7 ms·

> Interestingly they’re all quite fast — hundreds of kilometers per second I bet that the milky way could only capture these ultra fast moving stars because of

> Interestingly they’re all quite fast — hundreds of kilometers per second I bet that the milky way could only capture these ultra fast moving stars because of dark matter.

AnimalMuppet 2y ago
But they're in the halo. Can they actually be gravitationally bound at that distance at those velocities? Is even the (computed by other means) amount of dark matter enough for them to be captured?
- Keysh 2y ago
  If they weren’t gravitationally bound, they would have left the Milky Way billions of years ago. Also, “in the halo” doesn’t necessarily mean “far from the center of the galaxy”; it means “not orbiting in the disk”.[1] Some of these stars are closer to the galactic center than the Sun. [1] And further away from the center of the galaxy than the bulge — but the bulge is much smaller than the orbit of the Sun.
klyrs 2y ago
This echoes a thought I had recently -- stars traveling at relativistic speeds should look deceptively young due to time dilation. But while this is certainly speedy, mere hundreds of km/s isn't enough to significantly prolong the observed lifetime of a star.
- malfist 2y ago
  > should look deceptively young due to time dilation That's not how we date stars. We typically date the star by it's metallic content. More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars.
  - Starlevel004 2y ago
    > More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars. It's actually the exact opposite, but yes.
    - malfist 2y ago
      Yes thank you, got it backwards
  - pfdietz 2y ago
    There's some weird caveats, like so-called "Lead Stars". These are aged stars with very large amounts of lead relative to other heavy elements. This happens because lead is the end nuclear of the so-called s-process, where neutrons are captured at a slow rate in large stars whose internal processes produce some neutrons by (alpha,n) reactions. In lead stars, there are so few seed nuclei (those metals) that each of the seed nuclei that are there can capture many more neutrons.
  - jddj 2y ago
    Does it matter in this context whether it's metals or tree rings or wrinkles around the eyes? It's all time, right?
    - klyrs 2y ago
      > It's all time, right? Sure, but we can measure time dilation between the peak of a mountain and the base of a mountain, due to the differing velocities. Time is relative!
- pdonis 2y ago
  > stars traveling at relativistic speeds So far, to my knowledge, we have not observed any. As you note, hundreds of km/s is way too small to have any appreciable effect.
  - ChrisGranger 2y ago
    "The fastest star ever seen is moving at 8% the speed of light" https://www.space.com/fastest-star-ever-moves-8-percent-light-speed.html https://www.space.com/fastest-star-ever-moves-8-percent-ligh...
    - AnimalMuppet 2y ago
      0.08c translates into a gamma of 1.0032. That's not enough for relativity to be doing much...
      - bloopernova 2y ago
        How much relativity effects occur at 0.2c? A sci-fi book series I'm reading has warships unable to hit other ships if their relative velocity is above 0.2c. I was wondering if that was realistic.
        klyrs 2y ago
        I found this to help me not sound completely ignorant in this thread https://www.omnicalculator.com/physics/time-dilation https://www.omnicalculator.com/physics/time-dilation
        luyu_wu 2y ago
        As the lorentz factor calculated above, not much! Time would only be dilated by that much, and length by the the same amount. It'd definetly be hard to hit something at that speed, but not due to relativity!
        pdonis 2y ago
        The gamma factor at 0.2c is only 1.02, which isn't much of an effect. The main difficulty with aiming at something moving that fast wouldn't be time dilation, it would be light travel time delay. For example, suppose the other ship is one light-second away from yours and is moving at 0.2c. You're seeing the other ship where it was one second ago, and in that time it has moved 1/5 of a light second. Plus, supposing you're firing a laser at it, the laser will take another second to get to the target, so you have to aim 2/5 of a light second away from where you're seeing the other ship. And during that time the other ship could change direction and spoil your aiming calculation. The sci-fi book series probably is making some assumptions about how fast the ships can change direction, how fast and accurately they can aim, and typical distances between ships during combat to come up with the 0.2c number.
- chasil 2y ago
  The stars that are going beyond our cosmological horizon are effectively traveling away from us faster than their light towards us. https://en.wikipedia.org/wiki/Cosmological_horizon https://en.wikipedia.org/wiki/Cosmological_horizon "Or, more precisely, there are events that are spatially separated for a certain frame of reference happening simultaneously with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred at the same location in space that happened in the distant past. "While we will continue to receive signals from this location in space, even if we wait an infinite amount of time, a signal that left from that location today will never reach us."
jajko 2y ago
Sun is moving cca 230 km/s around Sagittarius A*, so something even further from the center would have even higher speed if rotating at same speed (don't know if that's the trend in our galaxy, very much a tourist in astronomy)... doesn't sound that unusual unless its those 999km/s corner cases
- deciduously 2y ago
  Except these stars are rotating the other direction
- ashes-of-sol 2y ago
  Orbital velocity increases as you get closer to the middle, not the other way around. An example closer to home, our orbital velocity around the Sun is 29.8km/s. Mercury is 47.9km/s (on average, it actually varies throughout its orbit). Neptune is 5.4km/s.
  - shrimp_emoji 2y ago
    This doesn't apply to stars in the Milky Way. Unlike planets around a star, stars in the Milky Way don't follow Keplerian physics in their orbit around the galactic center. https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve ``` The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. In contrast, the orbital velocities of planets in planetary systems and moons orbiting planets decline with distance according to Kepler’s third law. ```

> Interestingly they’re all quite fast — hundreds of kilometers per second I bet that the milky way could only capture these ultra fast moving stars because of dark matter.

AnimalMuppet 2y ago

But they're in the halo. Can they actually be gravitationally bound at that distance at those velocities? Is even the (computed by other means) amount of dark matter enough for them to be captured?

Keysh 2y ago
If they weren’t gravitationally bound, they would have left the Milky Way billions of years ago. Also, “in the halo” doesn’t necessarily mean “far from the center of the galaxy”; it means “not orbiting in the disk”.[1] Some of these stars are closer to the galactic center than the Sun. [1] And further away from the center of the galaxy than the bulge — but the bulge is much smaller than the orbit of the Sun.

klyrs 2y ago

This echoes a thought I had recently -- stars traveling at relativistic speeds should look deceptively young due to time dilation. But while this is certainly speedy, mere hundreds of km/s isn't enough to significantly prolong the observed lifetime of a star.

malfist 2y ago
> should look deceptively young due to time dilation That's not how we date stars. We typically date the star by it's metallic content. More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars.
- Starlevel004 2y ago
  > More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars. It's actually the exact opposite, but yes.
  - malfist 2y ago
    Yes thank you, got it backwards
- pfdietz 2y ago
  There's some weird caveats, like so-called "Lead Stars". These are aged stars with very large amounts of lead relative to other heavy elements. This happens because lead is the end nuclear of the so-called s-process, where neutrons are captured at a slow rate in large stars whose internal processes produce some neutrons by (alpha,n) reactions. In lead stars, there are so few seed nuclei (those metals) that each of the seed nuclei that are there can capture many more neutrons.
- jddj 2y ago
  Does it matter in this context whether it's metals or tree rings or wrinkles around the eyes? It's all time, right?
  - klyrs 2y ago
    > It's all time, right? Sure, but we can measure time dilation between the peak of a mountain and the base of a mountain, due to the differing velocities. Time is relative!
pdonis 2y ago
> stars traveling at relativistic speeds So far, to my knowledge, we have not observed any. As you note, hundreds of km/s is way too small to have any appreciable effect.
- ChrisGranger 2y ago
  "The fastest star ever seen is moving at 8% the speed of light" https://www.space.com/fastest-star-ever-moves-8-percent-light-speed.html https://www.space.com/fastest-star-ever-moves-8-percent-ligh...
  - AnimalMuppet 2y ago
    0.08c translates into a gamma of 1.0032. That's not enough for relativity to be doing much...
    - bloopernova 2y ago
      How much relativity effects occur at 0.2c? A sci-fi book series I'm reading has warships unable to hit other ships if their relative velocity is above 0.2c. I was wondering if that was realistic.
      - klyrs 2y ago
        I found this to help me not sound completely ignorant in this thread https://www.omnicalculator.com/physics/time-dilation https://www.omnicalculator.com/physics/time-dilation
      - luyu_wu 2y ago
        As the lorentz factor calculated above, not much! Time would only be dilated by that much, and length by the the same amount. It'd definetly be hard to hit something at that speed, but not due to relativity!
      - pdonis 2y ago
        The gamma factor at 0.2c is only 1.02, which isn't much of an effect. The main difficulty with aiming at something moving that fast wouldn't be time dilation, it would be light travel time delay. For example, suppose the other ship is one light-second away from yours and is moving at 0.2c. You're seeing the other ship where it was one second ago, and in that time it has moved 1/5 of a light second. Plus, supposing you're firing a laser at it, the laser will take another second to get to the target, so you have to aim 2/5 of a light second away from where you're seeing the other ship. And during that time the other ship could change direction and spoil your aiming calculation. The sci-fi book series probably is making some assumptions about how fast the ships can change direction, how fast and accurately they can aim, and typical distances between ships during combat to come up with the 0.2c number.
chasil 2y ago
The stars that are going beyond our cosmological horizon are effectively traveling away from us faster than their light towards us. https://en.wikipedia.org/wiki/Cosmological_horizon https://en.wikipedia.org/wiki/Cosmological_horizon "Or, more precisely, there are events that are spatially separated for a certain frame of reference happening simultaneously with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred at the same location in space that happened in the distant past. "While we will continue to receive signals from this location in space, even if we wait an infinite amount of time, a signal that left from that location today will never reach us."

jajko 2y ago

Sun is moving cca 230 km/s around Sagittarius A*, so something even further from the center would have even higher speed if rotating at same speed (don't know if that's the trend in our galaxy, very much a tourist in astronomy)... doesn't sound that unusual unless its those 999km/s corner cases

deciduously 2y ago
Except these stars are rotating the other direction
ashes-of-sol 2y ago
Orbital velocity increases as you get closer to the middle, not the other way around. An example closer to home, our orbital velocity around the Sun is 29.8km/s. Mercury is 47.9km/s (on average, it actually varies throughout its orbit). Neptune is 5.4km/s.
- shrimp_emoji 2y ago
  This doesn't apply to stars in the Milky Way. Unlike planets around a star, stars in the Milky Way don't follow Keplerian physics in their orbit around the galactic center. https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve ``` The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. In contrast, the orbital velocities of planets in planetary systems and moons orbiting planets decline with distance according to Kepler’s third law. ```