Archive for the The Universe and Stuff Category

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.

 

Cosmology Talks: Marika Asgari on Kids 1000

Posted in Cardiff, Maynooth, The Universe and Stuff with tags , , , , on January 18, 2021 by telescoper

It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting. Since I haven’t posted any of these for a while I’ve got a few to catch up on – this one is from September 2020.

In this talk Marika Asgari tells us about the recent Kilo-Degree Survey (KiDS) cosmological results. These are the first results from KiDS after they have reached a sky coverage of 1000 square degrees. Marika first explains how they know that the results are “statistics dominated” and not “systematics dominated”, meaning that the dominant uncertainty comes from statistical errors, not systematic ones. She then presents the cosmological results, which primarily constrain the clumpiness of matter in the universe, and which therefore constrain Ωm and σ8. In the combined parameter “S8“, which is constrained almost independently from Ωm by their data they see a more than 3σ tension with the equivalent parameter one would infer from Planck.

P. S. The papers that accompany this talk can be found here and here.

R.I.P. Kumar Chitre (1936-2021)

Posted in The Universe and Stuff with tags , on January 11, 2021 by telescoper

Shashikumar Madhusudan Chitre

Yet more sad news to pass on, I’m afraid.

I heard today of the death at the age of 84 of Indian theoretical astrophysicist Sashikumar Madhusan Chitre. Kumar worked on diverse subjects from solar physics to gravitational lensing (and many things in between) and had a very strong interest in science education. He was a frequent visitor to Queen Mary College when I worked there and I remember very many interesting discussions with him there, and then later when we met up at the Tata Institute for Fundamental Research in Mumbai and spent a very pleasant evening chatting to him over dinner at his home.

Kumar held a number of Visiting Professorships including the Universities of Cambridge, Sussex, Columbia, and Amsterdam, as well as Queen Mary, so many people around the world will miss not only his scientific expertise but also his wonderfully warm, kindly, humorous and generous personality.

R.I.P. Professor Sashikumar Madhusan Chitre (1936-2021).

Two Job Opportunities in Quantum Computing/ Quantum Information Processing in Maynooth!

Posted in Maynooth, The Universe and Stuff with tags , , , on January 11, 2021 by telescoper

Gratuitous Graphic purporting to represent Quantum Information Processing

Regular readers of this blog may recall an announcement that the Department of Theoretical Physics scored a notable success, or rather Dr Professor Jiri Vala did, in securing funding as part of a project called Quantum Computing in Ireland: A Software Platform for Multiple Qubit Technologies. To be eligible for this kind of funding, projects must involve businesses and this particular project includes IBM Ireland Ltd, MasterCard Ireland, Rockley Photonics and Equal 1 Laboratories, the latter two being SMEs based in the Dublin area. The project also involves the Tyndall National Institute (Cork); University College Dublin; and Maynooth University. This is the first large collaboration in Ireland in this area.

Well, now that the funds have actually arrived, I  thought I’d use the medium of this blog to pass on the information that the Department of Theoretical Physics at Maynooth University is looking two appoint not one but two theoretical physicists as postdoctoral researchers to work on this project.

The first position (for which you can find further details here) is  to work on modelling and simulations of quantum photonic systems for quantum information processing.

The second position (further details here) is to work on the development of compilers, quantum control protocols and algorithms for quantum information processing in quantum photonic systems.

The deadline for both positions is Sunday 31st January 2021!

Please feel free to pass this on to anyone you think may be interested!

 

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:

Congratulations to the RAS Medallists!

Posted in The Universe and Stuff with tags , , , , , , on January 9, 2021 by telescoper

Given all the doom and gloom going around I thought I’d take the opportunity to share some good news and also offer my public congratulations to the all the winners of medals and awards announced yesterday by the Royal Astronomical Society. Let me draw particular attention to the three, purely on the grounds that I know them and their work personally.

First, Dame Jocelyn Bell Burnell who receives the Gold Medal. For some reason the citation doesn’t mention that she should have won a share of the Nobel Prize in 1974.

Second, star cosmologist Hiranya Peiris who gets the Eddington Medal.

And third, Steven Smartt of Queen’s University Belfast, who gets the Herschel Medal.

Congratulations to them and indeed to all the winners of awards and medals, a complete list of whom may be found here.

And the most viewed paper at the Open Journal of Astrophysics in 2020 is…

Posted in Open Access, The Universe and Stuff with tags , , , , , , on January 6, 2021 by telescoper

Yesterday I was looking at the Publishing Analytics tool on the Open Journal of Astrophysics to see which paper(s) had attracted the most interest in 2020. The winner in terms of  page views is  this paper, A Beginner’s Guide to working with Astronomical Data. Here is a grab of the overlay:

You can find the arXiv version of the paper here.

The author is Markus Pössel of the Haus der Astronomie at the Max Planck Institute for Astronomy in Heidelberg (Germany). This is a long paper – 71 pages with over a hundred figures – that gives a comprehensive introduction to the various kinds of astronomical data and techniques for working with such data. This paper has obviously attracted a lot of interest from many different kinds of people, especially  students doing undergraduate projects involving astronomical data (and their supervisors). It has had more than three times as many views as the runner-up.

It’s interesting to note that this paper has not yet obtained any citations from academic papers through the Crossref system and it may never that because of the kind of paper it is. Nevertheless, I think this is a valuable resource for the astronomical community and I am very glad we published it. I do hope, however, that anyone who does use this paper does remember to cite it!

It is perhaps also worth mentioning that we do not track download statistics for the papers we publish. This is because the PDF files are held on the arXiv, which does not publish download statistics for individual papers.

R.I.P. Sir Arnold Wolfendale (1927-2020)

Posted in Biographical, The Universe and Stuff with tags , , , on January 4, 2021 by telescoper

I’ve just heard the sad news that former Astronomer Royal Sir Arnold Wolfendale passed away on December 21st 2020 at the age of 93. There’s a full tribute to him here from Durham University, where he spent most of his very distinguished career as a cosmic ray physicist and played such an important role in developing a worldwide centre of excellence in Astronomy.

I remember Arnold Wolfendale very well from many trips to Durham over the years, starting with the SERC School for new postgraduate students in Astronomy I attended in 1985. He was an avuncular and extremely friendly presence there who went to a lot of trouble to talk to studdents; you can see him in the front row of the now (in)famous group photograph taken there:

 

Rest in peace, Sir Arnold Wolfendale (1927-2020)

The Day of Perihelion

Posted in The Universe and Stuff with tags , , , , , , , , on January 2, 2021 by telescoper

Earth’s elliptical orbit viewed at an angle (which makes it look more eccentric than it is – in reality is very nearly circular).

Today (Saturday 2nd January 2021) at approximately 13:50 GMT the Earth reaches at the point on its orbit, which which it is at its closest to the Sun, i.e. at its perihelion. At this time the distance from the Sun’s centre to Earth’s centre will be 147,093,163 km. This year, aphelion (the furthest distance from the Sun) is at 23.57 GMT on July 5th 2021 at which point the centre of the Earth will be 152,100,527 km from the centre of the Sun. You can find a list of times and dates of perihelion and aphelion for future years here.

At perihelion the speed of the Earth in its orbit around the Sun is greater than at aphelion (about 30.287 km/s versus 29.291 km/s). This difference, caused by the Earth’s orbital eccentricity, contributes to the difference between mean time and solar time I blogged about when discussing the Winter Solstice a couple of weeks ago.

It surprises me how many people think that the existence of the seasons has something to do with the variation of the Earth’s distance from the Sun as it moves in its orbit. The fact that perihelion occurs in the depth of winter should convince anyone living in the Northern hemisphere that this just can’t be the case, as should the fact that it’s summer in the Southern hemisphere while it is winter in the North.

The real reason for the existence of seasons is the tilt of the Earth’s axis of rotation. I used to do a little demonstration with a torch (flashlight to American readers) to illustrate this when I taught first-year astrophysics. If you shine a torch horizontally at a piece of card it will illuminate a patch of the card. Keep the torch at the same distance but tilt the card and you will see the illuminated patch increase in size. The torch is radiating the same amount of energy but in the second case that energy is spread over a larger area than in the first. This means that the energy per unit area incident on the card is decreases when the card is tilted. It is that which is responsible for winter being colder than summer. In the summer the sun is higher in the sky (on average) than in winter. From this argument you can infer that the winter solstice not the perihelion, is the relevant astronomical indicator of winter.

That is not to say that the shape of the Earth’s orbit has no effect on temperatures. It may, for example, contribute to the summer in the Southern hemisphere being hotter than in the North, although it is not the only effect. The Earth’s surface possesses a significant North-South asymmetry: there is a much larger fraction of ocean in the Southern hemisphere, for example, which could be responsible for moderating any differences in temperature due to insolation. The climate is a non-linear system that involves circulating air and ocean currents that respond in complicated ways and on different timescales not just to insolation but to many other parameters, including atmospheric composition (especially the amount of water vapour).

The dates when Earth reaches the extreme points on its orbit (apsides) are not fixed because of the variations in its orbital eccentricity so, in the short-term, the dates can vary up to 2 days from one year to another. The perihelion distance varies slightly from year to year too.

There is however a long-term trend for perihelion to occur later in the year. For example, in 1246, the December Solstice (Winter Solstice for the Northern Hemisphere) was on the same day as the Earth’s perihelion. Since then, the perihelion and aphelion dates have drifted by an average of one day every 58 years and this trend will continue. This means that by the year 6430 the timing of the perihelion and the March Equinox will coincide, although I will probably have retired by then…

The Mechanics of Nursery Rhymes

Posted in Cute Problems, The Universe and Stuff with tags , , , , on December 30, 2020 by telescoper

I’ve always been fascinated by Nursery Rhymes. Some people think these are little more than nonsense but in fact they are full of interesting historical insights and offer important advice for the time in which they were written. One such story, for example, delivers a stern warning against the consequences of placing sleeping babies in the upper branches of trees during windy weather.

Another important role for nursery rhymes arises in physics education. Here are some examples that students of elementary mechanics may find useful in preparation for their forthcoming examinations.

1. The Grand Old Duke of York marched 10,000 men up to the top of a hill and marched them down again. The average mass of his men is 65 kg and the height of the hill is 500m.

(a) Estimate the total work done in marching the Duke of York’s men up to the top of the hill.

(b) If, instead of marching down again, the men take turns sliding down a frictionless slide back to where they started, estimate the average speed of a man when he reaches the bottom of the hill.

(You may assume without proof that when they were up they were up, and when they were down they were down and, moreover, when they were only half way up they were neither up nor down.)

2. By calculating the combined rest-mass energy of half a pound of tuppenny rice and half a pound of treacle, and assuming a conversion efficiency of 10%, estimate the energy released when the weasel goes pop. (Give your answer in SI units.)

3. The Moon’s orbit around the Earth can be assumed to be a circle of radius r. A cow of mass m is standing on the Earth (which has mass M, and radius R). Derive a formula in terms of r, R, M, m and Newton’s Gravitational Constant G for the energy the cow needs in order to jump over the Moon.

(The Earth, Moon and cow may be assumed spherical. You may neglect air resistance and udder frictional effects. )

Feel free to contribute similar problems through the Comments Box.