Stars that race through space at nearly the speed of light
scientificamerican.com
scientificamerican.com
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
[2] http://english.cas.cn/newsroom/research_news/phys/202012/t20...
[3] https://iopscience.iop.org/article/10.3847/1538-4365/abc16e
P.S. HN needs a better way to make links.
(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.)
Oh jeez. First I find out about Gliese 710, now this.
I'm not sure I'm helping. >.>
Death of the individual is a completely natural part of the cycle of life and should not be dreaded beyond the fact that it hurts like hell when it happens to someone you love.
Worrying about death is like going to a movie, and being so sad throughout the movie it will end at some point that you fail to enjoy the movie.
The later is much less likely to lash out, when his "protective" bubble against existentialist fear is penetrated. Its also indicative of a curious mindset, if you stare into your own emotional blindspot, to find out what you are made of. Of course, it can become a self-destructive behaviour, destroying ones ability to function, which suppressing the fear in the end is.
- The suggestion that meditation can reduce your fear of death
- The suggestion that if it can do so, that's a good thing
- The implication that things which are natural are not to be feared
I generally consider not being anxious about future events you cannot avoid a good thing. The lack of anxiety does not mean that death would feel like a good thing, nor does lack of anxiety invoke irrational behavior.
"Natural" was maybe a too loaded word. Maybe "unavoidable" would be better.
Do you have any more specific advice you'd want to give someone who would like to be at peace with the thought of it?
I'm not an expert on death anxiety. There is the "normal" kind, and then there are conditions that probably require a therapist.
I am not suggesting life should not be cherished, nor that one should not consider each death a loss.
Lots of people have written about getting rid of it. You just probably need to skim through self-helpy books until you find some rationale to appease you.
Three points that helped me: It is a huge privilege to be born in the first place. The number of living people is infinitesimally smaller than the number of people who will ever exist (I'm referring to the fact that there is about 70B ways fathers and mothers genome can combine and you are only one of those options - as is any human of their parents genomes who has lived).
Being dead is just like being before you were born. Renaissance doesn't cause anxiety in me so I don't know why a future centuries from now on without me would frighten me.
There is the "imagine you are dead" visualization. Imagine you are dead, buried to the ground and worms are eating your corpse. Really vividly, focus for a few minutes. Then stop and continue your day. Notice the thought of being dead does not seem so frightening now you've imagined it.
Some mythologies purport an afterlife, if you prefer in believing the continuity of consciousness go ahead and do that - I have nothing against it.
I think everybody agrees patterns that are related to of us will remain in the world like ripples in the pond after we are gone. But beyond that I have no reason to believe in any sort of continuity.
Edit. Here it is https://learn.donaldrobertson.name/p/the-stoic-contemplation...
If you die, nothing is a problem for you anymore.
The rest of the universe would look quite different, though. Stars "ahead" of the solar system - in the path of its motion - would be noticeably blue-tinted and clumped together. Stars behind the system would be red-shifted. There are some great visualizations at [0].
[0] https://math.ucr.edu/home/baez/physics/Relativity/SR/Spacesh...
Stars might pass through their lifecycle in a matter of weeks. Constellations might deform over days.
But ya, you'd have to be going really #*$&#@ fast.
In the frame of the planet orbiting the hypervelocity star, the rest of the universe is moving at 0.5c, so the planet would measure all clocks in the rest of the universe to be running slower by a factor of 1/sqrt(1-0.5^2) = 1.15. Astronomers on this planet would measure stellar lifecycles (and every other physical process) to proceed slower, not faster.
You have it backwards (easy to do with such a thought experiment!). The direction of time dialation originally stated was correct. Clocks on a body moving at ludicrous speeds will be slower (to the observers present, who are able to look at those clocks) than clocks on a body not moving at ludicrous speeds. Which is to say, if you did a loop around the galaxy at light speed (hypothetical, of course) and came back to Earth and compared your clock with a terrestrial clock, your clock would be waaaay behind, and you would be younger than the people you left behind. This effect is measurable and provable even in the orbit of our own planet (albeit, at a miniscule scale). And so while your time did in fact move more slowly, the overall effect is that you time travelled into the future because everyone else was moving much faster. This of course assumes that you are able to escape intact, which is most unlikely (again, this is just a thought experiment, for argument's sake).
In your defense, I know how easy it is to get turned around in such a thought experiment. Theoretically speaking from the perspective of the par-luminal traveller, it's easy to think "my clock moves slower, so am I slower?" In a way, yes, you are! Your reality moves slower relative to theirs, and for those not travelling at light speed time passes more quickly relative to your time. Objectively speaking, a human lifecycle on a very fast moving planet will equate to many, many lifecycles on a slow moving planet, if all other things are equal. And so while you are truly travelling much faster, your time moves slower, relatively speaking.
Remember as a kid being told about a hypothetical traveller that enters a black hole, and at one point they become a long spaghetti string and then simply freeze in time? That's because they are moving much faster than us, and from the outside looking in they appear to be frozen. The light emitted from their mass is but an echo. They have moved far beyond the light that we see. From the inside looking out, the universe would move much faster.
Again, this is an extreme example used for clarification, but it might not be as extreme as you think. Consider a solar system that gets caught within the grasp of a black hole. Initially the tidal forces tear it apart, but over eons that mass might begin acting like a normal solar system again, and harbor life once more.
If you are travelling fast in a straight line, you will observe the clocks in the entire universe around you running slower. Every clock you watch through a telescope on your ship will be running slow. The entire universe around you will be moving in slow motion.
However, everyone else observing you speeding by in your ship will observe your clocks running slow. They will observe you moving in slow motion.
This is an apparent paradox - in fact, it is the quite famous twin paradox.
The resolution is that this only holds for non-accelerated motion. Flying in a loop around the galaxy requires not just high speed but also a constant acceleration, giving you the additional effects of general relativity, which complicates the situation by quite a lot, but resolves the paradox.
However, an inhabitant on a star moving at relativistic speed through the universe still falls under the original case - the inhabitants on the star will see the rest of the universe moving in slow motion, while the rest of the universe will see them also moving in slow motion.
Very interesting response! Thank you for that. I do see why my comment was misleading now. If you don't mind, could you clarify a few things? I'm just really interested in the subject.
I understand your point about travelling in a straight line: If a traveller were to travel at the speed of light speed away from a theoretical stationary point then both the observers at the stationary point and the observer within the travelling vessel would appear to eachother as frozen (assuming the traveller had equipment to measure the place they left from), because light wouldn't be able to catch up in either direction. That makes sense. When I said "From the inside looking out, the universe would move much faster" I shouldn't have used the term "looking out", that was definitely misleading now that you've mentioned it.
> However, an inhabitant on a star moving at relativistic speed through the universe still falls under the original case - the inhabitants on the star will see the rest of the universe moving in slow motion, while the rest of the universe will see them also moving in slow motion.
Your last statement got me thinking... so let's say there are two equally sized vessels that are somehow able to communicate instantaneously (for arguments sake, let's say the travellers are immortal wizards who have a telepathic link).
Let's say that one of the vessels is caught in the orbit of a black hole and is travelling at nearly the speed of light, and is constantly accellerating as it gets closer to the singularity (ignoring that tidal forces would destroy the vessel, because of it's wizard shielding).
The other vessel is lacadaisically making it's way around a star the size of our own. And let's say that the star and the black hole are slowly drifting away from eachother.
My question is this: Will the wizard inside of the vessel travelling around the black hole age more slowly than the wizard travelling around the normal star? Keep in mind that they will never meet (i.e. never share the same world line), but they do still have a telepathic link and can compare their clocks.
We talk about the "speed of light" being a constant, but this is misleading. The "speed of light" is actually the "speed of causality". Breaking the speed of light, with some theoretical instant communication, would be breaking causality.
In the earlier example, if those two stars each had an emergency beacon that sent you an instant message just as they exploded, you would receive both simultaneously. But the person travelling past you would be very confused about how that could happen, since he clearly observed them exploding at different times.
I do understand what you're saying, and while I agree that observations differ based on the location of the observers, there's one thing that still baffles me. Quantum entanglement.
Let's say that we replace the wizards in my example with a pair of entangled particles, and in fact, for this thought experiment let's pretend that humans and "observers" don't even exist anymore - there is noone left in the universe to observe what is happening anywhere.
Wouldn't our current understanding state that the wave function would be identical between the particles regardless of where those particles are located in spacetime? In effect, wouldn't the wavefunction be "happening at the same time"? Mind you, I'm not suggesting that the particles are communicating, simply that their waveforms are identical. It's really baffling to me that one particle could be in an area subjected to massive gravitational distortion while the other is not (a completely different world line), and yet they are always in sync. Thoughts?
I've read that as well, and I concur that popular opinion states that quantumly linked particles do not appear to violate causality, because we don't believe they are actually communicating at all, let alone communicating faster than light.
The way I interpreted this claim is that those particals are more akin to a deterministic algorithm, which is to say, once you set it free it will always result in the same answer based on the seed. I'm still very confused though about how those particles could remain in sync in vastly different spacetimes. That to me, is a flaw in the conjecture, and one that cannot be tested. We've based our understanding of quantum entanglement only upon that which we can observe, and so we truly don't know if a quantumly entangled particle will be "behind" if it is in a denser gravitational field. It's unprovable, because even if we were to travel into that spacetime, we could not escape to share the results with other observers.
Thank you again for your input. Your argument forced me to mentally reconcile "observable reality" from "actual reality", and that was a fun and intriguing thought experiment, albiet probably pointless since none of it can be proven. Our observations do not define reality, they only measure it to the best of our ability. And most of the time when we measure reality, apparently we fuck it up (i.e. breaking quantum links, or just having incorrect assumptions).
No. When you go fast time slows. Thus the rest of the universe appears to be going faster.
So if you fly away on a spaceship at relativistic speed, then people observing you will see you moving slowly. However, if you observe them from your ship, you will think it is they who are moving slowly.
When he returns all of his friends have aged 50 years. But he has only aged 1 year.
So you see, his friends were aging 50x faster.
Assuming the planet miraculously hung around for the ride, after the initial acceleration, it would be no different then what we experience here on earth.
Remember that everything is relative and perspective is key. A question about speed depends on who you’re asking.
To us, the other star and it’s planets are traveling at near light speeds away from us. To that stars system, it’s Earth that’s traveling at near light speed away from them.
Space flight would be the same and so would gravity and time. To them, nothing changes within their stars system because it’s all within the same reference frame.
I guess the night sky would be the main difference for people on that planet. They’d see the galaxy they left behind as red shifted and anything they’re speeding towards as blue shifted.
Since they’ve left the galaxy and was red shifted, they probably wouldn’t see as many stars with their naked eyes.
This has always bothered me a bit, everything being relative. You can validly say that we're the ones moving at near light speed and those stars we're talking about are actually the stationary objects; there's literally no way to tell the difference.
Couldn't you say the same thing about the earth and the sun? Why do we insist that we are moving around the sun, and not the other way around if everything is relative? Well I understand the reason actually, and it seems that there is good reason to believe one interpretation of relative motion is the correct one, conforming to physics.. and the other view of relative motion isn't actually what's happening in reality.
A planet orbiting the sun is accelerating toward the sun.
How do we know that the sun isn't orbiting the fixed earth, and accelerating toward the earth? Everything would look _exactly_ the same to everyone.
Yes, the math works out much easier and more neatly, so that is likely what is happening in reality. But from a purely relativistic framework.. you can consider any point in space the frame of reference, and all motion relative to that "fixed" point.
It makes me think that relativity is actually describing a more subjective experience, rather than the objective reality where we "know" the earth orbits around the sun.
Well, yes.. because Einstein's theory beautifully makes all the math work out, explaining one interpretation of motion as the real one. But what it did at the same time was show that there is no "fixed grid" of space independent of the objects themselves. Which is what leads to my uneasy feeling of how we ever can say one interpretation is more real than another -- i admit it may just be a nonsensical perspective.
As for what all physicists mean by relativity in general... I can highly recommend this old series from the National Science Foundation:
That wouldn't work out. You can take the earth as a fixed point and describe the Sun's motion relative to it, and it would be perfectly valid, but it wouldn't look as the Sun orbiting the Earth in any kind of almost constant speed elliptical orbit, it would look like a very different kind of motion.
So yes, it is correct to say that the sun revolves around the earth or the other stars are stationary while the sun is moving.
So while both relative views are equally true, the laws of physics (or at least mathematics) don't seem to hold both those views with equal esteem.
However with the introduction of the mathematics of general relativity by einstein in 1915; we can prefer whatever reference frame we like and we get the same correct predictions. Predictions that are more accurate than Newton's.
Technically, the earth doesn't rotate around the sun either -- they both rotate around the center of mass of the solar system, which happens to be very close the the center of the sun, so it looks like everything rotates around the sun.
Clearly you aren't aware of what the principle of general relativity is. It is not a good idea to argue about a topic where you lack basic knowledge.
Try to express the trajectories relative to earth of other planets in our solar system and you will quickly understand why we use a heliocentric model.
Because it makes the math easier - that's all. If you fix your frame of reference to the Sun, the orbits of all other bodies become almost perfectly elliptical. If you try to pin the reference frame to any other body, you end up with complicated curves. But they model the same thing[0]. So the whole thing about "Earth orbits the Sun" is that it gives same results, but is much easier to work with.
Technically, for the easy math, the point you're after is the barycenter[1] of the Solar System - the center of mass, which, per Newton's First Law, can be used to center the reference frame, because it's not accelerating[3]. As it turns out, the barycenter of the Solar System spends most of the time within the volume of the Sun[2], and otherwise is very close to it. So for most calculations, you may just as well pin the reference frame to the Sun.
And then, when you fix your sights at the barycenter, you'll notice the movement of celestial bodies fall out pretty much straight from joining Newton's Second Law with the Law of Universal Gravitation - m₁a₁ = Gm₁m₂/r². Your model simplifies - you now realize the movement of celestial bodies is governed by the same laws movement on Earth is, and all the complexity of geocentric model was caused by needless coordinate transformation, due to a bad choice of the reference frame.
Also worth noting that historically, humans have developed the geocentric model to a very impressive level of precision - to the point that the "upstart" heliocentric model initially was worse at predicting movement of planets. It took some extra insights for the heliocentric model to beat the old ones[5] - and only then Newton came along, and people connected the effect with the cause.
--
[0] - If I recall correctly, if you were to take the path of a planet in a geocentric model and do a Fourier transform on it - that mathematical operation which represents a function of time as a possibly infinite sum of sine waves - you'd notice that the path of your planet is essentially a sum of two periodic functions. One would correspond to the movement of Earth around the Sun, the other to the movement of the planet around the Sun. This would give you a strong hint that your model is needlessly complicated, and can be recreated using much simpler curves.
[1] - https://en.wikipedia.org/wiki/Barycenter
[2] - https://en.wikipedia.org/wiki/Barycenter#/media/File:Solar_s...
[3] - Only forces from outside the considered system could cause it to move, which we're by definition not considering when talking about our system in isolation. And besides, they add up to negligible amounts anyway. Nice thing about forces in our reality scaling like 1/r^2 or worse[4] is that they very quickly add up to nothing with distance, which makes it easy for us to treat systems as isolated in calculations, and have the results match up to reality with extreme accuracy.
[4] - https://en.wikipedia.org/wiki/Inverse-square_law applies to gravity and electromagnetism; weak and strong forces drop much faster with distance.
[5] - Like using ellipses instead of circles as the fundamental curve, because your competition that used circles moving on circles could just keep adding circles - they were doing a Fourier transform without knowing it, and each circle added a frequency component, increasing the accuracy of approximating the actual ellipse.
It actually annoys me that we still teach kids that the Earth revolves around the Sun and the ancients with their silly earth-centric system were wrong. Just the math is (kinda [1]) simpler, thats all.
First, its a lie. And second it breeds contempt for the ancients that said and discovered many great things. I dont mind telling them that heliocentism is a great (amazing) approximation. But I really dont appreciate how from K-University previous work that has been outdone is dismissed [2]
[1] the higher order terms are sill there in the heliocentric system, they're just smaller in magnitude.
[2] Another example is atomism and how the physicists who rejected it are mocked in undergrad classes. Those guys were bloody titans and their models and methods are the cornerstone of modern engineering (try designing a bridge w/out continuum mechanics but using MD)
I wonder if this will work on trying to talk yourself out of a speeding ticket?
Edit: according to [0], you would have to be driving at 30855km/h (2237mph) to see a red semaphore as green, which comes up to ~3 millionths of c.
[0] http://www.astronomy.ohio-state.edu/~ryden/ast143/ps3_soln.p...
As sibling comments describe, any planetary orbits that happen to survive would continue operating as usual, but if they were outside the heliosphere they would be subjected to interstellar gas traveling at half light speed in the other direction.
As such in a cosmic void planets around such stars would be fine. Though it seems extremely unlike for such planets to exist.
It's negligible compared to our sun. All this from the top of my head (so double check if you really care ;) but our sun in winter at surface level is roughly about 100W/m^2 and in summer about 1kW/m^2.
To make this comparison more concrete, maybe we could consider how much alpha particle energy is present inside of a nuclear reactor running at peak energy output. I'd be surprised if it was as much as 1 W/m^2.
Next solar wind actually becomes significant protection at those densities. It varies significantly and simulating what happens gets complicated, but it shouldn’t be ignored at those densities.
Anyway, the outer fringes of earths atmosphere for example is almost exclusively Hydrogen and Helium as it gets sorted by atomic weight. Free neutrons decay in a matter of minutes and therefore would almost entirely end up as more hydrogen. What’s a much larger risk is stripping the atmosphere off of any planet.
The outer fringes of the Earth's atmosphere are hydrogen, but the density there is so low that these cosmic ray particles would be unlikely to interact there. Also, the neutrons here are FAST neutrons (the (n,p) reaction on 14N is a fast neutron reaction) so they are traveling at very high, if not relativistic, speed, and would not have time to decay before they react (or are thermalized and become irrelevant to 14C generation from 14N).
As to the density of earths hydrogen, that’s a function of earths atmosphere. Under sustained bombardment, assuming the planet kept an atmosphere, it’s going to have significantly less nitrogen in the upper atmosphere as that’s destroyed by collisions with relativistic hydrogen/helium.
All of those stars have velocities < 1000 km/s relative to the Milky Way, which is less than 1/3 of one percent of the speed of light. Calling that "relativistic" is just hyperbole.
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.
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
>Purpose of use: In my Air Force career flying for over 21 years, curious as to how much I actually aged.
Nice use case :D
You see a lot, but your face would be caked over with bugs.
IOW, a planet like that would have a much larger chance of colliding with interstellar debris. Then again, there is probably very little of that.
You may be confusing it with red shift.
If a hypervelocity star passed by us, we would see them in slow motion because they are moving at high speed relative to us. At the same time, they would see us in slow motion because we would be moving at high speed relative to them. The effect of “the other clock is slower” is reciprocal.
The weirder and non-reciprocal effects would occur when the hypervelocity star is first slingshotted out to high speed. At that point they would be close to a black hole (deep in a gravitational well) and accelerating (non-inertial frame of reference). Under those conditions, their clock would run slower than our clock for observers on both sides — i.e., we would see them in slow motion and they would see us in fast forward.
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?
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.
Here’s a minute physics video that shows how acceleration explains the twins paradox (the “common” explanation) https://youtu.be/0iJZ_QGMLD0
After you watch minute physics video, you might feel like you understand. Nope, can’t have any of that. Now, you must watch this Fermi lab video that says the common explanation is “not fundamentally correct” https://youtu.be/noaGNuQCW8A
After watching these incorrect videos it was still unclear why acceleration even matters, because with 1g accel in space you can reach subluminal speed in a matter of days, and then, at the distant location, turn back with just 2x long 1g again, and then “brake” with 1g again to land on earth. While the twin on earth experienced that 1g all the time. One may even pick a distance and a (probably hyperbolic-y) route with rotations so that all of the journey would consist of a constant 1g for a flying twin, exactly as a sitting one experiences. Put them both into opaque boxes, knock senseless for a couple of hours at launch, and neither of them would even tell who is where, until opened.
I still don't get it though
If you were to ask which of the twins is older at the time the second twin reaches the far away star, the answer is that the question doesn't make sense. They are in different places, so there is no way to compute a fixed time.
Imagine each twin is broadcasting a video signal of their face for the entire duration of the trip. The twin on earth puts a big red dot on the screen the moment that the traveling twin had reached the distant star in their frame of reference; while the twin on the ship would start putting a big red dot on their video the moment they reach the star in their own. Because the speed of light is limited, over the whole duration of the departure trip, for both twins, the latest image they see of their sibling will be younger than themselves.
When the twin on the ship reaches the star and starts emitting the red dot, they will have aged L/gamma * v years. By the time the twin on Earth sees this image, they will have aged L/v + L/c years = L(c+v)/c*v years, so they are [L(c+v)/c*v] / (L/gamma * v) = gamma(c+v)/c years older.
The twin on Earth will start emitting the red dot after L/v earth years, or L/gamma*v ship years. This signal will meet the ship after L/(gamma*(c - v)) ship years from the moment it was sent - so in total, L/gamma*v + L/gamma*(c-v) = L*c/gamma*v*(c-v) ship years. So, they are [L*c/gamma*v*(c-v)] / (L/gamma*v) = c/(c-v) ship years older than their sibling, which is gamma*c/(c-v) earth years.
While both see that they are older than their sibling at the time they first see their sibling's red dot, there is an asymmetry coming from the fact that one of them is at the same position as they were initially, while the other one is at a different position, relative to the star. Equivalently, there is an asymmetry that is caused by their different definitions of "the moment the ship reaches the star".
> 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.
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.
(boom tish)
(pi1.3e9m^2 cross sectional area 3e9 m/s velocity) / (1e-27 kg/m^3 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+*...
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.
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.
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.
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.
> 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.
For the star, from its point of view its mass has not changed at all. So yes fusion still works.
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.