Episode 5: Islands in a sea of darkness

WRITTEN BY: AURORA SIMIONESCU

You may imagine that the Universe is full of strange and complicated physics, and you’d be right. But that’s not the most difficult thing about it. Understanding the sheer size of our cosmos probably poses a bigger challenge to our minds than the most complicated physics any one of us can think of. The distance to the nearest star, called Proxima Centauri, is more than 2500 times the distance traveled so far by the Voyager space probes, in more than 35 years of hurling through space. There are hundreds of billions of stars in our own Galaxy, and it would take light about a hundred thousand years to cross the Milky Way. If a civilization on the other side of the Galaxy were able to watch what is happening on Earth, they would only now receive images of the first modern humans appearing on the planet, and might still be getting their last glimpses of Neanderthals before their extinction.

And that’s just the beginning. Even before having any observational evidence to support this idea, several philosophers in the early 1700s believed that, just as there are many stars like the Sun, there are also many galaxies like the Milky Way in the vastness of space. The first to propose this idea was Swedish philosopher Emanuel Swedenborg in 1734, though perhaps the best-known endorser is Immanuel Kant in his work published in 1755.

Astronomers knew about the presence of many small diffuse objects in the sky, which might have been exactly the distant Milky Ways that these philosophers envisioned. Charles Messier made a catalog of 110 such objects in 1771, to help him distinguish between fuzzy stuff that was always there in the same spot versus comets which appear and disappear and move across the sky. He never achieved his dream of discovering a new comet, but became famous for the Messier catalog which is still in use today. Not knowing how far away these objects were, however, astronomers initially could not tell how big they really were: they could equally well have been very distant large galaxies like the Milky Way, or nearby gas clouds, or small groups of closely packed stars (it turns out the Messier catalog in fact contains a mix of all of the above).

The first solid evidence that at least some of Messier’s fuzzy objects must be very far away came only in the 1920s, through the pioneering work of Edwin Hubble and his assistant, Milton Humason. Humason’s personal story is quite impressive by itself, though rarely mentioned. He dropped out of high-school at the age of 14 to become a mule-skinner, helped carrying equipment up Mount Wilson as the observatory was being built, then became a janitor at the observatory, and volunteered as a night assistant just out of interest. Despite the lack of schooling, he proved to be a very talented, meticulous, and successful observer. So think of him next time you’re afraid you don’t know enough math to understand astronomy.

With Humason’s help, Hubble put together a list of a particular kind of stars in M33 (M stands for Messier, as the creator of the catalog). The brightness of these stars, known as Cepheids, was known to vary with time, with a period that depended on how luminous they were. The idea is pretty simple. Imagine you have two lightbulbs, say, 7W and 20W. At night, the bright lightbulb placed far away from you would look the same as the fainter one that is closer, so you’d have no way of knowing how far the lightbulb is unless you could figure out which one of the two you are looking at. Now, suppose both lightbulbs have a defect, so they flicker – the 7W one flickers every second, the 20W every three. Now you can look at the lightbulb in the night, and if you see it flicker every three seconds you’ll know it’s the 20W one, so you’ll be able to guess how far away it is based on how bright it appears to you: the further away it is, the fainter it will seem. This is more or less how Hubble used the periods of the Cepheids to figure out how far M33 was – and came up with a distance about ten times the diameter of the Milky Way. So, M33 was clearly not part of our own Galaxy, but a different galaxy in its own right, now known as the Triangulum galaxy. It turns out, incidentally, that Hubble was pretty far off in his calculations: the latest estimates put M33 three times further from the Earth than he estimated. But his conclusion is still correct: the philosophers of the 1700s were right in believing that there are many galaxies in the Universe apart from our own, and many other diffuse objects in the sky soon turned out to be distant versions of the Milky Way.

Not all of the galaxies in the Universe look the same, though. In fact, galaxies come in roughly three kinds of shapes. Some look like a more or less flat disk, and on this disk one can see alternating bright and faint structures which give it a whirlpool, or spiral-like, appearance. These are fittingly called “spiral galaxies”. Our Milky Way, as well as our neighbors, the Andromeda and Triangulum galaxies, are of this type. Other galaxies look like much more uniform balls of stars – round or elliptical, without any alternating bright and faint features. These are called “elliptical galaxies”. A lot of the galaxies in the Virgo constellation are of this kind (for example, M87, M86, M84, M49 – all from the Messier catalog). The third broad category are irregular galaxies, which look essentially like none of the above, and more like a giant mess with no symmetric structure. The Large Magellanic Cloud is such a galaxy. If you’d like to try your own luck at distinguishing between spiral, elliptical, and irregular galaxies, while at the same time helping professional astronomers with their current research, please check out http://www.galaxyzoo.org/

A beautiful image taken with the Hubble Space Telescope showing one spot on the sky where many galaxies of all types are present. The large galaxy on the upper left side is an elliptical, the second-largest galaxy in the lower right corner is a spiral, and the bluish galaxy near the center of the image in the lower half is an irregular galaxy.

Why do galaxies have different shapes? Well, a lot of research is still being done on this topic, but the very general idea is that many galaxies start out as spirals. This is because things which are spinning like to end up being disk-shaped: if you’re a girl and you spin around really fast while wearing a skirt, it will flare up into a disk around your waist, or as much of a disk as the amount of fabric and width of the skirt allow for – and it will even have a little waviness to it, sort of like the spiral arms (but only sort of). If they happen to collide with each other, the disks initially get very messed up, creating irregular galaxies. After the collision, everything settles down into an elliptical ball of stars. Computer simulations of such collisions were able to reproduce this process of transforming two spiral galaxies into an elliptical one (as you can see in the following video). It also turns out that galaxies who live closely packed together, in so-called clusters of galaxies like the Virgo Cluster, tend to be elliptical – presumably because, in such dense environments, collisions happen more often. Lonelier galaxies, like the Milky Way, are less likely to collide with another galaxy and thus remain spiral-shaped for a longer time. But enjoy it while it lasts – the Milky Way is on a collision course with the Andromeda galaxy; the two will meet probably in 4 billion years’ time.

Hubble’s conclusion that the Milky Way is just one of many islands of stars in a sea of darkness was followed by a possibly even more profound realization. Hubble and Humason not only measured the distances to many nearby galaxies, but were also able to calculate howfast each of these galaxies are moving with respect to us, through something that is called the Doppler effect. You’ve experienced this if you ever watched Formula 1: when the cars are moving relatively fast, the sound has a noticeably different pitch when the cars are coming towards you than when they are going away from you. Something like this happens to light as well: if a yellow lightbulb is traveling towards you really fast, then it will appear more bluish, while if it’s going away from you, it will look more reddish. It would have to be traveling really fast (at a fraction of the speed of light) for you to notice this by eye, so you probably never experienced this, but with more sophisticated instruments, scientists can measure this effect quite accurately.

So, Hubble put together the distances and velocities of many nearby galaxies and realized that, in general, the further a given galaxy was, the faster it was moving away from us. This isn’t because we’re somehow in a special place in the Universe, but rather because the Universe itself is expanding. An analogy that is often used is to think of baking a raisin cake: the raisins are the galaxies and the dough is the space between them. As the yeast works its magic, the dough grows, and if you were a raisin (any raisin), you’d see all the other raisins moving away from you. The further from you they were, the faster they’d appear to be moving – just like what Hubble found about the galaxies in the Universe. When researchers were able to push Hubble’s measurements to more and more distant galaxies, it was realized that there is really a very precise relationship between the distance and speed of each galaxy – so much so, that professional astronomers today use distance and “redshift” (how much the light from the galaxy is shifted towards the red) as if they were practically synonyms. Our Universe isn’t just unimaginably big; it’s still growing – and its size is probably getting more and more difficult to comprehend by the minute.