Where exactly are we located in the milky way?
If you were in an unfamiliar city, and they showed you a map of that city with no other
directions, would you be able to indicate your location?
The answer can only be NO, unless you owned a lot of other information, such as the street
name or the proximity of particular, immediately recognizable monuments or buildings.
We encountered the same difficulty when, decades ago, we inhabitants of planet Earth set out
to figure out what the position of our solar system is within the framework of our host
galaxy: the Milky Way.
You know those cute little cartoons with a galaxy drawn, an arrow pointing to a spot
... and the words, "You are here"?
Here--since the beginning of the last century, astronomers have been working hard to succeed
in doing something like that.
Putting 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 specify our planet's location in a galaxy of hundreds
of billions of stars is like trying to guess your location in a forest while tied to one
of the trees.
And given our historical propensity to always consider ourselves at the center of everything,
figuring out where we are can become even more challenging.
In 1584, the Italian philosopher Giordano Bruno, in his
book "About infinity, universe, and worlds," hypothesized that stars were like other suns
and that planets, probably even Earth-like ones, might orbit around them.
Although initially branded as heretical, the hypothesis gained credibility over the following
centuries and reached general acceptance in the astronomical community in the 18th century.
After all, it was not until 1838 that the distance to a star was first measured.
The honor fell to 61 Cygni, which German astronomer Friedrich Bessel calculated to be about 10
light-years away.
And until a century ago, we did not even know for sure that other galaxies existed outside
our own...
Before the advent of the telescope, in fact, only the existence of a bright spot in the
direction of the constellation Andromeda was known; visible without difficulty to the naked
eye, but whose real nature was completely unknown....
so much so that until not too many years ago it was still called the "Great Nebula of Andromeda."
Indeed, with early instruments, a large number of vaguely diffuse objects were identified,
which were cataloged simply as "nebulae."
Which could include objects actually composed only of gas and dust belonging to our own
Milky Way, but also what we now know to be very distant galaxies.
To astronomers, they were all nebulae, precisely, although some were presented with a strange
vortex structure...
And after all, those were times when it was thought that everything that could be observed
in the sky was part of the Milky Way.
Our galaxy was basically the entire universe!
In 1845, Irishman William Parsons, better known as Lord Rosse, built a telescope large
enough to enable him to distinguish individual stars in some of these nebulae, lending credence
to the hypothesis of German philosopher Immanuel Kant, who believed that some nebulae were
actually galaxies outside the Milky Way.
Despite this, there was still debate in the early 20th century 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 controversy culminated in the famous Great Debate 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 that the solar system
was on 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 in the center of the
Milky Way.
They were both a little right and a little wrong, because we now know that spiral nebulae
are galaxies in their own right and that the Sun is, if not on the periphery, then in an
intermediate position.
Definitive confirmation came in those same years from Edwin Hubble, who using so-called
"standard candles," i.e., objects such as novae, supernovae, and Cepheid variables,
which reach a well-known absolute brightness at their maximum, was able to derive the first
crude estimate of the real distance of the "Andromeda Nebula": 900,000 light-years!
A much lower estimate than true, since the distance we know today is more than 2.5 million
light-years, but still more than enough to prove the existence of galaxies far beyond
our own.
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With conclusive proof of the existence of at least one other galaxy, a question then
became more pressing: if we are in just one galaxy like many, with a well-defined shape
and boundaries ... 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.
The last time the Solar System was in this position in the Milky Way, there were still
dinosaurs on Earth.
Next time, who knows?
Humanity might be extinct or it might have evolved into something else.