The Taurus Molecular Cloud is, more or less, what it sounds like. It's a cloud made of molecular gas (mostly molecular hydrogen) in the constellation Taurus. Well, that was simple.
OK, OK, I'll actually do a complete episode on the Taurus Molecular Cloud. First of all, the cloud is not constrained to just the constellation Taurus; part of it falls within the constellation Auriga, and you might encounter references to it as the "Taurus-Auriga Complex" or something like that. Second, the object was not originally known as the Taurus Molecular Cloud. Instead, it had previously been referred to as the Taurus Dark Cloud, because it was first identified as a series of dark, wispy cloud-like nebulae in the sky. The reason the cloud looks dark is in part because it also contains a lot of interstellar dust that obscures the starlight from both stars within the cloud and stars behind the cloud and in part because the hydrogen gas and other molecules in the cloud do not emit very much light in the visible part of the electromagnetic spectrum. The cloud also doesn't contain any really big, really bright stars that are going to illuminate the gas within the cloudn in the same way that, for example, really big and really bright blue stars illuminate the Orion Nebula.
Dark clouds were generally only discovered with the development of astrophotography. E. E. Barnard was a notable pioneer in the field of astrophotography at the end of the nineteenth and beginning of the twentieth centuries, and Barnard wrote multiple papers about these objects that included discussions about whether they could be holes in the sky or whether they could be interstellar clouds [1]. Barnard also compiled a catalog of these dark clouds that included parts of what are now known to be the Taurus Molecular Cloud [1].
Eventually, people figured out that things like the dark clouds in Taurus were regions containing interstellar gas and dust. Quite honestly, it's kind of freaky to think about holes in the Milky Way that are completely devoid of stars, and I think this is why a lot of astronomers other than E. E. Barnard thought that these things were actually clouds in space. However, that was only the beginning of the deep and lengthy studies into the Taurus Molecular Cloud.
In the 1940s, Alfred Joy at Mount Wilson Observatory defined a class of variable stars called T Tauri stars [2], which were named after the star T Tauri because what could be more confusing than naming a class of objects after one of the objects in that class (other than having a last name that is a three letter word for an emotion). Anyway, Alfred Joy had defined T Tauri stars as varying irregularly in brightness, as having specific types of spectra similar to the Sun's, as having relatively low brightnesses, and as being associated "with dark or bright nebulosity", which meant that the stars were surrounded by their own small clouds of gas [2]. What was rather noteworthy was that Alfred Joy found many T Tauri stars within the Taurus Molecular Cloud, including T Tauri itself, and he concluded that these stars tended to be associated with dark clouds []
Since Alfred Joy's discovery, we have now learned that T Tauri stars are actually stars that are still in the process of forming. They are surrounded by disks of gas and dust that is either falling into the stars or that is forming planets around the stars. The variability in the brightness from these stars is related to a combination of instabilities in the disk of gas falling into the star, flares on the stellar surfaces, and huge starspots that appear and then disappear as the stars rotate [3]. I will talk more about these stars later, but for now, back to the cloud.
While astronomers in the 1940s knew that the Taurus Molecular Cloud (which, just to remind you, was being called the Taurus Dark Cloud) contained lots of interstellar dust and lots of T Tauri stars, they would need to wait until the 1960s to detect the molecular gas in the cloud. Molecules in the interstellar medium are generally difficult to detect in the visible part of the electromagnetic spectrum, but they do emit lots of emission with wavelengths ranging from a fraction of a millimeter to multiple centimeters in size. The centimeter-sized radiation is usually referred to as radio emission, but the shorter wavelength radiation is called millimeter or submillimeter emission depending on whether the wavelength is shorter or longer than a millimeter. To detect this emission from interstellar molecular gas efficiently, astronomers needed to develop the radio telescope receivers to do this, and that would not be done until the 1960s.
However, just to make astronomy even more challenging than it needs to be, the hydrogen molecule itself does not emit much emission at all for reasons involving quantum mechanics. Consequently, if astronomers want to find interstellar molecular gas, they have to look for virtually any molecule other than the hydrogen molecule. In the Taurus Molecular Cloud, as far as I can tell, the first molecules detected in the cloud was hydroxyl radical, which has the chemical equation OH (yes, one oxygen atom joined to one hydrogen atom). That doesn't seem like a thing that would be stable on Earth's surface, at least in a non-ionized state, but it is common in interstellar molecular gas, and it was detected in the Taurus Molecular Cloud in 1967 by Carl Heiles [4], which was a landmark achievement at the time. However, if you ask most professional astronomers what they think is the easiest molecule to detect in space, most of them will tell you that it's carbon monoxide, or CO. This molecule emits at relatively high frequencies (or very short wavelengths), so it wasn't until the 1980s that people began to map the carbon monoxide in the Taurus Molecular Cloud [5, 6, 7].
Once people starting mapping the actual molecules in the Taurus Molecular Cloud, they discovered that, first of all, it looks rather filamentary, and second, it's big (as in it covers a really big area of the sky). With a size of roughly 120 light years, it's not necessarily a physically large object in astronomical terms, but since it is located very nearby at a distance of about 460 light years (about 140 pc) [8], it looks large. It doesn't have well-defined edges, but it more or less has a length of about 15 degrees [7, 9]. That's roughly half the width of the constellation Taurus itself. For reference, the Moon as seen from Earth is half a degree in diameter, so it would be possible to place 30 Moons across the nebula. Another way to think of the size of this cloud would be how large it looks as projected onto the Earth's surface, something that I normally discuss towards the end of a typical podcast episode. Anyway, the center of the Taurus Molecular Cloud would cover an area that includes most of the southern half of Pakistan as well as parts of the states of Rajasthan and Gujarat in India and parts of southern Afghanistan. (For reference, the coordinates used in this episode, which came from the Simbad Astronomical Database, point to an off-center location that corresponds to the desert in Rajasthan.)
It's also worth pointing out that carbon monoxide and hydroxyl are not the only molecules that people have found in the Taurus Molecular Cloud. I found quite a few references describing the detection of other molecules. Some of them are relatively simple things that you might have lying around your home, like ammonia [10]. Some are relatively simple but weird, like sulfur monoxide [11], or diazenylium [11], which is a molecular ion consisting of two nitrogen and one hydrogen atoms, or thioxoethenylidene [12], which consists of two carbon atoms and a sulfur atom and which also has a name which I am certain no one ever says out loud. However, some really weird and really complex molecules have also been found in the Taurus Molecular Cloud, such as deuterated cyanodiacetylene [13] or ethynylbutatrienylidene [14], both of which are so complicated that I am not going to attempt to describe their chemical equations in this podcast.
The Taurus Molecular Cloud is also recognized as containing lots of newly forming stars, and very specifically, stars that are roughly the size of the Sun or smaller. The cloud is basically a giant reservoir of raw material for forming new stars, which is why it would be the first place to go if you want to find lots of these young stars or lots of smaller clouds called prestellar cores that have not yet begun to form into stars. I found one very recent paper that identified 532 individual stars and protostellar objects within the Taurus Molecular Cloud [15]. Many of those objects are famous, too, and I could create individual podcast episodes on a couple dozen of them. One object I will recommend looking up online is HL Tauri. In 2014, the Atacama Large Millimeter/submillimeter Array (ALMA) created a submillimeter image of the dust surrounding this protostar and found a series of concentric rings, which were most likely created by protoplanetary objects forming within the disk [16].
Anyway, I am going to end this episode here, in part because the amount of information available about the Taurus Molecular Cloud is about as large as the cloud itself. The one key thing to remember about the Taurus Molecular Cloud is that it's a cloud made of molecular gas (mostly molecular hydrogen) in the constellation Taurus.