The Fascinating Andromeda constellation: Facts, myth and location
One of the most meaningful things astronomy teaches us is we can always try to understand
a bit more of the world around us. Think, for example, of a constellation. We
can spot it in the night sky, and we can spend some time admiring it with the people we love.
It is surely romantic stuff, but if you see the world with the eyes of an astronomer,
everything acquires a new, deeper meaning. Astronomers know that a constellation is always
more than just stars connected one to each other by means of imaginary lines.
They know that some exotic objects might be hiding within the constellation, some objects
that we are not able to see by the naked eye. Maybe one of the stars that make up the constellation
is actually a multiple star system, only we can't see it because of the huge distance
between us and them. Or maybe some constellations host a galaxy.
Today we are going to talk about the Andromeda constellation, which actually has both: a
multiple star system and a galaxy hiding within it.
Let's find out!
======================================= The Andromeda
constellation is made up of 16 stars, an it is visible in the northern sky.
In fact, the constellations that you can see in the sky depend on where you are located.
In short terms, different latitudes means different skies.
The constellation of Andromeda is called so after the Andromeda princess.
She was a mythological character in ancient Greeks' culture.
Of course, as you've probably already heard, the Andromeda constellation hosts the Andromeda
Galaxy, the nearest full-fledged galaxy to our own Milky Way.
The Andromeda Galaxy and the Milky Way are actually the two most prominent galaxies in
the so called local group. The Local Group is a galaxy group with a
total diameter of roughly 3 megaparsecs, which correspond to10 million light-years,
and a huge total mass of the order of thousands of billions solar masses.
The local group has a dumbbelll shape,which lobes are given from the Milky way and its
satellites on one hand, and tha Andromeda Galaxy and its satellites in the other.
Perhaps the most interesting – and amazing – thing isabout these two lobes is that
they are moving toward one another with a velocity of 123 km/s. This means that,
sooner or later, they will collide. In particular, the collision between the Andromeda Galaxy
and the Milky Way has already started.
The exact number of galaxies in the Local Group is unknown as some are occluded by the
Milky Way; however, at least 80 members are known, most of which are dwarf galaxies.
Going back to our constellation, why is it so important?
Well, first of all, Andromeda was one of the first constellation ever described.
In fact, the ancient greek astronomer Claudius Ptolemy, in his work “The Almagest”; described
48 constellations. And one of them was Andromeda. The Almagest can be seen as a mathematical
and astronomical treatise, and it talks about stats, planetary paths and assumes a geocentric
model. It is therefore one of the most influential
scientific texts in history, but also a key source of information about ancient Greek
astronomy.
However, such constellations had already been known by Greeks, Babulonins and Egyptians.
Ptolemy's work was important though, because it made the first official description of
constellations. Andromeda is one of the largest constellations
described in the Almagest, and starting from the 48 constellations of Ptolomy, another
40m were added over the centuries. Many of these additions reside in the southern
sky, which was not visible to Ptolemy, who spent most of his life in Egypt.
================================================= Now I want to ask you: how many times did
you happen to look at the stars and try to connect dots between them, to recognize a
figure, a pattern, but failed? Well, this will never happen because now I'm
going to explain to you how to admire Andromeda. First of all, for the best vision you need
to be in the Northern Hemisphere. Thids is because, changing latitude and going towards
south, Andromeda moves lower toward the horizon. In short terms, the farther south you stand,
the lower it is. And, if you live at about 40 degrees south
latitude, I have bad news for you! At those latitudes, Andromeda disappears completely.
So be sure to check your position on Earth before starting looking for Andromeda!
Well, once you set the place, and you are sure you can see the right stars, you need
to understand WHEN. And this all depends on the period of time during the year.
First of all, you should look for it during the night, because during the day you won't
be able to spot it! ….Did you like this joke? ….
Being serious now, there are some months in which you will have very good observations.
As a rule of thumb, if you live in the Northern Hemisphere, Andromeda is best observed from
August to Februayry. Instead, if you live in the southern hemisphere, the best period
is the one that goes from October to December.
From August to September, in the Northern Hemisphere, Andromeda emerges on the north-eastern
horizon at about 10 p.m. local time, then gradually rises overhead. From October to
November, it emerges in the eastern sky at around 8 p.m. From December to January, Andromeda
rises at around 6 p.m. and moves along the northwestern horizon.
Andromeda is not hard to find. It's actually pretty easy, considering that yo can find
it starting from one of the most beautiful and well known constellations of the northern
hemisphere. We are talking about Cassiopeia.
It is a w-shaped constellation, which makes it quite recognizable, and it is named after
the vain queen Cassiopeia, mother of Andromeda and king Cepheus' wife, according to the Greek
Mythology. When her mother bragged that Andromeda was
better-looking than the famously beautiful Nereid sea nymphs, the sea god Poseidon sent
his some sort of pet sea monster, called Cetus, to destroy Cepheus' kingdom.
And the gods said that sacrificing Andromeda could save the country and its people.
So, the loving parents had their daughter chained to a rock by the sea, so that the
sea monster could get her easily. Fortunately, the charming prince Perseus flew past on his
winged horse Pegasus and fell in love with Andromeda's beauty.
The two of them got married, and lived happily ever after.
The Cassiopeia constellation points directly at Andromeda.
You can easily see it, you just need a bit of imagination!
You can see the constellation as a sprawled figure with its arms extended.
The figure's feet point in the direction of the Perseus constellation, right next to
Cassiopeia. The figure's "head", instead, connects the constellation to the neighboring
square of Pegasus.
Well, now you know everything on how to spot Andromeda in the night sky, depeding on the
time of the year you are looking for it, and on you location on earth.
What about the main features of this constellation? What can we say about its stars?
Are they boring, usual stars, or do they have special characteristics?
Well, we can start by saying that three of the Andromeda's stars are brighter than magnitude
3.00, which puts them into the top 100 brightest stars in the sky. This alone could already
their uniqueness, but there's actually a lot more to say.
Let's start with the brightest star. It is Alpheratz, also called Alpha Andromedae.
Alpheratz is essentially Andromeda's head, but it's also part of the neighboring square
of Pegasus. This star is located 97 light-years from
Earth and it is a so called binary star, because it consists in a system of two stars orbiting
a common center of mass. The two components have different masses, and the bigger one
has a unique chemical composition in terms of mercury and manganese.
It is in fact the star with high levels of these two elements.
The smaller star is instead is about 10 times as luminous as the sun and orbits the larger
star every 97 days.
The second brightest star is Mirach, also known as Beta Andromedae.
As it usually happens, stars belonging to the same constellation are at different distances
from the Sun. So it won't be weird for you to know that
beta Andromedae is twice more distant than alpha andromedae.
Its distance from us is about 200 light-years. It is a giant star 1,900 times as bright as
the sun, and three to four times as massive. Within the Andromeda Constellation, Mirach
forms the chained figure's left hip.
The third star, Almach, or Gamma Andromedae, is Andromeda's foot.
It is a multi-star system featuring a central giant orbited by a pair of white dwarfs.
The star's central giant is 2,000 times more luminous than the sun! Can you believe it?
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Anyway, in addition to the visible stars, one can find many intriguing objects within
the Andromeda constellation that are only visible with telescopes or binoculars. Something
that our eyes can't unveil, binoculars and telescopes can show.
For example, as we said, somewhere in the Andromeda constellation it is hidden the Andromeda
Galaxy, the closest full-fledged galaxy to the Milky Way.
The galaxy, is located some 2.5 million light-years away from Earth, and it is barely visible
to the naked eye but can be found with binoculars.
If you have binoculars and you want to find it, you can do it by looking fora misty patch
on the edge of the Andromeda Constellation to the right of the sprawled figure's right hip,
which is right next to Mirach. And once again, the w-shaped Cassiopeia constellation
helps us, because it points vaguely in the direction of the galaxy.
The great thing about Andromeda Galaxy is that it is currently racing toward our Milky
Way at a speed of about 113 km per second. Given its distance from the Milky way, our
merger will occur five billion years from now, so you don't have to worry about it,
because unfortunately you will be long dead at the time the collision occurs.
However, don't lose hope, because in a certain sense, the collision has already started!
In August 2020, the peer-reviewed Astrophysical Journal published new research revealing
that the collision between our galaxies is already underway.
The news about the Andromeda galaxy came from Project AMIGA, which uses the Hubble Space Telescope to
look at the deep-space surroundings of the Andromeda galaxy. AMIGA stands for Absorption
Map of Ionized Gas in Andromeda. NASA called it the most comprehensive study of a halo
surrounding a galaxy. The Andromeda galaxy, our Milky Way and other galaxies all sit enshrouded
in a large envelope – called a galactic halo – which consists of gas, dust and
stray stars. The halos of galaxies are faint, so faint, in fact, that detecting them is
not an easy feat. These astronomers measured the size of the halo of the Andromeda galaxy
by looking at how much it absorbed light from background quasars. They were surprised to
find that the Andromeda galaxy’s halo stretches much, much farther beyond its visible boundaries.
Indeed, it extends as far as half the distance to our Milky Way (1.3 million light-years)
and even farther in other directions (up to 2 million light-years).
The thing is: we cannot be sure that the two galaxies's halos are touching yet.
This is mostly because we cannot easily measure the characteristics of our galaxy's halo.
It is much easier to study Andromeda's halo, because our vantage point inside the Milky
Way is not the best place for understanding how our galaxy is made.
Think about it: You observe an external galaxy, and you clearly see its shape. But how do
you find the milky way's shape, if you are orbiting inside of it? It needs way more study
and understanding. However, because the two galaxies are so similar
in size and appearance, scientists assume that the halo of the Milky Way would also
be similar.
Are you curious to know how Andromeda and Milky Way merger would look like?
NASA released these images below in 2012. They are artist’s concepts of what someone
on Earth might see as the Andromeda galaxy hurtles toward us. Going from the first row
left panel, to the 4th row on the right, we see the evolution of the collision from the
present day to 7 billion years. As you can see, in 7 billion years, the merged
galaxies form a huge elliptical galaxy, with its bright core dominating the nighttime sky.
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