Tuesday, October 31, 2006

Usra Major: The oldest constellation?

Two quick but fascinating pieces regarding the constellation Ursa Major (aka the Big Dipper, aka the Great Bear, aka the Plow, aka the Wagon) came to my attention this month.

(1) Scientific American had an article by the archaeo-astronomer Bradley E. Schaefer regarding the origin of the Greek Constellations (note: only a preview is available online unless you are a SciAm Digital subscriber). The majority of the constellations are not that old - the official constellations used today date back to an IAU resolution of 1922, and archaeological evidence shows that very few constellations were named by 1300 BC, with a big burst of constellation-naming occurring at ~ 1100 BC.

But what I found really interesting is that one of the constellations, the Great Bear, might be many times older than that.

"The wagon name must, of course, have come after the invention of the wheel (roughly the fourth millennium B.C.), but the bear name is undoubtedly much older. Early societies throughout Eurasia recognized the Great Bear stars and myth. The most common version was the the four stars in the bowl of the dipper were the bear, which was perpetually chased by the three stars in the handle, which represented three hunters. The Greeks, Basques, Hebrews and many tribes in Siberia had this basic star/myth combination. Surprisingly, the same bear stars and stories surfaced throughout North America. With some variations, many tribes of the new world - including the Cherokee, Algonquin, Zuni, Tinglit and Iroquois - share the interpretation of the bear followed by three hunters... [he discusses various ways of interpreting this synchronicity, before concluding] ... The Bear is unlikely to be an independent invention, because the stars do not look like a bear... The most logical [remaining] explanation to connect the traditions holds that the first settlers of the New World carried the basic myth across the Bering Straight [i.e. about 14000 years ago]."

(2) Space.Com has an interesting article about a powerful optical illusion where the Great Bear appears to shrink as it rises away from its winter position (near the horizon).

Hubble repair mission gets the go-ahead from NASA

NASA has officially announced that the shuttle will be used to repair and upgrade the Hubble Space Telescope, in mission STS-125 set for 2008. Hopefully Hubble will survive until then...

From the BBC article:

Dr Griffin's decision reverses that of his predecessor, Sean O'Keefe, who cancelled the planned visit in the wake of the Columbia shuttle disaster in 2003.

This is certainly the correct decision - both benefiting science and making sense economically. The indefinite delays arising from the last Shuttle disaster have cost NASA a huge amount of money that would otherwise have been spent on doing other astrophysics, and has caused real funding problems for US astronomers (in particular those not associated with HST-based science).

No doubt Maryland's hard working and influential (democratic) Senator Barbara Mikulski has had a large part to play in making this repair/servicing mission a reality.

Sunday, October 22, 2006

Failure in the self-correcting system? Maybe not.

The NYT magazine has a fascinating article by Janeen Interlandi on the case of Eric Poehlman, a medical researcher at UVM who was discovered faking his data by a student at his lab. Eventually he was sentenced to 1 year and 1 days jail time for scientific fraud, but he'd been getting away with it for years - indeed he was hired to UVM based on work he'd faked and seemed never to have stopped faking data until he was investigated.

The scientific process is meant to be self-correcting. Peer review of scientific journals and the ability of scientists to replicate one another’s results are supposed to weed out erroneous conclusions and preserve the integrity of the scientific record over time. But the Poehlman case shows how a committed cheater can elude detection for years by playing on the trust — and the self-interest — of his or her junior colleagues.

... Not only does any research touched by tainted data have to be re-examined, but high-profile cases of misconduct can also shake public confidence. “We already have a large subculture in society of people who don’t trust science to begin with,” says John Dahlberg, one of the Office of Research Integrity investigators who oversaw Poehlman’s case. “This doesn’t help at all.”

Peer review and repeatability in science are self correcting, but deliberate fraud is harder to guard against. If you get a result that disagrees with a colleagues you don't immediately think "fraud" - you check your analysis, try to see how they might have analyzed or interpreted the data differently. It might even be a typo in their manuscript, or put in the journal. And thats if you're using the same data, often you're not. Deliberate fraud of the "making up data" variety is probably rare, but this case shows that it is caught... eventually. The self-correcting part of science is necessarily speedy.

Another interesting part of the article is when it asks how did he get away with it for so long?

The length of time that Poehlman perpetrated his fraud — 10 years — and its scope make his case unique, even among the most egregious examples of scientific misconduct. Some scientists believe that his ability to beat the system for so long had as much to do with the research topics he chose as with his aggressive tactics. His work was prominent, but none of his studies broke new scientific ground. (This may also be why no other scientists working in the field have retracted papers as a result of Poehlman’s fraud.) By testing undisputed assumptions on popular topics, Poehlman attracted enough attention to maintain his status but not enough to invite suspicion. Moreover, replicating his longitudinal data would be expensive and difficult to do [emphasis mine].

“Eric excelled at telling us what we wanted to hear,” Matthews, Poehlman’s former colleague, told me.“ He published results that confirmed our predisposed hypotheses.” Steven Heymsfield, an obesity researcher at Merck Pharmaceuticals in New Jersey, echoed Matthews’s sentiments and added that Poehlman’s success owed more to his business sense and charisma than to his aptitude as a scientist.

Friday, October 20, 2006

Friday Catastrophysics: Battered Galaxies part 1

Our cosmic neighborhood is not as quiet and suburban as you might imagine. New research using the Spitzer Space Telescope suggests that the nearest giant spiral galaxy (i.e. similar in type and size to our own Milky Way galaxy) took a direct hit from another galaxy, triggering an ripple of star formation that has now propagated 30 000 light years outward.

The image to the left is an optical image of M31 and its satellite galaxies: M32 (the bright white dot at center left) and M110 (the fuzzy blob toward the bottom right hand side), taken by Robert Gendler.

Block et al (2006, Nature; also see astro-ph/0610543) base this interpretation on mid-infrared imaging at a wavelength of 8 micron, which is sensitive to warm dust (and starlight, but they can remove the starlight using 3 micron imaging which is only sensitive to starlight). The resulting image of M31 is shown in red here, along with a the optical image rotated and scaled (by eye) to match the orientation of the Spitzer image.

Although there is something of a weak distorted spiral arm pattern to M31, the 8 micron image shows closed ring-like structures. The outer red ring is a region of hot dust, heated by the young massive stars that have recently formed there. This star-forming ring has been know about for some time, but it is the inner dust ring that is the new discovery.

Star-forming rings can be produced by two mechanisms. The more standard one is for a stellar bar to create inner and outer rings, but M31's bar is relatively weak and not as large as the 10-kiloparsec radius (30 000 light year) outer ring, nor is it aligned with the inner dust ring. The other way to get a large-radius ring of gas and dust in a spiral galaxy is for another galaxy to pass through the disk on a polar orbit. Here the culprit is suspected to be M32, the smaller of the dwarf galaxies visible in the image of M31.

This form of galaxy collision has been seen before, and in the more extreme cases (where the colliding objects are closer in size and mass) the disruption of the impacted galaxy can be severe, as can be seen below in the image of the Cartwheel Galaxy. These objects are typically called ring galaxies. Another example worth a look at is NGC 4650A.

An an aside, Astroprof had a blog post about our (the Milky Way) not-so-imminent collision with M31 just the day before the Block et al results were released.

Wednesday, October 18, 2006

Fascinating Fungi

The ever interesting Carl Zimmer has a fascinating post up about Fungi. Go read.

[Update: picture from a hike we did in Pennsylvania back in 2003.]

Monday, October 16, 2006

Cosmic Rays and Climate Change: RealClimate's analysis

The always interesting RealClimate blog has an interesting analysis of a paper by Svensmark et al [Svensmark, H., et al, 2006, Proc. Royal. Soc. A., xx, 1364] investigating whether Cosmic Rays can promote low altitude cloud formation, a paper with the snappy title "Experimental evidence for the role of ions in particle nucleation under atmospheric conditions"

The paper itself (go read it) is pretty innocuous and hardly applies their results to anything, but the press release isn't shy about explaining the (supposed) implications:

"A team at the Danish National Space Center has discovered how cosmic rays from exploding stars can help to make clouds in the atmosphere. The results support the theory that cosmic rays influence Earth’s climate."

Here are some more choice bits from the press release:

The experimental results lend strong empirical support to the theory proposed a decade ago by Henrik Svensmark and Eigil Friis-Christensen that cosmic rays influence Earth’s climate through their effect on cloud formation. The original theory rested on data showing a strong correlation between variation in the intensity of cosmic radiation penetrating the atmosphere and the amount of low-altitude clouds. Cloud cover increases when the intensity of cosmic rays grows and decreases when the intensity declines.

It is known that low-altitude clouds have an overall cooling effect on the Earth’s surface. Hence, variations in cloud cover caused by cosmic rays can change the surface temperature. The existence of such a cosmic connection to Earth’s climate might thus help to explain past and present variations in Earth’s climate.

Interestingly, during the 20th Century, the Sun’s magnetic field which shields Earth from cosmic rays more than doubled, thereby reducing the average influx of cosmic rays. The resulting reduction in cloudiness, especially of low-altitude clouds, may be a significant factor in the global warming Earth has undergone during the last century. However, until now, there has been no experimental evidence of how the causal mechanism linking cosmic rays and cloud formation may work.

Its not quite claiming that ALL climate change is due to changes in the Cosmic Ray irradiance, but its not making any attempt to say this might only be a small effect.

If you're interested in a discussion of the physics, and why Gavin at RealClimate isn't buying the PR spin, then go over to RC.

What I'd like to know is who chose to put the emphasis in the press release on climate change? It certainly sounds like it was written by someone who doesn't believe that human activity is responsible for the unprecedented climate change in the 20th century.

Buts its not clear to me as yet that we know who put this spin into the press release.

Within the last year we've seen the case of a unqualified political hack who somehow got a job in the NASA press office editing scientific press releases to reflect his personal beliefs, and attempting to cut of press access to a prominent climate scientist. Perhaps something similar has happened in this case?

Saturday, October 14, 2006

Debate or Denial? What constitutes a valid argument?

I was not going to discuss the Hopkins/Iraqi mortality study recently published in the Lancet (as its methodology is outside my experience in statistics, and there are already a lot of good analyses of the fallacies in the naysayers arguments), but I saw an interesting comment by Zeyad of Healing Iraq (that This ModernWorld had picked up on) that is actually quite a nice explanation of what makes for a real scientific debate verse simple (unscientific) denial:

One problem is that the people dismissing – or in some cases, rabidly attacking – the results of this study, including governmental officials who, arguably, have an interest in doing so, have offered no other alternative or not even a counter estimate. This is called denial. When you have no hard facts to discredit a scientific study, or worse, if you are forced to resort to absurd arguments, such as “the Iraqis are lying,” or “they interviewed insurgents,” or “the timing to publish this study was to affect American elections,” or "I don't like the results and they don't fit into my world view, therefore they have to be false," it is better for you to just shut up.

This is just as true in the physical sciences, e.g. astrophysics, as it is in the medical/social sciences. In astronomy, much of it an observational science, there are as many claims made based on human interpretation of images ("by eye" as we'd say) as there are based on quantitative measurements based on the data.

I've just been mulling over one such issue, which I'll present part of just for fun as to get some real superwind stuff into this blog (and as an antidote to all the complaining about the woes of the space program).

For me the issue is whether the soft X-ray emission in superwinds (which are galaxy sized winds of gas flowing out of star-forming galaxies, I'll do a better into some time later) comes from the stuff drivin the wind itself, or is just a tracer of its interaction with the ambient medium. By way of analogy, think of a dust devil or tornado. What you see in a tornado is only a tracer of the actual thing powering it, you see debris (dust, water vapor, leaves, bits of houses etc) carried along by the motion of the wind - you don't actully see the air molecules themselves.

This image is of a dust devil. Notice how there seems to be more dust a the left and right-hand edges of the devil than in the center? In astronomy we would call this limb-brightened (limb as in edge, not as in arm or leg). If you imagine looking down on the devil from above you might seem an approximately circular structure, with a central circle of nearly clear air (little dust) surrounded by an annulus of dusty air.

Now that was an interpretation based purely on a qualitative argument - I looked at the image and interpreted it based on my scientific experience and came up with an hypothesis based on the dust devil looking (by my eye) to be limb brightened.

But could I test this hypothesis rather more quantitatively? Well, I could construct a mathematical model of the 3-dimensional geometry and distribution of dust, calculate how that would look projected into 2-dimensions (i.e. an image) and compare that to the data (the image).

What if someone said that the dust devil didn't look limb brightened to them? Say they though it was pretty uniformly dusty from left to right and that they though this meant it was uniformly filled with dust. Well, rather than a meaningless he said/she said argument what we'd have to do is measure the amount of dust from left to right [Note: A uniformly filled cylinder would actually appear to have more dust in the center when seen in projection].

Just for fun I've rotated the dust devil image by 5 degrees (to make the dust devil more nearly vertical) and written it out as a FITS file using the gimp, so I can use some standard astrophysical software (the wonderful fv) to probe the image quantitatively. Then I've just taken a horizontal cut accross the evil and plotted the brightness in a X-Y graph. As the dust is brighter than the sky then the image brightness is roughly a measure of the amount of dust.

As you can see the edges are brighter (have more dust) than the center, so this is evidence for limb-brightening. One side is slightly brighter than the other, which tells us there are assymetries in the dust distribution, but to first order its OK just to say its limb-brightened.

In astronomy we often have to compare data taken with difference telescopes at different times, which makes things a bit more complicated. The different telescopes often will differ in sensitivity, in the wavelengths of light they're most sensitive too, and in their spatial resolution (how sharp the images are), not to mention that the astronomical source itself might have actually changed its state in the time between the different observations.

Imagine if we'd taken a picture of the dust devil with a rather blurry camera. Then it would be harder to see evidence for limb-brightening.

Just for fun, here is that image again, now blurred so as to represent a lower spatial resolution observation (I haven't added the noise you'd actually get in a new observation). You can see the same general shape as before, but note that the evidence for limb-brightening is weaker - the peak to trough amplitude (dust devils edges to center) in the profile on the right hand side is much less than the earlier full-resolution image. If I'd blurred the image even more you wouldn't see any evidence for limb brightening. But it doesn't mean it isn't there - in this case by blurring the image we're using an effective spatial resolution too low to be able to test the hypothesis of limb-brightening.

Could we use data from the second camera to argue against the limb brightening seen in the first camera? The answer, which might surprise you, is a qualified yes. Sure the instrument itself might be not as well suited to the question we're trying to anser, but nonetheless if we got really good data we could try to quantitatively compare the two images, if we make sure to accounht for the difference in instrument capability. But you would need to work harder to prove it, you couldn't just say "well, doesn't look limb brightened in our lower resolution data, therefor the interpretation using the better camera is wrong".

Of course, no practising professional scientist would be so lazy as try a flat-out unsubstantiated denial, would they?

Friday, October 13, 2006

Seeing through dust

The Hubble Space Telescope is the public face of astronomy, but despite the beautiful pictures it produces it alone can only show use a tiny fraction of the Universe - specifically those parts that can be seen in the visible part of the electromagnetic spectrum (although its vision does extend into the near ultra-violet and near infra-red, unlike human vision).

But even a small change in the wavelength of light can make a big difference - what appears opaque to the human eye can be completely transparent to IR or X-ray radiation.

Astronomers need telescopes covering the full range of the EM spectrum. Without that coverage, even with a Hubble or JWST, we're really handicapped. Try to image medicine without X-rays, CAT scans, PET, MRIs and just relying on one technique, say ultrasound.

A native Hawaiian was leading a tourist trip up Mauna Kea that I was on last year asked "Why do these astronomers need all these telescopes? Why can't they all just get together and share one?" Its a fair question [particularly if it is your sacred mountain that is being built on] - one answer is one type of instrument can only detect a limited range of wavelengths, and hence you need multiple observatories to be able to cover as much of the spectrum as possible.

If we astronomers were limited to just the optical it wouldn't really save money, it actually be a waste of money as we could not fully understand and make use of the limited data we'd get. Unfortunately the public, and politicians, don't understand this. They tend to think that if they scrap a billion dollar mission they've saved a billion dollars (which can then be spent on a pork-barrel bribes to the public), but in fact they've just reduced the bang-for-buck of the remaining observatories.

As an example of what a small change in wavelength does, compare the picture at the top (the Hubble optical image of the Eagle Nebula) to this one here: a near IR image taken with the VLT ISAAC. I've tried to scale and rotate the VLT image to match the HST image (as closely as is possible by eye).

Seen in the near-IR the once imposing "Pillars of Creation" have half vanished. Look at the left-hand-most pillar, for example. Background stars are visible through the region that appeared almost black in the Hubble image. Only the densest conglomerations of dust and gas still block the background star (IR) light near the tips of the pillars.

In fact it is these dense clumps that create the pillar-like structures by shielding the less-dense gas in their shadow from the destructive UV radiation of the young massive stars in the cluster (they're outside the field of view to the upper right). In between the dense clumps the atomic gas has been photo-dissociated and effectively evaporated away.

Cosmic Chesire Cats in M16

Given the tradition of Friday cat-blogging I thought I should start presenting some astronomical cats. The most famous of which is the one lurking in M16, aka the Eagle Nebula, aka the Pillars of Creation.

The cat visible in the image to the left has an angular size of about 15 arcseconds (lengthwise). M16 is about 2.0+/-0.1 kpc distance from us (about 6500 light years), so the physical size of the structure that looks like a cat is (15/3600) * (pi/180) * 2000 = 0.145 pc long, or very roughly about 2.8e12 miles (say three thousand billion miles) long.

I was reminded about the Eagle Nebula by Gagne et al's (2006) preprint on arXiv.org regarding a Chandra observation of M16. Unfortunately the Chandra X-ray image is rather dull looking (to me) as you don't see the nebula (its only a HII region/PDR, so the temperatures in the gas are too low for X-ray emission) but only the young stars.

The cat's ears are actually tipped by EGGs (Evaporating Gaseous Globules), which either are the same thing as Proplyds, or will evolve into them (there seems to be some argument about this). Proplyds are thought to be young solar systems (maybe with planets, but mainly they're comprised of a very young star still in the process of forming), and M16 is covered in EGGs/proplyds (hence the Pillars of Creation moniker).

Thursday, October 12, 2006

Earth's orbit and extinction patterns in spanish rodents

In a study that is bound to get tons of press, a paper by van Dam et al (published in Nature, I'll add the exact reference later) claims that long-term periodic changes in Earth's oribital tilt "offers a plausible explanation for the characteristic duration of more or less 2.5 million years of the mean species life span in mammals.” They were looking at rodent species, which is odd as naively you'd assume rodents are pretty much impossible to exterminate!

I shall remain skeptical for now. There have been many claims of periodicity in extinction rates over the years, and for pretty much any time scale you can find a similar astronomical timescale, or make one up (e.g. the orbital period of Nemesis, the hypothesized stellar companion of the Sun).

David Raup even wrote a very convincing (but probably wrong) book exploring whether ALL extinctions might be due to astronomical causes.

Correlation does not imply causation, and at present van Dam et al's results are intriguing but not compelling. Much more evidence will be required to prove such a link between orbital variations and species extinction. Still, this will spur more work by people eager to disprove this hypothesis.

Wednesday, October 11, 2006

The Iraqification of Space?

Although it has received little press the US now has a new National Space Policy, replacing one dating back to 1996. Secrecy News has an informative write up that compares the old Clinton and (the non-secret parts of the) new Bush policies.

The new parts of the policy appear to be mix of grandiose objectives ("extending human presence across the solar system") and dangerous unilateralism ("The United States will oppose the development of new legal regimes or other restrictions that seek to prohibit or limit U.S. access to or use of space").

Tuesday, October 10, 2006

Michael Steele may like puppies, but he isn't into the truth

If you live in Maryland you'll probably have seen Ben Cardin's amusing attack ad "Its nice that Michael Steele likes puppies" whether you want to or not. Its taken a while for Steele to respond (perhaps too long for someone who continues to trail Cardin by about ten points in the polls), but tonight I got to see Steele's response... about 3 times in the same ad break.

It contained a rather stunning claim:

"First….Ben Cardin's team hacks into my credit report…steals my Social Security number. Oh yeah: they pled guilty in federal court." These exact lines come from Steele press release about his new ad.

Wow - that sounds bad! Illegal behaviour is definitely not acceptable to me as a liberal, whether or not the person involved shares my political beliefs or not. But is what Steele claims the truth?

The substantive claims in Steele's ad are that:
  1. Operatives working for Ben Cardin obtained Steele's credit report by hacking;
  2. Said operatives stole (i.e. obtained illegally) Steele Social Security number;
  3. and that they plead guilty in federal court to the above deeds.
Steele's press release quotes the Washington Times, which as Sun Myung Moon's propaganda organ is hardly a trustworthy source of information.

Of these three claims it turns out that none of them are actually true. Yes, there is (or was) a scrap of truth in there originally, but its been as heavily distorted as Bush's claims about Iraq. The Washington Post's John Wagner covered the case back in March 2006.

  1. Yes, Steele's credit report was obtained fraudulently, but not by any one on Ben Cardin's campaign. It was instead a researcher working for the Democratic Senatorial Campaign Committee in DC who obtained Steele' credit report without authorization (presumably by using an online service like freecreditreport.com I presume, the reports don't say how). No nefarious computer "hacking" involved. So charge number one is false.
  2. But what about the stolen (identity theft!) Social Security number? From Wagner's article we find this: "Personal finances were considered fertile territory for researchers looking into Steele, who acknowledged financial difficulties when he ran for statewide office in 2002. Sources familiar with the episode said Steele's credit report was obtained with the use of his Social Security number, which was found on a public court document. [emphasis mine]". So no theft.
  3. OK, but they plead guilty right? Well... the researcher involved plead guilty to a misdemeanor, and the charge will be dismissed in a year. They certainly didn't plead guilty to hacking and theft, as Steele's ad implicitly implies. Neither the DSCC's director of research (who along with the researcher had resigned, in deed the resignations drew the FBI's attention in the first place) nor the DSCC itself were charged.
So Steele's big come-back ad against Cardin is based on total distortions of events that are not tied to Cardin in any way! Great work there you Steele staffers - nothing like incompetence to help your candidate along. You are really helping the GOP lose one of the few seats they stood a vague chance of taking from the Democrats...

But it gets better... Cardin's ad strategy is simply to make Steele look (a) ineffectual (which Steele's puppy ads only help solidify) and (b) like another rubber-stamp republican toadying up to Bush.

So what is Steele's great rejoinder against attacks of this type? What proof will he show of his resolve and independence?

Now he says I'm in the President's hip pocket.

Listen to me Mr. Cardin: I think for myself.

Wow - that was convincing! Remember, Steele is the guy who was happy to criticize Bush when he thought it was off the record, but didn't have the guts (or sense) to stand by it after his comments were released and attributed to him in the press.

[I've just discovered Oliver Willis' excellent blog on Maryland politics, lots of great stuff to read through if you're interested.]

Friday, October 06, 2006

Those old computer monitors were good cat beds

Final Friday cat blogging as a batchelor. This is a 2003-era picture of Macio keeping himself warm on my viewsonic monitor.

Future Airport Hell due to NASA funding woes?

DailyTech has an interesting article on how NASA's current funding problems impact aviation research, such as the next generation of air traffic control (note that NASA itself says there isn't a funding problem).

You might end up stuck in horrendous airport delays in a few years time thanks to Shuttle cost overruns and the Moon/Mars program. Not the kind of social impact we'd like the space program to have eh?

[update - fixed typos]

Tuesday, October 03, 2006

COBE team get Nobel in Physics for verifying the Big Bang

The Nobel Prize committee have made an excellent choice in awarding the 2006 Nobel Prize in Physics to John Mather and George Smoot for their work leading to the NASA COBE (Cosmic Background Explorer) satellite. SciAm has a nice summary of Mather and Smoot's work.

COBE was an Explorer-class NASA mission (*) (i.e. pretty small and low cost compared to HST or Cassini) launched in 1989 that measured the microwave background radiation reaching Earth. This link will take you to a NASA page with more info on COBE.

This radiation arises from warm dust in the Milky Way and also the left over glow of the Big Bang itself, now cooled by the expansion of the Universe to a temperature of about 3 degrees above absolute zero. Detecting this background and showing that it has an almost perfect black body spectrum is pretty much inescapable proof of the Big Bang.

The picture shown here is comes from the COBE data, and shows the separate components to the detected microwave emission:

  • The horizontal structure (top two panels) is emission from the disk of our Milky Way galaxy. Filaments and spurs are also Galactic emission.
  • The blue-ish curvy structure seen in the top image is microwave emission from inter-planetary dust within our Solar system - the curve actually marks the (physically flat) plane of the ecliptic within which the planets orbit (Pluto's orbit is rather inclined away from the ecliptic, which shows you that its formation or history are significantly different from the other planets).
  • The isotropic extra-galactic emission left behind when you remove the previous two components is the T=2.7 Kelvin microwave background left over from the Big Bang, as seen in the bottom panel. The minute variations in the temperature of the background from different parts of the sky also carry cosmologically-important information regarding the shape of space, the mass density of the Universe, and the formation of the first galaxies... stuff WMAP is doing in greater detail now (maybe Chuck Bennett will eventually get a Nobel for his work on WMAP?).
It is good to see astrophysics recognized by the Nobel Prize committee. Historically astronomers have received a much smaller fraction of Nobel prizes in Physics than they probably should have given the contribution of astronomy to our understanding of the Universe.

(*) There are several types of Explorer class missions, e.g small (SMEX), medium (MIDEX) and so on. There have been significant changes to the classes of space mission NASA funds since the new Moon/Mars push was imposed, but I'll have to look up the details.

Monday, October 02, 2006

The speed of sound in an ideal gas

Reading through today's astronomy preprints on astro-ph I couldn't help but notice one of my pet peeves: the wrong speed of sound!

For an ideal gas, assuming adiabatic compression, a sound wave travels at a speed c_s = sqrt(gamma*pressure/density), where gamma is the ratio os specific heats at constant pressure and constant volume (gamma=5/3 for a monatomic gas).

So for a typical astrophysical plasma with a temperature of T=1,1604,500 Kelvin (kT=1 keV) the sound speed is 511 km/s (the speed of sound in air at STP is about 0.3 km/s). Yet its not uncommon to see the speed of sound for this temperature given as being around 300 km/s. Why the big difference?

Its because many astronomers simply insist on using the isothermal speed of sound, c_s_iso = sqrt(Pressure/density), i.e. ignoring the gamma.

What exactly is the difference?

The isothermal speed of sound is valid in conditions where the period of the distance (i.e. 1/frequency) is long compared to the cooling time of the plasma. You compress the plasma, it heats up adiabatically, but radiation carries the heat away and the plasma cools back the ambient temperature before the next pulse of the sound comes along. Another way of looking at it is that the ratio of specific heats is now ~1.

Basically in any situation where the temperature of the plasma is independent of the density you should use the isothermal sound speed. A good example is an HII region, where the plasma temperature is set by photoionization from the embedded stars.

But if the plasma temperature is not set by an external process and the cooling time is long then then gamma = 5/3 and the isothermal sound speed IS NOT VALID. Yet time after time I see papers always applying the one formula they learnt at grad school (which is typically the isothermal version) irrespective of the actual physical conditions in the plasma. Doesn't anyone actually pay attention to what Spitzer says instead of just using the formulae? Here is Wolfram on the case as well.

OK, I know using the wrong sound speed is not quite as bad a thing as ignoring warnings about increasing terrorist activity and then keeping that information from the 9/11 commission (yes Condi, I mean you), but its still wrong.

[Update] Forgot to add labels to the post.

Cat Astrophysics? Late cat blogging...

Argh! --- I just realized that catastrophysics (catastrophy/astrophysics, get it?) could also be interpreted as cat (as in feline) astrophysics. No wonder I liked the word...

This catastrophy is Karma, sadly still losing badly on kittenwars, rather strangely as she was extra cute as a kitten (I think). Must be Dielbold running the voting...