Institute of Astronomy


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Radio emission from black holes

Published on 09/04/2014 

I've been reading about astronomical radio sources and I've noticed something that doesn't seem to make sense to me.
It is my understanding that gamma rays are the highest energy form of electromagnetic radiation, is it the shorter wavelength that gives it more energy?
If this is true then why do planets radiate mostly in the infrared and paradoxically, extreme energetic processes like those around supermassive black holes radiate in the radio spectrum even though infared wavelengths are shorter than radio wavelengths? 

You are quite correct that gamma rays are at the top end of the electromagnetic energy scale, and that a shorter wavelength corresponds to higher energy.

Part of the difference between the two scenarios you mention, radiation from planets and processes related to supermassive black holes, is what is generating the radiation.  For thermal radiation, that is light produced as a result of the heat of the object, the wavelength at which the emission peaks gets shorter as the object gets hotter.  This is why emission from Earth peaks in the infrared, while sunlight peaks in the visible.  The accretion disks around supermassive black holes are actually so hot that the radiation they produce peaks in X-rays.

Active supermassive black holes are also however often associated with strong emission in the radio, as in radio galaxies, like you've noticed.  This radio emission is much stronger than would be expected from thermal sources.  Instead, the radio waves produced by radio galaxies are the result of energetic electrons (accelerated by the black hole) spiralling around magnetic field lines.  We call this sort of radiation synchrotron radiation, so called because it was first associated with synchrotrons - devices that use powerful magnetic fields to bend beams of extremely energetic particles.  The spectrum of synchrotron radiation has a very different shape to thermal radiation. 

Flying near the edge of darkness

Published on 20/01/2014 

On a flight to Greenland on the 18th  January 2014, we were approx 30 mins from Kangerlussauq, arriving at 9.40 local time.

When we looked out of the right hand window of the aircraft the sky above was blue - daylight - underneath that there was a layer of darkness where we could see the moon and a star and underneath that the 'land' which was a cloud layer.

Out of the left hand widows the sun was rising and looked normal.

Is that possible due to flying close to the 'night and day line' on the earth?

This sort of effect is possible when flying close to the terminator (the line between day and night).  At the time you were over Greenland and saw this effect I presume you were probably at cruising altitude 10-12km up.  Line of sight and the curvature of the Earth means that from that height you could see the Sun, although on the ground below the Sun would not yet have risen. Similarly in the sky on the 'nightward' side of the aircraft the Sun would have been shining high up in the atmosphere, but not nearer the ground, and being high up yourselves you could see the effect more clearly.  On the ground this is why the sky toward the horizon where the Sun has either recently set or is soon going to rise appears lighter.

Plane of the solar system

Published on 15/01/2014 

This has somewhat been playing on my mind for a little while now, We've all seen the posters in school telling us the order of the eight planets and they're all neatly put in a straight line and it came to me, that seriously cannot be how the planets orbit the sun in a straight line some must be off in a tilt. So I went and tried to do some research and most sources do put all the planets in a somewhat near line not really varying from a straight rotation around the sun... So I was wondering is that image correct do all the planets tend to rotate around the sun on an even plane if so then our solar system must be extremely flat with huge vast spaces closely above and below planetary rotations that are never occupied.

The solar system is indeed very flat, the orbits of all of the planets are within a few degrees of the same plane.  This plane is also very close to the plane of the Sun's equator.  The flatness of the solar system is one of the pieces of evidence that suggests that the planets formed within a disc around the young Sun.

The space above and below the plane in which the planets lie is not entirely unoccupied, as many asteroids and comets have much higher inclinations.  The main part of the asteroid belt for example reaches up to about 20 degrees above and below the plane of the planets and some of the outlying groups can reach 30 degrees.  Long period comets, like Hale-Bopp, come into the inner solar system pretty much evenly from all directions, so we believe that the Oort cloud, where they originate, is roughly spherical.  All of these small bodies probably didn't form with such inclined orbits though, they were scattered by the planets (particularly Jupiter) to reach their current orbits.

Not all planetary systems show the same amount of flatness though. Some of the new planetary systems that we have been finding around other stars are somewhat different.  Some of them are actually even more flat than the solar system, but then there are others that where the opposite is true and there are huge differences between the planet orbit and the stellar equator.  We suspect that the cases where the orbits are not aligned are systems that have had much more violent histories than the solar system with close encounters between some of the planets.

What happens to light falling into a black hole?

Published on 15/01/2014 

I can't find an explanation that my brain likes for my query.....
My logic thinks that light should 'pause' at the event horizon of a black hole, to an observer away from the outside of the event horizon.  I'm imagining firing a flare away from me into a black hole from a distance.  As the flare moves further away from me it becomes smaller, but once it reaches the event horizon it's size will remain the same from the distance where the observer remains stationary/fired the flare.  If observer moves closer to the light it will become larger as if the flare was coming back to me.
Now, going back to the initial query... being light has the fastest speed, at the point of the event horizon would the speed of light not be countered with the 'speed' of the gravitational pull of the black hole and just pause?

One of the things about relativity that can be a bit difficult to get your head around is that light always seems to move at the same speed, no matter how fast you are travelling, or in what direction. What happens instead is that the Doppler effect changes the wavelength of the light, similar to the way that the pitch of a siren on an ambulance or a police car seems higher as it is coming toward you, and then lower as it is moving away.

With light, if you are moving toward the light source then the light will seem bluer, whereas if you are moving away from it the light will seem redder.  Gravity has the same effect, so what happens to the flare as it approaches the event horizon is that it seems redder and redder, until at the moment that it actually crosses the event horizon the wavelength has been stretched so much that is undetectable.

Adaptive Optics - Tip/Tilt correction for amateur astronomy

Published on 14/01/2014 

In our local astronomy club we are debating the effectiveness of available amateur tip-tilt adaptive optics at removing the effects of seeing. The promotional literature shows tighter stars with higher peaks using the AO. Could it be that the AO is only compensating for errors in the telescope mount? Or is it possible that AO can partially compensate for bad seeing? They advertise 5 to 10 corrections per second.

Adaptive Optics (AO) systems measure and correct any distortions coming into your telescope/detector. It does this by measuring in real-time the distortion on the incoming 'wavefront' and applying a correction to it, flattening out the distortions so you obtain a sharper image on your detector. Typically AO is used to measure and correct the atmospheric effects (i.e. bad seeing) on a wavefront coming into a telescope system but it can be used to correct systematic problems. As such, you could use it to correct for errors in the mount providing they are not too severe although as these effects are most likely repeatable, mounts can either be trained or the use of an auto-guider may be as effective and cheaper.

If you are trying to remove the majority of the atmospheric distortions you would need a system running at many hundred times a second and this is what many of the professional large telescopes around the world do today. It is however possible to get a correction with a much simpler system by thinking about turbulence being made up of different scales of distortion. You have larger scales i.e. tip-tilt, defocus etc. which will affect the whole of your image while much smaller scales will only effect a smaller part. This means you can think of these large scale distortions causing most of the spread of the PSF and if you can remove these effects from the system, you can improve your image. For tip-tilt, typically you will get a factor of two improvement in image quality but this improvement will only be as good as the measurements and corrections of the AO system - think of it more as a 'theoretical' limit. If you can also remove the defocus term, by that point you'll have image quality by about a factor of 5. The more corrections you do, the greater this factor becomes but to do these extra corrections adds complexity, cost etc.

Regarding timescales and seeing - the timescale of correction is set by how fast the atmospheric turbulence changes. This is principally set by the 'wind crossing time' of a telescope (i.e. how long it takes cell of air (think of it as a cubic metre) to move from one side of the telescope entrance aperture to the other) and how many 'turbulent cells' you have across an aperture. Turbulent cells of air are typically 10-20cm across in size (this is seeing dependent - on a clear night it could be bigger) and so across the diameter of a 4 metre telescope we have approximately 20. As such, you need to correct at least 20 times a second on a 4m telescope and for smaller telescopes, you can work slower than that. The slower you can go, the easier the system is as the correcting mirror can move more slowing and you can collect more photons from your reference star before needing to make a measurement i.e. improving sensitivity. There is however a limit to the level of corrections you can make - the correcting mirror will have a limited 'stroke' on each element which is the maximum movement possible. As such, if the tip-tilt value is greater than this (at the edge of the mirror) you will be unable to correct the distortion. This is why typically AO systems on large telescopes do not work well with seeing greater than 1-1.5" (there is additional complexity) although on smaller telescopes some level of correction should still be possible.