If you are a stargazer, you have to love early autumn.
The summer Milky Way rises early. By the time it’s fully dark, you see it stretched across the sky if you are observing from a dark, rural location. My first view of it left me hungering for more. That’s a hunger that has not been satisfied in the half a century since then. Yes, it’s that good.
The Milky Way is, of course, your galaxy. You’d think we’d know a lot about it already and we do, I suppose, but not as much as you might think.
The difficulty is that we are in it. Its very proximity, size, and density made it difficult to estimate its structure.
It’s easy enough to tell that it’s disk shaped and that we are near the outside of the disk. That’s what makes it look like a streak of light across the sky.
It seems obvious enough in retrospect. However, an amazing fact about astronomy is how little we knew about the universe until relatively recently. Humans have been staring at the night sky since our dim beginnings, yet we only have had an inkling of the size and grandeur of the universe since the 1920s.
Early in the 20th century, some astronomers assumed that the Milky Way constituted the whole universe.
Soon thereafter, astronomers discovered the most important fact about our universe, then or since. A drumroll, please …
The universe is lumpy. It is composed of galaxies, pinwheel- and egg-shaped conglomerations of hundreds of billions of stars. The Milky Way is simply one of the lumps.
Even the lumps are collected together in larger clumps. Our own galaxy is part of a group of perhaps tens of thousands of galaxies called the Virgo Supercluster. At 100 million light-years wide, the Virgo cluster is huge by any standard. (A light-year is 6 trillion miles, Earthlings.)
Within the Virgo cluster is a smaller clump, a tiny island of 40 or so galaxies called the Local Group, which might be called our cosmic neighborhood.
The best and the brightest of the Local Group is not our home galaxy, the Milky Way. Sorry.
The Andromeda Galaxy takes the honors at 150,000 light-years wide. That’s 50% wider than our Milky Way, with more than twice the stellar material. Our Milky Way has perhaps 300 billion stars. Andromeda tops the charts at one billion.
The Andromeda Galaxy is bright enough to be visible to the unaided eye from dark, rural skies even though it is 2.5 million light-years away, er, 2.4 million light-years away. Well, actually, it’s both.
The pinwheel shape of the galaxy is tilted at about a 75-degree angle with respect to Earth. Because we see it practically edge-on, the far edge is almost 100,000 light-years farther away than the close edge. We are seeing the trailing edge 100,000 years later than the closer one, so our view of the far edge is 100,000 years older.
How big is Andromeda? It’s so big that we can’t see the whole galaxy at the same time.
We don’t have that problem with another nearby member of the Local Group. M33, sometimes called the Pinwheel Galaxy, in the constellation Triangulum (the Triangle) is a bit farther away at 2.6 million light-years.
Because we see it face-on, its beautiful spiral shape shows up in medium-sized amateur telescopes. Look for a faint, fat S shape. Although it’s not visible to the unaided eye, binoculars will show it as a small fuzzy patch to the west of the most acute tip of the triangle.
M33 is considerably smaller than its larger cousins, the Andromeda Galaxy and the Milky Way. At only 50,000 or so light-years wide, it contains only enough stellar material to make up 8 billion suns. (Compare that with the 300 billion or more stars that make up the Milky Way.)
Like all spiral galaxies, M33 is slowly turning. Astronomers estimate that stars out on its edge take up to 200 million years to make one turn around the galactic periphery.
The rest of the galaxies in our Local Group are mostly tiny, so-called “dwarf galaxies,” which hover around the larger ones like bees around celestial hives. They are mostly invisible except in powerful telescopes.
Beyond our Local Group are the thousands of galaxies of our Virgo Supercluster. Beyond the Virgo cluster are hundreds of thousands, perhaps millions, of other superclusters, which are unimaginable, brain-melting distances away.
As you gaze at the faint fuzzy patch called M33, you are still sticking only a single toe into the water as you stand at the edge of an immeasurably vast cosmic ocean.
We know as much as we do about our Milky Way galaxy by looking at those other lumps. Astronomers actually found out as much about the overall structure and functioning of our galaxy by looking at other galaxies than by observing our own.
You can make some of those observations yourself in a typical backyard telescope. An example is M51 in Ursa Major, the Big Bear. The galaxy is a classic spiral, a child’s pinwheel of a hundred billion stars.
Of course, the big question when you see typical characteristics in other galaxies is whether they are really present in your own. That seems to be the case, and often in surprising ways.
A larger backyard telescope reveals a smaller smudge near M51. It appears to be about the same distance as the larger galaxy. An even larger telescope reveals a bridge of light between the two. M51’s more powerful gravity is apparently sucking the stars right out of the satellite galaxy.
In fact, such galactic mergers seem fairly common. Larger galaxies like the Milky Way and M51 move through intergalactic space, acquire dwarf galaxies as satellites, and then often gravitationally suck them up.
Given the large distances between galaxies these days, such galactic interactions seem unlikely. However, one implication of the Big Bang theory is that galaxies were a lot closer to each other 10 billion years ago.
Big galaxies were bound to collect smaller brethren. Gravity has kept them together all this time.
Our own satellite galaxies, the Magellanic Clouds, are twisted and distorted — bent out of shape — by the intense gravitational power of the Milky Way.
But has our own Milky Way absorbed other galaxies? The answer turns out to be a big yes!
The most stunning evidence that our own galaxy is the product of such a galactic merger is right there in front of your nose. The brightest star in the nighttime sky right now is Arcturus in the constellation Bootes, the Herdsman. You’ll see it setting in the west just after dark.
One of the reasons our galaxy is shaped like a spiral is that its stars are rotating around the center of the galaxy. Our own star, the sun makes a spin in 100 million years or so. But the orbit of Arcturus is weird — tilted and elongated. Some astronomers speculate that its odd orbit is a result of it being a star from another galaxy that was absorbed by our own a very long time ago.
Astronomers have detected two small “dwarf” galaxies that are being absorbed into the Milky Way as you read these lines.
In fact, galactic merger features in our future in a big way. Watch the skies, my friends! Keep looking at M31, that naked-eye galaxy in the constellation Andromeda. It will continue to get bigger because it is screaming closer to us. In a scant four billion years, the 2.5 million light-years between us will evaporate and the two galaxies will merge irrevocably and permanently.
Because of the enormous distances between the stars, it is unlikely that any of the stars of the Milky Way will bump into any of the stars of M31. However, their clouds of unused hydrogen will compress our clouds of unused hydrogen.
The resulting rounds of star births will light up both galaxies for a long time after they pass completely through each other and go for a time on their merry ways, somewhat the better for the experience.
But their velocities will have been considerably slowed by the pass-through. Their mutual gravitational will pull them together and through each other again, and perhaps a second time. Finally, their central black holes will merge, and they will become a single galaxy, which some astronomers have dubbed Milkdromeda and others Milkomeda.
M33 will go into orbit around the new galaxy, Milk-whatever, for a time until it too is absorbed.
If you’re worried about what will happen to your blessed planet, take heart. By the time the two galaxies are merging, the sun will have swollen into a red giant star and heated the surface of the planet to about 6,000 degrees.
Astronomers predict that the enormous distances between the stars mean that no star in the Andromeda galaxy will come close enough to rip the sun’s planets from their orbits.
However, the sun stands a very good chance of being ejected out of the Milky Way into intergalactic space. It and its planets will become hobos forever separated from their home galaxy. The night sky from Earth will be empty. No stars will light up the night as our sun and its planets get lost in the inky depths of intergalactic space.