Ask Science

9096 readers

1 users here now

Ask a science question, get a science answer.

Community Rules

Rule 1: Be respectful and inclusive.

Treat others with respect, and maintain a positive atmosphere.

Rule 2: No harassment, hate speech, bigotry, or trolling.

Avoid any form of harassment, hate speech, bigotry, or offensive behavior.

Rule 3: Engage in constructive discussions.

Contribute to meaningful and constructive discussions that enhance scientific understanding.

Rule 4: No AI-generated answers.

Strictly prohibit the use of AI-generated answers. Providing answers generated by AI systems is not allowed and may result in a ban.

Rule 5: Follow guidelines and moderators' instructions.

Adhere to community guidelines and comply with instructions given by moderators.

Rule 6: Use appropriate language and tone.

Communicate using suitable language and maintain a professional and respectful tone.

Rule 7: Report violations.

Report any violations of the community rules to the moderators for appropriate action.

Rule 8: Foster a continuous learning environment.

Encourage a continuous learning environment where members can share knowledge and engage in scientific discussions.

Rule 9: Source required for answers.

Provide credible sources for answers. Failure to include a source may result in the removal of the answer to ensure information reliability.

By adhering to these rules, we create a welcoming and informative environment where science-related questions receive accurate and credible answers. Thank you for your cooperation in making the Ask Science community a valuable resource for scientific knowledge.

We retain the discretion to modify the rules as we deem necessary.

founded 2 years ago

MODERATORS

Tell me about the physics of material "falling onto a neutron star and emitting hard x-rays" (lemmy.world)

submitted 2 years ago by j4k3 to c/askscience

2 comments fedilink hide all child comments

https://piped.video/watch?v=simuXjzxlGI&t=380

This video by Scott Manley is about object impact physics. The relevant clip is 6:20~6:50. Scott collides a neutron star with a simulated Earth. He mentions the ejecta should be falling onto the neutron star and emitting hard x-rays.

How would forces balance in real life?

I'm trying to mentally picture how the objects interact as gravity wells and how this might interact with the impact and velocity. Is the gravity well even relevant at high velocity?

Any neutron star is as close as possible to a black hole. Its gravity well should be capable of accelerating material at a significant fraction of the speed of light right? So does the Roche limit really obliterate the planet long before the impact as simulated? What prevents all of the planetary matter from collapsing onto the neutron star? Why were the "hard x-rays" specifically mentioned?

top 2 comments

sorted by: hot top controversial new old

[–] [email protected] 3 points 2 years ago (1 children)

Roche limit is not really relevant here. That's for orbiting bodies, like a satellite around a Jupiter-like planet whose orbit spirals inward due to tidal forces, and eventually crosses the Roche limit, whereby the moon disintegrates into a cloud of rocks that spreads out and forms a ring. Yes, the hyperbolic orbit of the collision trajectory here is a "type" of orbit, but really the video is about the collision itself. There is not enough time for the planet to meaningfully disintegrate under the neutron star's gravity. "What's that? The ground is kinda shaking. Could that be the tidal force from that neutron st-ACK!!!".

In the video you can see the surface of the Earth bulge out towards the star under its gravity in the last second, but most of the kinetic energy of the explosion is imparted by direct physical interaction (i.e. electromagnetic) between the matter of the earth and the matter of the star, and in particular between the matter of the earth that has already been accelerated and the matter of the earth lying farther out.

Or at least it would be if the impactor really was just a chunk of iron with the density slider cranked up. This fluid simulator can't imagine anything else of course, but you are right that it remains a question of whether a neutron star or a black hole could impart any kinetic energy onto the greater earth at all. Maybe it will just pass through and leave a circular hole, sweeping the material in front of it onto itself. The tunnel would immediately collapse, and the crust would be messed up from tidal sloshing, but maybe the ball of the earth itself will remain intact.

The hard x-rays I believe is a reference to thermal radiation of infalling matter. Just like a bullet that hits a wall while staying intact is hot to the touch because its kinetic energy got 100% converted into heat, or a meteoroid that hits the Moon creates a flash of light visible from Earth because for a second the cloud of collision debris is as hot as the filament of a lamp, the earth material impacting the surface of the star gets really hot. The impact velocity is at minimum the escape velocity of the star, which is thousands of km/s, which means the peak of thermal radiation is in the x-ray range.

[–] [email protected] 2 points 2 years ago

As a quick calculation using the Boltzman formula:

E = 3/2 k_B T

Say we imagine that the entire kinetic energy of bulk material from Earth (let's say iron) impacting the star at 10000km/s is converted into thermal kinetic energy of individual iron atoms (atomic weight 56).

1/2 m v**2 = 3/2 k_B T
T = 1/3 m v**2 / k_B
k_B = 1.38e-23 J/K
m = 0.056 kg / 6.02e23
v = 1e7 m/s
T = 1/3 * .056/6.02e23 * 1e7**2 / 1.38e-23

T = 225 GK

Looking at the black body temperature chart that 225 gigakelvin corresponds precisely to gamma rays from neutron star collisions.