What is our place in the Milky Way? And our place in the Universe?
In ancient times, many people had the idea our planet Earth to be at the centre of the Universe,
as stated by Aristotle and Ptolomeus in their ptolemaic – aristotelic concept of universe:
according to this model, Earth is at the center of the universe and all the other celestial
bodies orbit around it. Today lots of people think the same. But is this really the case?
To answer this question, let’s try to to a travel in the universe, through space and time; we will
start our travel from our planet to reach, in the end, the extreme boundaries of the universe.
During the 1600s, Galileo Galilei, the famous Italian astronomer, was one of the first people,
during modern age, to have some doubts about the geocentric model of universe:
thanks to telescopic observations, he was able to demonstrate our Earth is not at the rotation
centre of planets and the Sun, but really it is the Sun itself. Moreover, observing planet
Jupiter, he discovered that the giant planet is the rotation center for its moons. So,
Galileo became aware that the center of the Solar System was the Sun, not the Earth!
The Solar System is made by a star, the Sun, eight planets and different types of minor celestial
bodies, like comets, asteroids and dwarf planets. Well, the Earth isn’t at the center of the Solar
System, maybe is the closest planet to our Sun? No it isn’t, because it is only the third planet
from the Sun: the closest planet to our star is Mercury, followed by Venus and then Earth. The
Earth moves around the Sun, our star, just like all the other celestial bodies in the Solar System
do: this implies that the Sun, and not our planet, is the center of rotation of the Solar System!
The Earth takes a year, 365 days, to travel its orbit, and its average distance from the Sun is
150 million kilometers, which is the measure unit of distances in the Solar System known
as the astronomical unit and abbreviated AU. Why do we talk about average distance?
Because the orbit traveled by the Earth around the Sun is not circular but elliptical, and this
means that there will be an aphelion (i.e. the point of the Earth's orbit farthest from the Sun,
just over 1 AU away from it) and a perihelion (the point of Earth's orbit closest to the Sun,
just under 1 AU). An alternative way to define the astronomical unit passes through the light time,
in particular we can say that the average distance Earth - Sun is equal to about 8 light minutes:
this means that sunlight takes 8 minutes to arrive on Earth, so that the sunlight we see
at a certain moment is not that of that moment but it is the sunlight which left from the Sun
8 minutes earlier! In other words: if the sun went out for example at 2.30 pm, we would only notice
it at 2.38 pm! Or again: if you could travel aboard the Star Wars Millennium Falcon it would
take you only 8 minutes to travel from the Sun to the Earth (when in reality it takes a few years).
To give a more concrete idea of the dimensions of the Solar System: if the Sun were a sphere
with a diameter of 14 cm, Pluto would be at 700 m from the Sun, like seven regular soccer fields!
The nearest celestial body to Earth is the Moon, our satellite: to reach it you should take
three days off! It’s the same time taken by Apollo astronauts to cover the distance of nearly
400 thousand kilometers that separate Moon and Earth. But if you had Star Trek Enterprise,
and travel at maximum curvature, you would only take less than 2 seconds to reach the Moon!
Now let’s begin to move towards the outer Solar System: at a distance of 2 – 3 AU from the Sun,
between the orbits of Mars and Jupiter, we would meet the Asteroids Main Belt.
Asteroids are those small rocky bodies, whose dimensions span from a few kilometers to a few
hundred kilometers. They are so small that they aren’t visible through unaided eye, but only
through telescopes. We could reach asteroids in a few years, with a traditional spaceship or probe.
Let’s move on: we would need around 10 years and 40 AU to fly-by Pluto,
one of the outermost bodies of Solar System. Try to imagine it in your mind:
if you were on Pluto’s surface and had a look to our planet, you would only see a pale blue dot,
that is the same sight captured in 1990 by Voyager 1 NASA probe, while it was leaving Neptune behind.
Around Earth, only the dark vastness of the universe, barely illuminated by star light.
Now we have left behind our Solar System, and we are going to enter the interstellar space.
To get Proxima Centauri, the closest star to Earth, we have to cover a bit more than
4 light years. We remember a light year is the distance covered by light in a year in the vacuum
and is equal to about 10000 billion kilometers. Distances begin to become big, since Millennium
Falcon would take more than 4 years, travelling at light speed, to reach Proxima Centauri!
And the Sun? Okay, it's at the center of the Solar System,
but is it also at the center of the Milky Way? If you continue to travel onboard your beautiful
spaceship you will discover it. But it’s a very very long trip, so you will need much patience!
The Milky Way is our galaxy: it is an immense group of stars held together by the force of
gravity; it has the shape of a flattened disk, with a bright core in the center, where the star
density is very high, and spiral arms around it. Around the core there is a spheroidal halo,
which extends outside the galactic planet. At this point it could be useful to introduce the parsec,
equivalent to 3,26 light years, to measure the dimensions of our galaxy. So, our Milky way
has a diameter of around 30000 parsec, which are equivalent to 100000 light years. So, if we were
back on the Millennium Falcon again and had to cross the Milky Way from one side to the other
traveling at the speed of light, the fastest thing there is, it would still take us 100000 years to
cross it all ... or to put it in other way, if you were to send a Whatsapp message to your friends
on the other side of the Galaxy to decide what to do on the weekend, your message would take
about 100000 years to arrive; assuming that your friends respond immediately to the message, the
response would take another 100000 years to get back to you! Keeping the same analogy seen before,
with the Sun represented as a 14 cm diameter sphere, our Milky Way would have a dimension of
100 million kilometers, that is equivalent to make 2497 turns of the Earth along the equator!
The galactic halo has a diameter of the order of 30 kiloparsec,
and is populated by old stars, that are stars with an age double that of the Sun. In the
spiral arms instead there are younger stars, like the Sun, located within the arm of Orion,
a secondary arm of the Milky Way between the arms of Perseus and Sagittarius.
Our star, the Sun, is only one of the extimated 200 billion stars in the Galaxy!
Galaxy with capital initial letter because it is the Galaxy par excellence, our galaxy:
in fact its name derives from the Greek "galaxias", "the way of milk",
referring to an ancient Greek myth for which the Milky Way would represent nothing other
than the milk sprinkled out of the breast of the goddess Hera while nursed little Hercules.
If it happened to you to find yourself at the mountainside during a clear summer night,
looking up at the sky you will certainly have noticed that whitish, milky band that crosses
the whole sky: it is the Milky Way. Since the sun is inside one of the spiral arms of the Milky Way,
from the Earth we see the Galaxy as a streak precisely because we are looking at the galactic
plane edge-on and from inside it! It’s like to try to observe all our condominium from our flat:
we will only see just our neighbour’s flat, not the entire condominium because we are in it.
But is the Sun at the center of the Galaxy? No: its distance from the galactic center is about
8000 parsec, or, if you prefer, 26000 light years. Our star takes around 250 million years to go
along its orbit around the galactic center: this means that since its formation it only
made around 25 orbits around the center of our Milky way, traveling at 215 km/s!
And it is good that the Sun is not in the center of the Galaxy, because in it there
is a supermassive black hole: Sagittarius A *, a monster with a mass equal to about 4.3 million
times the solar mass and an immense gravitational field, such that even light is not able to escape!
This huge black hole is located in the direction of the constellation of Sagittarius,
a constellation observable in summer for the inhabitants of the northern hemisphere and in
winter for those in southern hemisphere. Since a black hole does not emit light in the visible band
(the same we see with our eyes), it is not visible directly through a telescope,
but can be detected through indirect methods: 1) Radiation emitted by the accretion
disk around the black hole: a black hole is surrounded by an accretion disk made of matter
that orbits spiraling around it; this matter sooner or later falls into the black hole.
Before ending up in it, however, it is heated to very high temperatures, of the order of million
degrees, producing electromagnetic waves of very high energy, such as X-rays and gamma rays.
2) Study of the orbits of the stars closest to the black hole:
studying the movement of the stars in the area of the galactic center,
it was noted that they seemed to rotate around "nothing", because apparently there was no visible
celestial body in that area. Studies conducted by research groups led by astrophysicists Andrea Ghez
and Reinhard Genzel (who won the 2020 Nobel Prize in physics together with Roger Penrose)
over the course of many years have shown the existence of a celestial compact body at the
center of our galaxy, right in the area where the black hole Sagittarius A * is located.
And what about the Milky Way? Is it at the centre of the universe? Even in this case, the answer is
negative: the Milky Way belongs to the Local Group of galaxies, with a dimension of 10 million light
years or 3,1 Mpc (Megaparsec). We remember that 1 Mpc is equal to 1 million pc, and that 1 pc equals
to 3,26 light years. The Local Group includes more than 70 galaxies, most of them of small size. The
center of mass of the Local Group is between the Milky Way and the Andromeda Galaxy M 31. And the
Local Group, together with the Eridanus Cluster, Fornax Cluster and Virgo Cluster of galaxies,
belong to the Local Supercluster, with a diameter of around 110 million light years.
It is thought that our universe contains around 200 billion galaxies,
everyone with 200 billion stars. Do you imagine that, with satellites,
space probes and telescopes, we have explored so far only a very little percentage of the Universe?
In conclusion: our Earth is not at the center of the Galaxy, neither of the universe, but only a
small planet around a small star which orbits in a peripherical zone of a spiral galaxy
similar to many others. Citing Carl Sagan, is only “a mote of dust suspended in a sunbeam.”