Its difficult not to be awed by the stunning beauty of the “grand design” spiral galaxies, so exquisitely detailed in Hubble images. These incomprehensibly immense discs of dust and gas and stars, structured on a sinuous backbone of fluid spiral arms, spin in a continuous whirlpool of stellar birth and death. They scatter the universe in the hundreds of billions, in any and every direction we look, for as far as we can see.
However, before we knew of the true nature of galaxies, the universe was a much smaller place. A mere century ago, our Milky Way galaxy was considered to be the one and only galaxy. All these nebulous swirls in the night sky that were being discovered in ever-greater numbers were assumed to be gas and dust in the process of stellar formation through gravitational collapse. And, most importantly, within the confines our Milky Way galaxy—the entirety of the universe at that time.
However, Edwin Hubble, using the new 100-inch Hooker telescope in 1924, was finally able to resolve individual stars within Andromeda, our closest neighbouring galaxy. Most importantly, he was able to detect Cepheid variable stars, a recently-discovered variable with a known period-luminosity ratio that could be used as a “standard candle” to measure distance. And the distances were immense! Overnight, a fledgling theory of the era (that these spiral nebulae were actually monumentally huge galaxies, just like our own Milky Way, and very far away) was confirmed. Quite suddenly, the universe became incomprehensibly huge and far grander than ever imagined, with “island universes” (the pet name for galaxies at the time) studding the night sky.
Our knowledge of galaxies has grown immensely in the intervening decades. Research has revealed that the strikingly beautiful spiral arms, which are a manifestation of the intense gravity of the supermassive black hole way down at the center of galaxies, are vast star-forming regions. Gas and dust entering these arms slows down and “thickens” up, increasing density and generating prodigious stellar birth.
Stellar formation in M51: (1) Gas and dust entering the spiral arms slows down, creating dark lanes of increased density, triggering prodigious stellar formation. (2) Glowing pink clouds of ionized hydrogen attest to massive new stars still buried within the dark lanes. (3) Countless hot new stars exit the spiral arms.
Messier 51 (left), the stunning face-on spiral galaxy located just off the handle of the Big Dipper, is an excellent display of stellar birth within spiral arms. The spiral arms rotate with the disc of the galaxy, but much slower, usually less than half the speed. Eventually, all the matter of the disc that’s in orbit around the galactic center catches up to the spiral arms. There, just like a traffic jam on a busy freeway (mercifully, someplace far south of here) everything slows down and bunches up, creating dark lanes of dense gas and dust, where even light cannot escape. It is here, within these dense clouds, that millions of new stars of every size are being born through gravitational collapse.
The largest of these new stars first reveal themselves while still deep within the cocooning gas and dust from which they were born. Massive stars initially emit tremendously powerful radiation, particularly in ultraviolet, which begins to blast the surrounding clouds away, sculpting a cavity within. The intensity of this radiation is powerful enough to ionize hydrogen by stripping the electron off the hydrogen atom and giving the remaining proton a positive charge. In this state, hydrogen has the capacity to emit its own light, on the red end of the spectrum. All the beautiful pinkish regions in the image represent vast clouds of glowing hydrogen, ionized by massive stars still buried deep within the dark lanes. Most of those regions in the image are hundreds of light years across and contain many stars.
Eventually, all the brand new stars emerge along the trailing edge of the spiral arms, breathtakingly obvious in the image of M51. Here, the “traffic jam” of stars and matter begins to speed up and spread out again. Notice how this bright glow only hugs the outside edge of the arms. The stars responsible for this light have very short lives of only a few million years. Considering that it takes a star almost 250 million years to orbit the galactic center once, these massive stars barely make it out of the spiral arms before expending all their nuclear fuel and, in one cataclysmic way or another, turn off. Our healthy, medium-massed Sun, at 4.5 billion years young, has orbited the galactic center almost 20 times.
So much more to tell on the fascinating workings of galaxies, but it will have to wait for another time. For I haven’t yet told you anything of our beautiful “island universe” next door. The Andromeda galaxy, M31 in the Messier catalogue, is, to me, the crowning jewel of said catalogue. It is the largest, and the furthest of all the Messier objects that can be seen with the naked eye. And, at 2.5 million light years (ly), it is staggering to me that we can see it at all!
Andromeda is the big daddy of galaxies amongst our Local Group of about 50 galaxies. At over 200,000 ly in diameter, Andromeda is more than twice the diameter of our Milky Way. It contains close to a trillion stars. And surprise—our neighbour is coming for a visit and we’re on the menu!
When we talk of the universe expanding, we are referring to clusters of galaxies moving away from each other in all directions. Each cluster is a collection of galaxies that are near enough to each other to be gravitationally bound. Within clusters, galaxies can move toward or away from each other. Andromeda and the Milky Way happen to be moving toward each other. However, even at the approach speed of 110 km/sec, it won’t be here for another 4 billion years. And even when our two galaxies finally do collide, because of the enormous distances between the stars, it will be extremely unlikely that any of the trillions of stars will actually collide. So relax. We have the more immediate problems of global warming and world peace on which to focus.
Here is a link to a short, but revealing simulation of this upcoming galactic collision https://www.youtube.com/watch?v=4disyKG7XtUnd . Note the clock in the lower corner, clicking by, I think, faster than a 100 million years per second! Also notice how the two galaxies initially miss the first time they meet, causing their spiral arms to tidally splay far out from their centers. The end result, when the two supermassive black holes finally merge into one, is a giant elliptical galaxy, without any spiral arms, and therefore very little new star formation.
We are nearing the end of the season for viewing Andromeda, but it can still be found in our northwest sky. The best way to flush out Andromeda now is by starting from Cassiopeia, the W-shaped constellation. Downloading the March sky chart from skymaps.com will assist you in finding Cassiopeia. Notice in the chart here, how the right “V” (well, the “U” to be precise) of the “W” points to the vicinity of the Andromeda galaxy. In fact, it should lead you directly to the star Mirach, in the Andromeda constellation. From there, it’s just a short, two-star hop back to the Andromeda galaxy. It is always good to have your binoculars along when searching Messier objects out. With Andromeda, you’ll be impressed by the clarity and dimensions of its oval shape.
Congratulations! You’ve just found the oldest light that can be seen without optical aid. Those 2.5 million year old photons striking your eyeballs right at that very moment left our neighbouring “island universe” when Australopithecus was strutting his stuff on our humble little planet. Enjoy!
In the realm of the globulars – Ancient companions of the Milky Way
