Archive for James Webb Space Telescope

In the Name of JWST

Posted in LGBT, Politics, The Universe and Stuff with tags , , , , , on October 25, 2022 by telescoper

JWST – nice telescope, shame about the name

I’ve blogged before about the problematic naming of the James Webb Space Telescope. Its name was changed in 2002 from the Next Generation Space Telescope to the James Webb Space Telescope after James E. Webb, a civil servant who was NASA’s chief administrator from 1961 to 1968.

It’s not uncommon for scientific space missions like this to be named after people once the proposal has moved off the drawing board and into serious planning. That happened with the European Space Agency’s Planck and Herschel to give two examples. In any case Next General Space Telescope was clearly never anything but a working title. Yet naming this important mission after a Government official always seemed a strange decision to me. Then news emerged that James Webb had enthusiastically cooperated in a McCarthyite purge of LGBT+ people working in government institutions, part of a wider moral panic referred to by historians as the Lavender Scare. There have been high-profile protests (see, e.g., here) and a petition that received over a thousand signatures, but NASA has ruled out any change of name.

The main reason NASA give is that they found no evidence that Webb himself was personally involved in discrimination or persecution. I find that very unconvincing. He was in charge, so had responsibility for what went on in his organization. If he didn’t know then why didn’t he know? Oh, and by the way, he didn’t have anything to do with infrared astronomy either…

I still think it’s a shame that this fantastic telescope should have its image so tarnished by the adoption of an inappropriate name.

Anyway, yesterday I saw that the Royal Astronomical Society has issued a statement about this issue, which I encourage you to read in full. It begins

At its meeting in July the governing council of the Royal Astronomical Society (RAS) took a decision to write to the UK Space Agency, the European Space Agency (ESA) and NASA to express its concerns about the original JWST naming process, the apparent failure to investigate James Webb’s background and the dismissal of requests to rename the telescope.

Until that investigation takes place and the results are made public, the RAS now expects authors submitting scientific papers to its journals to use the JWST acronym rather than the full name of the observatory. In this case, the previous requirement for the acronym to be spelled out at first mention will not be observed. This change will also be reflected in our communications more generally.

This does at least acknowledge the problematic nature of the name and the message it sends to LGBT+ scientists around the world and it the statement as a whole is to be welcome.

I think I’ll continue to use the name James Webb Space Telescope on this blog, though, as a reminder that the name should just be changed. Even in shorthand it’s an insult.



Life, the Universe and the Drake Equation

Posted in The Universe and Stuff with tags , , , , , , , on September 3, 2022 by telescoper
Picture of Frank Drake with his equation

Frank Drake and the Drake Equation (Picture credit:

I heard last night of the death at the age of 92 of astronomer Frank Drake, one of the pioneers of the Search for Extraterrestrial Intelligence (SETI). He was best known to most people for formulating the Drake Equation, so since it’s a rainy Saturday morning I thought I’d commemorate him here by presenting a brief discussion of that equation and what it means.

Our Universe is contrived in such a way as to make life possible within it. After all, we’re here! But just because it is possible, that doesn’t mean that it is commonplace. Is life all around us, or did it only happen on Earth? It fascinates me that this topic comes up so often in the question sessions that follow the public lectures I give on astronomy and cosmology. Do you think there is life on other worlds? Are there alien civilisations more advanced than our own? Have extraterrestrials visited Earth? These are typical of the kind of things people ask me when I give talks on the Big Bang theory of the origin of the Universe. It often seems that people are more interested in finding out if there is life elsewhere than in making more serious efforts to sustain life in the fragile environment of our own planet. But there’s no doubting the effect that it would have on humanity to have proof that we are not alone in the cosmos. We could then accept that the Universe was not made for our own benefit. Such proof might also help release mankind from the shackles currently placed on it by certain fundamentalist religious cults. But whatever the motives for seeking out life on other worlds, this is undoubtedly a subject worthy of serious scientific study.

Our understanding of the origins of terrestrial life still has important gaps. There is still no compelling direct evidence that life has existed elsewhere in the Solar system. Conditions may, for example, have been conducive to life earlier in the history of Mars but whatever did manage to evolve there has not left any unambiguous clues that we have yet found. The burgeoning new field of astrobiology seeks to understand the possible development of life far from Earth, and perhaps in extreme conditions very different from those found on our planet. This is, however, a very new field and it will be a very long time before it becomes fully established as a rigorous scientific discipline with a solid experimental and observational foundation. What I want to do in this discussion is therefore not to answer the question “Are we alone?” but to give some idea of the methods used to determine if there might be life elsewhere, including the SETI (Search for ExtraTerrestrial Intelligence) industry which aims to detect evidence of advanced civilizations.

The first ever scientific conference on SETI was held in 1961, in Green Bank, West Virginia, the site of a famous radio telescope. A search had just been carried out there for evidence of radio signals from alien intelligences. This conference didn’t exactly change the world, which is not surprising because only about ten people showed up. It did, however, give rise to one of the most famous equations in modern science: the Drake Equation.

The astronomer Frank Drake was setting up the programme for the inaugural SETI conference and he wanted to summarize, for further discussion, the important factors affecting the chances of detecting radio transmissions from alien worlds. The resulting equation yields a rough guess of the number of civilizations existing in the Milky Way from which we might get a signal. Of course we can’t calculate the answer. The equation’s usefulness is that it breaks down the puzzle into steps, rather than providing the solution. The equation has been modified over the years so that there are various versions of it addressing different questions, but its original form in all its glory was


N=R× fp × ne × fl × fi × fc × L


The symbols in this equation have the following meanings. The left hand side N is the number of transmitting civilisations in our Galaxy, which is what we want to determine  The first term on the right hand side is R, which is the birth-rate of stars in our Galaxy per year. We know that the Milky Way is about 10 billion years old, and it contains about 100 billion stars. As a very rough stab we could guess that the required birth-rate is therefore about ten stars per year. It seems unlikely that all stars could even in principle be compatible with life existing in their neighbourhood. For example, very big stars burn out very quickly and explode, meaning that there is very little time for life to evolve there in the first place and very little chance of surviving once it has. Next in the equation is fp, the fraction of these stars having planets, followed by ne, the typical number of planets one might find.  This is followed by fl, the fraction of all planets on which life in some form does actually evolve. The next term is fi, the fraction of those planets with life on them that have intelligent life on them. Finally we have two factors pertaining to civilization: fc is the fraction of planets inhabited by intelligent beings on which civilizations arise that are capable of interstellar communication and L is the average lifetime of such civilizations.

The Drake equation probably looks a bit scary because it contains a large number of terms, but I hope you can see that it is basically a consequence of the rules for combining probabilities. The idea is that in order to have a transmitting civilisation, you must the simultaneous occurrence of various properties each of which whittles away at the original probability.

To distil things a little further we can simplify the original Drake equation so that it has only four terms

N=NH × fl × fc × fnow

The first three terms of the original equation have been absorbed into NH, the number of habitable planets and the last two have become fnow, the fraction of civilized planets that happen to be transmitting now, when we are trying to detect them. This is important because many civilizations could have been born, flourished and died out millions of years in the past so will never be able to communicate with them.

Whichever way you write it, the Drake equation depends on a number of unknown factors. Combining factors multiplicatively like this can rapidly lead to very large (or very small) numbers. In this case each factor is very uncertain, so the net result is very poorly determined.

Recent developments in astronomy mean that we at least have something to go on when it comes to NH, the number of habitable planets. Until relatively recently the only planets we knew about for sure were in our own Solar System orbiting our own star, the Sun. We didn’t know about planets around other stars because even if there were there we were not able to detect them. Many astronomers thought planets would turn out to be quite rare but absence of evidence is not evidence of absence.  Observations now seem to support the idea that planets are fairly common, and this also seems to be implied by our improved understanding of how stars form.

Planets around distant stars are difficult to detect directly because they only shine by light reflected from their parent star and are not themselves luminous. They can, however, be detected in a number of very convincing ways. Strictly speaking, planets do not orbit around stars. The star and the planet both orbit around their common centre of mass.  Planets are generally much smaller than stars so this centre of mass lies very close to the centre of the star. Nevertheless the presence of a planet can be inferred through the existence of a wobble in the stars’ path through the Galaxy. Dozens of extrasolar planets have been discovered using this basic idea. The more massive the planet, and the closer it is to the star the larger is the effect. Interestingly, many of the planets discovered so far are large and closer in than the large ones in our Solar System (Jupiter, Saturn, Uranus and Neptune). This could be just a selection effect – we can only detect planets with a big wobble so we can’t find any small planets a long way from their star – but if it isn’t simply explained away like that it could tell us a lot about the processes by which planets formed.

The birth of a star is thought to be accompanied by the formation of a flattened disk of debris in the form of tiny particles of dust, ice and other celestial rubbish. In time these bits of dirt coagulate and form larger and larger bodies, all the way up in scale to the great gas giants like Jupiter. The planets move in the same plane, as argued by Laplace way back in the 19th century, because they were born in a disk.

As an aside I’ll mention that when I started my PhD in 1985 there were no known extra-solar planets -exoplanets for short – so as a field exoplanet research hadn’t really started. Now it’s one of the biggest areas of astrophysics and is set to grow even more with the launch of JWST, which has just made its first direct image of an exoplanet:


Of course, while planets may be common we still do not know for sure whether habitable planets are also commonplace. We have no reason to think otherwise, however, so we could reasonably assume that there could be one habitable planet per system of planets. This would give a very large value for NH, perhaps 100 billion or so in our Galaxy.

The remaining terms in the Drake Equation pose a bit more of a problem. We certainly don’t have any rational or reliable way to estimate fl. We only know of one planet with life on it. Even Bayesians can’t do much in the way of meaningful statistical inference in this case because we do not have a sensible model framework within which to work. On the other hand, there is a plausibility argument that suggests fl may be larger rather than smaller. We think Earth formed as a solid object about 4.5 billion years ago. Carbon-isotope evidence suggests that life in a primitive form had evolved about 3.85 billion years ago, and the fossil record suggests it was abundant by 3.5 billion years. At least the early stages of evolution happened relatively quickly after the Earth was formed and it is a reasonable inference that life is not especially difficult to get going.

It might be possible therefore that fl=1, or close to it, which would mean that all habitable planets have life. On the other hand, suppose life has a one-in-a-million chance of arising then this reduces the number of potentially habitable planets with life actually on them to only a millionth of this value.

The factor fc represents the fraction of inhabited planets on which transmitting civilizations exist at some point. Here we really don’t have much to go on at all. But there may be some strength in the converse argument to that of the previous paragraph. The fact that life itself arose 3.85 billion years ago but humans only came on the scene within the last million years suggests that this step may be difficult, and fc should consequently take a small value.

The last term in the simplified Drake equation, fnow, is even more difficult because it involves a discussion of the survivability of civilizations. Part of the problem is that we lack examples on which to base a meaningful discussion. For present purposes, however, it is worth looking at the numbers for terrestrial life. The Milky Way is roughly 10 billion years old. We have only been capable of interstellar communication for about 80 years, initially accidentally through through stray radio broadcasts. This is only about one part in 200 million of the lifetime of our Galaxy. If we destroy ourselves in the very near future, either by accident or design, then this is our lifetime L as it appears in the original Drake equation. If this is typical of other civilizations then we would have roughly a one in 200 million chance of detecting them at any particular time. Even if our Galaxy had nurtured hundreds of millions of civilizations, there would only be a few that would be detectable by us now.

Incidentally, it is worth making the comment that Drake’s equation was definitely geared to the detection of civilizations by their radio transmissions. It is quite possible that radio-based telecommunication that results in leakage into space only dominates for a brief stage of technological evolution. Maybe some advanced form of cable transmission is set to take over. This would mean that accidental extraterrestrial communications might last only for a short time compared to the lifetime of a civilization. Many SETI advocates argue that in any case we should not rely on accidents, but embark on a programme of deliberate transmission.  Maybe advanced alien civilizations are doing this already…

In Drake’s original discussion of this question, he came to the conclusion that the first six factors on the right-hand-side of the equation, when multiplied together, give a number about one. This leads to the neat conclusion that N=L (when L is the lifetime of a technological civilization in years). I would guess that most astronomers probably doubt the answer is as large as this, but agree that the weakest link in this particular chain of argument is L. Reading the newspapers every day does not make me optimistic that L is large…

How big were the biggest galaxies in the early Universe?

Posted in Biographical, Cardiff, The Universe and Stuff with tags , , , , , , , on August 23, 2022 by telescoper

Once upon a time (over a decade ago when I was still in Cardiff), I wrote a paper with PhD student Ian Harrison on the biggest (most massive) galaxy clusters. I even wrote a blog post about it. It was based on an interesting branch of statistical theory called extreme value statistics which I posted about in general terms here.

Well now the recent spate of observations of high-redshift galaxies by the James Webb Space Telescope has inspired Chris Lovell (who was a student at Cardiff back in the day then moved to Sussex to do his PhD and is now at the University of Hertfordshire) and Ian Harrison (who is back in Cardiff as a postdoc after a spell in the Midlands), and others at Cambridge and Sussex, to apply the extreme value statistics idea not to clusters but to galaxies. Here is the abstract:

The basic idea of galaxy formation in the standard ΛCDM cosmological model is that galaxies form in dark matter haloes that grow hierarchically so that the typical size of galaxies increases with time. The most massive haloes at high redshift should therefore be less massive than the most massive haloes at low redshift, as neatly illustrated by this figure, which shows the theoretical halo mass function (solid lines) and the predicted distribution of the most massive halo (dashed lines) at a number of redshifts, for a fixed volume of 100 Mpc3.

The colour-coding is with redshift as per the legend, with light blue the highest (z=16).

Of course we don’t observe the halo mass directly and the connection between this mass and the luminosity of a galaxy sitting in it is likely to be complicated because the formation of the stars that produce the light is a rather messy process; the ratio of mass to light is consequently hard to predict. Moreover we don’t even have overwhelmingly convincing measurements of the redshifts yet. A brief summary of the conclusions of this paper, however, is that is some of the big early galaxies recently observed by JWST seem to be a big too big for comfort if we take their observed properties at face value. A lot more observational work will be needed, however, before we can draw definite conclusions about whether the standard model is consistent with these new observations.

Recalibration of Ultra-High-Redshift Galaxies

Posted in Astrohype, The Universe and Stuff with tags , , , , on August 10, 2022 by telescoper

Remember all the recent excitement about the extremely high redshift galaxies (such as this and this; the two examples shown above) “identified” in early-release JWST observations? Well, a new paper on the arXiv by Adams et al using post-launch calibration of the JWST photometry suggests that we should be cautious about the interpretation of these objects. The key message of this study is that the preliminary calibration that has been in widespread use for these studies is wrong by up to 30% and that can have a huge impact on inferred redshifts.

The new study does indeed identify some good candidates for ultra-high-redshift galaxies, but it also casts doubt on many of the previous claims. Here is a table of some previous estimates alongside those using the newly recalibrated data:

You will see that in most – but not all – cases the recalibration results in a substantial lowering of the estimated redshift; one example decreases from z>20 to 0.7! The two candidates mentioned at the start of this post are not included in this table but one should probably reserve judgement on them too.

The conclusive measurements for these objects will however include spectroscopy, and the identification of spectral lines, rather than photometry and model fits to the spectra energy distribution. Only with such data will we really know how many of these sources are actually at very high redshift. As the philosopher Hegel famously remarked

The Owl of Minerva only spreads its wings with the coming of spectroscopy.

Now a Galaxy at z>16?

Posted in The Universe and Stuff with tags , , , , , , on July 26, 2022 by telescoper

It’s less than a week since I posted an item about an object which is possibly the highest redshift galaxy ever observed (with z ~13) and now along comes a paper describing an object that may be of even higher redshift (with z~16.7). The abstract of the new paper – lead author of which is Callum Donnan of the University of Edinburgh – is here:

As with the previous object the redshift of this one is not obtained via spectroscopy (which usually involves the identification of spectral lines) but via fitting a spectral profile to photometric imaging data seen in different bands. The process for this galaxy is illustrated by this diagram from the paper:

There are 7 images along the top showing the source through various broad band filters. Suitably calibrated these can be converted to the flux measurements shown on the graph. Notice the first three images are significantly fainter than the others, so the first three points on the left of the graph are lower.

If this is a galaxy its spectrum is expected to possess a Lyman Break resulting from the fact that radiation of shorter wavelength than the Lyman Limit (912 Å) is absorbed by neutral gas surrounding the regions where stars are formed in the galaxy. In the rest frame of a galaxy this break is the ultraviolet region of the spectrum but because of the cosmological redshift it is observed in the infrared part of the spectrum for very distant galaxies. In this case the best fit is obtained if the break is positioned as shown, with the first three fainter points to the left of the break and the rest to the right. The break itself is straddled by two observational bands. Employing a number of different estimates the authors conclude that the redshift of this galaxy is z=16.7 or thereabouts.

There is no direct evidence for the sharp edge associated with the Lyman Break – and no spectral lines are observed either – so this all depends on the object being correctly identified as a high-redshift galaxy and not some other object at lower redshift. You have to assume this to get a redshift, but then all inferences are based on assumed models so there’s nothing unusual about this approach. The authors discuss other possibilities and conclude that there is no plausible alternative source. Take away the green template spectrum and you just see a spectrum that rises to a peak and falls again. The authors claim that there is no plausible low-redshift source with such a spectrum.

Anyway, here is a composite colour image of the source:

So is this now the earliest galaxy ever observed? And what object will I be asking this question about next week? One thing I can predict is that there are going to be many more such objects in the very near future!

The Earliest Galaxy we’ve seen?

Posted in Astrohype, The Universe and Stuff with tags , , , on July 20, 2022 by telescoper

The red smudge in the centre of this image is thought to be a galaxy with a redshift of around z=13, as seen by the NIRCam instrument on the James Webb Space Telescope. This redshift estimate is based on photometry so the object remains a candidate rather than a confirmed high-redshift galaxy, but if confirmed spectroscopically this would be the highest-redshift galaxy yet observed.

For more details on the observations and their implications see the preprint on arXiv here. It’s interesting (and challenging) that there are such bright galaxies at such an early stage of cosmic evolution, assuming of course that the redshift is correct. Photometric redshift estimates have been wrong before.

If we take the estimated redshift at face value and adopt the standard cosmological model, the lookback time to this galaxy (GLASS-z13) is about 97.6% of the current age of the Universe so we’re seeing it as it was just 330 million years after the Big Bang. It could therefore be the earliest galaxy we have seen. It isn’t very accurate to say that it is the oldest galaxy we’ve seen, as we are probably seeing it as it was when it was very young.

These observations come from JWST Early Science Release Programmes so are just a taster of what is to come. No doubt we’ll hear much more about high-redshift galaxies from JWST in future and there’s every chance that they will change our view of the high-redshift Universe in dramatic ways.

I’ll just mention here that I’m old enough to remember going to conferences where “high redshift” meant z=0.5! In those days the highest redshift objects were quasars, but they have long since been overtaken.

Characterization of JWST science performance from commissioning

Posted in The Universe and Stuff with tags , , , on July 14, 2022 by telescoper

I don’t suppose it will take very long for science papers based on the first data from JWST to start appearing on arXiv but I haven’t seen any yet. There is however a very important with an uncountable number of authors, led by Jane Rigby, that describes the commissioning process. Uncountable by me, that is.

Here is the abstract:

This document characterizes the actual science performance of the James Webb Space Telescope (JWST), as known on 12 July 2022. Following six months of commissioning to prepare JWST for science operations, the observatory is now fully capable of achieving the discoveries for which it was built. A key part of commissioning activities was characterizing the on-orbit performance of the observatory. Here we summarize how the performance of the spacecraft, telescope, science instruments, and ground system differ from pre-launch expectations. Almost across the board, the science performance of JWST is better than expected. In most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.

Although it’s very long (60 pages) it’s well worth reading for an account of how meticulously the various calibrations etc were done. Various objects make cameo appearances, including Jupiter:

James Webb: the wrong name for a Space Telescope

Posted in History, LGBT, The Universe and Stuff with tags , , , on July 13, 2022 by telescoper

Following yesterday’s excitement about the new images from the James Webb Space Telescope I thought I’d share this video documentary that explains why the choice of name for this facility is highly inappropriate and should be changed. This is a matter I’ve blogged about previously, in fact, but the video is new.

Those First Results from JWST

Posted in The Universe and Stuff with tags , , , , , on July 12, 2022 by telescoper

As promised in my post earlier today, we gathered in a small lecture theatre in Maynooth to watch the “reveal” of various new images and other data from the James Webb Space Telescope. The images are indeed wonderful and spectacular, but the video stream was excruciatingly bad to watch, with more technical glitches than I’ve had hot dinners. It was like an astronomical version of Acorn Antiques!

Anyway, you can find them all the new results together with explanations and descriptions here so Ill just put up a gallery here:

Those are the four results released today. This image was previewed last night and appeared in my post earlier today:

I couldn’t resist, however, adding this spectrum of a faint reddish galaxy in the above image:

This spectrum is taken using the NIRSpec instrument on JWST. The observed wavelength along the horizontal axis is measured in microns. If I’ve got the line identifications correct I think this galaxy is at an amazing redshift of about z=8.5. Amazing. High redshift galaxy spectra obtained are usually a lot rattier than this. I think this demonstrates that JWST is going to revolutionize the field of galaxy formation.

The First Deep Field from JWST

Posted in Astronomy Lookalikes, The Universe and Stuff with tags , , , , , on July 12, 2022 by telescoper

I have to say that I didn’t stay up to watch the live stream of last night’s preview of this afternoon’s release of the first images from the James Webb Space Telescope. It started very late and I got sick of listening to the dreary music on the feed so went to bed. Nevertheless here is the first picture:

Credits: NASA, ESA, CSA, and STScI

This is a deep field image taken using JWST’s NIRCAM (Near-Infrared Camera). Note that the artifacts you see around some objects are diffraction spikes which occur around bright sources; their six-fold symmetry reflects the hexagonal structure built into the JWST’s mirror assembly. Sources sufficiently bright and compact enough to cause these spikes in deep field images are foreground stars: the extended, fainter objects are all much more distant galaxies.

The description from the NASA page is:

NASA’s James Webb Space Telescope has produced the deepest and sharpest infrared image of the distant universe to date. Known as Webb’s First Deep Field, this image of galaxy cluster SMACS 0723 is overflowing with detail.

Thousands of galaxies – including the faintest objects ever observed in the infrared – have appeared in Webb’s view for the first time. This slice of the vast universe is approximately the size of a grain of sand held at arm’s length by someone on the ground.

This deep field, taken by Webb’s Near-Infrared Camera (NIRCam), is a composite made from images at different wavelengths, totaling 12.5 hours – achieving depths at infrared wavelengths beyond the Hubble Space Telescope’s deepest fields, which took weeks. 

The image shows the galaxy cluster SMACS 0723 as it appeared 4.6 billion years ago. The combined mass of this galaxy cluster acts as a gravitational lens, magnifying much more distant galaxies behind it. Webb’s NIRCam has brought those distant galaxies into sharp focus – they have tiny, faint structures that have never been seen before, including star clusters and diffuse features. Researchers will soon begin to learn more about the galaxies’ masses, ages, histories, and compositions, as Webb seeks the earliest galaxies in the universe

Here is a close-up of one of the distorted galaxy images and othe features produced by gravitational lensing:

We’re having a special viewing in Maynooth this afternoon of the press conference which will unveil more new images from JWST – nice telescope, shame about the name. I may add comments on here if anything particularly exciting turns up. You can watch it here:

Let’s hope this one starts on time!