Star eaten by a black hole: still blasting away

[Seshmeister]

http://blogs.discovermagazine.com/badastronomy/2011/08/26/star-eaten-by-a-black-hole-still-blasting-away/


In late March of 2011, an extraordinary event occurred: a black hole in a distant galaxy tore apart and ate a whole star (I wrote about this twice at the time; here’s the original post, and a followup article including a Hubble image of the event).
Now, there’s more info: the black hole, lying at the center of a galaxy nearly 4 billion light years away, has about 8 million times the mass of the Sun. When it tore the star apart, about half the mass of the star swirled around the black hole, forming twin beams of matter and energy that blasted outward at a large fraction of the speed of light. The folks at NASA’s Goddard Space Flight Center made a great animation to show this:


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The star was ripped apart by tides. The thing about black holes is, they’re small: this one was probably about 15 million kilometers across. A typical star is about a million km across (the Sun is 1.4 million kilometers in diameter, for comparison). This means the star could get really close to the black hole, and that’s why it was doomed. The force of gravity drops with distance, so as the star approached, the side of it facing the black hole felt a far greater force than the size facing away. That stretched the star, and the stretching increased as the star got closer. At some point, the force was so great it exceeded the star’s own gravity, and it could no longer hold on to its material. The black hole won — as they usually do.

http://blogs.discovermagazine.com/badastronomy/files/2011/04/star_torn_apart.jpg


The material from the shredded star formed a disk around the black hole, and near the center heated up to millions of degrees as it swirled around at near the speed of light. For reasons not entirely understood, this forms the beams of matter that jet away from the hole, and as it happens one of these beams was aimed pretty much right at us (not to worry, though, since at that vast distance the light was so diminished it took Hubble to see it in visible light at all).

That’s what alerted us to the event in the first place; it was detected by the Swift satellite, which was designed to look for high-energy blasts from space.

Normally, things like this fade pretty quickly, but in this case, amazingly, it’s still pouring out energy and will probably be detectable even into 2012. That is partly due to relativity: because we’re looking straight down the beam of material, we see its clock ticking more slowly. This effect works better when the material is moving at high speed, and radio observations show that the blast is still expanding away from the black hole at half the speed of light! And that’s after it slowed down by ramming into material floating in between the stars in that galaxy. It started out moving at more than 90% the speed of light.

The energy it takes to do this is mind-numbing: we’re talking about roughly an octillion tons of matter screaming outward at well over one hundred thousand kilometers per second!
It literally makes the hair on the back of my neck stand up. I’m glad this happened billions of light years away.

Astronomers will continue to observe this event to learn more about it. It probably happens all the time in the Universe, but this is the first time we’ve had the equipment to really get a good look (even if we have to crane our necks a bit from 4 billion light years away). We’re not really sure how often something like this happens, or how it affects the galactic environment. I’ll note I’m not terribly worried about it happening in our Milky Way (we have a 4 million solar mass black hole in the center of our galaxy), since, after all, we’re here. If this happened often enough to be dangerous, we wouldn’t be here to talk about it!

But it might make an interesting movie plot. Hmmmmm…

Credits: Video: NASA; Artist’s illustration of star and black hole: NASA/CXC/M.Weiss.

[Seshmeister]

http://blogs.discovermagazine.com/badastronomy/2011/08/26/star-eaten-by-a-black-hole-still-blasting-away/

Normally, things like this fade pretty quickly, but in this case, amazingly, it’s still pouring out energy and will probably be detectable even into 2012. That is partly due to relativity: because we’re looking straight down the beam of material, we see its clock ticking more slowly.

I'm finding this bit a little tricky to get my head around...

[Seshmeister]

Always best to go to Mr Sagan for explanations...

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[binnie]

Fascinating stuff.I

I often wonder how much of this is 'known' and how much is 'surmised'. As far as I can tell, satellites and telescopes have detected an 'event' which has occured millions of light years away - scientists then have to interpret what that 'event' is/was. The result of that interpretation is what we have here.

I'm always curious to know how they arrive at that result, and how certain anyone can be about it. Oftentimes, the thought-process which these people undergo is far more interesting than their hypotheses.

[Sensible Shoes]

I always enjoy these big discussions of space that I cannot understand about energy that can potentially "eat" the earth. If we can't grasp how big it is it will certainly be a painless and unknowing death.

[Seshmeister]

I find it amazing that the sat nav in your car has to adjust for relativity.

Time passes slower on the satellite than it does on earth because of it's speed so your sat nav has to include this to calculate your location otherwise it would 'lose' 100 feet every day.

[ashstralia]

whoa yeah! love the astronomy. time, distance, the speed of light. all connected by this elegant theory.



http://www.haydenplanetarium.org/universe/duguide/app_light_travel_time_dista.php

[ashstralia]

Fascinating stuff.
I'm always curious to know how they arrive at that result, and how certain anyone can be about it. Oftentimes, the thought-process which these people undergo is far more interesting than their hypotheses.

the astronomy has a long and interesting history, bin. i reckon it's like the king of science because it encompasses.... everything.:)

[Sensible Shoes]

I find it amazing that the sat nav in your car has to adjust for relativity.

Time passes slower on the satellite than it does on earth because of it's speed so your sat nav has to include this to calculate your location otherwise it would 'lose' 100 feet every day.

I wonder if this is the same thing that causes the delay in "live shots" done over a satellite. You always see the reporters delay a second or two in responding - that's how long it takes them to hear the studio.

[Seshmeister]

No that's just the time it takes for the signal to travel, the time dilation effect from a satellite is very very small .

Sat navs use very accurate clocks so time slowing down even a tiny amount has an effect.

The satellites used for TV are usually geostationary, i.e they are put in a position whereby they stay at the same point relative to the Earth.

http://www.paulandliz.org/Technical_Pages/Relativity/Default.htm


The GPS Navigation System
Did you know that Relativity has real consequences in our everyday life? Well, it does, and one key area in which it affects us is in the global positioning system commonly used these days for aircraft navigation, trekking, sat-navs in cars etc. If you are flying in a commercial airliner, the pilot and crew are navigating to your destination with the aid of the Global Positioning System. Further, many luxury cars now come with built-in navigation systems that include GPS receivers with digital maps, and you can purchase hand-held GPS navigation units that will give you your position on the Earth (latitude, longitude, and altitude) to an accuracy of 5 to 10 meters that weigh only a few ounces and cost around £75.

GPS was developed by the United States Department of Defence to provide a satellite-based navigation system for the U.S. military. It was later "released" to other organisations for civilian navigation uses. The current GPS configuration consists of a network of 24 satellites in high orbits around the Earth. Each satellite in the GPS constellation orbits at an altitude of about 20,000 km from the ground, and has an orbital speed of about 14,000 km/hour (the orbital period is roughly 12 hours - contrary to popular belief, GPS satellites are not in geosynchronous or geostationary orbits)! The satellite orbits are distributed so that at least 4 satellites are always visible from any point on the Earth at any given instant (with up to 12 visible at one time). Each satellite carries with it an atomic clock that "ticks" with an accuracy of 1 nanosecond (1 billionth of a second). A GPS receiver in an aeroplane determines its current position and heading by comparing the time signals it receives from a number of the GPS satellites (usually 6 to 12) and triangulating on the known positions of each satellite. The precision is phenomenal: even a simple hand-held GPS receiver can determine your absolute position on the surface of the Earth to within 5 to 10 meters in only a few seconds (with differential techniques that compare two nearby receivers, precisions of order centimetres or millimetres in relative position are often obtained in under an hour or so). A GPS receiver in a car can give accurate readings of position, speed, and heading in real-time!

To achieve this level of precision, the clock ticks from the GPS satellites must be known to an accuracy of 20-30 nanoseconds. However, because the satellites are constantly moving relative to observers on the Earth, effects predicted by the Special and General theories of Relativity must be taken into account to achieve the desired 20-30 nanosecond accuracy.

Because an observer on the ground sees the satellites in motion relative to them, Special Relativity predicts that we should see their clocks ticking more slowly. Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion. Further, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth's mass is less than it is at the Earth's surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away. As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.

The combination of these two relativistic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometres each day! The whole system would be utterly worthless for navigation in a very short time.

The engineers who designed the GPS system included these relativistic effects when they designed and deployed the system. For example, to counteract the General Relativistic effect once on orbit, they slowed down the ticking frequency of the atomic clocks before they were launched so that once they were in their proper orbit stations their clocks would appear to tick at the correct rate as compared to the reference atomic clocks at the GPS ground stations. Further, each GPS receiver has built into it a microcomputer that (among other things) performs the necessary relativistic calculations when determining the user's location.

So, as you can see, Relativity is not just some abstract mathematical theory: understanding it is absolutely essential for our global navigation system to work properly!