By Chris Fellows, Serenity Mobile Observatory
Hey, everyone. I hope you are all staying warm. It has started to get chilly here in Utah at night, so I am going to pull up stakes and head south to the dark skies of Rodeo, New Mexico, and Rusty’s RV Ranch for a month or two. Although it isn’t the best time of year to observe or photograph my favorite celestial objects, galaxies are always in the sky and some of the most spectacular and closest specimens will be up at night this winter. I can’t wait to see them again under the pristine conditions of Rodeo, NM.
It still astonishes me to think that just a single human lifetime ago we had no good idea about these objects or the vastness of the cosmos. It was only in 1929 that Edwin Hubble discovered that the Andromeda Nebula, as it was known at the time, wasn’t just a collection of stars and gas in our own Milky Way, but an entire and separate “island universe” in a much larger universe than we had ever imagined. He went on to map the distance, trajectory and velocities of several “nearby” galaxies and posited from that data that the universe was not static or contracting, like EVERYONE at the time assumed, but was expanding. The universe grew by many orders of magnitude that day, and Edwin Hubble set his place in history as an important contributor to human knowledge.
So, what is a galaxy? My search for a definition returned “a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.” I think that is a good description but it leaves out the planets, moons, comets and asteroids that are certainly out there, and it also neglects supermassive black holes that seem to be at the center of every galaxy.
These strange objects have never been directly observed – they are black after all – but can be “seen” by the effect they have on things we can see around them. This brings up a chicken-and-egg question surrounding galaxies and their associated black holes: Which came first? Was it the black hole which later accreted gas and dust from the surrounding cosmos forming a galaxy, or did the galaxy come first and its in-falling matter become so dense a black hole was formed at its center? Cosmologists are working on the question but we don’t have a verified answer yet.
Galaxies come in every form and size imaginable. From the beautiful disk-like structures we call spiral galaxies, to huge spherical blobs dubbed elliptical galaxies, to strange smears of all shapes and dimensions called irregular galaxies. Due to modern telescopes, including the aptly named Hubble Space Telescope and many more, we know today that there are billions of galaxies in the cosmos and they are spread out uniformly in every direction. We also know that they are almost all racing away from us and that the farther away a galaxy is, the faster it is receding.
Another important observation – that the universe is made up of the same stuff everywhere – was made when we analyzed the light from these distant collections of stars. A technique called spectroscopy is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Close examination of the rainbow bands of colors that are produced when white light is shined through a prism shows dark lines within those bands. Called Fraunhofer lines, after the German physicist Joseph Ritter von Fraunhofer who discovered them, it turns out, if you burn an element, let’s say oxygen, and analyze the light coming from that fire, you will see a very distinctive pattern of these dark lines. If you burn nitrogen you will get a different pattern. Using this method, you can determine what a celestial object is made from even from billions of light years away. All the stars we have measured, millions of them now, are made from the same exact elements we find here on Earth. The universe seems to be one thing made of the same stuff and having a single origin.
Of course these are just the glowy bits of the cosmos – the stuff we can see with our telescopes. In the 1970s American astronomer Vera Rubin noticed that the galaxies she was studying were spinning way too fast to hold together. At the rates she observed, galaxies should fly apart throwing their stars into inter-galactic space. Although dark matter had been speculated about as early as 1884 by Lord Kelvin, Vera’s observation was the first evidence of this strange substance which we still haven’t directly detected today. If you add a halo of dark matter to your galactic model adding mass to the system, the spin rates make perfect sense. We now think that the cosmos is permeated with this stuff and that it makes up about 85% of the matter in the universe.
Well, this is starting to get a little long and I think I covered at least the stuff that makes up a galaxy. There is a lot more to learn, so get online and do a little research for yourself. Let me know if I left anything out of this description because I want the stage properly set for the next and final level we will explore: the large scale structure of the universe. But that is a story for another day.
Till next time!
Chris Fellows, Serenity Mobile Observatory
Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)