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How are distances in the Universe measured ?
First, a few figures.
The Moon is approximately 400 000 km away from us
and the Sun is approximately 150 million km away.
The Solar System expands beyond 10 billions km away from the Sun.
Beyond the Solar System,
we no longer use kilometres to estimate distances,
but the light years.
A light year is the distance covered
by the light in one year.
The speed of light being about 300 000km/s,
the Moon is at a little more than a light second away,
the Sun is at a little more than 8 light minutes away,
and the edge of the Solar System are at 4 light days away.
Then, the closest star is at more than 4 light years away.
The Milky Way, our Galaxy,
(the group of stars which we are part of)
has a 100 000 light years diameter,
and the next big galaxy is at 2.9 million light years away.
The most distant galaxies observed by the telescopes
are at more than 13 billions light years away from the Earth.
By the way,
how do we know that?
How the distance of such distant objects can be measured?
For really close objects, no problem,
direct measures can be done.
For instance,
the distance Earth-Moon has been measured
with a laser telemeter,
working in a similar way than the one used by surveyors.
And for the close stars,
we use the parallax method:
when an object is close,
the viewing angle changes as we move.
It is the same for the stars:
when the Earth moves along its orbit,
the position of close stars in the sky
changes in comparison to distant stars (far more numerous).
Then, some formulae of trigonometry just have to be applied
to find the distance.
But beyond 200 000 light years,
the angular differences become too small
and this method can no longer be used.
A luminous point seems less and less luminous
as we move away from it.
Actually,
the luminosity from a source of light
decreases along with the square of the distance.
For example,
a 60-watt bulb located 10 metres away
seems a hundred times less luminous
than the same bulb 1 metre away.
So the visible luminosity of a bulb can be used
to calculate the distance.
The problem is that we need to know the bulb power:
a 6000-watt bulb 10 metres away
will have the same visible luminosity
than a 60-watt bulb 1 metre away.
Obviously,
we do not know the power of a star that we are observing
(for a star, we talk about absolute luminosity).
Fortunately,
there are the Cepheids:
they are stars which luminosity periodically varies,
and which period is related to their power.
When we observe the Cepheids,
and we measure the visible luminosity,
we can therefore deduct their distance,
and so, the distance of galaxies to which they belong.
Once we know the distance of the galaxy,
we can easily deduct its size
from the direct observation with a telescope
(still the trigonometry).
This method works
as long as the telescopes are powerful enough
to see the Cepheids,
i.e. near 100 million of light years.
It is the case for the Hubble space telescope for instance.
By the way,
did you know that this telescope
is named after the astronomer Edwin Hubble?
who has also discovered a law
that will allow us to measure the distance
of galaxies yet more distant?
The Hubble's law says that
galaxies move away from each other
with a speed proportional to their distance.
This is due to the expansion of the Universe.
The farther a galaxy,
the higher the speed at which it is moving away.
When a galaxy is really distant,
the speed at which it is moving away
is so high that we might neglect the disturbances
due to the proper motions.
So the speed just has to be measured
so that the distance can be deducted.
And a speed is easy to measure.
We do exactly as roadside radars:
the light colour emitted by those stars
is shifted because of the speed.
Purple tends to become more blue,
blue more green,
green more yellow,
yellow more orange
and orange more red.
While measuring this shift towards red,
we deduct the speed of the galaxy,
and then the distance.
Production: Unisciel/ University of Lille 1
Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin
Editing/Special effects: Benoît Leleu
Music: Sébastien Ride, « Thunder Chacha » (SR Music)
Presentation: Maxime and Nina Beaugeois