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| The Milky Way Galaxy |
Introduction
If you were in an unfamiliar city, and they showed you a map of the city with no other directions, would you be able to pinpoint your location? The answer can simply be no, unless you own lots of other information like street names or special, instantly recognizable monuments or buildings. We faced the same difficulty when, decades ago, we Earthlings set out to determine the position of our solar system within the framework of our host galaxy: the Milky Way.
You know those cute little cartoons, where a galaxy is drawn, an arrow points to a space... and the words, you're here? Here--since the turn of the last century, astronomers have been working hard to succeed in doing just that. Place an arrow on the image of the Milky Way to answer the question, What is our place in the universe? Someone has said that trying to pinpoint the position of our planet in a galaxy of hundreds of billions of stars is like trying to guess your position in a forest while tied to a tree.
And given our historical tendency to always consider ourselves at the center of everything, figuring out where we belong can become even more challenging. In 1584, the Italian philosopher Giordano Bruno, in his book On the Infinite, the Universe and the World, hypothesized that stars were like other suns and that planets, perhaps even Earth, could orbit them.
Although initially characterized as heretical, the hypothesis gained credibility in later centuries and gained general acceptance among the astronomical community in the 18th century. After all, it wasn't until 1838 that the distance to a star was first measured. That honor goes to 61 Signi, which German astronomer Friedrich Bessel calculated to be about 10 light-years away. And until a century ago, we didn't know for sure that other galaxies existed outside our own...
Before the advent of the telescope, in fact, a bright spot toward the star Andromeda was known to exist; Visible without difficulty to the naked eye, but whose true nature was completely unknown....so much so that many years ago it was still called the Great Nebula of Andromeda. In fact, with early instruments, a large number of faintly diffuse objects were detected, which were simply cataloged as nebulae. Which may actually include objects made up of gas and dust in our own Milky Way, but which we now know to be very distant galaxies.
Nebula
To astronomers, they were all nebulae, although some presented with a strange vortex structure... and after all, there were times when it was thought that everything visible in the sky was part of the Milky Way.
Our galaxy was basically the entire universe! In 1845, the Irishman William Parsons, better known as Lord Ross, built a telescope that enabled him to distinguish some of the individual stars in these nebulae, giving credence to the hypothesis of the German philosopher Immanuel Kant, who believed that some of the nebulae were actually galaxies. Outside the Milky Way. Despite this, in the early 20th century there was still debate as to whether all observed stars and nebulae belonged to our galaxy or whether certain nebulae were island universes, as Kant had already suggested.
The debate culminated in the famous Great Controversy that took place on April 26, 1920 at the Smithsonian Museum of Natural History in Washington, DC: a bitter public confrontation between astronomers Harlow Shapley and Heber Curtis. The former argued that the universe coincided with the Milky Way and the Solar System was at the periphery of our galaxy, while the latter argued that the spiral-shaped nebulae were independent galaxies, but also that the Solar System was right at the center of the Milky Way.
They were both partly right and partly wrong, as we now know that spiral nebulae are their own galaxies and that the Sun is in an intermediate position, if not in the periphery. Definitive confirmation came in those same years from Edwin Hubble, who, using the so-called standard candle, was able to achieve the first, objects such as novae, supernovae and Cepheid variables, which reached a well-known absolute luminosity at their maximum.
Crude estimate of actual distance to the Andromeda Nebula: 900,000 light years! A much lower estimate than the truth, since the distance we know today is more than 2.5 million light years, but still enough to prove the existence of galaxies beyond our own.
With conclusive proof of the existence of at least one other galaxy, one question then becomes more pressing: if we are in a galaxy like many, with a definite shape and boundary...
Where then is our solar system within it?
Knowing the approximate shape of the Milky Way would already be a first step toward solving the problem, but how to accomplish this by being able to observe it only from within? Oddly enough, an idea of its shape can be obtained simply by looking at the sky on a clear, very dark summer night. We will then be able to witness one of the most exciting of all the spectacles offered by the natural world: a misty arc of light stretching from one side of the horizon to the other--an irregular band of light about 15 degrees wide, marked by more or less pronounced condensations of light.
It seems impossible, but even such a trivial observation would be able to suggest to us that our galaxy must necessarily be in the shape of a very thin disk and that we are located somewhere within that disk. It can not be so... For example, if the galaxy were spherical, we would see its glow all over the sky, not just collected in a narrow band. And if the Earth were far above or below the plane of the disk, we would not see it split the sky in half: the glow of the Milky Way would in fact be brighter on one side of the sky than on the other! OK, that's something... but how to quantify it more precisely?
The English astronomer William Herschel, the discoverer of Uranus, was the first to attempt to describe the shape of the Milky Way and the position of the Sun within it. In 1785 he made a painstaking count of the number of stars in six hundred different regions of the Northern Hemisphere, noting that the stellar density increased as one approached a certain area of the sky, coinciding with the center of the Milky Way, in the constellation Sagittarius.
His son John then repeated the measurements in the southern hemisphere and came to the same conclusions. Based on these data Herschel senior was able to draw what must be considered the first true, if very approximate, depiction of the Milky Way in its proper form. The only flaw is that of considering the Sun at the center of everything. A first formidable refutation to the centrality of our solar system in the galaxy came from Harlow Shapley himself, through his analysis of the observed distribution in the sky of spherical clusters of stars known as globular clusters.
There are about 150 known globular clusters in our galaxy and probably as many that have not yet been discovered. Through the observation of the Cepheid variables in them, Shapley calculated that the distance of these clusters from the Sun was between 20,000 and 200,000 light-years, is hypothesized that they moved in a sort of shell around the center of the galaxy, like gnats around the light of a street lamp.
That, coupled with the fact that the swarm of globulars observed from Earth did not appear evenly distributed across the sky, as it would be if the Sun and Earth were at the center of the galaxy, led Shapley to infer that the solar system was 50,000 light-years away from the center. This overestimated value was later refined over time by other observational methods, but it is the cornerstone of every subsequent study of the solar system's position. From the fact that the galaxy has the shape of a disk, for example, astronomers inferred that it must rotate on itself.
Since the galaxy is not a solid object but is composed of a large number of individual stars, it could not be expected to rotate rigidly, as a wheel would. Stars located near the gravitational center of the disk must rotate around it more rapidly than those further away, just as planets closer to the sun travel their orbits with greater speed.
Starting with the measurement of the relative motion of stars, it became possible to calculate the speed of rotation around the center of the galaxy, and from this their true distance from the galactic center. It was found that the sun had to orbit around the galactic center at a speed of 250 kilometers per second, making a complete revolution around in about 250 million years. From the orbits described by the examined stars, it was also possible to determine more accurately the position of the center around which they revolved.
Thus it was possible to confirm that the center of the galaxy lies in the direction of Sagittarius, as Shapley had concluded, but at a distance of only 35,000 light-years. In that new model, which has now been partially corrected, the thickness of the disk at the center was about 20 thousand light-years and gradually decreased toward the periphery; so that at the position where our Sun must have been, the thickness of the disk must have been about 3000 light-years.
But these were only rough estimates. Over the past 75 years, astronomers have obviously improved the information picture, using a variety of techniques based on radio, optical, infrared, and even X-ray astronomy. The advent of space telescopes has then helped to provide an almost definitive answer regarding our initial question: where are we located in our island universe?
Keeping in mind the difficulty in estimating the shape of an object from its interior, today we are nonetheless certain that the Milky Way is a barred spiral galaxy, that is, having a central elongated core from which two arms rich in stars, dust, and gas branch off at opposite ends. For a long time, the Milky Way was thought to have four spiral arms, but more recent investigations have established that there are only two called Scutum-Centaurus and Carina-Sagittarius, named after the constellations in which we see them projected.
The whole ensemble looks like a disk of matter 91,000 light-years in diameter, and 2-3000 light-years thick. Recent simulations suggest, however, that a halo of dark matter, also containing some visible stars, may extend to a diameter of nearly two million light-years. What about the population of stars that inhabit it?
Here the question is more delicate, as those of very small mass are many, and difficult to estimate. Based on this assumption, the most widely accepted hypothesis is that the Milky Way may contain up to 400 billion stars. The Solar System - and here we finally come to the data we are most interested in - is within about 25,600 light-years of the Galactic Center; which means that we are almost exactly halfway between the center and the edge of the disk.
Compared to the Galactic plane we are currently about 40 light-years above it. And we specify currently because it appears that throughout its orbit the Sun oscillates up and down relative to the Galactic plane approximately every 80 million years. Oscillations that often coincide with periods of large mass extinctions that have occurred on Earth, probably due to the increased risk of impact with foreign celestial bodies You may be surprised to learn that the Sun takes 250 million years to complete one rotation around the Milky Way-a period that is known as the Galactic Year or Cosmic Year. Thus, the Sun is thought to have completed about 20-25 complete orbits during its lifetime.
The orbital speed of the Solar System relative to the Galactic center was recently calculated at 227 km/s; at this speed, it takes the Solar System about 1400 years to make a shift equal to one light-year. Next time, who knows? Humanity may become extinct or it may evolve into something else.