After the basic introductory material of the first two weeks, it's time to start looking at the main subject of astronomy: the sky. If you go outside at night in a location with reasonably dark skies and you stay up all night, you will be able to see about 6,000 stars. That may look like a lot, but it's not: our constellation, the Milky Way, contains approximately 200 billion stars, and it is only one of billions and billions of galaxies in the visible universe. 6,000 is really not that much.
What we do see are only the nearest stars to our own, plus a few very bright distant ones. We also see other types of objects: for example, planets, easily recognised because they will change locations from day to day against the background of stars. In dark skies you can also see other galaxies: Andromeda, our nearest large neighbour, can be seen easily with the naked eye as a fuzzy blob, and the Large and Small Magellanic Clouds, two dwarf galaxies which were once thought to be orbiting the Milky Way, can be seen very clearly from southern hemisphere locations. And, if you look to the skies for a while, you will almost certainly see artificial satellites moving very quickly as they circle the Earth, and very likely the odd meteor will show up from time to time.
[/caption]One interesting aspect of the human brain is its ability — or need — to identify patterns in the world around it. That is the reason why we aggregate the stars into constellations: they are nothing more than patterns formed by connecting the "dots" in the sky. It is very important to know that the stars in any given constellation are not physically together; the pattern they form is an artefact of "projecting" a 3D universe into an apparent 2D sky. If you were to move a large distance from the Earth, the patterns in the sky would be significantly different from the ones we know.
In general, the constellations are figures formed by the brightest stars we can see; some do look like what they're supposed to represent (for example, Scorpius (pictured, right) or Orion), but some require a significant degree of imagination (such as Pisces or Lynx). Every culture on Earth invented its own set of constellations; and, as the sky we see today is not significantly different from that of hundreds or thousands of years ago, many of those are similar, but many more are entirely different. Some cultures (notably the Incas and the Australian aborigines) even recognised "constellations" that were in fact dark areas of the sky.
Our current set of 88 official constellations owes much to Ptolemy, who defined an original set of 48 constellations covering the North sky (which was all he could see), many of which survived to this day. When European explorers started moving south of the Equator, they started inventing new constellations with the new stars they saw. This brought a new set of themes to the sky; mythical themes still abounded, but some biblical terms also made their way onto the sky during these period, as did some pieces of modern scientific equipment (Horologium, the clock, is one example). At some point, over 150 constellations were known and widely used, with some overlap happening between them.
In the late 1800s, the organisation that was to become the International Astronomical Union defined the current set of 88 constellations, including their well-defined boundaries. This allows us to refer to objects that are not part of the figure of a constellation as still belonging to it if they fall within its boundary. Also, the whole sky is covered by these 88 constellations.
As a result of this, many constellations were abandoned and forgotten; my personal favorite is Felis, the cat. Others, such as Argo Navis, the ship of the Argonauts, were split into several (in this case, Carina, the keel, Puppis, the poop, and Vela, the sails).
It's worth noting that most cultures also had individual names for the brightest stars in the sky, and that most of those stars are still referred to by name. Most of the surviving names are Arabic (Rigel, Aldebaran, Deneb...), but Greek (Sirius, Arcturus...) and Latin (Polaris, Spica...) are also common.
For unnamed stars, the most commonly used designation is the Bayer designation, which uses a letter of the Greek alphabet followed by the Latin name of the constellation to which the star belongs, in the genitive form; letters are assigned in order of brightness, alpha (α) being the brightest. Most people have heard of the star "Alpha Centauri", and now you know that this refers to the brightest star in the constellation Centaurus, the centaur.
However, the strict order of brightness is not always followed for historical reasons; when the letters were originally assigned (by the German astronomer Johann Bayer, in the early 1600s) there was no way of accurately measuring the brightness, so constellations with similarly bright stars may have them in the wrong order. Orion is a particularly good example of this, with its beta (β) star (Rigel) being the brightest and its delta (δ), Mintaka, being the sixth, rather than fourth, brightest.
This concludes our first look at the universe outside our planet. Starting from the next week, we will spend quite some time closer to home, looking at the Earth, the Moon and the rest of the solar system before venturing out towards the stars.
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