Westerlund 1 is an open cluster, which means that it looks like a scattered but overdense gravitationally-bound collection of stars in the sky that all formed from the same nebula at the same time. The cluster is about one tenth the diameter of the Moon [1], which means that it's rather small-looking as seen from Earth. It's located in the constellation Ara, which is supposed to represent an Atlar, and both the cluster and the constellation are located sufficiently far south that they are difficult to see from areas north of the tropics. Because Westerlund 1 is located in Ara, it is also called the Ara Cluster, which is weird because it's not the only cluster in the constellation. Additionally, doing a Google search on the phrase "Ara Cluster" will turn up several websites related to an unofficial server for the game ARK: Survival Evolved, which is a multiplayer survival game involving riding around on prehistoric creatures. Most professional astronomers seem to call this thing "Westerlund 1", so I'm going to use the name "Westerlund 1" for this cluster for the rest of this episode.
As you may have already guessed, Westerlund 1 was discovered by a person named Westerlund, or more specifically, the Swedish astronomer Bengt Westerlund [2]. Westerlund found the cluster named after him in 1961 while surveying the Milky Way using a telescope called the "Uppsala Southern Station", which was so named because it was located south of Uppsala, Sweden. More specifically, the Uppsala Southern Station was located on Mount Stromlo in Australia, which is indeed very far south of Uppsala. Bengt Westerlund would go on to discover another cluster that was named Westerlund 2 [3], and if I were to believe what Wikipedia says on this topic, Westerlund discovered a third cluster named Westerlund 3 [4]. However, I didn't find any mention of Westerlund 3 outside of Wikipedia, so I don't believe what Wikipedia says on this topic.
Anyway, back to Westerlund 1. What Bengt Westerlund noticed right away was that this cluster appeared to be heavily obscured by interstellar dust [1], which is probably why the cluster was not discovered until 1961. The reason why the cluster is so obscured is in large part because the cluster is located 13800 light years (4.23 kpc) away from the Earth in the plane of our galaxy [5], which is where most of our galaxy's interstellar dust is concentrated. Interstellar dust affects starlight in two ways. First, it absorbs starlight, making stars behind the dust look dimmer. Second, it tends to make stars look redder in color. Since the light from the stars in Westerlund 1 has to pass through over 10000 light years of this interstellar dust, it ends up looking rather red, even though many of the stars are actually very blue in color. A few years ago, I would have thought that few people would have actually seen or noticed this phenomenon on Earth. In recent years, however, both Australia and the West Coast of the United States have experienced severe forest fires that filled the sky with smoke and turned the sky red. This is the rather frightening hell-on-Earth equivalent of what happens when the light from distant stars passes through dust in space.
Given how severely interstellar dust affects the light from the stars in Westerlund 1, it is almost surprising that professional astronomers would want to actually study the object. However, Westerlund 1 has turned out to be a truly special open cluster. It is, in fact, the largest open cluster in the Milky Way that we know of. Its total mass is somewhere around 50000 to 100000 times the mass of the Sun [6, 7, 8], which means that it could be expected to contain roughly that many stars. That, in and of itself, makes it cool. Anything that is the "largest" is always going to be at least interesting, and that includes the largest species of whale living in the ocean, the largest building ever constructed, the largest pumpkin ever grown, and the largest ball of twine ever created for a roadside attraction (although several locations claim that their ball of twine is bigger than everyone else's [9]). However, Westerlund 1 is not cool just because it's the largest known open cluster in our galaxy but because of what that means scientifically, but to understand this, I need to explain some things that happen when a cluster of stars forms.
When a cloud of interstellar gas collapses to form new stars, that gas cloud fragments in a way that results in more smaller stars forming than larger stars. So, if one star the size of the Sun forms from a gas cloud, that gas cloud will also produce about 5 red dwarfs with half the mass of the Sun and about 40 red dwarfs with one-tenth the mass of the Sun. Meanwhile, for every 200 stars the size of the Sun that form from a gas cloud, one blue hydrogen-burning star with 10 times the mass of the Sun forms, and for approximately every 13000 stars the size of the Sun that form, one blue hydrogen-burning star with 60 times the mass of the Sun forms. So, when stars form from interstellar gas, relatively few of these stars are very large stars. Additionally, the really big blue hydrogen-burning stars have relatively short lifespans before they first transform into red supergiants and then explode as supernovae, so they tend to be rare.
Hence, if astronomers want to find lots of big blue stars, they are going to search for a star cluster that is both really big and really young. As I said before, Westerlund 1 is the biggest open cluster than anyone knows about, and it appears to be less than 10 million years old [10, 11, 12], which is rather young when considering that the Sun is about 4.5 billion years old. That makes Westerlund 1 a great place to find lots of young blue stars and things associated with young blue stars. In fact, it is a great place to find lots of stars that are really large, really rare, and really weird. So, if astronomers want to find blue supergiants, they can find loads of them in Westerlund 1 [10, 13]. If astronomers want to find big blue stars that have transformed into red supergiants or things called yellow hypergiants, they can find loads of those stars in Westerlund 1 as well [6, 14]. If astronomers want to find Wolf-Rayet stars, which are evolved blue stars that have blown away their outer hydrogen gas layers to reveal shells of helium fusing into carbon, they can find many of these stars in Westerlund 1 [6, 15]. If astronomers want to find the latest videogame console, then Westerlund 1 has quite a few, although the shipping times are a bit long. If astronomers want to find magnetars, which are pulsars with really strong magnetic fields that form after really massive stars explode as supernovae, Westerlund 1 has one [16, 17]. Heck, Westerlund 1 is even a source of gamma rays, which are extremely difficult to find either on Earth or in space.
As you can tell, Westerlund 1 is like a giant stellar superstore. If astronomers want to go shopping for some sort of really massive but really rare star, they are going to stop off at Westerlund 1 first.