How to distinguish constellations and star groups?

Constellation and star group (asterism) are two concepts that are quite difficult to distinguish. For many years, these two concepts were not clearly distinguished, leading to confusion and imprecision.

A constellation is a collection of groups of stars

Star group (asterism) is a general concept, it refers to any group of stars with imaginary lines forming any image. The images formed by these stars can be from a certain major culture, can also be from a division of any author, or even just a fun convention for yourself in your mind. statues of children.

As we know, the number of stars in the celestial sphere that humans can see with the naked eye in one night is just over 6,000 (if conditions are ideal), and about 20,000 if the total number of stars visible in a year is calculated. .

Simulation image of constellations in the universe.

In other words, easier to understand, our entire sky is almost fixed (with changes due to the motion of the Solar System around the galactic center and the birth/death of some but very small stars). ), which means that regardless of region, culture and imagination, the number of stars in the celestial sphere and their arrangement are always the same for the observer.

When an imaginative observer joins relatively close bright stars together and assigns them a particular image, the grouping of stars becomes an 'asterism'.

Initially, when mankind was not yet developed, human perception depended only on certain scientists, these joined groups of stars also mostly belonged to famous thinkers and scientists. These 'asterisms' are collectively known as constellations.

However, it was not until the early 20th century, when the convention and division of constellations were agreed upon by astronomers, that the concept of constellations (constellation) became more rigorous.

By modern convention, constellations are not just imaginary seams between bright stars, but each constellation represents a region of the sky it occupies.

According to the common convention of today's world, our list of constellations has 88 constellations, which divide the celestial sphere into 88 regions of the sky with different areas and shapes. Any star (including those later discovered through telescopes) in a certain constellation's sky is counted as a member of that constellation.

Misunderstandings

Thus, a constellation is a collection of star groups (asterism), and in today's astrophysics, only constellation lists are officially recognized. As defined, asterism can be imagined by anyone from any culture or profession, so their number is limitless. Below, I would like to give a few typical examples of star groups (asterism) that are often misunderstood in Vietnam.

The group of 7 northern bright stars forming a chair (or some people see it as a bucket of water) belongs to the constellation Ursa Major (Big Bear / Great Hero) corresponding to an Eastern constellation, the Big Dipper. Many people mistakenly believe that Big Dipper and Big Bear are one, but in fact that statement is not true.

The constellation Ursa Major has many stars and occupies an area much larger than the Big Dipper of the East, the seven stars of the Big Dipper being only its brightest. In Western culture, people also only have a concept of only these 7 stars, which is the Big Dipper. The Big Dipper and the Big Dipper coincided and today they are asterism, not constellation.

The star cluster M45, also known as the Pleiades, occupies a sizable area of ​​the sky, making it the most visible star cluster with the naked eye. Some documents in Vietnam translate it as 'seven-star constellation', which is a misleading translation because it is not recognized in the list of 88 constellations of modern astronomy today. Taxonomically, it is a star cluster - an area of ​​stars directly bound together by gravity, and apparently it is also a group of stars (asterism).

The Scorpio group, which includes most of the stars in the constellation Scorpius (one of the zodiacal constellations) becomes Scorpio, while their stars do not coincide.

Shen Nong is just a group of stars called by the ancient Vietnamese people, because the stars do not correspond, so it does not correspond to Scorpius (Scorpius), so it is just a group of stars, not a constellation, and because So of course it can't be said that it belongs to the Zodiac.

The constellation Archer (Sagittarius) is one of the constellations of the Zodiac, commonly known as the constellation Sagittarius, derived from the naming of Vietnamese astronomers before. This constellation has the image of a centaur with a bow, but this naming presents a problem because it shares the same name and is easily confused with the constellation Centaurus, which also bears the image of a centaur. Vietnamese authors overcome this error by adding the word 'sell' in front of the name of the constellation Centaurus, that is, the name when it was 'Semi Sagittarius'.

It should be said that the phrase 'half centaur' has no meaning, as the image of centaur is very full, not half. Second, in order to unify with the world, we should care about language unity. All official international astronomical documents translate into English from Sagittarius as Archer, and Centaurus as Centaur.

In addition, ancient Eastern astronomy has set forth a lot of conventions about 'constellations', the most famous and most well known are the 28 'constellations' belonging to the pentagram group.

Today, with the utmost respect for Eastern culture, and to avoid unnecessary naming, we can still refer to the widely recognized groups of stars in the East as 'constellations'. , it should be known, however, that in the mainstream astronomical texts there are only groups of stars.