Photopic Sky Survey

Astronomy has a rich and extensive history of work done by both amateurs and professionals. Given its long history and the ease of access the study of the night sky can be done by anyone with the desire to ponder what lies in the great beyond. This has become even more true with the advent of mass production of telescopes and reasonably priced high performance digital cameras. With the additional aid of advanced computer software, amateur astronomers can take images that used to be relegated to the realm of professional astronomers.

Such is the case with today's example of a superb amateur project the Photopic Sky Survey. An amateur astronomer from Seattle, Nick Risinger, spent a year collecting 37,400 exposures of the night sky to make a complete map of the sky at 5 Gigapixels. The resultant image shows what the sky looks like in natural colors, the colors our eyes would naturally see if they could see faint enough objects. This is in contrast to the majority of professionally made images which have their colors changed to emphasize things of scientific interest.

The survey is a stunning piece of astrophotography which illuminates what has been lost in the light pollution of modern society. While the image shows objects and features far fainter than our eye can see, it does illustrate the richness of the night sky which has been seen from time immemorial if only we stop to appreciate it. An article describing the project can be found below (thanks to my dad for pointing it out):

http://seattletimes.nwsource.com/html/localnews/2015040399_nightsky12.html

Einstein Right Again

Not necessarily that surprising that this point, but still it is always fun (and extremely important) to have experimental/observational evidence to back up theoretical expectations. Courtesy of today's APOD, which as evidence of this blog will show is my number 1 source for cool astronomy stuff, we have the results of Gravity Probe B (astronomers and physicists are terrible at naming things).

So what theory of Einstein's did Gravity Probe B confirm? It confirmed one of the predictions of General Relativity (GR) known as frame-dragging. I will admit, this one is a little bizarre and incredibly tough to test but I will do my best to explain. Now let's say you were a gyroscope. It turns out for gyroscopes once they are pointed in a direction they remain pointed in the same direction forever unless acted upon by a force. This is actually true for any spinning body, which is why gyroscopes are so useful for navigation. You always know where the gyroscope is pointing so you can figure out the way you are pointing by looking at which direction the gyroscope is.

Now let's say you were in space, orbiting the Earth. You had a gyroscope pointed at a particular star, let's say IM Pegasi. In regular Newtonian gravity you would expect that the gyroscope would continually point in the same direction as you orbited the planet, never changing. However, this is not true for GR. In GR a mass physically warps the fabric of space-time, creating a sort of divet. This in turn creates a small force which will perturb our gyroscope causing it to precess (geodetic precession in the image on APOD). Not only that, since the Earth is rotating it creates an additional force called frame-dragging. Frame-dragging causes an additional force which causes additional precession. Think of frame dragging as similar to magnetism. In electromagnetism if you rotate a charged particle you generate a magnetic field. The same is true for GR, instead the charge is the mass of the object. The mass effectively pulls space-time with it as it rotates creating a sort of spiral effect in the fabric of space-time.

Both of these precession effects are super small, but can be predicted from GR. Thus Gravity Probe B did the above experiment to detect these two precession effects. To do this experiment was a heroic effort in astronautical engineering and precision physics. First the team made 4 nearly perfect spheres which were spun up to create the gyroscopes. Then they had to make a near perfect laboratory in space in order to make sure they didn't add apply any force to the gyroscopes once they were spinning. This required them to build a drag-free space craft, exclude the Earth's magnetic field from the area where the gyroscopes were, and measure the gyroscope without touching it. Details on how they accomplished this can be found here.

The sum total of there effort culminated in the most exacting test of GR to date. The precession they measured was consistent with the rate predicted by GR. With this confirmation, we can be confident in applying GR corrections to our every day items such as GPS, atomic clocks, communications satellites, etc. It also gives numerical astrophysicists confidence in their predictions which use GR, such as black holes and other compact objects.

Magnetars may fuel Gamma-ray bursts

Its an interesting news which came in spacenews on Nov 2010 (had been thinking of posting it for a long time, but procrastination won!). According to this new finding, magnetars may be the driving force behind a larger population of gamma-ray bursts (GRBs) than scientists had earlier thought. 

Magnetars that last several minutes before they collapse under their own weight to form black holes had previously been invoked to explain the formation of some long GRBs (bursts that last for more than 2 seconds). Calculations by Paul O’Brien and Antonia Rowlinson of the University of Leicester in England and their colleagues show that a magnetar could also account for a short GRB, specifically a 36-millisecond flash (GRB 090515) observed on May 15, 2009, by NASA’s Swift satellite. About 10% of all bursts are believed to be short GRBs. If the magnetar model proves correct, it may indicate that a larger reservoir of stars than previously estimated could power GRBs, O’Brien said.

Gamma-ray burst aficionados cite two reasons for their interest in magnetars. First, the enormous rotational energy of these stars, which can spin hundreds of times a second, is sufficient to power some gamma-ray bursts. In addition, a short-lived magnetar could account for the brief but steady X-ray afterglow that immediately follows some long bursts. Unlike a smoldering ember, these unusual X-ray afterglows radiate at a constant brightness until they abruptly die away. The duration of the steady emission matches the lifetime of the magnetar, theorists propose.

In analyzing the afterglow of the short burst GRB 090515, the team led by O’Brien and Rowlinson found that the X-ray emission was unusually steady until it dropped off sharply a few hundred seconds later, just like the afterglows of 10 long gamma-ray bursts they had also recently examined. Aside from the magnetar, “there are no other sensible models” to explain a steady X-ray afterglow that suddenly drops to zero, said theorist Péter Mészáros of Pennsylvania State University in University Park. 

Researchers agree that because magnetars have a maximum rate of rotation, these spinning stars are limited to powering bursts less energetic than 3 times 10^52 ergs — about the energy that would be unleashed if 3 percent of the sun’s mass were converted into energy. Bursts with higher energies would require a black hole to power them, said Brian Metzger of Princeton University.

Here's the paper featuring these findings.

Holmberg's Light Bulb N-body Simulation

Ever thought to use one of these to simulate gravity? Erik Holmberg did back in 1941. Seeking to understand how galaxies interact Erik Holmberg developed a new method for studying these complex systems which were too difficult to study analytically.

It turns out that the flux of light emitting from a point source falls off with radius the same way as the force of gravity, 1/r^2. Since gravity is so weak one could not make a scale model of an interacting system in the lab that worked via gravity. However, if one substituted light one could do a crude simulation of how the system worked. For example, let us consider the Earth and the Sun. If we wanted to simulate how these two bodies interact with each other via gravity using light instead we could do the following:

1. Scale the relative brightnesses of the light bulb to be the ratio of the masses between the Earth and the Sun. For reference lets set the Earth mass to be equivalent to a 60 Watt light bulb, which means the Sun would be a 20 MWatt light bulb.
2. Now that we have our light bulbs we can set up a light detector at each of them to detect how much light they receive from the other bulb. The amount of light received equals the amount of force felt. Since the Sun is so much brighter than the Earth, the Earth will detect more light from the Sun and thus more force. Meanwhile the Sun will feel very little force from the Earth.
3. Now that we know the forces we can calculate the accelerations and move our lights a short distance to new positions based on those calculated forces.
4. After we move them we can recalculate the forces and repeat the process.

If one gave the Earth and appropriate starting velocity one could simulate the Earth going around the Sun by this method. This type of technique is called a N-body simulation.

Now Holmberg didn't just simulate the interaction of two bodies, but of 37 bodies interacting with each other. The 74 bodies were arranged to simulate 2 galaxies colliding with each other (37 light bulbs for each galaxy). Each light bulb stood in for a collection of stars, and the mass of those stars was indicated by the brightness of the light bulb. Each galaxy then is approximated by a collection of light bulbs, which serve as proxies for the collection of stars that make up galaxies.

Much pains taking work was needed for this calculation, but the results were stunning. Holmberg notice tidal tails in his simulations, as well as the ejection of stars from the interacting pair (take a look at the paper for details). This gave him insight into the complexities of these types of interactions that served as a pioneering step forward in simulation.

Now a days N-body simulations are the norm of astrophysics and are done on supercomputers not via light bulbs. However Holmberg's simulation occurred 20 years before any attempt was made to simulate these types of interactions via a computer. Truly a remarkable accomplishment.

50 Years of the US in Space

A little less than a month after Yuri Gagarin launched the era of human space flight, his US counterpart Alan Shepard followed him in to the great beyond becoming the first US citizen in space. His vessel, the Freedom 7, is the subject of today's APOD commemorating the 50th anniversary of this momentous event. When Shepard was asked after his successful mission about what was going through his head while sitting in his ship he replied, "The fact that every part of this ship was built by the low bidder."

While not nearly as impressive as Yuri's complete trip around the Earth, with Alan's flight only lasting 15 minutes, travelling up 188 km and travelling downrange only 486 km, it was the start of something huge. Unlike Yuri's flight Alan's was televised live across the world and observed in person by almost half a million people. With the US having sent a man into space it kicked off the Space Race between the USSR and USA, the eventual goal of which was the Moon. It was this event that catalyzed US President Kennedy to make his famous call for the US to send astronauts to the Moon and return them safely before the end of the decade. The rest is history.

For more insight on today's events take a look at this article which contains an excerpt from the book Moon Shot, co-written by Alan Shepard and Deke Slayton. In it you will get a feel for how Shepard, his wife and those who observed the launch felt on that day. Fun fact, Alan Shepard actually ended up going to the Moon on Apollo 14. This flight earned him another distinction of being the only man to play golf on the Moon.

The Great Red Spot by Voyager 1

Sorry for the gigantic image, but it was just too cool to pass up (Edit: The image has been resized to fit better. NASA loves to show things at uber-resolution, unfortunately that doesn't work too well when showing it in a blog. The full res image can be found at APOD). Courtesy of today's APOD we have an exquisite picture of Jupiter's Great Red Spot taken by Voyager 1 as it passed by in 1979. To be honest this picture looks more like a painting than an actual image of a real event, but such is the way of many astronomical images.

The Great Red Spot is an incredible storm that has been raging for over 400 years on Jupiter. No one is quite sure how it started or why it is so long lived. It is huge though being so large that two Earths could easily exist inside of it, with winds topping out at about 400 km/h.

While it looks like a giant hurricane, the Great Red Spot is actually anticyclonic. This means the Great Red Spot rotates the wrong way with respect to the expected Coriolis forces. Instead the Great Red Spot is a fantastic example of a classic fluid instability called the Kelvin-Helmholtz instability. In this instability you have two fluids adjacent to each other, each flowing in opposite directions. This forms a shear layer between the two fluids. If one perturbs the boundary between the fluids, one creates and instability which will grow mixing the fluids. This mixing pattern can form into a stable configuration known as a cat's eye, of which the Great Red Spot is an excellent example. Simulations of this instability can be found here.

The probe that took these pictures, Voyager 1, is now long gone from Jupiter. It is now on a trip out of the Solar System and is investigating other interesting astrophyisical fluid dynamics. In this case it is looking at how the wind generated by our Sun interacts with the surrounding Interstellar Medium (ISM). Currently Voyager 1 has entered what is known as the Heliopause. In this region the Solar wind slows down and stops as it interacts with the ISM. This creates a contact discontinuity between the Solar Wind and the ISM. When Voyager 1 leaves this area it will finally enter into interstellar space for the first time. This will be our first chance to directly probe what interstellar space look like rather than relying on telescopes. Voyager 2 is also on the same path and will join its sibling out in interstellar space soon after Voyager 1 gets there.

On a side note, in my previous post I mentioned Endeavour's impending launch. The launch has been delayed due to a heater malfunction. The earliest date for a retry won't be until May 8th. However, that is subject to change as any further problems are found. Once a firm date is set I will try to post a note here.

Endeavour's Final Flight and Asteroid Collisions

Despite some crazy storms rolling through the US south-east, the final flight of the space shuttle Endeavour is slated to go at 3:47pm EDT today. The above picture was taken during the night before the launch. While lightning and rocket fuel don't normally mix, NASA has lightning rods stationed all over the place to absorb the impacts and keep the shuttle safe.

Endeavour is carrying, among other payload, the AMS (Alpha Magnetic Spectrometer). This 7 ton instrument will be attached to the International Space Station and has a primary science mission of 3 years. The goal of the AMS is to detect and identify the types of cosmic rays in space. This is important because ground detectors of cosmic rays have a difficult time identifying the specific types of particle a cosmic ray is. It can generally say whether it was an electron, photon or atomic particle, and generally how massive it is. However it cannot identify which specific element it is, nor can it figure out if it is something more exotic like antimatter.

The AMS is basically a like taking a smaller version of one of the Large Hadron Collider particle detectors and flying it in space. So it has the capability to detect the type and energy of the particles that pass through it. Since it is so big it will also be able to probe high energies, which is extremely useful for cosmic ray physics and particle physics in general. The AMS will also provide a very accurate measure of what the radiation environment in space is like. This will be crucial for planning future manned space flights, especially interplanetary flights as precautions need to be taken to protect the astronauts from the dangerous radiation that permeates space.

You can join the other 750,000 people who will be watching the launch by watching it live on NASA TV. Any launch, particularly a shuttle launch is a sight to behold. Unfortunately I will be judging a science fair at the time so I can't watch.

Another interesting story came of NASA yesterday regarding a collision that happened between two asteroids. While the event itself was observed with a smaller telescope, both Swift and Hubble took a look at it shortly after the collision. The images show dust and debris being flung off of the impacted asteroid called Scheila. From the impact and comparing it to the Deep Impact event we know that the impactor was about 30 meters across and moving at about 11,000 mph. Scheila itself is about 70 km and lives in the Asteroid Belt between Mars and Jupiter. This is the first time this type of event has been observed so soon after the impact, excluding of course man made impacts. These type of impacts tell us a lot about the object that got hit as it excavates material from the asteroid that we would normally not see. Analyzing the debris can tell us about what the asteroid is made of. A link to the press release is below:

http://www.nasa.gov/topics/universe/features/asteroid-collision.html