CapitalistImperialistPig

Galaxies may or may not have a social life.  Many, like our Milky Way, live in groups of a few dozen.  Others are loners, and still others live in clusters of hundreds or thousands.  Loners and group members tend to be actively forming stars, and consequently have the large, extremely bright young blue stars that only live for a few millions or tens of millions of years.  By contrast, most of the galaxies in large groups, especially the big elliptical galaxies, aren't forming stars, haven't for  perhaps a few billion years or so, and consequently have only old, red, and small stars - they are red and dead in the professional parlance. We know why, in a sense.  They lack the cold molecular gas clouds where stars form, and instead, are embedded in hot ionized gas at a million or so kelvins.  However, this presents another puzzle.  Galaxy clusters typically have one or more of the very large cD ellipticals at their center.  Their large gravity...

In the current universe, stars form in large molecular clouds, usually having masses thousands of times that of the Sun. Consequently, they usually form in clusters of hundreds or thousands of stars. The discovery of thousands of extra-solar planets in the last couple of decades has demonstrated that many of these stars have planets. So how do these planets form? The molecular clouds out of which stars form are turbulent, and consequently the blobs that condense to form stars have angular momentum - quite a bit more angular momentum than a star, and more, in fact, than a typical stellar system with planets. One way to deal with the angular momentum is to form a binary or multi-star system, two or more stars orbiting each other, and this is extremely common. Another way is to produce a planetary system, and for our solar system, most of the angular momentum is in the planets - mostly in Jupiter. When a overdense "core" region of a cloud stars contracting under gravity, ...

I used this book by Cathie Clarke and Bob Carswell to study for the fluid dynamics portions of my Astrophysical Dynamics and Fluid Dynamics course, and found it very useful. The authors base the book on lectures they have given to third year students at Cambridge. For me, the level was about right. It assumes no fluid dynamics but expects reasonable proficiency in vector analysis. Nearly all physics equations are carefully derived, usually with no missing steps that were too difficult for me to fill in. Most of the book is devoted to inviscid compressible fluids, with a strong focus on astrophysical applications, although the last three chapters (which I haven't studied) treat viscous astrophysical fluids and plasmas. I worked my way through much of the book, usually deriving every equation, and it's pretty amusing if you like that sort of thing. There is always a payoff in physical insight. Convection, hydrostatic equilibrium, sound waves, supersonic flow, shock waves...

And I'm not talking about Trump's inauguration yet. Well, maybe that's part of it. My actual subject is my reaction when I saw the first homework assignment in my Astronomy class in Dynamics and Hydrodynamics. Of course it wasn't based on the class material, since there wasn't any yet. Instead it was more of a basic math pretest: Differential equations, analyzing the behavior of integrals and deriving vector identities - stuff I hadn't done, for the most part, in fifty years. I panicked when I couldn't see how to get the inhomogeneous solution to the very elementary first differential equation. It reminded me of the feeling I had when I first saw the problem set on my PhD comprehensive and realized that there was not a single problem on it that I knew how to solve. However, just as on that long ago comprehensive, once I pondered the problems a bit I gradually realized that I did have the tools, in this case rusted, dull, and buried deep, for solving ...

Sean has some more cool stuff on the LIGO accomplishment. A favorite quote: The fact that Einstein’s prediction has turned out to be right is an enormously strong testimony to the power of science in general, and physics in particular, to describe our natural world. Einstein didn’t know about black holes; he didn’t even know about lasers, although it was his work that laid the theoretical foundations for both ideas. He was working at a level of abstraction that reached as far as he could (at the time) to the fundamental basis of things, how our universe works at the deepest of levels. And his theoretical insights were sufficiently powerful and predictive that we could be confident in testing them a century later. This seemingly effortless insight that physics gives us into the behavior of the universe far away and under utterly unfamiliar conditions should never cease to be a source of wonder. The equations that Einstein wrote down predicted lasers, black holes, and gravitational ...

Lumo has an excellent post on the GR wave discovery. Highly recommended if you want some modestly technical detail. Here is a quote on the energy release: Kip Thorne has described the unbelievable power of the black hole merger differently. During the peak power, the black hole merger releases 50 times more watts in gravitational waves than all damn stars in the visible Universe combined. This is just shocking.

Exploding Superstars: Understanding Supernovae and Gamma Ray Bursts by Alain Mazure and Stephane Basa is a very well-written and interesting book, somewhat marred by flaws which I prefer to attribute to the evil that has come to dominate Springer. If you prefer to avoid the rant and get to the recommendation, skip the next paragraph. The book is a translation from the French original, and, so far as I can tell, excellently done, but the title is a bit misleading. Although Supernovae and Gamma-Ray Bursts are prominently featured, the real subject is cosmology, as indicated the the original French title, which was something like "the Universe in all its glory". The text makes frequent mention of twenty or so color plates - these do not make an appearance in this English edition - a considerable loss. There are also many dozens of figures and diagrams many of which appear to have originally been done in color but have been reproduced by some idiotic process which destroys ...

Joshua S. Bloom's What Are Gamma Ray Bursts is the second book in the Princeton University Press Frontiers in Physics Series. It shares the same concise and compact format as the earlier volume on the First Stars and Galaxies in the Universe, together with the same annoyingly small type face. It's also very reasonably priced. Gamma Ray Bursts, first discovered as a side effect of a program to monitor the nuclear test ban treaty, are extraordinarily intense and very brief, with the duration of the gamma ray pulse being anything from less than a second to several seconds. During this time they are thousands of times brighter than a quasar and millions of times brighter than a supernova or a galaxy. Bloom traces the history of our understanding of this phenomenon, and discusses the physics believed to be involved in the phenomena. There are still many uncertainties, but it is generally believed that there at least three different types of GRBs. The so-called soft gamma r...

How Did the First Stars and Galaxies Form? By Abraham Loeb This is intended to be a short introduction to the interface between cosmology and astrophysics governing the first galaxies and stars. Allegedly it is aimed at the undergraduate with a science background or the non-specialist scientist, but I found the level of presentation rather uneven. What kind of student, I wondered, would need to have the terms "star" and "galaxy" defined but still be able to decipher "Polarization is produced when free electrons scatter a radiation field with quadrupole anisotropy Q?" This short book packs a lot of information into it, though I'm not sure that I agree with the title. It's mostly not about those events but rather about the modeling of the growth of cosmological density perturbations which ultimately gave rise to those stars and galaxies. This is a technical book, with lots of equations, but derivations of those equations are mostly absent or ex...

Metals, in Astro speak, are any elements other than hydrogen and helium. Since they weren't synthesized in the big bang (except for traces of lithium) the cosmos started out without any of them. The metals (in the astronomical use of the word) out of which we and our planet are made were created inside of stars and later dispersed in the interstellar medium through stellar winds and supernova explosions. The oldest stars in our current universe, some of them over twelve billion years old, have much smaller metallicities (metal contents) than our own star, the Sun, which wasn't formed until the Universe was already about 9.4 billion years old. None of those stars, however, have the zero metallicities expected from the first born stars, so it looks like none of those survived to the present. Since a star with a mass of 85% of that of the Sun (or less) would be expected to live longer than the present age of the Universe, that suggests that no such stars were made in that fir...

Via the Lumonator , some interesting news about the search for dark matter. Dark matter, you may recall, makes up about 80% of the matter content of the universe and about 25% of the total energy content. It's the second most mysterious component of that universe, trailing behind its bigger, stranger, brother, dark energy, thought to make up the other 70% of the cosmos. It is clumped by gravitation, but appears to interact scarcely at all with ordinary matter via the electromagnetism or the strong force. Its condensation under gravitation seeded the cosmic web and galaxy formation. The most popular theory of its nature is the so-called WIMP hypothesis, for Weakly Interacting Massive Particle. Since nothing is known about WIMPs, this doesn't pin them down much, but presumably that arose when the universe was hot enough in approximately equal numbers of particles and antiparticles. After the temperature cooled enough, they went their separate ways, influenced only by gravi...

Actually, I want to talk about the so-called Lyman-alpha forest. Suppose we look at a distant Quasar. It puts out a broad continuum in the UV, which gets red-shifted depending on how far it is away. On its way, it is extremely likely to encounter clouds of ionized intergalactic hydrogen, as well as more minor ingredients. Even at a cosmically "modest" red shift of 1, corresponding to a look back time of about 7.5 billion years, back in the strapping youth of the cosmos (5.5 Gyr.) there will be a forest of absorption lines in that emission. That forest consists almost entirely of differently red shifted versions of the Lyman-alpha line. The various incarnations are due to differently red shifted clouds of hydrogen along the path from quasar to Earth, with the least red shifted due to nearby clouds and the more red shifted to clouds farther away. The farther the source is away, the more intervening gas clouds until the "forest" becomes an impassible jungle of t...