When you look up at the night sky, the first thing that catches your attention is that you can trace imaginary lines joining the stars and you can create any shapes you want. It is very easy to imagine fantastic animals or all kinds of imaginary characters with a thousand and one shapes. Then, you just have to let your imagination fly and endow the imagined shape with a life, a past, and events that have caused it to end up in the sky represented by the chosen stars. <\/p>\n\n\n\n
The marvellous part is that humanity has spent its entire existence doing precisely that. There has always been someone who has looked up at the sky and imagined a world full of creatures and beings that, whether for their goodness or wickedness, have ended up fixed among the stars. Culture and era do not matter. Everyone has an imaginary represented in the celestial vault and it is incredibly rich. For good reason. The human imagination has no limits and that is absolutely marvellous. In reality, the underlying idea of creating these imaginary stellar figures is to try to bring a little order and make it easier so that when someone talks about a part of the sky, others know which part they are referring to.<\/p>\n\n\n\n
These celestial forms receive the generic name of asterism. This word comes from Ancient Greek (asterism<\/em>what does literally mean \u201c<\/em>star cluster \u201c<\/em>stellar configuration\u201d. Asterism derives from star<\/em> (star<\/em>), which means star. The term later passes into Latin as asterism<\/em> and, from here, it enters modern languages (Catalan, Spanish, French, English...) keeping its original meaning almost intact.<\/p>\n\n\n\n It appears that the first graphic representation of an asterism would be in Lascaux Cave, France<\/a>. Among thousands of paintings of various animals, painted about 16,500 years ago before the present, in the middle of the Mesolithic, there would be some black dots that could be the representation of the Pleiades<\/a> and of the Hyades<\/a>, both star clusters located in the constellation of Bull<\/a>. Not everyone agrees, however.<\/p>\n\n\n\n Instead, the first written reference would be much more modern and would be represented by what appears to be humanity's first astronomical treatise. This is composed of two clay tablets written in cuneiform of Babylonian origin and which would describe different aspects of the astronomy and astrology of this Mesopotamian culture. These tablets are known as MUL.APIN<\/a> and, according to some recent astrometric work, would have an age of around 3,370 +\/- 100 years before present<\/a>. While it would seem that the name refers to the asterism of Cassiopeia<\/a>, the treatise would describe 17 asterisms, among which would stand out Bull<\/a>, Perseus<\/a>, Orion<\/a> i Auriga<\/a>, among others.<\/p>\n\n\n\n Most of the most identifiable asterisms in the northern hemisphere in Western culture reach us through Greco-Roman tradition, although many of them have much older roots, especially in the Mesopotamian cosmology of the 3rd\u20132nd millennium BC. Proof of this are the names of Orion, which in Mesopotamian culture is named SIPA.ZI.AN.NA and could be translated as Shepherd of Heaven, and which in Greek culture becomes the giant hunter we already know. In fact, Greek culture inherits this Mesopotamian cosmology and does three fundamental things:<\/p>\n\n\n\n Over time, with technological and scientific progress, we now know that asterisms are nothing more than curiosities from the past that we use to amuse ourselves. The great advantage is the same as it was in the past: they allow us to orient ourselves in the sky and explain it to others when we do an outdoor astronomy session. Now we know that the stars that make up the asterisms are not all in the same horizontal plane but are each at a different distance from Earth. The part that has always interested me is knowing how far away these stars are.<\/p>\n\n\n\n The European Space Agency's Gaia project<\/a>, with the launch in 2013 of satellite<\/a> which bears the same name, made it possible to achieve something that until then was unthinkable: to measure astrometric data such as position, luminosity, distance, temperature, chemical composition, and movement of nearly 2 billion celestial objects located around us with unprecedented precision. This project, still ongoing even though the satellite ceased to be operational on January 15, 2025, made it possible to create a very detailed map of our galaxy, the Milky Way, in an effort to decipher its origin and future evolution. The data produced within this project have been progressively released to the entire scientific community and the general public. The first data release took place on September 14, 2016, under the name of Gaia Data Release (DR) 1<\/a>. On April 25, 2018, Gaia DR2 was released, and on December 3, 2020, it was Gaia DR3<\/a>. Gaia DR4 is expected around December 2026, and the final data release (Gaia DR5) is not expected before 2030.<\/p>\n\n\n\n Since Gaia's data is publicly accessible, you just need a little bit of programming to download the data you are interested in to your computer so you can \u00abplay\u00bb with it. And that is what I have done. Using the R programming language<\/a> (free and multiplatform software program) I wrote three scripts that allowed me to download distance, luminosity, and radial velocity data of more than 200 stars to make 3D graphs of the main asterisms of the northern hemisphere. <\/p>\n\n\n\n The first script allowed me to relate the common name of the stars with Gaia DR 3. The objects in the Gaia database are identified with a code. Therefore, if I wanted to reconstruct the 3D geometry of the Taurus asterism, I first had to know what the Gaia codes were for the stars that make it up. What I did is search in the SIMBAD database<\/a> correspondence with Gaia (for example, Delta Tau and Gaia DR3 3314024566919613952<\/a>) and extract the coordinates and the rest of the properties (such as the Bailer-Jones distance<\/a>, which is the most reliable way to compute the distance of stars) directly from Gaia. The problem came when seeing that not all stars are present in both databases. Although Gaia DR3 contains a huge amount of stars, it doesn't have all of them. <\/p>\n\n\n\n The second script allowed me to use the parallax data<\/a> presents a SIMBAD of the stars that had no correspondence in Gaia DR3 and derives their distance. Even so, there were stars, such as Nu Dra, Zeta Lyr, and Kappa Boo, that also did not have parallax data available in SIMBAD. Then, I extracted the parallax data from ESA using the star's coordinates by adding 10 arcseconds as a search radius and telling it to keep the data of the star closest to these coordinates as long as it matched the known magnitude of the star I was looking for. In this way, I prevented it from attributing erroneous properties to the stars I was searching for. This was especially important for very distant stars with very large parallaxes.<\/p>\n\n\n\n The third script allowed me to make the 3D plot of the asterism I wanted from the R library \u00abplotly\u00bb<\/a>. If you look at the axis units, you'll find they aren't expressed in declination, right ascension, and light-years, but in degrees relative to Xi (\u03be) and Eta (\u03b7) and in parsecs (pc). Perhaps the easiest part to understand is the distance expressed in parsecs, since 1 pc = 3.26156 light-years. Therefore, the conversion is very easy. The other two axes arise from the complication of representing spherical coordinates\u2014namely declination (expressed in degrees, minutes, and seconds) and right ascension (indicated in hours, minutes, and seconds)\u2014in a Cartesian system defined by two-dimensional planes. Then I defined a function that allowed me to convert spherical celestial coordinates into local Cartesian coordinates on a tangent plane using an exact gnomonic projection centered on a chosen reference position. A gnomonic projection<\/a> it is a type of geometric projection that represents points of a sphere onto a tangent plane by means of lines passing through the center of the sphere. <\/p>\n\n\n\n You will see that, apart from the stars that make up the asterism itself, a series of colored dots appear. These are real stars extracted from Gaia. Their color corresponds to the star's temperature. Blue colors indicate hot stars, while red colors indicate cooler stars. You'll find the color legend and the approximate star type correspondence in the legend at the top left. If you hover over one of the stars that make up the asterism, a tooltip will appear showing the star's temperature in color and its main characteristics, such as its distance from us, its radial velocity, Gaia code, etc.<\/p>\n\n\n\n You'll see that once the graph loads, you can move it in any direction simply by clicking on it and dragging. In the upper right, you'll find a series of buttons that let you rotate, zoom, and capture a specific image of the graph.<\/p>\n\n\n\n Orion<\/strong>:<\/p>\n\n\n\n\n