 5y agoWhat fraction of c qualifies as "relativistic" in this case?
- ineedasername 5y agoGenerally it's whatever speed that forces you to account for relativity for you desired precision. However, I've seen 10% referenced as a lower limit.
- jessriedel 5y agoThat's generally true, but it's not meaningful in this case. If I say "One in 10,000 stars in the Milky way is a hypervelocity star moving at relativistic speeds", there is no context to provide a threshold for the magnitude of relativistic corrections that are important. (And if it's just about escape velocity, they should just say that. The Milky Way's escape velocity is 550 km/s = 0.002 c, for which relativistic corrections are extremely tiny: \gamma ~= 1.0000017.)
- tomatotomato37 5y agoMakes me wonder, if a relativistic star passes through a decently thick dust cloud could its bowshock achieve fusion and glow brighter than the star? That would be quite the impressive lightshow
- jcims 5y agoI did some napkin math and a star the size of our sun passing through intergalactic space (not a dust cloud obv) at the speed of light would only encounter about 1kg of matter per second on average. (pi1.3e9m^2 cross sectional area 3e9 m/s velocity) / (1e-27 kg/m^3 density)
- EForEndeavour 5y agoYour result is only about twice my own napkin math, so it checks out. That formula as written has a typo: to get mass per unit time, multiply (not divide) cross-section-speed by mass density. Other fixes: - solar radius is 7e8 m, not 1.3e9 m - c is 3e8 m/s, not 3e9 m/s I get about 0.5 kg/s https://www.wolframalpha.com/input/?i=%28pi*%287e8m%29%5E2+*+c%29+*+%281e-27+kg%2Fm%5E3%29 https://www.wolframalpha.com/input/?i=%28pi*%287e8m%29%5E2+*...
- jcims 5y agoThanks for the correction, in either case the number is way way smaller than I would have ever imagined.
- gmuslera 5y agoWould they qualify as stars? Would they be able to maintain integrity with the relativistic change of mass (across its volume) while still running the fusion process?
- zlynx 5y agoIn relativity, everything is relative. You can choose any "rest frame" that you like and all the math still works out. For the star, from its point of view its mass has not changed at all. So yes fusion still works.
- jniedrauer 5y agoFrom the perspective of the star, what impact does relativistic speed have on the rest of the universe? Does it also gain mass? Does that mean that an object accelerating to relativistic speed increases the mass of the universe from both frames of reference?
- akomtu 5y agoIndeed, in their local coordinate system, their mass hasn't changed, while from a still observer's point of view, such a star gains that extra relativistic mass and should in theory collapse once it reaches a certain speed. Of course, it would be strange if the same star would be collapsed or normal in different corrdinate systems. I guess the solution is that the extra relativistic mass isn't real, but a curious artifact that manifests as the higher than normal kinetic energy.
- XorNot 5y agoBlack hole's aren't really defined by mass though - they're defined by spacetime curvature. Specifically, inside the event horizon of a blackhole, every possible vector winds up at the singularity. A very speedy object however doesn't look like this: objects it passes have vectors which take them away from it. So while it can warp spacetime with that mass, it can't form an event horizon - there are vectors which lead things away from it and thus can escape. [EDIT: I had to go do some reading to check up on this and it actually has an analogue to electromagnetism: moving objects develop gravitational effects perpendicular to their direction of motion, essentially this is one of the origins of gravitational waves - but it's also important to the overall point, which is that even if you had infinite gravitational acceleration perpendicular to your direction of travel, you've got numerous other degrees of freedom where this doesn't happen and thus escape vectors - so no event horizon though you might suffer a lot of damage and be spagettified anyway] Black holes only work when you have a point force exerting uniform acceleration in all directions towards itself - beyond a key point, no attainable velocity ever results in an object not moving towards you. Whereas a speedy object has an obvious one: just jump off the back of it (i.e. accelerate slightly against it's vector) and your trajectory now points you away from it.
- mulcahey 5y agoCould we use stars like this as intergalactic vehicles!? If (BIG if) we some how figure out how to launch a mass the size of a star or planet into an orbit around two galaxies. You could then launch your starship into its orbit around the apogee, ride the mass to the next galaxy, and de-orbit burn around perigee! In this way the orbit would practically be a store of energy as well.
- ashtonkem 5y agoSeems improbable. The Delta V required to catch up with these things would also be an appreciable percentage of the speed of light. The star’s gravity would help, but it’s only going to take the edge off what will be an immense acceleration budget. Given that we basically haven’t hit even 0.01C, getting a planetary mass up to this speed seems impossible. Plus, once you get wherever you’re going, you’d have to pay back whatever boost the star’s gravity gave you as you both escape its gravity well and spend down the velocity that the star itself was going.
- XorNot 5y agoOn the other hand spending your intergalactic voyage orbiting a star would be very convenient from a resources and fuel perspective.
- ashtonkem 5y agoResources maybe, since you’d get sunlight from the star plus whatever material is caught in its orbit. Fuel, no. You’d spend the exact same amount of fuel catching up to this star and eventually leaving its orbit as you’d spend just travelling across the universe alone at the same speed. You’d only save fuel if you used the star for a slight shot maneuver, where you steal a tiny amount of its energy to accelerate you, but this would preclude you spending the voyage in the orbit of this rogue star.
- ncallaway 5y agoIt could be fuel, if you consider fuel to be one of the resources a star can provide you. If your fuel is something that is inside the star (or star's orbit), and you have a means of extracting it, then you can use the star as a refueling point. You're right that using the star wouldn't require less fuel overall, but it could provide you access to extra fuel along the way.
- nostromo 5y agoIt’d be fun to live on a planet around one of these stars. Life would be normal for you. But the rest of the universe would be running on fast forward. Maybe a civilization could use these stars to spread itself to other parts of the galaxy or, if fast enough, to other galaxies.
- vermilingua 5y agoEven better, half the universe would be in fast forward, everything behind you in slow motion! Watching a galaxy collission in accelerated time and a supernova in slowmo would be a unique experience.
- dnadler 5y agoI don't believe this is the case, I think you may be confusing the effects of time dilation and redshift.
- deleted 5y ago[deleted]
- p1mrx 5y agoSome events would appear faster because you're moving toward them, but once you account for that, all other clocks in the universe would tick slower from your perspective.
- MichaelApproved 5y agoTime dilation isn’t dramatic as you might think. For example, at 66% the speed of light, you only get a 1.3 (or 30%) time difference. You have to get into the 90% range for something meaningful. Even at 96% the speed of light, it’s a 4x speed difference. 99.99% is 80x time difference. 99.999% is 240x time diff. To have time speed up to the point of civilizations rising and falling in a matter of days, you’d need to be traveling at 99% (with a few 9s in the decimal places). Though at relative speeds that high, it would be difficult to actually see what’s happening. Here’s a calculator that’ll show you time dilation at different speeds https://keisan.casio.com/exec/system/1224059993 https://keisan.casio.com/exec/system/1224059993
- MichaelMoser123 5y agocould a black star swing around and accelerate wandering asteroids and comets; that would be a more frequent event, wouldn't it? what would be the likelyhood of running into an object like that in our solar system? wouldn't the friction with interstellar medium brake an object like that before it gets out of a galaxy? > searches for extraterrestrial intelligence should check for radio signals coming from riders of hypervelocity stars howso? wouldn't the ejection/acceleration process disrupt the planets around a star? Still amazing how much action is happening in astrophysics these days.
- MichaelMoser123 5y ago.. and how would they stop such a spaceship, once it arrives at its point of destination?
- mrfusion 5y agoThere was a novel where they had turned a star into a giant spaceship by redirecting light and gasses in one direction.
- m3kw9 5y agoWhat would a black hole that move at relativistic speed be like? That’s some exotic physics to think about
- exporectomy 5y agoLike a normal black hole. They all move at relativistic speed relative to something.
- plushpuffin 5y agoThis is the setting for a sci-fi novel by Frederik Pool called The World at the End of Time https://en.wikipedia.org/wiki/The_World_at_the_End_of_Time https://en.wikipedia.org/wiki/The_World_at_the_End_of_Time
- syspec 5y agoIf a star is traveling that fast (let's say 85% the speed of light), and it emits light, is the light traveling only 15% faster than the star?
- taneq 5y agoDepends on your reference frame. From the point of view of the star, no. From our point of view, yes.
- ketralnis 5y agoNo. The speed of light is measured as the same constant by all observers. If that sounds counterintuitive, it is https://en.wikipedia.org/wiki/Speed_of_light https://en.wikipedia.org/wiki/Speed_of_light
- gigatexal 5y agoI hate how the article plays loose with the wording: “nearly” the speed of light is not half the speed of light as comments have suggested about hypervelocity stars.
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- turtleofdeath 5y agoYou seem to be lost.
- freddealmeida 5y agoThat can't be true by any sense if you accept relativity. Now if you believe in an absolute frame. Well that would be something.
- slowhand09 5y agoMind blown; "It must be thrilling to live on a planet orbiting one of these ejected stars and to witness its trip through space."
- AtlasBarfed 5y agoSo a star moving that fast would last far longer from our frame of reference than a star in "normal" frames of reference? So it might be a fast-forward to the end of the universe if we speed up to catch it and hang out there for a billion years.