Star distances: how far away from Earth? Stars have always been considered from a
romantic and poetic perspective. They make humans wonder about their destiny since from prehistory.
As a matter of fact, cavemen understood moon periodic cycles and depicted them in a symbolic
way in the world-wide known Lascaux caverns. Maybe the main reason why stars are so
loved and admired by us is their shining beauty that seems totally unattainable,
it’s seen like something that can’t be reached. Hundreds of tales and myths revolve around
the possibility of getting near these bright and mysterious objects in space.
As an example think about the story of Icarus, who died in his existential flight to the Sun.
Moreover, even the history of society is full of events linked to space missions that will always
be seen like something memorable. Think about the launch of Sputnik in 1957 or the first human
step on the Moon made by Armstrong in 1969, two incredible events that will never be forgotten.
Even the remote possibility of getting a bit closer to stars will always make
us feel amazed and emotionally touched. But how far are stars? Do they really are so
distant from us? And how can we measure such tremendous distances? Are there
any valid techniques? A lot of surprising things can be said about these questions.
Do you want to know more about it? Stick with me and I’ll tell you everything in a moment.
Is it possible to calculate star distances? Definitely yes, but some further explanations
must be clarified. Obviously, we can’t measure such a huge gap with the normal methods we are
used to on earth. We must calculate distance in an indirect way. What does it mean?
We need to measure two different quantities that are related to each other through distance;
then, with a simple equation, we can easily derive how far the star is from us.
Let’s make an example: if we want to measure how many kilometers are there between two cities,
we can drive at a constant velocity and measure directly how much time does the
journey take. After that we can just get information about the distance.
Similarly, physicists utilize the different types of brightness of a star that depend on
distance to know how far the star actually is. Before going on more rigorously on this subject
we must list the major units of measure used to identify the distance of a star.
The first one is astronomical unit, which is exactly the gap that separates Earth from the sun,
corresponding to 1011 meters or 8 minute at the speed of light.
The other one is parsec, nearly around 1016 meters or 3,26 light years, which represents the distance
that causes a change of one degree in the sky over an entire year. This phenomenon is also
known as parallax, but we’ll discuss it later. In order to understand the major techniques
used in astronomical researches, we need to take a little step back and
define some important physical quantities. The first one is absolute luminosity, which
is the radiation power of a light emitting object. Consider it as the measurement of its brightness:
a small lamp has a lower luminosity than a lighthouse. Moreover, we can define another
quantity that derives by the previous one and it is named apparent luminosity.
It is proportional to the inverse of square distance. Shortly, apparent luminosity is
higher when the star is closer and viceversa. A lighthouse is certainly brighter than a lamp,
but if it’s 10 or 20 times more distant the lamp will “seem” having a greater power.
Apparent luminosity is a sort of superficial density of power.
In parallel, physicists defined apparent magnitude which is greater when luminosity is lower and
smaller when luminosity is higher. As a matter of fact, the brightest star visible from Earth,
which is known as Vega, has a conventional value of zero. Obviously, it is dependent
on distance and can be considered as an equivalent of apparent luminosity.
In addiction, another quantity called absolute magnitude has been introduced by scientists:
it measures the magnitude that all stars would have at a distance of 10 parsec.
Similarly to what we did before with velocity and time, astronomers usually utilize absolute
and apparent magnitude to derive distance. Since apparent magnitude can be measured through
telescope observations, it is necessary to find objects that have a well-known absolute magnitude.
These stars or galaxies are also called “standard candles” are they’re
crucial for measuring distances of stars. The first object that is very useful for this
method is a cepheid. Its major characteristic is the emission of electromagnetic waves:
its intensity varies with time according to specific laws. Consequently, these stars are
also known as “variable Cepheids”, which turn to be a crucial feature for the measurement
of star distances. Why is the radiation of Cepheids so important? Because by measuring
the pulsation it is possible to derive the true luminosity (and magnitude) of the star.
This is the main reason why Cepheids are important indicators of cosmic benchmarks for
scaling galactic and extragalactic distances. This robust characteristic
of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying
thousands of variable stars in the Magellanic Clouds. This discovery allows one to know the
distance of a star from the relation between observed brightness and the true luminosity
given by the measurement of the variable emission. How far can we find star distances thanks to this
method? Around 1 kpc, just the closest ones can be detected using Cepheids as standard candles.
As opposed to this technique, there’s even another way of revealing star distances at this scale just
by using geometric properties. Did you know that? Its name is Parallax.
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Parallax is probably the most famous method for measuring star distances. Practically, it is
based on the displacement of the observed object related to a change of position of the observer.
Does it seem difficult? Let's try to make a simple example. Try to position one finger
near your face and observe it while closing the left eye. Now close the right one and watch the
finger with the left. Have you noticed something? The finger has slightly changed position. Why
does that happen? Because we are observing the object from a different point in space.
The same effect can be observed with stars: their coordinates in space
undergo some small changes as the Earth changes positions orbiting around the Sun.
How can we measure distance through this technique? By knowing the change of
position of the observer and measuring the angular displacement of a star in the sky
we can easily derive the distance using some simple trigonometric laws.
The definition of the unit parsec is derivable by this technique:
1 parsec is the distance that is necessary for observing a parallax with one degree of
angular displacement in the sky. Approximately, parallax can give
information of distances lower than 500 parsec. Therefore, there are some different kind of
techniques that are used according to the scale of the length we want to measure.
Firstly, this type of parallax we just described which is totally based on Earth proper motion
is called “trigonometric” and is perfect for deriving distances under the limit of 200 parsec.
Secondly, a new type of parallax, known as “secular”, was introduced to reach the
enormous distance of 500 parsec. Why is it different from the previous one? It exploits
the proper motion of the Sun relatively to the displacement of the star in the sky.
Finally, the “statistic” parallax has been developed in in the range of 500 parsec.
It is based on the hypothesis that a group of stars is almost at the same distance on the Earth,
and focuses on calculating the relative movement of the group
comparatively to that of the observed star. However, if we want to move our research further,
we need to use other methods in order to detect star distances. The principle is the
same of cepheids, exploiting the properties of a standard candle with known true luminosity.
Some great candidates for this role are Supernovae type IA. What are they?
Supernovae are terrible explosions which very high energy, some of the most powerful events in
the entire universe! Where do they originate from? They are caused by the implosion of massive stars.
In a previous video on this channel we have gone through the evolution of different types of stars.
Primarily, the life of a star is based on its mass: if it’s relatively small, like less than
eight times that of our Sun, the star will last more time as the “fuel” that propels energy
(which is the nuclear fusion that happens in the center) will be used at a slower rate.
On the other hand, if a star is very massive its life will be much shorter since it consumes
nuclear energy at a higher rate. A simple comparison can be made with cars: a Ferrari
will finish its fuel before than a little Fiat because its engine is way more powerful.
According to that, a massive star at the end of its life, when the nuclear fusion has stopped
because there’s no more energy available, will implode and then explode with an
enormous amount of energy, called Supernova. However sometimes even smaller stars like our Sun,
which usually turn into white dwarf at the end of their cycle (small and ultra-dense
objects with a relative low temperature), can cause a particular kind of event,
known as Supernova type Ia. These explosions have an absolute known magnitude, and its
apparent luminosity can be easily measured through telescopes of different wavelengths, providing us
with the distance from our Earth. Although they can be used with a wide range of distances,
that can even reach the tremendous values of billions of light-years (almost one giga-parsec),
these types of events are rarely observed. Another important technique for measuring
star distances is the Hubble Law. Discovered by Edwin Hubble in the 1929, it’s one of the
most famous laws of astronomy. It states that the faster a star or a galaxy recedes from us,
the greater its distance. Distance and velocity have a linear relation through a constant, called
Hubble constant (that was initially calculated with a very low precision by the astronomer).
But wait a moment, we have just changed the problem, how can we measure the velocity of a
star? Although it seems very difficult, physicists have elaborated a very good method to reach this
goal. Usually, they analyze the spectrum of an object, which is like a catalogue of the different
wavelengths emitted by the star. If some gas of the stellar or galactic atmosphere is positioned
between our telescope and the star, it will absorb some light at specific lengths depending on the
elements of the atmosphere. These causes some black lines in the spectrum and scientists have
calculated the value of wavelengths of each single element. If a galaxy recedes from us,
the light waves will be partially stretched (due to the famous Doppler effect,
the same that makes the alarm of an ambulance change in frequency if it’s reaching us or
going away) and will have greater wavelengths, translated towards red. This phenomenon is called
redshift. The measurement of the displacement of the absorption lines towards red gives us
the velocity of the stars, and consequently its distance, thanks to Hubble Law. Therefore,
further studies have proven that the redshift is not caused by the movement of the galaxies,
but by the expansion of the universe itself. They are not moving relatively to us, it’s
the spacetime that is stretching and growing the relative distance from Earth and other Galaxies.
In conclusion, we have seen that just little changes in some well-known quantities as the
position of a star in the sky or a color of a light wave can surprisingly give us
such a precious information about the distance of a star or a galaxy. It’s really incredible
how powerful science can be! "This video ends here! Thanks
for watching everyone! Did you find this topic exciting? Are you amazed by
the possibility of measuring indirectly star distances? What is your favourite technique?
Did you know all of them? Is there anything more you want to hear about star distances? Let me know
in the comments below, be sure to subscribe, and I'll see you next time on the channel!"