The constellation Perseus (Perseus - Per)

The constellation Perseus



Perseus is a northern constellation, visible in our skies in an area very rich in objects to observe, between Auriga, Cassiopeia, Andromeda and Taurus, but above all crossed by the Milky Way: a guarantee for those who want to show their friends the beauty of the universe.

All of us, good or bad, know the Perseids, the Tears of St. Lawrence, the shooting stars that can be observed every year a few days after August 10: when I convince some intrepid observer in the mountains to expose themselves to a nice icy breeze to observe something new, not infrequently these same people confess that they have always heard of shooting stars, but have never seen one. Years later, some of these people still thank me for the opportunity I had given them to catch these fleeting apparitions, in some cases welcomed by a chorus of "ooohhh" absolutely spontaneous and all in all liberating! In the lucky years we usually end up organizing a race for the most observant, if only to keep the attention of the most distracted, before we freeze and eventually run to warm up...


The name, the story, the myth of Perseus...


Perseus is one of the most famous heroes of mythology, and he is precisely the hero who killed the Medusa, able to implore anyone who crossed his eyes. Son of the father of the gods Zeus and Danae, in turn daughter of the king of Argos Acrisius, he was the husband of Andromeda, by whom he had many children including Electrion and Gorgophone.


The antecedent of Perseus' exploits lies in King Polidette's purpose to seduce his mother Danae. With the secret intent to get rid of her, the king imposed on the young man the apparently desperate task of bringing him the head of one of the Gorgon women.

Medusa was protected by the Grays, her sisters, with only one tooth and one eye passing each other from time to time.

Perseus faced the Graces with the help of a helmet donated by Hades (Pluto) able to make him invisible, a bronze shield donated by Athena (Minerva) and a diamond sword built by Hephaestus (Vulcan), (in some versions Athena gives him a sharp sickle).So armed she effortlessly killed the Grays and, approaching Medusa, the only one among the Gorgonites to be mortal, while she was sleeping, looking at her reflected in her bronze shield, she cut off her head.

The head was then used by Perseus to petrify the monster that was supposed to kill Andromeda, tied to a rock.

At the end of the story, Perseus marries Andromeda.

Perseus for the ancient and modern

Let's see how Perseus was represented in antiquity and how he is shown today. Both Hevelius

Perseus according to Hevelius

that Bayer, in his Uranometry

Perseus in Uranometry

they represent him from behind while triumphantly holding with his right hand the head of the Medusa, just cut off with the sword that holds with the left. Stellarium instead depicts him


Perseus according to Stellarium

with a shield in her right hand and Medusa's head in her left.

A couple of stars close by, a big star and a variable

The constellation of Perseus has inside it just two stars whose distance is less than the threshold of 60 light years (al), the one I used in all my articles: I remember that 60 al is about the distance from which our Sun would have a magnitude equal to 6 and therefore it would no longer be visible to the naked eye. In this case we have ι Per mail at 34 al distance and θ Per situated at 37 al and in both cases travelling with Celestia to the two stars individually, the Sun is seen as a fifth star, at the edge of the Milky Way (ι Per and θ Per), in the company of stars belonging to constellations of our southern hemisphere, such as the Wolf, Scorpio and Centaur, in addition to the well known Sirius, which from there has lost much of its brightness.

comparison diagram of Perseus' stars

As we can see in the usual diagram, made by me to have a comparison between the sizes of the biggest stars of the constellation with respect to other stars encountered gradually in the series of articles, in Perseus there are ten stars larger than 40 times the Sun's radius, all of different star classes: on all stands out η Per, with a diameter equal to 156 times our Sun, followed by a trio placed at 62-63 times Per, ψ Per and 17 Per), by three stars below the value 50 (58 Per, ρ Per and α Per), to get to another trio with a diameter between 50 and 40 times the solar one (f Per, c Per and μ Per).


η Persei from 10 UA distance

As usual, just to give a visual sensation of what the biggest star of the group (η Per, class K3) would look like from a safe distance of 10 UA, I boarded my spaceship Celestia to visit my friends Etapersi, known all over the galaxy to be very distracted and to forget here and there personal objects: it seems that in the city they have warehouses full of umbrellas lost here and there by these nice gentlemen. However, since it never rains on their planet Lost (coincidentally), you might wonder why all these umbrellas: we drop a pitiful veil on the story.

Rather let's talk about the star β Per, called Algol (the daemon), very well known eclipse variable star, which presents a variation of 2.3 to 3.5 in a period of little less than 3 days: the variability of this star (binary) is due to the fact that the two components are one brighter than the other and when the less bright transits in front of the other component, eclipsing it, the brightness of the pair is drastically reduced, while when the hiding star is the brightest then the total brightness varies little. This pair of stars (in reality it is a ternary system) is 93 light years from the Sun and a source (wikipedia) reports that more than 7 million years ago it was a little less than 10 light years from us, appearing as the brightest star in the firmament with a -2.5.

Deep sky objects

Let's go now to discover some objects present in the constellation of Perseus, whose photos are always made by that marvel of Technology that is the Hubble Space Telescope (I remember that clicking on the photo we will see the image itself in high resolution). Let's start from a very well known double cluster (h-χ Per), visible to the naked eye like a cotton ball, but that already with a pair of binoculars appears in all its splendor: observing this photo we can't but remain open-mouthed for the beauty of these two open clusters, catalogued as NGC 869 and NGC 884.

the double heap h-χ Per

In this other picture we see the open cluster M34, definitely less crowded with stars

the M34 open cluster

while here we can see the M76 nebula, also called Little Dumbbell (small dumbbell) or Cork Stopper, catalogued as NGC 650 and 651: nice!

the planetary nebula M76

What to say now about the wonderful NGC 1499, the famous California Nebula?

the California Nebula

Much stranger is instead the reflection nebula called NGC 1333, which for some is known as Chaotic beauty.

the strange NGC 1333 nebula.

Finally, let's close with the galaxy NGC 1260, within which, in 2006, the supernova SN2006gy exploded, studied in depth by observers such as Chandra in X-rays.

supernova SN2006gy in galaxy NGC 1260

This supernova for some time had the record of the brightest stellar explosion ever recorded, but absolutely invisible to the naked eye since the host galaxy, NGC 1260, is at a monstrous distance of 238 million light years, almost one hundred times the distance that separates us from the well-known galaxy of Andromeda (M31).

The names of the stars

In this constellation we find a dozen stars with names associated with them:

  • Mirfak (α Per): elbow
  • Algol (β Per): the demon
  • Atik (ζ For): space between shoulders
  • Miram (η Per): the pulse
  • Misam (κ Per): the wrist
  • Menkib (ξ Per): shoulder of the Pleiades
  • Alatik (ο Per): space between shoulders
  • Misam al Thurayya (χ Per): pulse of the Pleiades
  • Gorgonea II, III and IV, ρ and ω Per): head of the Gorgon
  • Seif (φ Per): unknown meaning

Constellation visibility

Having said that the northern part of Perseus is circumpolar and therefore visible at any time of the year, the central part is low on the north-eastern horizon towards the end of August (always at the canonical and convenient time, 9 p.m.), reaches the zenith at the turn of the New Year, while at the end of May it is low on the north-western horizon.

Ultimately, there is plenty of time to observe it.

A freewheeling chatter

Yeah, let's have a laugh...

The name Perseus has always lent itself to puns: it seems that in the course of a battle he was wounded in such a way as not to be recognizable, to the point that one of his comrades began a (terrible) exchange of jokes: "Are you Perseus?" and he "Thirty-six"... Whatever...

the world-famous star 6 Per

Continuing in this wave of factions, we must remember that all the stars of the constellation have a name of the type xx Per, where "Per" must be read in Latin, "Persei": this fact gives the constellation of Perseus the enviable title of mathematical constellation par excellence. All the stars with a numerical name (4, 1, 28, 12, etc.) in this case become 4 Per, 1 Per, 28 Per, 12 Per etc. in a sort of cosmic table... The progenitor of all these stars can only be 6 Per, that here we see (seriously!) placed right on the border between Perseus and Andromeda: no astronomer has ever dared to rename it Thirty-six. And neither do I!



Audio Video The constellation Perseus (Perseus - Per)
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