What unique phenomena would be observed in a system around a hypervelocity star?

Question

A star is ejected at a high velocity from the Milky Way Galaxy, going towards Andromeda at a significant speed. Around said star is a stable planet - that either at the time of ejection or is made to be during transit towards the Andromeda Galaxy - is habitable.

What kind of observations would a person have on the surface of such a planet? Would seasons be stable, and could they function similarly to Earth? On Earth, we perceive the night sky rotating, how would night sky observations be different in such a system?

Generally, I want to know anything of note that people can think of that would be different.

• For the purposes of this question, we'll say that this planet has been placed in an ideal situation for human survival by an advanced civilization
• The planet is on a course towards the Andromeda Galaxy
• To clarify, this is not about a rogue planet, without any star, but a star system with at least one planet
• If doing some math on this is necessary, just go with any data you can find on the velocity of hypervelocity stars.

Answer 1

Hypervelocity stars are created through close gravitational encounters with other bodies, which are likely to strip away planets, especially those particularly far from the star. You have declared that the planet is "stable", and I'm happy to work with this, but that means you can essentially choose what happened to the system's planets and moons, and that's all that will matter here.

Seasons have nothing to do with the interstellar environment or the star's motion relative to the galaxy — they're a function of the planet's axial tilt and its orbit around its star. Since star systems in our galaxy are already only negligibly affected by forces from outside them, and this system is outside the galaxy and thus even further away from any outside influences, whatever's influencing seasons will be internal to the system. Just as in any other case, a moon may help stabilize the planet's axial tilt and keep its seasonal strength stable, while other planets in its system might perturb its orbit over millions or billions of years. Exactly what is in the system to influence these is entirely your call, since you've said that the result is stable and habitable by fiat, and has nothing to do with the star's hypervelocity nature.

The star's velocity also doesn't matter for anything related to energy balance or zodiacal light. Intergalactic space is essentially empty; the numbers typically thrown around are on the order of 1 proton per cubic meter. The total energy of these particles on a planet sweeping through at typical-to-high hypervelocity speeds of 1,000 km/s is roughly 1 nW/m^2, which is much fainter than Earth's zodiacal light from solar system particles and would be totally drowned out in a similar system even before accounting for the star's heliosphere or the planet's magnetosphere protecting it.

In intergalactic space, there will probably be no visible stars. The Milky Way and Andromeda, and possibly other small Local Group galaxies, may appear in the sky as fuzzy blobs, allowing possibly for some amount of celestial navigation. Because the "night sky rotating" is a function of Earth's rotation and its orbit around the Sun, and a star moving quickly through space does not directly impact either of these things, one would expect the exact same cycle — this planet's star rises and sets as it spins, and the night sky is always visible on the other side, while over the course of its orbit the visible half of the night sky sweeps out around the ecliptic in a circle.

Andromeda's half of the sky will be slightly blueshifted, by an increasing factor centered towards Andromeda — if it's approaching at 1,000 km/s (1/300 c), then it'll be by a factor of about 1/300, or about 2nm in typical visible wavelengths, enough for spectrometers to easily notice but not enough for a human to tell. The Milky Way's half of the sky will be similarly imperceptibly redshifted.

Answer 2

What a wonderful and weird question. The first thing to consider is how fast the earth is already moving. We calculate that the solar system is rotating around the galactic core at 700,000 km/h. Additionally, the milky way travels at 2.2 million km/h compared to the cosmic background. Yet we "feel" nothing.

Another thing to consider is how vast the chasm between the milky way and andromeda really is. Andromeda is 2.5 million light years away.

At any non-relativistic speed, the ejected solar system would have very similar qualities to our own. The stars would be fixed in the sky, but dimmer as they would be farther away. However, instead of seeing a galactic arm in the sky, you would see the whole galaxy (depending on how far your system is from each galaxy when the story takes place.

At relativistic speeds things would get interesting. And I can only begin to guess at the phenomena on the system. I suppose orbits would be unstable and the light from the star would have a relativistic doppler effect.

Answer 3

The Milky Way and Andromeda galaxies are already on collision course at a speed of a similar magnitude to hyper velocity stars. So the hyper velocity star would arrive at Andromeda a lot sooner than the Milky Way but it would still take billions of years to get there.

The seasons and all other aspects of our world could* continue except for greatly reduced tidal action and the appearance of the night sky.

The night sky would change over time. Starting from roughly what we see minus the Moon (which would be gone in few days or weeks).

Then after hundreds of millions of years the stars should clear away from one side of the sky to provide a magnificent view of the entire Milky Way from one hemisphere depending on season, latitude and time of night. Whilst the other hemisphere would be dark except for one bright star / smudge that would be Andromeda.

After perhaps a billion years or so the Milky Way would shrink to a much smaller but very apparent galaxy in one hemisphere whilst Andromeda would appear similarly in the other hemisphere again depending on the season, latitude and time of night.

Hundreds of millions of years after that Andromeda would grow massive in the night sky as the Milky Way Shrank to a bright star.

Eventually after perhaps 2 billion years the night sky would very roughly approximate what we see today, but with a totally different placement of stars (from Andromeda) and a small faint smudge somewhere just visible to the naked eye that would be the Milky Way.

*The event that propelled the star to hyper velocity would surely destroy, disrupt or at the very least eject all of its planets in all directions so it is very unlikely that a hyper velocity star would have any planets. However it is perhaps possible, if very unlikely (detailed calculations would be required to be sure) for the star and the planet to be flung out in the same direction and for the star to recapture its planet again.