The most comprehensive course ever

kw: book reviews, nonfiction, science, astrophysics, cosmology, physical universe, galaxies

As a student of geophysics, I occasionally remarked that the subject's bailiwick was "from the center of the Earth to the end of the Universe." The same could be said for astrophysics. Geophysics and astrophysics are a kind of tag team, covering the same realm from different perspectives. Astrophysics deals in part with how stars forge the elements that wind up in planets, while geophysics deals in the main with what happens to those elements once they form a solid or semisolid body (e.g. a gas giant planet).

I have great interest in both subject areas, so it was a real treat to read Welcome to the Universe: An Astrophysical Tour by Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott. The book is a distillation of material from a course taught by these three men at Princeton University, to non-astronomy students.

• Part I: Stars, Planets and Life, was written (and I presume taught) primarily by Dr. Tyson with certain sections by Dr. Strauss.
• Part II: Galaxies, was written (and presumably taught) entirely by Dr. Strauss.
• Part III: Einstein and the Universe, was written (and presumably taught) entirely by Dr. Gott.

You could say that Tyson deals with stellar and condensed matter, Strauss with galaxies and their formation, and Gott with the gamut of cosmological theories. For me, given my lifelong love of reading astrophysical books, both popular treatments and texts and monographs, there was little I would call "new to me." But these scientists are writing at the top of their form, and present their subjects in a most enjoyable way. I had certain take-away's from each author:

• Chapters 7 and 8 [Tyson], "The Lives and Deaths of Stars", parts I and II, are a good summary of the different types of stars based on their masses, certain features of their internal dynamics that are a result of their mass, and the fate of each type. I did not note a discussion of the first stars, those that were entirely metal-free (Astronomers call all elements heavier than helium "metals", which is understandable from a statistical viewpoint: of the 88 natural elements beginning with lithium, and also the two synthetic elements among the first 92, all but 18 are metals). Perhaps it would have been confusing, because such "zero-metallicity stars" could not have had "careers" that fit well into the Hertzsprung-Russell Diagram that does such a good job classifying all known stars in the present universe.
• Chapter 16 [Strauss], "Quasars and Black Holes", provides a clear summary of the spectral evidence that led firstly to the discovery that quasars are receding at phenomenal rates and are thus very distant (up to more than 90% of the way to the Big Bang some 13.8 billion years ago) and thus extremely luminous; and secondly that they must be powered by matter streaming into enormous black holes at the centers of galaxies. Nearly all quasars are more distant than a few billion light years. The closest is 600 million l-y. Quasars are the highest energy "active galactic nuclei" (AGN's), and since it seems that every galaxy hosts a supermassive black hole (from millions to billions of solar masses), any galaxy could host an AGN whenever a clump of matter finds its way to the galactic center.
• Chapter 24 [Gott], "Our Future in the Universe", discusses what has happened to the whole universe since the Big Bang, and what is expected to happen, according to current theories. It is on a sort of super-logarithmic scale, highlighting 15 events ranging from the first 10^-44 second to (very approximately) 10¹⁰⁰ years in the future. In the text other possible events are mentioned, and one is as far off as a number of years described by a number with 10³⁴ zeroes! That number of zeroes equals the number of hydrogen atoms in about 17 billion kilos of hydrogen. There will never be enough paper to "write" it down.

I was eager to see how Dr. Gott discussed Dark Energy and the (alleged) accelerating expansion of the universe. In the seven chapters he wrote, from time to time he discusses one or another mathematical principle that seems to require cosmic inflation (near the very beginning) or accelerating expansion (ongoing). I have yet to see an explanation of accelerating expansion that makes sense to me. The "evidence" for such acceleration is the anomalous brightness of some very distant supernovae. I have read recent articles that question both the data and the interpretation.

For my own part, I have yet to see an analysis of Type 1a supernovae that originate with a C-O white dwarf that accretes material of very low metallicity, as we would expect of very ancient objects at very great distances. Accretion, however, is not certain as a mechanism; WD-WD collisions are thought to produce the more prevalent type of supernova. The mass limit that must be crossed to yield a supernova is 1.44 solar masses. Thus the product of a collision will momentarily have a mass in the range 1.44 (plus a little) to 2.88 (minus a little). So, how "standard" is the standard candle known as a Type 1a supernova?

Well, that question did not get addressed, but for now that is OK. Astrophysicists and cosmologists are not single "voting bloc" in this regard, and I continue to read with interest the work being reported in this area.

Fascinating subjects, excellent writing: I expect this book to become a classic in its field.

How full of galaxies is the sky?

kw: references, galaxies, astronomy

This is just a quick reference note, to bring together a few useful quantities. I began to wonder just how many galaxies there might be in each square arc-minute or arc-second of the sky. I was surprised at the result.

First, the entire sky contains 4 x pi x (180/pi)² = 41,252.96 square degrees. This comes to 148.51 million square arc-minutes or 534.62 billion square arc-seconds.  The best resolution under "good seeing" with a ground-based telescope, and not enhanced with wavefront correction technology, is about one arc second. Thus, just recording and managing those half a trillion "sky pixels" is a big job already. The Hubble Space Telescope has a resolution in blue light of 0.05 arc-sec, so its potential number of "sky pixels" is 400 times greater.

How well populated is the sky? The Atlas of the Universe contains these figures:

• Number of large galaxies in the visible universe: 350 billion.
• Number of dwarf galaxies (ditto): 7 trillion.

Thus, an average set of three one-square-second bits of sky must contain two large galaxies. Each square second must contain about 13 dwarf galaxies. It takes a long time to gather enough light to see them all, clear out to just over 13 billion light years, but there is a lot going on in every square arc-second!

How big will they appear? A typical dwarf galaxy of 1,000-light-year size, 13.4 billion light years away (right after the universe cleared up enough to see that far), has an angular diameter of 0.015 arc second. So even Hubble won't be able to do more than register its existence in a particular pixel. Back up to 13 billion light years, when larger galaxies were forming up, and a 100,000 light-year-sized galaxy would subtend 1.5 arc seconds. Hubble can see a little detail in such an object. We can expect, as wavefront correction improves for large, and multiple-instrument, ground-based telescopes, that clearer images of such faraway objects will be obtained.

It is going to take a while, however, for a multi-exabyte database of the sky to be gathered and curated. Astronomers have a never-ending subject to explore!

As usual, favorites are rare

kw: astronomy, photographs, galaxies

I have a folder full of galaxy images as my current screen saver. Looking at it in a contemplative moment, I was struck again by how we prefer the big spirals to the much more common ellipticals. This image of M74 exemplifies what people think of as a galaxy:


By contrast, elliptical galaxies are simply bright blobs with a brighter center, and hardly any other features. This image of the center of cluster Abell SO740 is dominated by a very large elliptical galaxy. The large spiral to the lower left is probably a bit larger than our Milky Way galaxy, perhaps the size of the Andromeda galaxy (M31).


In this image, I can pick out two larger spirals and six or seven smaller ones which are probably part of a more distant cluster in the background. By contrast, there are dozens of elliptical galaxies of all sizes visible (again, many of these probably belong to a more distant cluster). Beauty may be in the eye of the beholder, and in this case, that eye seems to like a certain amount of detail and contrast. Compared to a richly detailed spiral galaxy, an elliptical galaxy is only slightly more interesting than a round, featureless spot.

Time to switch the screen saver

kw: hobbies, photographs, astronomy, galaxies

You really ought to click on this image to see the larger version (~1,000 px across); this 400 px version is just a bit puny.

I really appreciate the many observatories, including the space ones, that post large, high-resolution images on their web pages. A number of "wallpaper" web sites also post lovely images in their Nature sections. I downloaded these images and others for a specific purpose (my enjoyment), and cropped or resized them to the same dimensions in a 1.6:1 format.

I recently decided it was time to go for a new look with my screen saver. I like the "My Pictures Slideshow" supplied by Microsoft, and I have several folders that I switch among from time to time. For several months I've been showing pictures of various mineral specimens, some that I took of my own minerals, and some downloaded from museum and rock store web sites. Now that I've been classifying galaxies for several weeks in Galaxy Zoo, I decided to show some galaxies that are close enough to home (all less than half a billion light years) to make good subjects for "astro cheesecake".

Although spiral galaxies are the most interesting from a visual perspective, most galaxies are elliptical, such as the first thumbnail in the second row above. Galaxy clusters are typically dominated by one giant elliptical galaxy that is busy consuming its neighboring galaxies, and heavily populated by medium-sized to small elliptical galaxies. About 40% are spirals. The Local Group that includes The Galaxy, the Milky Way where we are, does not have a giant elliptical within it, but contains a number of smaller ones, and only three large spirals, the Milky Way, the Andromeda galaxy M31 and the Triangulum galaxy M33. The Local Group is part of the Virgo Supercluster, which is centered around a pair of giant elliptical galaxies. The fifth and sixth thumbnails in the second row above show central portions of the Virgo Cluster and the Coma Cluster.

When I was a child, I was fascinated by galaxies, and learned to "draw" rather realistic ones on a chalk board by rubbing colored chalk on an eraser and then blowing the dust onto the board. It is quite time consuming. None of these ephemeral artworks was photographed, more's the pity.