A Beginning and an End

China successfully launched the first piece of its upcoming space lab on Thursday.  Named Tiangong 1 (Heavenly Palace), it is intended to serve as a testing base for space technologies that China will need in the future.  This makes China the third country to successfully have launched their own space station, following Russia and the US.  The station module is currently unmanned but will see several visitors in the coming months including China's first female astronaut.

In other news, the Tevatron shuts down for the final time tomorrow.  After nearly 25 years of operation, it has seen its last ion particle beams.  Tevatron bridged the gap between the cancellation of the Superconducting Super Collider and the building of the Large Hadron Collider (LHC).  With the closure of the Tevatron the US will lack a high energy particle collider on the scale of the LHC, meaning that US particle physicists interested in high energy collisions will have to go to CERN to do their experiments.  The Tevatron itself may be recommissioned as a muon collider making it one of the first in the world to collide leptons at super high energy.

Particles faster than the speed of light???

This surprising and confusing possibility was brought up due to recent results from OPERA (Oscillation Project with Emulsion-tRacking Apparatus), a neutrino experiment at Gran Sasso Laboratory (the paper can be found here).  The experiment works by firing neutrinos from CERN at Gran Sasso through the ground.  The original intent of the experiment was to study neutrino oscillations, but they ended up with a curious result.

When the timed how long it took the neutrinos to reach the detector at Gran Sasso from CERN they clocked in at 987.8 ns (Gran Sasso is 730.085 km away from CERN).  When they checked how long light would have taken in a vacuum they got 1048.5 ns, a difference of 60.7+/-6.9(statistical)+/-7.4(systematic) ns.  In other words the neutrinos appear to be traveling 0.00248+/-0.00028(statistical)+/-0.00030(systematic) percent faster than the speed of light.  The statistical and systematics after the numbers are the calculated errors on the measurements.

This is a mind boggling result, contrary to all that we expect from physics as we know it.  Thus the physicists who work on OPERA have asked other neutrino experiments such as T2K and MINOS to confirm this result.  Naturally they are also searching for any errors in their calculations.  If it is not confirmed then the measurement by OPERA is suspect and there is likely an error in their analysis.  However if confirmed, things will get a little weird to say the least.

Like the OPERA physicists I will refrain from speculating about what this might entail for physics if true.  This result is too significant to speculate on.  We need to know for certain if this is true.  Odds are definitely on the side of it being flat out wrong. Too much of modern physics works too well, which casts significant doubt on this measurement.  Not to mention that a significant delay was not seen between the neutrinos arriving and us observing the explosion of Supernova 1987A.  This single event already ruled out this result, but only for electron neutrinos, the neutrinos in this case are muon and tau neutrinos.  Also there is history to consider, in 2007 a similar claim was made in Chicago but it had a huge margin of error that ruled out the measurement being significant.  This measurement though is way above the normal 3 sigma level of significance (basically 3 times the error bar size) usually needed for scientific data so it deserves close scrutiny.  Naturally the entire scientific community will be holding its collective breath until it is either confirmed or falsified.

Why is the Sun Yellow?

This classic childhood question actually has the same answer as, "Why the sky is blue?'', which is actually not the same answer as many teachers give when teaching introductory Astronomy courses, including myself.  This was pointed out in an article, written by Jonathan M. Marr and Francis P. Wilkin at Union College, which appeared on arXiv today.

The usual explanation for why the Sun is yellow is given by the use of Wien's Law which describes where the peak of a blackbody spectrum is given the temperature of the body.  Using Wien's Law and the surface temperature of the Sun, 5800 K, we end up with a wavelength of 500 nm, which we perceive as green.  However, the Sun isn't green by yellow.  Thus the explanation goes that the sky preferentially scatters blue light, via Rayleigh scattering, which shifts the color towards yellow.  Plus the way the human eye perceives the light and the shape of the blackbody curve naturally favor a yellow color rather than green.

This explanation turns out to be partially correct.  While Wien's Law does give the peak emission for a blackbody curve, it only does so for the intensity, B_{\lambda}, and only when done in wavelength.  If one does the emission in frequency, which is inversely related to wavelength one ends up with the blackbody peak being at 880 nm, which is red.  One can't have two peaks so what is going on here?

This ends up being a pretty subtle, but power piece of radiative transfer (I apologize in advance this gets a little technical).  It turns out what matters is the units one looks at.  If one looks at the proper units for the emission, the peaks line up.  The proper units to look at are not intensity but what is termed the spectral energy distribution (SED).  Whereas intensity, B_{\nu} in this case, is in ergs/sec/cm^2/str/Hz, the SED is measured in \nu B_{\nu} which is ergs/sec/cm^2/str.  With the per frequency dependence gone, the peak now lines up properly with the peak gotten from doing the analogous thing with \lambda B_{\lambda}.  This is because the frequency and wavelength are inversely related.  This shifts the respective intensities in different ways.  The wavelength version shifts the intensity down to smaller wavelength, higher frequency.  The frequency version does the opposite by shifting the frequency down and increasing the wavelength.  The SED formulation removes this dependency, reconciling the two.

Using the SED version of the Planck function, another name for a blackbody distribution, the peak is now in between the two at around 633 nm, which is red.  The Sun isn't red though so what's the deal?  It turns out that astronomers have been abusing Wien's Law.  The Planck function isn't sharply peaked around its peak.  Rather it slowly falls off in the immediate region surrounding the peak wavelength.  Over the visual range, which is actually quite small in terms of wavelength space at 390-750 nm, the SED of light between the red and blue end only varies by about 1% which is less than the eye can discern.  Thus the eye would see the unmodified light of the Sun as white, and not any color.

Then why does the Sun look yellow?  Well the atmosphere preferentially scatters blue light via Rayleigh scattering as noted before.  This gives the sky its blue color.  All the blue light that comes from the sky has to be removed from the light coming directly coming from the Sun.  As a result this leaves only the red and green light, which our eyes perceive as yellow.

Thus from now on whenever I, or any other astronomer for that matter, are asked why the Sun is yellow or why the sky is blue, we can give the same answer from now on.

Planet found around Double Suns

This iconic scene from Star Wars may not be too far from reality.  It turns out that Kepler-16, has a planet orbiting around a binary star.  While the planet is much, much larger than the fictional Tatooine, about the size of Saturn, it illustrates that it is possible to have planets in stable orbits around a close binary star.

Why are orbits around binary stars thought to be unstable?  Much for the same reason that Jupiter acts like a giant scatterer in the outer Solar System.  Large bodies orbiting each other tend to push and pull orbits that come into their gravitational influence.  Actually NASA regularly uses this effect to change the orbits of satellites.  The larger the planet, or star, the larger the influence.  This can cause orbits to become unstable and if a close encounter occurs the body in question may be ejected in to interstellar space.  This is thought to be the cause of high velocity stars that we see flying through our galaxy.

However, in the case of Kepler-16 we have a close binary that also has a planet that is fairly close in, at only 0.7 AU.  Even more interesting is the fact that is has a low orbital inclination, meaning that the planet likely originally belonged to the star system when it formed.  Thus Kepler-16 likely shows that binary stars can have stable planets.

Not that this is entirely unexpected, theorists have been able to model stable orbits around binary stars for quite a while.  Observationally, though, this is the first time that a binary has been confirmed to have a planet.  Thus it should constrain the theoretical models of how planets form and orbit around binaries, giving us an even better chance of finding a planet in a similar situation to Tatooine.

SDO Views an Eclipse

One might think it weird to position a Sun observing telescope such that it would experience an eclipse of that celestial body.  However, that is what is happening now to the Solar Dynamics Observatory (SDO) as the Earth moves towards the Autumnal Equinox on September 23rd.  For the next 3 weeks around the equinox the Earth will block out the Sun for the SDO for up to 72 minutes.

So why did NASA decide to do that?  Well it was to put the SDO in a convenient position to be able to transmit data to Earth constantly to the same ground station.  This was deemed to be more important than the ability to monitor the Sun 24/7, especially since the eclipses are not very long.

Similar to many communications satellites, weather satellites and the GPS system, the SDO sits in geosynchronous orbit (GEO).  At that orbit you go around the Earth at the same speed as the Earth rotates which makes you appear stationary with respect to the Earth's surface.  The orbit is at a radius of 42,164 km from the center of the Earth, which is much closer than the Moon.  In order to be truly geosynchronous the orbit has to be inclined the same amount as the Earth's tilt (24.3 degrees) such that it hovers over the equator.

This is tilt means that for most of the year the Earth will not block out the Sun for the SDO. The Earth has only an apparent angular size of 17 degrees from the point of view of the satellite.  Since the inclination of the SDO's orbit follows the Earth's tilt, it is usually far enough away the ecliptic, the plane of the orbit of the Earth around the Sun, when it is behind the Earth not to have the Earth block its view.  This is due to the Earth's north pole tilting towards the Sun in the northern hemisphere's summer, which moves the orbit behind the Earth above the ecliptic, and away during the winter, which moves the orbit behind the Earth below the ecliptic.  This is the same reason why we don't get Lunar and Solar eclipses all the time, because the orbit Moon is inclined with respect to the ecliptic.

However if one is at GEO there are two times of year when the Earth's axis is not tilted away or towards the Sun, the equinoxes.  At those points the orbital section behind the Earth is in the ecliptic and the shadow of the Earth falls on the satellites in that orbit.  Thus the SDO is eclipsed for a short period of time during these sections of the Earth's orbit.  Not that the SDO can't do interesting science during these times. It will have to opportunity to look at the Earth and its surroundings during this time, not to mention that eclipses give a great chance to study the corona of the Sun.

In other news the US Senate has proposed an appropriations bill for NASA which continues funding for JWST.  Thank you to all those who contacted their representatives and senators.  The bill is not final however and still needs to be reconciled with the House version as well as voted on.  So continued support is appreciated.

AREPO vs. Gadget-2

Remarkably the image posted here is not of actual galaxies but rather those simulated by two codes.  The top is from a code called AREPO while the bottom is from the code Gadget-2.  Even more remarkable is that this simulation was for a code test of AREPO to see how it compared to a well established code like Gadget-2.

Both codes were developed by Volker Springel at Heidelberg University.  Both codes solve hydrodynamics and gravitational collapse.  However, the two codes could not be more different in terms of how they solve the hydrodynamics. 

Gadget-2 solves hydrodynamics via smoothed particle hydrodynamics (SPH).  SPH codes solve hydrodynamics in a similar way to N-body simulations.  Fluids after all are just a collection of particles whose collective evolution can be explained by the Navier-Stokes equations.  SPH just assumes that the particles in question are very large.  Since fluid mechanics a scale free, so long as your particles are not too large they will act the same as a fluid with much smaller particles.

SPH codes are popular for cosmology simulations because they allow you to solve gravity in a similar way to normal N-body codes.  This is convenient because the most computationally expensive part is usually solving the gravity.  There are a number of tricks one can use with N-body simulations to reduce the expense which can be applied to SPH as well.

The downside is that SPH does a poor job of capturing shocks and other fluid discontinuities.  These are important to track as these control fluid mixing as well as heating of the gas.  Thus SPH usually gets the general structure correct but it looks clumpy, due to the particles, and shocks are much more diffusive than one would like.

In order to capture shocks and discontinuities properly one typically falls back on grid based codes.  These codes treat the fluid in its continuum limit rather than treating it as a collection of particles.  Having a grid allows one to solve the equations of fluid dynamics directly at each grid interface, leading to much sharper discontinuities.  The problem with these types of codes is that gravity is difficult to and the resolution does not adjust with the flow, unlike in SPH where the resolution naturally follows where the gas is.

One can alleviate this problem by doing adaptive mesh refinement (AMR), which adapts the structure of the grid to the flow pattern.  However, you still have a problem that the code is Eulerian rather than Lagrangian.  In an Eulerian calculation one fixes a frame with respect to the gas and then calculates the flow with respect to that frame.  This can lead to large errors if the flow is moving fast through the frame in question.  On the other hand Lagrangian calculations follow a gas element of a specified mass.  The fluid equations are solved in the frame of that gas, reducing errors due to its motion.  This is another advantage SPH has.

Ideally one would like to have both the advantages of SPH and grid based codes.  Well AREPO was developed to do just that.  AREPO has what is known as a unstructured mesh, which is common in engineering circles.  However, AREPO takes it one step further by allowing the mesh to move with the flow in a psuedo-Lagrangian way.  This allows the grid to resize and shape itself as the fluid evolves, putting higher resolution in denser areas and reducing the errors due to fluid motion.

This leads to the much sharper and better looking galaxies in the AREPO image.  One can see movies showing individual galaxies, here.  It's uncanny how realistic these simulations look compared to real observations.

All is not roses for AREPO though as it is an expensive code to run, taking 50% longer to execute than Gadget-2.  Plus the memory overhead is large for these type of codes.  Nonetheless, the pay off one gets in terms of resulting simulation is quite impressive.

One more thing to wrap up this ridiculously long post (congrats to making it this far).  Fermi released their 2 year all sky map.  It shows even more sources than the 1 year map and with greater detail.  Very cool stuff.

HH47 Puts on a Show

This amazing video was released recently by Hubble of a jet coming from a young star.  Taken over 14 years from 1994 to 2008, these images give us a rare look at the time evolution of an astrophysical system.  Normally it is impossible to see time variability in large astronomical objects as their shear size and distance give them evolution times that are much longer than any human could hope to see.  However, some objects are close enough, small enough or moving fast enough that we can make out their motion as shown in the above clip.

The above object is called HH47.  HH is the designation for a Herbig-Haro object.  HH objects are now known to be glowing portions of jets that are ejected by a young star when it forms.  These jets originate at the accretion disc of the star and then are collimated by magnetic fields and rotation.  The clumpiness and structure in the jet is currently a subject of much study in the astrophysical fluids community.  The nature of the clumps and their formation can tell us a lot about the formation of the jet and its propagation.