Archive for Astrophysics

That was the Astrophysics & Cosmology Masterclass that was

Posted in Education, Maynooth, The Universe and Stuff with tags , , on March 25, 2021 by telescoper

I’m a bit late getting round to writing something on the blog today because it has been yet another hectic day. Between my usual lecture this morning and Computational Physics Laboratory session this afternoon we also had our long-awaited Astrophysics & Cosmology Masterclass (held via Zoom).

This event had been delayed twice because of Covid-19 so we were glad that it went ahead today at last!

We were a little nervous about how well it would go but as it happened I think it was a success. We had approaching a hundred schools tuning in, from Wicklow to Tralee, Longford to Monaghan, Donegal to Cork and many places between. The level of engagement was excellent. We held a question-and-answer session but were a little nervous in advance about whether we would actually get any questions. As it turned out we got a lot of questions with some very good ones among them. Reaction from students and teachers was very good.

For those who couldn’t make it to this morning’s session we did record the presentations and I’ll make the video available via YouTube in due course.

Now, I’ve been Zooming and Teaming (with a bit of Panopto thrown in) all day so if you don’t mind I’ll now go and vegetate.

Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , on February 23, 2021 by telescoper

Regular readers of the blog – both of them – may remember that we planned to present a Masterclass in Astrophysics & Cosmology on January 14th 2021 but this had to be postponed due to Covid-19 restrictions. After today’s announcements by the Government of  a phased return to school starting on March 1st we have now decided to proceed with a new date of March 25th 2021.

This will be a half-day virtual event via Zoom. It’s meant for school students in their 5th or 6th year of the Irish system, who should be returning to classrooms on March 15th, but there might be a few of them or their teachers who see this blog so I thought I’d share the news here. You can find more information, including instructions on how to book a place, here.

Here is the updated official poster and the programme:

I’ll be talking about cosmology early on, while John Regan will talk about black holes. After the coffee break one of our PhD students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the study!

Physics versus Astrophysics

Posted in The Universe and Stuff with tags , on January 21, 2021 by telescoper

I don’t often post memes on here, but I couldn’t resist this one. Credit here.

On the Colours of Stars

Posted in The Universe and Stuff with tags , , , on January 20, 2021 by telescoper

There’s an interesting paper on the arXiv by Harre & Heller with the title Digital Color Codes of Stars. Here’s the abstract:

Publications in astrophysics are nowadays mainly published and read in digitized formats. Astrophysical publications in both research and in popular outreach often use colorful representations of stars to indicate various stellar types, that is, different spectral types or effective temperatures. Computer generated and computer displayed imagery has become an integral part of stellar astrophysics communication. There is, however, no astrophysically motivated standard color palette for illustrative representations of stars and some stars are actually represented in misleading colors. We use pre-computed PHOENIX and TLUSTY stellar model spectra and convolve them with the three standard color matching functions for human color perception between 360nm and 830nm. The color matching functions represent the three sets of receptors in the eye that respond to red, green, and blue light. For a grid of main sequence stars with effective temperatures between 2300K and 55,000K of different metallicities we present the red-blue-green and hexadecimal color codes that can be used for digitized color representations of stars as if seen from space. We find significant deviations between the color codes of stars computed from stellar spectra and from a black body radiator of the same effective temperature. We illustrate the main sequence in the color wheel and demonstrate that there are no yellow, green, cyan, or purple stars. Red dwarf stars (spectral types M0V – M9V) actually look orange to the human eye. Old white dwarfs such as WD1856+534, host to a newly discovered transiting giant planet candidate, occur pale orange to the human eye, not white. Our freely available software can be used to generate color codes for any input spectrum such as those from planets, galaxies, quasars etc.

This reminded me of a post I wrote in 2011 about why you never see any green stars. They say a picture paints a thousand words so here’s an illustration from the above paper:

This shows that although there are stars in the Main Sequence whose spectra peak at wavelengths corresponding to green light, none of them look green. The paper also claims that there are no yellow stars either. The Sun can look yellow when viewed from Earth but that is to do with scattering in the atmosphere: from space, the Sun looks white.

For another discussion of the use of colour representations in cosmology, see here.

 

Postponed: Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , on January 10, 2021 by telescoper

Regular readers of this blog may recall that the Department of Theoretical Physics at Maynooth University  planned to host a Masterclass in Astrophysics & Cosmology on January 14th 2021 (i.e. next Thursday). Unfortunately the closure of schools in Ireland until at least the end of January has given us no alternative but to postpone this event. It’s not cancelled though and we intend to run it as soon as possible: the date is now set provisionally for 25th February.  Limited places remain available and bookings are still open. You can find more information, including instructions on how to book a place, here.

Here is the official poster and the programme (timings still apply, but not the date..):

I’ll be talking about cosmology early on, while John Regan will talk about black holes. After the coffee break one of our PhD students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the courses.

For updates please follow the Department’s on twitter-feed:

Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , on November 24, 2020 by telescoper

Stealing the idea from our long-running Particle Physics Master Class (which sadly had to be cancelled this year for Covid-19 related reasons, but will resume in 2021), the Department of Theoretical Physics at Maynooth University has decided to launch a Masterclass in Astrophysics & Cosmology. The first one will be on January 14th 2021.

This will be a half-day virtual event via Zoom. It’s meant for school students in their 5th or 6th year of the Irish system, but there might be a few of them or their teachers who see this blog so I thought I’d share the news here. You can find more information, including instructions on how to book a place, here.

Here is the official poster and the programme:

I’ll be talking about cosmology early on, while John Regan will talk about black holes. After the coffee break one of our PhD students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the study!

Machine Learning in the Physical Sciences

Posted in The Universe and Stuff with tags , , , , , on March 29, 2019 by telescoper

If, like me, you feel a bit left behind by goings-on in the field of Machine Learning and how it impacts on physics then there’s now a very comprehensive review by Carleo et al on the arXiv.

Here is a picture from the paper, which I have included so that this post has a picture in it:

The abstract reads:

Machine learning encompasses a broad range of algorithms and modeling tools used for a vast array of data processing tasks, which has entered most scientific disciplines in recent years. We review in a selective way the recent research on the interface between machine learning and physical sciences.This includes conceptual developments in machine learning (ML) motivated by physical insights, applications of machine learning techniques to several domains in physics, and cross-fertilization between the two fields. After giving basic notion of machine learning methods and principles, we describe examples of how statistical physics is used to understand methods in ML. We then move to describe applications of ML methods in particle physics and cosmology, quantum many body physics, quantum computing, and chemical and material physics. We also highlight research and development into novel computing architectures aimed at accelerating ML. In each of the sections we describe recent successes as well as domain-specific methodology and challenges.

The next step after Machine Learning will of course be Machine Teaching…

Lectures and Lava Lamps

Posted in Education, Maynooth, The Universe and Stuff with tags , , , on November 7, 2018 by telescoper

Teaching at Maynooth University has resumed after the Study Break, and yesterday I gave my first lecture on Astrophysics & Cosmology after a gap of a week. I still haven’t got onto the Cosmology bit yet, but am most of the way through a set of half-a-dozen lectures or so on stellar structure and evolution.

In past incarnations I’ve deployed a lava lamp as a prop to illustrate convection, one of the ways that heat can be transported from the core of a star (where it is generated by nuclear fusion) to its surface (whence it is radiated). The simple demonstration of how a temperature gradient can lead to convective motion always proved popular with students. In fact, more-or-less the only complimentary comments I ever got about my lectures on this topic were about how nice the lava lamp was.

Anyway, no longer having access to the official Cardiff University School of Physics & Astronomy Lava Lamp, I thought I’d just show a video chosen from the many available on youtube. They seem quite popular, perhaps because they are rather restful:

Unfortunately, however, the fates had it in for me yesterday. The Powers That Be decided to update the version of Windows on all the PCs in all the teaching rooms on campus during the study break. When I tried to show the video the computer crashed and would not restart. I had to run back to the office to get my laptop, which I eventually got to work, but I had lost so much time that I skipped the video. Hopefully I’ll get to show it properly at some point in the future.

 

A Python Toolkit for Cosmology

Posted in The Universe and Stuff with tags , , , , on December 14, 2017 by telescoper

The programming language Python has established itself as the industry standard for researchers in physics and astronomy (as well as the many other fields, including most of those covered by the Data Innovation Research Institute which employs me part-time). It has also become the standard vehicle for teaching coding skills to undergraduates in many disciplines. In fact it looks like the first module I will be teaching in Maynooth next term is in Computational Physics, and that will be delivered using Python too. It’s been a while since I last did any significant hands-on programming, so this will provide me with a good refresher. The best way to learn something well is to have to teach it to others!

But I digress. This morning I noticed a paper by Benedikt Diemer on the arXiv with the title COLOSSUS: A python toolkit for cosmology, large-scale structure, and dark matter halos. Here is the abstract:

This paper introduces Colossus, a public, open-source python package for calculations related to cosmology, the large-scale structure of matter in the universe, and the properties of dark matter halos. The code is designed to be fast and easy to use, with a coherent, well-documented user interface. The cosmology module implements FLRW cosmologies including curvature, relativistic species, and different dark energy equations of state, and provides fast computations of the linear matter power spectrum, variance, and correlation function. The large-scale structure module is concerned with the properties of peaks in Gaussian random fields and halos in a statistical sense, including their peak height, peak curvature, halo bias, and mass function. The halo module deals with spherical overdensity radii and masses, density profiles, concentration, and the splashback radius. To facilitate the rapid exploration of these quantities, Colossus implements about 40 different fitting functions from the literature. I discuss the core routines in detail, with a particular emphasis on their accuracy. Colossus is available at bitbucket.org/bdiemer/colossus.

The software can be downloaded here. It looks a very useful package that includes code to calculate many of the bits and pieces used by cosmologists working on the theory of large-scale structure and galaxy evolution. It is also, I hope, an example of a trend towards greater use of open-source software, for which I congratulate the author! I think this is an important part of the campaign to create truly open science, as I blogged about here.

An important aspect of the way science works is that when a given individual or group publishes a result, it should be possible for others to reproduce it (or not, as the case may be). At present, this can’t always be done. In my own field of astrophysics/cosmology, for example, results in traditional scientific papers are often based on very complicated analyses of large data sets. This is increasingly the case in other fields too. A basic problem obviously arises when data are not made public. Fortunately in astrophysics these days researchers are pretty good at sharing their data, although this hasn’t always been the case.

However, even allowing open access to data doesn’t always solve the reproducibility problem. Often extensive numerical codes are needed to process the measurements and extract meaningful output. Without access to these pipeline codes it is impossible for a third party to check the path from input to output without writing their own version assuming that there is sufficient information to do that in the first place. That researchers should publish their software as well as their results is quite a controversial suggestion, but I think it’s the best practice for science. There isn’t a uniform policy in astrophysics and cosmology, but I sense that quite a few people out there agree with me. Cosmological numerical simulations, for example, can be performed by anyone with a sufficiently big computer using GADGET the source codes of which are freely available. Likewise, for CMB analysis, there is the excellent CAMB code, which can be downloaded at will; this is in a long tradition of openly available numerical codes, including CMBFAST and HealPix.

I suspect some researchers might be reluctant to share the codes they have written because they feel they won’t get sufficient credit for work done using them. I don’t think this is true, as researchers are generally very appreciative of such openness and publications describing the corresponding codes are generously cited. In any case I don’t think it’s appropriate to withhold such programs from the wider community, which prevents them being either scrutinized or extended as well as being used to further scientific research. In other words excessively proprietorial attitudes to data analysis software are detrimental to the spirit of open science.

Anyway, my views aren’t guaranteed to be representative of the community, so I’d like to ask for a quick show of hands via a poll…

…and you are of course welcome to comment via the usual box.

R.I.P Leon Mestel (1927-2017)

Posted in Biographical with tags , , on September 18, 2017 by telescoper

Leon Mestel FRS, photographed by Lucinda Douglas-Menzies

I heard this weekend the very sad news that on Friday 15th September 2017, we lost one of our great astrophysicists. Professor Leon Mestel FRS, pictured abvove, passsed away, peacefully in his sleep, at Addenbrooke’s Hospital. He was 90 years old.

Leon Mestel was a scientist of the highest distinction. Elected a Fellow of the Royal Society in 1977, his research interests were very broad, encompassing, but not restricted to, the areas of star formation and structure, especially stellar magnetism and astrophysical magnetohydrodynamics. Among his contributions in other areas were important papers on gravitational collapse and equilibrium in the context of galaxy dynamics, of which the classic `Mestel Disk’ is just one example. He has been awarded both the Eddington Medal (1993) and the Gold Medal (2002) of the Royal Astronomical Society. He had great physical insight which was backed up with prodigious mathematical skill and an encyclopedic knowledge of astrophysics. He also had great powers of concentration and the determination to tackle the kind of extremely challenging problems that scared off lesser intellects. Leon  was an ‘old school’ theoretical astrophysicist who was held in very high regard across the astrophysics community, and he will be greatly missed.

Others more expert than me will be able to pay proper tribute to his scientific work, so I’ll restrict myself here to a few personal reminiscences.

Leon Mestel was Professor in the Astronomy Centre at the University of Sussex when I joined it to start my DPhil in 1985. We new postgraduate research research students were required to take four courses on various aspects of astronomy, and pass oral examinations on them, before being allowed to progress beyond the first year. One of these courses was a course on Stellar Structure, taught by Leon Mestel. His lectures were pretty intense – and, I have to say, not made any easier to understand by his truly terrible handwriting on the blackboard! – but I learned a huge amount from them. I still have the lecture notes I took, in fact.

I had a root around in my old files this morning and found this evidence that I once knew something about stars!


All of us found Leon very scary to start with. He was intellectually quite intimidating and seemed to be a rather fierce and irascible character. That opinion changed when, a few weeks into term, he invited us to his house in Lewes for a social evening. We were all a bit nervous on the way there, unsure of what to expect, but as it turns out Leon was a marvellously entertaining and avuncular host. He had a wicked sense of humour and a seemingly inexhaustible stream of jokes, across a spectrum from groan-worthy puns to very risqué stories, including a  liberal measure of archetypal Jewish humour.  Leon’s father was a Rabbi, actually.  That evening broke the ice and we all realised that the was one of the good guys. That he came across as grumpy sometimes was because he was concentrating very hard, but it was rather easy to make him laugh and bring that twinkle to his eye that we will all remember.

His sharp brain and very broad knowledge meant that Leon could spot bullshit at a  considerable distance and, while he often seemed to be snoozing through  our weekly seminar,  he invariably woke up at the end and asked a penetrating question. Since one of his main interests was the role of magnetic fields in astrophysics, a subject that sends many astrophysicists screaming from the room, he would often choose something about magnetism as a question. When I was there his main focus was on the fiendishly difficult problem of building a self-consistent model of the pulsar magnetosphere. He was, however, genuinely interested in all branches of astrophysics and always recognised good work when he saw it, especially from younger scientists.

During my time as a PhD student I had some problems that required me to take quite a lot of time off. Leon was extremely kind and supportive during this period, and he even bent the rules a bit to avoid putting me through the formal process of interrupting my studies. When I was back at work and just about finishing my thesis in 1988 it was Leon who came to see me in person, with a big smile on his face, and offered me a postdoctoral position at Sussex to follow my graduate studies. I nearly fell off my chair with surprise and gratitude.

After I joined the staff later in 1988, it became a bit of a ritual for us to visit the Senior Common Room (which was situated in what is now Bramber House) for lunch, followed by coffee. It turned out that Leon liked to do the Times crossword with his post-lunch coffee. He wasn’t at all averse to a collaborative effort on tricky research problems, and it was thus with crosswords too. We both preferred the Guardian puzzle, actually, but he saved that one for after work and did the Times one because the paper was provided free in the SCR. There was also a Chambers dictionary.

I left Sussex in 1990 and Leon retired in 1992. I didn’t see as much of him after that, except for the occasion when he and my former DPhil supervisor John Barrow organized a meeting in 2004 about Eddington at which I was honoured to be asked to give a talk about the 1919 eclipse expeditions. That was a very nice occasion at which Leon was in sparkling form. Thereafter I saw him occasionally at the RAS Club, but in recent years he didn’t come so often as he found it increasingly difficult to get around.

Leon Mestel was not only a great astrophysicist but also a great character.  I’m so very sorry I can’t attend his funeral (which is being held tomorrow), but I send heartfelt condolences to his family, friends and colleagues.

Rest in Peace Leon Mestel FRS (1917-2017)