Joel L Schiff - The Most Interesting Galaxies in the Universe

Beatrice Tinsley at Yale University in 1978. Credit: Encyclopedia of New Zealand.

Mankind has no doubt pondered the origin of the Universe, well, since the beginning of mankind. The notion that the world is borne on the back of a turtle who is borne on the back of another turtle, and so on in an infinite regress of turtles all the way down is a historically fanciful myth but nothing more. However, as it turns out, the standard scientific model of the Universe actually has rather extraordinary and unusual features. Indeed, any study of the Universe and its contents requires the reader to become a time traveler, peering back to periods of millions and even billions of years ago. Indeed, there is no experience of now elsewhere in the Universe, only then.

Moreover, even the very notion of space itself is somewhat bizarre, because it happens to be expanding like a rubber band. How can empty space expand? Well, it is, and pushing nearly all the galaxies in the Universe away from us. As well, all the evidence suggests that the expansion rate is actually increasing because so-called empty space is teeming with a mysterious force called dark energy driving this expansion.

When we add in the very strange feature of invisible black holes, supermassive ones that are millions to billions of times the mass of our Sun, that suck in anything in their vicinity never to be seen again, then the idea of turtles all the way down is not so strange after all.

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This book covers the fifty-one most interesting galaxies in the Universe according to the author. Of course, there is no such thing as most interesting since every galaxy is interesting in its own right simply by being that most extraordinary structure called a galaxy. So the choice is somewhat arbitrary, but in each case, it is explained what characteristics make that particular galaxy special. And believe me, many are indeed exceptional while some are still the subject of scientific discussion as to what we are truly looking at.

The first part of the book is a very brief introduction to astronomy in order to appreciate the science involved for each galaxy. This discussion must necessarily be very incomplete but any astronomy book can be a useful source to fill in the gaps. Only enough is explained in order to understand the discussions in the section on galaxies.

As astronomy is a very scientific pursuit, there is normally a considerable amount of mathematics involved. In this book, however, the author has attempted to give the flavor of the scientific arguments by sparingly using only the simplest of mathematical equations and calculations. Where some basic mathematics is involved (logarithms, algebra, etc), those calculations are reserved for the appendices. But again, they are very elementary in nature and the reader should not be intimated by the appearance of a few symbols. Nothing more than a high school acquaintance with some of the concepts involved is required and from such simple notions as the equation of a straight line and a logarithm, one can calculate the mass of a black hole at the center of a galaxy. Now that is pretty extraordinary as well.

A few astronomical terms

The one basic notion at the heart of all astronomy is distance. How far away is a star or galaxy? And because the Universe is so vast, the units we ordinarily measure distance with, such as kilometers or miles, are simply inadequate. So there is a more useful concept and that is the light-year, namely the distance that light travels in an entire year. Light (in a vacuum) has a constant speed of 299 792.458 km s1 (km/sec), which we will round off to 300 000 km s1. That is roughly 7 times around the Earth in one second! So you can see that in an entire year, light will travel some considerable distance, roughly 9.5 trillion kilometers or 5.88 trillion miles. So we will abandon kilometers and miles as they are astronomically useless and use the distance concept of a light-year.

Thus, the distance to the nearest star Proxima Centauri is 4.2 light-years, a nice and small number to work with. The distance to the nearby galaxy Andromeda is 2.5 million light-years, again a manageable distance value. On the other hand, astronomers find it more convenient for technical reasons to use a different distance unit instead of the light-year, called a parsec. Traditionally, it has been defined as the distance to an astronomical object that has a parallax of one arcsecond as seen from Earth. The name is derived from parallax and (arc)second and 1 parsec = 3.26 light-years. But throughout this book, we will persist with the more familiar light-year unit of measurement and if necessary make the standard conversion from parsecs to light-years. The notion of parallax will be discussed in the sequel because it is an important one in measuring the distance to stars and galaxies.

One thing that the reader must appreciate is that when one looks up at the Heavens, you are peering into a time machine. To say that Proxima Centauri is 4.2 light-years away means that the light reaching our eyes when we look through a telescope has taken 4.2 years to reach us. That is why the unit of light-years is so useful. It not only conveys the distance to an object but also how many years back in time we are actually seeing it. This is called the lookback time. So, if a galaxy is 40 million light-years away, any image of it shows it as it was 40 million years ago. In this sense, we are time travelers when peering into the Universe.

This can sound problematic as perhaps the galaxy is no longer even there after 40 million years. Indeed, some of the galaxies we consider are colliding with other galaxies and what their present state is