The Space Thread

All I know is... with each flyby we make of odd-shaped icy rocks, the odds of eventually finding an incredibly rude shape seem to go up. :/

Any of you watching the eclipse?

Can't. Wrong timezone. Sun's up for me during the eclipse

NASA is making its last effort to contact the Opportunity rover, which hasn't responded since it ran into a Martian dust storm in June. Winter is coming to the part of the planet it's on, and if it can't move away, then it means an end to the rover's 15 years of service.

#RIPOppy.

Fifteen years of stirling work. NASA would've been thrilled with just the five when at the blueprint stage, let's face it.

Oppy did good.

Just in case anyone missed it: astronomers have captured the first direct image of a black hole. What an incredible mark for science, I hope over the years we may get some more detailed pictures of this incredible phenomenon, now that they know its location.

Well, technically, it's not really a picture of the black hole itself...

But, I'm giving the getting of very good picks of the mess this nomming glutton is making (thereby throwing its usually-not-visible self into relief) a pass.

Imagine all the gravitational info and everything else we'll get off this beastie now we know exactly where it is. And what algorithms we can use to clear the scatter with.

Edited by Euodiachloris on Apr 11th 2019 at 11:47:47 AM

I know enough about black holes to know that you can't technically see a black hole because it absorbs all light, so what precisely is the orange colour we see around the black center?

Black holes actually release matter and radiation pretty much all the time (not everything gets attracted: plenty gets repelled or slingshotted away, and that's not accounting for quantum shenanigans, superhot accretion discs or radiation plumes/flares).

It's just that for most of that time, the levels of energy are far too low to easily detect from any distance away, even in radio. Although they are quite a bit easier to pinpoint in X-ray, but it's generally a tough job to get anything beyond what looks like a blob in either.

The trick is to catch a black hole you've deduced exists care of the star orbital movement around it while it's busy acquiring significant amounts of unfortunate mass from the neighbourhood. Then it'll splurge out in a broad array of detectable wavelengths.

And that one is going to be a busy piggy for a while: it's digesting a large cluster of stars' worth of matter. Which means a lot of it is being thrown about.

In short: black holes are messy eaters.

Edited by Euodiachloris on Apr 11th 2019 at 7:07:25 PM

In other words this star is surrounded by an accretion disk. Matter rarely falls into a black hole directly, usually its in a decaying orbit and if enough of it spirals in all at once it gets heated due to friction and it gets hot. Really hot. And as Euodiachloris said, this ones currently chowing down and as such is surrounded by an active accretion disk.

Phil Plait, astronomer and blogger, has a nice breakdown of the details in the linked blog post.

Veritasium has a great video about what we see in the black hole image. I highly recommend it. To simplify as much as possible, black holes distort spacetime in a variety of ways, but what we actually see when we look at them is based on several phenomena:

• The event horizon is the point (surface, technically) of no return, beyond which matter and energy are lost to our reality. We cannot see it, of course, because nothing escapes from it. ^note Except via Hawking radiation, which is too faint to detect for any black hole of stellar or greater mass.
• For a non-rotating, Schwartzchild black hole, at about 1.5 times the radius of the event horizon we find the photon sphere, which is the innermost stable orbit that light can achieve. Any light (photons) falling within this orbit inevitably spiral into the event horizon. Light just above this orbit will circle the event horizon, possibly several times, before escaping back into space. It can create some absolutely insane image distortions. The pitch black part of the image that we actually see is the shadow cast by the photon sphere.
• At about 3 times the radius of the event horizon, we find the innermost stable orbit for matter. Just like light at the photon sphere, any matter descending beneath this will inevitably fall into the black hole. Matter orbiting beyond this distance is what we call the accretion disc, and can have a velocity up to 90% (?) of the speed of light.
• Any rotating black hole may also have a magnetic field, and that field will collimate and eject matter and energy along its rotational axis. M87 has one of these jets aimed almost directly at us, which we can see in images of it.

Because the matter orbiting the black hole (the accretion disc) is moving incredibly fast, it is also incredibly hot, and it emits black body radiation in all frequencies of the EM spectrum. This is the glow we see around it. Since it's moving so fast, it is also significantly Doppler-shifted, so we see a brighter (bluer) image for the side rotating towards us, and a dimmer (redder) image for the part rotating away.

Even more amazingly, due to the weird optical distortions caused by the black hole, some of the accretion disc image will be from behind the event horizon.

I just found this thread while looking for a place to discuss the recent SpaceX launches. Is that a suitable topic? The Arabsat-6A mission last week was the first commercial launch of the Falcon Heavy rocket, and the first successful landing/recovery of all three stage 1 boosters. It was amazing to watch, and I never get tired of seeing it.

Edited by Fighteer on Apr 15th 2019 at 11:03:28 AM

I managed to get about 3/4 of the way through that video before my brain melted. I cant quite get the difference between the event horizon itself and the so-called "shadow".

OTOH- I think rocket launches are on topic for this thread.

Well, I've found a couple of YouTube channels recently that are very entertaining and very informative, as they discuss various aspects of space travel and rocket technology: Everyday Astronaut and Scott Manley.

Everyday Astronaut focuses on the technology and the launches. Among my favorite videos thus far is "The Biggest BOOMS in Rocket History", looking at [non-fatal] rocket failures and the magnificent explosions they create. My son enjoyed it as well, unsurprisingly. "Come watch rockets blow up." "Okay, Dad." Did you know that the Soviet N1 rocket failure in July, 1968 is the largest man-made, non-nuclear explosion in history?

Scott Manley also talks about technology and space news, with more animations and deeper dives, and he uses KSP a lot. This recent episode talks about the Event Horizon Telescope image.

Does anyone else have favorites to share?

It's very mathy, but to simplify as much as I can: the event horizon is the surface beyond which things vanish from our spacetime. However, there is a surface above the event horizon that marks the radius beyond which light cannot orbit fast enough to escape falling in. ^note Photons move at the speed of light, always, but if the photon is moving at an angle relative to the event horizon, only a part of its momentum vector is directly opposing the inward vector of the black hole's gravity. The most direct route to escape the event horizon is radially outward, but no incoming photon can ever have that trajectory, as it would mean it had to go through the black hole first.

Any photon coming in at a different angle will be bent around the event horizon and, if it's too close, will hit the horizon before its trajectory would carry it away. This defines the photon sphere, and you can show via diagrams that it creates a "shadow" that eats all light within this radius.

You do actually get emissions from matter falling inward towards the black hole, as some of these photons would take a straight-line trajectory away from the event horizon, but they would be red-shifted to the point where they would be almost undetectable against the glow of the accretion disc.

Edited by Fighteer on Apr 16th 2019 at 9:57:45 AM

To put it another way, the event horizon is the point at which nothing (not even light), can escape from the black hole's gravity. The "shadow" is the point at which nothing (not even light) can avoid hitting the event horizon.

Anything within the event horizon effectively no longer exists in our universe^note except as mass added to the black hole, anyway. Anything within the "shadow" isn't gone yet, but its orbital trajectory means that it will hit the event horizon and vanish forever. And since all of the light within the "shadow" inevitably hits the event horizon, it's impossible for us (as outside observers stuck halfway across the universe) to ever see that light, even though it hasn't hit the event horizon yet.

ultra tldr: stuff that passes the event horizon disappears, stuff in the "shadow" hasn't disappeared yet but will disappear soon, so none of that light is visible to us on Earth.

Edited by NativeJovian on Apr 16th 2019 at 12:13:12 PM

Technically, incoming mass can add to a black hole's angular momentum and electric charge too. Perhaps other properties as well - but by that point we are deep into the realm of quantum gravity.

And the holographic principle, which I've been learning about recently thanks to PBS Space Time. It's utterly fascinating, but well beyond the scope of analyzing the EHT image.

Note that matter falling into the black hole can emit photons that escape its gravity, up until the matter actually crosses the event horizon. There are some great videos that illustrate this.

Edited by Fighteer on Apr 16th 2019 at 12:45:57 PM

If a photon is in an orbit that will inevitably intercept the event horizon, then how can information about that photon ever reach us?

Rocket launches are the single most epic thing humans have ever done.

It cannot. That's why the photon sphere casts a shadow.

On the opposite(?) side to black holes, would a quasar outshine the stars of planets in their galaxy? I've heard quite impressive things about them. And that they're probably mostly dead by now, but that applies to a lot of things in space.

That's a good question, and I'm not certain of the answer. Quasars can have a variety of intensities, as can stars, and distance plays a factor as well — a solar system in the rim of the galaxy would be in a different situation from one in the core.

Quasars can outshine all the stars in their galaxy put together, so at the very least you'd get one hell of a light show at night, and possibly even during the day.

Would these quasars be capable of holding potential life? Because with that lightshow their sleeping cycle and mythology is going to be interesting

Umm, what? The quasar itself is an engine of pure destruction. Nothing could possibly live anywhere within thousands of light-years of it.

Life in a planet that is outside of the immediate death zone of the quasar would be... interesting.

@Fighteer- "It cannot. That's why the photon sphere casts a shadow."

I must be missing something. If information about the photon cannot reach us, is it not already lost to our universe?

Sorta, however, another photon in the exact same location but going a different direction could reach us.