## Wednesday, December 26, 2007

### Life and death of interstellar dust

In this part of the "dusty posts program" we will talk about formation and destruction of dust in the interstellar medium. Before we embark on this questions we need to know at least two things about dust: its size and composition.

As we mentioned in the last post, the dust betrays its presence by absorbing light, the absorption efficiency at some wavelength depends on the size of dust grains. Absorption is a rather complicated phenomenon because there are many physical processes that can cause absorption in our sightline, light might be literally absorbed by the dust grain or it might be scattered out of our sightline. Scattering is also dependent on the ratio between the wavelength of the scattered light and the dust grain diameter. If this ratio is small we talk of Rayleight scattering, otherwise the process is known as Mie scattering.

Detailed analysis of absorption curves leads us to believe that scattering is caused by small solid solid particles which have a diameter comparable to the wavelengths of visible light (this explains the 1/lambda behavior in the visible part of the spectrum), nonetheless we still need other components to explain some features of the absorption curve.

An average absorption curve, the far infrared is on the left and the ultraviolet on the right. There are some variations in different zones of the galaxy, but the shape of the curve is usually quite similar. We can see the peak of absorption are at 200 nm (or 1/lambda = 4.6 ).

Now let's move to the composition of dust. The Sun being so close to Earth is readily observed and we (somehow naively) assume that it is a typical star and its composition is assumed to be fairly typical of the Galaxy. We can then use the Sun abundance of elements as a guide for relative abundances between it and the interstellar medium.

We know that the relative abundances of the Sun and the interstellar medium are different. The main difference is that some elements have an underabundance of elements compared to the Sun. This elements are usually refractory, meaning that this elements can survive high temperatures without being sublimated, this directly points us to dust, as this elements can condense in dust grains that can survive in the vicinity of stars. Depletion is defined as

and as we can see below, this elements are among others Cr, Fe, Ni, Co, Ti. Despite that, the bulk of a grain is quite similar in composition to the dust you can find in your backyard and is mostly divied in to graphites, silicates and olivines.

Interstellar elemental abundances compared relative to hydrogen compared with the Sun. We can see that refractory elements are heavily depleted. The depletion is a logarithmic quantity defined, meaning that some elements like Ti are depleted by three orders of magnitude.

So, how are dust grains formed? The first idea is to think that they grow slowly in the interstellar medium. The problem with this idea is that actual calculations show that this will take too much time, suppose that at time t=0 the grain has a radius r(0), if we suppose the grain grows by addition of species i with mass m_i moving at a mean thermal speed v_i then the radius at a later time t will be

$r(t)=r(0)+\frac{\epsilon n_i m_i \overline{v_i}}{4s}t$

where epsilon is a "sticky coefficient" that meassures the probability that the m_i will remain attached to the grain and s is the density of the solid grain. Using the typical values for interstellar medium we find that for a typical grain (about the size of visible wavelengths) the time required is

$t=\frac{3 \times 10^9}{\epsilon}\mathrm{yr}$

which is too much time if we consider that the value of epsilon is smaller than one, and that the age of the universe is around 13 Gyr.

The solution comes from considering denser regions, in particular the atmospheres of cool stars. The material in stars is originally so hot that it did not contain any solids, but as it outflows in later phases of the life of the star it starts to cool with much higher densities than the interstellar medium and the atoms can arrange themselves in to the most stable molecules at that temperature (usually around 10³ K). When this outer layers are ejected in stellar winds the dust goes in to the interstellar medium. There is some controversy about the formation of dust in the early life of the universe, the most accepted view is that this dust was formed around active galactic nuclei.

Now let's move to grains destruction. The simplest way to destroy a grain is to heat it enough so it sublimates. That indeed happens in the vicinities of young, hot stars. The sublimation temperatures depend on the composition and size of the grain, this results in a stratification of the dust with large graphites closer to the star, followed by smaller graphites and large silicates, at larger distances from the star we can find smaller silicates, olivines, fused quartz and silicates with ice mantles.

A more interesting mechanism comes from accumulation of electric charges in the surface of the dust, if the grain is spherical with a radius a and is carrying a charge of Z electrons then the electrons capture cross section if the speed of the electrons is u_e is given by

$\sigma_e = \pi a^2 \left( 1+ \frac{2 Z e^2}{4 \pi \epsilon_0 a m_e u_e^2} \right)$

When enough electric charge accumulates on the surface the stress it causes on the grain as it tries to redistribute, causes the grain to literally explode! Quite an interesting consequence for the well known fact that charges redistributes on a surface!

It actually results that the capture of species by dust grains has a crucial importance in astrobiology, keep tuned!

## Monday, December 24, 2007

### Merry Christmas

Today it's Christmas eve. Receive the warmest regards from the "staff" of this blog. On this day, 39 years ago the crew of Apollo 8 entered into orbit around Moon, so this is also an special day for space exploration. Not only that, maybe there is a quite special gift under the tree this year: 2007 WD5.

2007 WD5 is an asteroid that will cross Mars orbital path in January 30. There is some chance (around 1 in 75) that it will impact Mars! This asteroid is around 50 meters wide and might create a half-mile wide impact crater. Despite this event is rather unlikely it will cast some serious fireworks in case it happens.

This is the orbital path of 2007 WD5, you can clearly see that it intercepts Mars orbit. The uncertainties in the measurement of 2007 WD5 path don't allow us to assure a collision will happen, actually it's quite unlikely it will occur. In a few days we will have more data.

Update: This asteroid has been recently identified in archival imagery, this has allowed astronomers to refine the orbital parameters, now the odds of a collision are of 1 in 25! Still a rather meager chance (4%), but we still need more data for a more accurate prediction.

## Monday, December 17, 2007

### Dust in the interstellar medium

Today I will give you a brief overview of a topic that's quite close to me: the effects of dust in interstellar medium. There are some regions in the sky that seem to be almost magically depleted of stars, even in very dense regions. The classical example of this "dark nebulae" is the coalsack in the Crux constellation.

The coalsack dark nebula is the best known region where interstellar dust is so dense that it blocks all the light (in the visible frequencies) in it's sightline.

This regions are not depleted of stars, but rather the light of the stars in the sightline is absorbed by dust. This is far from obvious, after all there is no a priori reason to discard the idea that this regions simply lack any stars.

The first conclusive evidence of light absorption in the interstellar medium came from the work of Robert Trumpler who measured the angular sizes of globular clusters trying to calculate its distance from Earth. The idea was simple, let's assume that globular clusters are roughly the same size, then the smaller a particular cluster seems to us, the farther it is. When Trumpler carried on this plan he realized that the there was a linear relation between the size and the distance, i.e. the clusters were bigger as the distance to them increased. This was quite unexpected and also seemed to point to the rather disturbing conclusion that Earth is located in a special place where globular clusters are smaller.

Rather than that, Trumpler realized that the distance was systematically overestimated (so the clusters weren't that big, after all). Then he assummed that the reason for this bias was extinction in the interstellar medium, meaning that something (presumably dust) was absorbing light on its way to Earth.

Further evidence for dust comes from light polarization. When light is reflected from a dust grain it is polarized depending on the alignement of the grain, if the grains are aligned with Milky Way's magnetic field, we can then use the dust polarization as a measure of the direction of the magnetic field.

Measurements of dust polarization, this measurements show that optical polarization is aligned with the magnetic field.

What is the composition of this dust? This depends on many factors. Nonetheless the most important one is the distance to a star, particullary to hot, young stars. This stars (known as early spectral type) are so hot that that dust sublimates in its vecinity. The grains composed of carbonites are more resistent (and bigger grains have a better chance of survival), then the silicates enter the stage. In colder enviroments larger molecules appear on the surface of the grains.

How are this grains formed? Why is dust relevant for astrobiology? Which are the effects of dust on ionized regions? Keep tuned for later posts!

## Friday, December 14, 2007

### Doomsday

What will happen to the Earth in the far future? Can humanity or it's successors live forever in it? By now most of you know the lore: the sun will eventually consume all of its "fuel" (hydrogen and later helium) in 5 billion years, expand in to a red giant that will devour Mercury, Venus and the Earth. After that the Sun will eject the outers layers of its atmosphere and leave a stellar corpse: a white dwarf which is small, extremely hot object supported by the pressure of electron degeneracy. The ejected outer layers will be ionized by the radiation of the white dwarf and form a nice planetary nebula, like the Helix.

The Helix nebula is one of the best known planetary nebulae, in the astro-lore it is usually asummed that the Sun will produce a similar nebula when it dies.

Well, I recently attended a talk by Klaus Peter-Schroeder, from University of Guanajuato, in his talk he disputed this scenario. Now, let's discuss the good, the bad and the evil ...

• The good: Previous models assumed a "naive" modeling of mass loss as the Sun ejects it's outer layers based on Reimer's Law, which is basically based on dimensional arguments. An improved version of this law applied to the Sun shows that it will loose enough mass to allow Earth's orbit to enlarge sufficiently to avoid the doomsday scenario.
• The bad: Well, that doesn't mean that Earth will be an habitable planet. It will be too uncomfortably close to the Sun, but you might still think that we have around 5 billon years to worry about that. Well, that's wrong. The Sun will increase it's energy output in around a billion years (still a lot of time) by a sufficient amount to increase Earth's temperature around 10 K. You might think that it will be an uncomfortable but still bearable change, nonetheless climate models predict that effects of such a change will be catastrophic.

In "The End of the World", the Doctor travels to Earth's last moments before being engulfed by the Sun. In the show the writers imagined that some "gravity satellites" held back the expansion of the Sun, we now know that it will be tidal dragging to blame for Earth's destruction.

• The evil: Well, life on Earth will end one day, but at least will Earth survive to total destruction? Not really, despite that the Sun won't grow enough to engulf Earth, a process known as tidal dragging (you can think of it as a sort of "tidal friction" assisted by dynamical drag) will cause the Earth to spiral into the Sun! The model is extremely sensitive to little details, but we can estimate that the doomsday will happen just 500 000 years before the Sun reaches the tip of the AGB branch.
So the lore is actually quite wrong in practically every detail, but wait! There is still a final twist to this plot. The nice planetary nebula that will act as a sort of mausoleum to the star that made life on Earth possible actually requires stronger stellar winds than the Sun can provide. This is still a point of controversy, but at least among the attendants the consensus was the Sun might produce an irregular planetary nebula at best.

Addendum: It results I was being very naive with the raise of temperature. In Klaus own words:

"It is 1 Gyr for a rise of 10% in the solar irradiance, which makes the Earth leave the habitable zone. But in order to raise the temperature by just 10K, we need much less time! It is difficult to compute because a detailed knowledge of the various positive (and negative) feed-backs is required, but it should be of the order of 100 mio years. That is still very long compared to the timescale of the current climate change....! "

## Saturday, November 17, 2007

### Modest Understanding of Lie Groups Part 0: U(1)

This semester I had the pleasure to take a very little nice course on mathematics, mathematics for physicists that is. What this means is that half the course we dealt with lie groups and the remaining month or so we studied path integrals. Now, why is this interesting? It just happens to be the sexiest mathematics available to me at this point.

In case you didn't know, finding symmetries in physics leads to a deeper understanding of the phenomena at hand. This is obvious to any undergraduate student facing for the first time electromagnetism. The most basic problem of this course is finding the electrical field a distance d above an very long line of uniform density charge. Needless to say, you want to know how much the line would pull (or repel) a charge, should you feel like putting one a distance d above it. Of course you don't need to understand much about physics to eventually see that it doesn't matter where you place it, as long as it is a distance d perpendicular to the cable. Clearly this is because the line of charge is very long and this places are practically the same to the line. From this information you then can guess that the electric field must only depend on the coordinate perpendicular to the line, a trivial conclusion, but proves the point just fine I guess.

Again, why am I talking about this? Turns out our most precious tool (for the moment) allowing us understanding the world, the Standard Model, is based on symmetry groups. Namely it is usually represented by SU(3)xSU(2)xU(1). Let's start by understanding the simplest part of this: U(1). Imagine a circle, or rather, its points:
In this figure, A and B are two points on the circle. All the points on this circle are characterized by some properties. For example, if a point on the circle is represented by the vector
$X = \begin{bmatrix}x\\y\end{bmatrix}$
the point
$\begin{bmatrix}x'\\y'\end{bmatrix}=\begin{bmatrix}\cos \theta & -\sin \theta \\ \sin \theta & \cos \theta\end{bmatrix}\begin{bmatrix}x\\y\end{bmatrix}$
is also on the circle! Having seen this, it's easy to see that the points on a circle with the operation of addition (since each point is characterized by an angle, we can understand it as the sum of their angles) form a group. If we see this circle on the complex plane, a point on the circle can be represented by a complex number
$e^{ia}$

and a rotation about the center of the circle will be given by multiplying this number by the following phase factor
$e^{i\theta}$
This will take us
$e^{ia}\rightarrow e^{i\theta}e^{ia}=e^{i(a+\theta)}$
that is, another point on the circle (closure). If this is new to you, try and find the identity and inverse elements.
We can see this phase factor as a 1X1 matrix, and call it U. It's clear then that in this case
$UU^*=1$
But in general for bigger matrices
$UU^\dagger=1$
I hope then to have explained how this implies that the complex numbers of norm 1 form a group under the operation of multiplication. This group is the most simple I can think of for now, the U(1) group (unitary matrices of rank 1, which satisfy the last equation).
Tune in next time for a brief explanation on all the other classic groups.

## Saturday, November 03, 2007

Last may Symmetry magazine launched a contest for inventing new particles, and the results are out in the latest edition. One of the particles is so relevant for this humble site that it deserves special mention: the blogino, it's creator Jacobo Konigsberg from Fermilab says about it:
Particles created by non-abelian Blog-Blog interactions. Bloginos typically are produced in a very excited state and with a high degree of spin. Even though all their properties have not yet been determined, it is commonly agreed that they exhibit considerable truthiness. They also have the annoying ability to propagate into extra dimensions, away from the blogosphere, and generate lots of phone calls.

The allmighty blogino, the coolest particle around only behind...

The rockon "discovered" by Ike Hall from Fermilab was, hands down, my favorite particle:

Responsible for such things as face-melting guitar solos, heart-pumping rhythms, screaming vocals, and hair bands. Observation of the rockon over the airwaves has been on the decline since 1995.

Yep, that particle really rocks. It's particullary close to me since rock is what I most like in life!

## Thursday, November 01, 2007

### Comet Holmes from OAN-SPM

Comet Holmes is an old folk for astronomers, discovered in 1892 and with a period of 5.9 years, it was discovered in an outburst during which the comet brigthened to a magnitude around 4-5.

A similar outburst happened last october, from magnitude 17 to 2.5! It is still visible at the dusk (or dawn) if you look at Perseus, maybe some binoculars will be necesary. I won't delve further into it because there are many posts about it in the blogosphere (1)(2)(3), rather I'll show you a nice pic of this comet.

Use this sky chart to look for Comet Holmes, it depicts the sky at a latitude adequate for most of North America and Europe.

Alan Watson, an astronomer at Centro de Radiastronomía y Astrofísica, UNAM was at OAN-SPM (National Astronomical Observatory at San Pedro Martir), and used the 1.5 meter telescope to take this nice picture of comet Holmes in the R band (meaning light was filtered to only allow "red" light arrive the detectors).

The exposition time was 10 seconds. Look at the displacement between the core and the coma.

## Wednesday, October 24, 2007

### Are the laws of physics emergent ?

Last monday I attended a fascinating conference by Robert Laughlin (who won a Nobel Prize for his remarkable work on fractional quantum hall effect). The meadow of his arguments is this: the behavior of the world is ultimately governed by emergent phenomena.

Let's say a kid wants to know how some simple device works, he will surely try one thing: dissasembling the device, look at each part and figure out how they work. This is what Laughlin calls the reductionist approach. Physicis has been carried this way for the past centuries, this is how we arrived to our knowledge of elementary particle physics, by dissambling the matter in to smaller chunks at increasing energies.

Emergent phenomena arraises when collective behavior suddenly becomes different from the behavior of it's individual parts, this pinpoints to one of the fundamenal characteristics of emergent phenomena: universality. This means that the collective behavior is esentially independent of the properties of it's individual parts.

This was exemplified in a funny way by Laughlin who showed Newton burried under a big pile of apples, which is an obviously different behavior from that single apple in Woolsthorpe. Note that for the sake of beeing buried under a big pile of something it won't make a difference to be buried beneath apples, watermelons or potatoes.

One of the really perturbing things mentioned by Laughlin was that Newton's first law is actually emergent and comes from a broken symmetry, this was totally unexpected for me, but maybe one enlightned reader can bring some light to this issue (please!).

But, are fundamental laws of physics emergent? There is no doubt that emergent phenomena is important and quite interesting. And of course, every reasonable physicist will tell you that we don't need to know the detailed behavior of quarks or QED for describing biology or weather.

Whatsoever I don't honestly believe we can have a complete/satisfactory knowledge can be acquired in this way, we actually need to know from where this laws emerge from. As an example, how are we going to figure (say) the properties of fundamental particles? It seems to me that if you keep asking "why?" you eventually need a detailed description, the sort of fundamental physics we have always think about.

And, this might be just a wrong perception but it seems to me that the "reductionist approach" is far more general, at least in the sense we only need to know the law of gravity (and dynamics, of course) to compute the behavior of an arbitrary number of apples, this also applies to any other system where gravity is acting. On the other side you need to have a "lot's of falling things law", "orbital's motion law", "single falling ball law", etc...

## Saturday, October 20, 2007

### Halley's debris

During the year, earth's orbit crosses the paths of hubris left behind by comets. When this debris enters the atmosphere it is heated by friction and it's temperature raises dramatically making them quite easy to spot, this are the so called shooting stars.

When the earth crosses a dense stream then the number of shooting stars (usually close to one per hour in the whole sky) meteor showers occur. Some well known meteor showers are the Perseids and the Leonids (usually boring, the Leonids can produce the most spectacular meteor showers evey 33 years including the meteor shower of 1833, usually considered the most spectacular ever).

Tomorrow's night (October 21-22) the Orionids will reach their peak, in the north you can expect 20 meteors per hour and 40 in the south.

Use this starmap (from meteorshowersonline) to watch the orionids, the stream of meteors will look as emerging from a radiant, the radiant is quite close to red bright star Betelgeuse.

Remember that this means 20 meteors in the full sky, and to be honest this shower isn't as reliable as the Perseids, nonetheless it is a nice weekend activity.

The source of the Orionids stream is the famous Halley's comet, so you are actually watching little pieces from this comet as they enter the earth, quite amazing if you think about it, there are even missions where high altitude planes collect this comet dust.

## Wednesday, October 17, 2007

### Cool Nerd King

Are you facing the ultimate question: Am I a nerd? The website NerdTests.com offers you the answers you always wanted. In particular in my trip to the oracle I got:

Oficially I am a cool nerd king, whatever that means. Submit your results to the comments section!

### First light ... and some Sudoku!

Well, hello to all of you, kind blog readers, whoever and wherever you are. This is my first post, or making reference to what observational astronomers say when a new telescope captures its first image ever, this is my first light.

I think Luis already mentioned the main points concerning myself. I've been fascinated with Physics, Astronomy and Computers since I was very little, so getting into numerical astrophysics was an inevitability of fate, it seems. In case I seem too geeky, I should note that I also enjoy movies, music, videogames and drinking beer/tequila like there's no tomorrow.

My current research is focused towards numerical models of DEM L316, a pair of supernova remnants in the Large Magellanic Cloud that were caught in action, ie: two supernovas that might have exploded near to each other and thus may now be colliding.

-

Now, time for some Sudoku! For those of you who don't know, Sudoku is a simple number game, where one must fill the blanks in an incomplete 9 x 9 number grid, following a simple rule: a number may only appear once in each row, each column and in each of the nine 3 x 3 blocks of the grid.

I spent most of my afternoon writing a Sudoku solver. Since I'm a bit rusty with my programming skills, I thought it'd be a good exercise. The program is written in C++ and I've compiled it under SuSE 10.3 (which I've just installed on my laptop and works wonderfully).

It's a gzipped tarball. I've also included a binary compiled in SuSE 10.3. In order to compile the source in your own distro, if you have the GNU C++ compiler just type "g++ -o sudoku sudoku.cpp". The grid to solve is entered in the data file sudoku.dat, which should be in the same directory as the binary. I've included the hardest grid I've found (for my solver) as example; it takes 605,484 trials to solve it.

The algorithm behind the program is simple. It's a brute-force recursive algorithm. This means my algorithm solves Sudoku by trying a valid number in the first empty position, then trying a valid number in the next grid position, then the next, etc ... all this done through a recursive function. When the function finds it cannot place a number in a particular position, it goes back to the last grid position, and tries the next valid number there... again, all this done recursively.

In the end, it works very well and fast enough (despite being an exhaustive algorithm). It takes well under a second on my C2D T7100, 2GB RAM laptop to find the solution to any grid I've fed the program. The program is nice since it makes use of recursion, which is a powerful feature of programming languages. As the creator of Ghostscript, L. Peter Deutsch, put it:

To iterate is human; to recurse, divine.

As is to be expected, the greater the number of blanks to fill, the more trial number placements the program must do to achieve a solution. However, there is no clear correlation between the reported difficulty of a particular grid and the trials needed by my code to solve it, other than the fact that harder puzzles usually have a larger number of empty squares to fill.

## Thursday, October 11, 2007

### And Now for Something Completely Different...

A man with string theory up his nose.

As our gracious host has already pointed out, I'm actually a physics student, so it's an honor to be able to write for this fine astronomy blog. In any case I'm beginning to understand the AdS/CFT correspondence, which as you may already know, deals with the apparent duality that exist between some string theories and quantum field theories.
What we can aspire to do with this approach is to investigate quantum field theories without relying on perturbation theory, which limit our understanding of nature. For example, Quantum Cromodynamics (QCD) enjoys asymptotic freedom at high energies, this means that since interactions are weak at those energy scales it's sane to use perturvative methods. However in cases where energy scales are small, we simply do not know how to perform calculations. As always numerical methods are useful to some extent, but even they have a hard time dealing with some situations (they involve dealing with some dynamical quarks, as far as I know). Other option is to study the AdS/CFT correspondence and gather bits of information about gauge theories in general. It all boils down to this: either, A) Find a string theory dual to QCD to be able to make calculations or B) Understand properties about gauge theories in general so you can make predictions about QCD. The second option is more viable and has had success in the past. But I'll explain that some other time.

Since I'm just starting studies on this subject I hope I'm not making some false statements, (I'm not, as far as I know). Feel free to comment on the subject should you feel otherwise.

On a side note, Toots Thielemans is playing tomorrow at Netzahualcoyotl Concert Hall, one of Mexico's most beautiful concert halls in my own opinion. This will be interesting...

### What makes this blog thick?

Counter services are one of those little modern wonders, allowing to track the number of visitors to a site, the country of origin, the browser they are using and most importantly what keywords they used to arrive to the site.

So, which are the more popular keywords in the last 100 visits? Let's take a look:

mexican scientist, mexican astronomer
Yes. That's just what we are.

mexican astronomy
You have just come to the right site, you can have a little glance at astronomy done in Mexico from firsthand participants, mostly students, but still firsthand participants.

iraf ubuntu, ubuntu iraf, ubuntu iraf package, install ds9 ubuntu
Fine, enjoy the scripts. You can still say you compiled IRAF yourself, just to impress your (geeky) friends.

astronomy package octave
Sorry, but octave is an interpreted numerical language quite similar to Matlab and it lacks astronomy specific packages (you can update me on that), maybe if you have some astronomy package for Matlab it will work on octave.

mexican scientist famous, important mexican scientist
Get back in a few years...

luis.sanchez
What!!! Who is google stalking me? I should lock the door right now...

### Claudio Toledo: our latest adquisition

Juan Claudio Toledo (also known as Meithan West in the underworld) has joined our blogging team. He is a grad student at Instituto de Astronomía, UNAM, where he works on astrophysical fluid dynamics and interstellar medium (yes, this blog is populated by ISM researchers).

He is currently working on numerical simulations of astrophysical fluids (this usually means a fluid that is self-gravitating and usually ionized), so hopefully he will bring his numerical expertise to this site.

With his addition this blog is now almost a chilango blog!!! Well, that should be remedied soon!

Welcome Claudio!

## Tuesday, October 09, 2007

### Eric joins the team

I proudly announce the addition of Eric Pulido (from a dual approach) to our blogging staff. He is currently a physics grad student (yes, this blog is driven primarly by students) at Universidad Nacional Autonoma de Mexico, as far as I can say his interests are in theoretical physics, mostly at string theory and cosmology (allowing me to call him an "astronomer").

Besides bringing his stringy expertise to this blog his addition will move the center of gravity of this blog a bit in to Mexico City.

Welcome Eric!

### Nobel to giant magnetoresistance

I have always been very impressed by this modern devices capable of storing vast ammounts of data in a quite reduced space.

This devices had been possible by giant magnetoresistance, an effect discovered in multilayered materials (Fe/Cr/Fe trilayers and Fe/Cr multilayers). The effect manifests itself as a significant disminution in the resistance of the material in the abscence of a magnetic field, this has been implemented in the reading heads of hard drives allowing the data to be stored in really weak magnetic fields.

Giant magnetoresistance is widely regarded as the birth of spintronics, in particular in giant magnetoresistance the spins of the electrons of the nonmagnetic metal align parallel or antiparallel with an applied magnetic field in equal numbers.

This work got the nobel to Albert Fert and Peter Grünberg, who had just won the Wolf Award this year, so this prize wasn't a complete surprise for anyone, and it is actually the recongnition to some really remarkable research that has lead in to devices that everyone uses on a daily basis.

## Monday, October 08, 2007

### Ok, I'll read the manual

In my latest procastination attack I stumbled over this masterpiece of contemporary humor. If you change the toaster by my computer then the situation starts to feels quite familiar for me. Check xkcd for more great comic strips (just in case you didn't knew it before).

## Sunday, October 07, 2007

### Nobel Hype 2007

The next week the winners of the Nobel prize will be announced. As usual there is always some fuzz speculating who will win. As usual I declare my own ignorance of the whole selection process. Whatsoever I can always say who are the obvious candidatates in the astro related areas: Alan Guth, Paul Steinhardt and Andrei Linde for inflation (although Andreas Albrecht was also crucial and a quite similar mechanism was proposed before by Starobinsky), and the leaders of the Supernova Cosmology Project and the High-z Supernova Search Team that found evidence of the acceleration of the expansion of the universe. Now, considering the prize went to cosmology the last year I don´t think it will be awarded again to a related area this year. Dark matter also deserves a nobel, but it's history is too long, although Vera Rubin is the usual suspect. Thomson scientific has a stadistical based prediction, saying that Martin Rees will win the prize.

What about other areas of physics? Well I am certainly less savvy about that, I have always believed that James Bjorken who found the scaling law for QCD deserves the prize and also Yoichiro Nambu along with Jeffrey Goldstone for the bosons that appear in simultaneously broken symmetries. The SNO experiment showing that neutrinos indeed oscillate is another crucial contrubution.

On the rest of physics I don't feel like making some sort of prediction, but of course, your predictions are always welcome.

## Monday, September 24, 2007

### Resurrection day

After a really long hiatus I have decided to bring this blog back to life, mainly because I really enjoyed writing it.

In the mean time I have been quite busy (which is why I haven´t posted for so long, of course), in particular with some calculations of the mechanism that keeps HCHII regions (relatively small clouds of ionized gas around young massive stars) confined. Expect some posts on this.

## Tuesday, January 16, 2007

### Getting IRAF the easy way

If you have ever reduced astronomic data, then is almost sure that you know the IRAF package. Despite being old and cumbersome this package is unvaluable for image reduction and analysis in astronomy.

However it's installation is a quite involved ritual, well, not anymore. Realizing that the entry where I discussed a debian package for IRAF was the most visited one, I am posting a much better solution: installation scripts, you can use this scripts for installing IRAF (and related packages like ds9) in ubuntu, and in Scientific Linux (this script should also work in Opensuse and Fedora). Just download them anywere and follow the instructions on screen. This scripts are only small modifications from the original one by amd77. Just make sure you have installed csh (it is in the universe repository in ubuntu and quite probably in the distribution media of Fedora and Opensuse) , you just need to run the comand: bash installiraf_* and enter your password in Ubuntu or the root password in other distro, so here are the scripts (links not working):
After running the script you should run the command mkiraf for creating a login.cl file, I have that file in my home directory but you can put it anywhere, just remember to start iraf (the command is cl) from the same directory.

Update:

Since I posted this (quite a long ago) there have been all sort of changes in of libraries/packages included in modern distributions. Since there is no simple way to address this isssue (there are 6 versions of Ubuntu, 6 of Fedora and 5 of opensuse in the past 3 years, and that not counting that both come in 32 and 64 bit versions) I declare the scripts dead.

Nonetheless, an iso image which includes an useful installer is now being offered at: http://www.astro.uson.mx/favilac/downloads/ubuntu-iraf/iso/IRAF_Ubuntu.iso. Just keep in mind that it only works for 32 bit kernels (which are the majority of installed kernels, anyway).

The same autor also offers a (rather outdated) set of rpms at http://www.astro.uson.mx/favilac/downloads/IRAF/ . Find out which one is better suited for your distro and add it to your repos. I have not tried to install from these rpms so I can not comment on them.

For users of 64 bit systems only solution I know is to actually download iraf from iraf.net and proceed to install as detailed in the installation manual.

## Wednesday, January 10, 2007

### News from the AAS meeting

This week in Seattle,Washington will be held the 209th meeting of the American Astronomical Society. This a very important event and there have been some quite interesting results already anounced.

The findings of the COSMOS team consisting of a weak lensing survey of a 1.6 square degree patch of the sky have gathered a lot of attention. This a rather big portion of the sky (around 9 full moons), this technique measures the distortions (gravitational lenses) produced by the mass between the source objects and the observer, I had already discussed it in relation with the Bullet Cluster. We now have a map of the dark matter in this region of the sky and it shows clearly how the dark matter is getting clumpier by the effect of gravity. I won't go into further details, mostly because many others (Clifford, Phil, Sean, Angela) have already blogged about it.

Another interesting announcement has been the detection of a Triple Quasar using the Keck telescopes and the VLT by a team lead by George Djorgovski. This system known as LBQS 1429-008 was already known to be a doble quasar, but the new deep images show a faint third member. This is interesting mostly because it supports the idea that quasars are more frequent in intereacting enviroments as the gravitational interactions throw large ammounts of material in to the central black holes of this galaxies. You can find more info here.

Stay tuned for more news, this week will surely have more interesting findings!

### Watch for comet McNaught, today!

Comet McNaught has brightened this week to the point it is readily observable, even in large cities! If you are living at northern latitutes and have a clear southwestern sky you can try to watch it, it might be a very nice sight. Depending on your latitude you can watch it up to the friday or saturday.

The comet should be very low, close to the horizon, at 15-20 degrees from Venus (depending of your latitude). For more detailed instructions go to SkyTonight.com site, they also have a nice photo gallery.