Our galaxy

 

Most intersting facts about our galaxy.

 

 

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.

Galileo

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).

Solar System

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

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

The Black Hole

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.”

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