The Milky Way And Large Magellanic Cloud: When Galaxies Collide Catastrophically

The two Magellanic Clouds are small, amorphous, starlit satellite galaxies in orbit around our own large spiral Milky Way. The Large Magellanic Cloud (LMC) is about 163,000 light-years from us, and it is the second- or third-closest galaxy to our own, after the Sagittarius Dwarf Spheroidal and the unconfirmed dwarf irregular galaxy named the Canis Major Overdensity. Classified as a Magellanic spiral, the LMC displays a stellar bar that is geometrically off-center, indicating that it was once a barred dwarf spiral galaxy before its spiral arms were violently disrupted by a probable tidal interaction with its neighboring Small Magellanic Cloud (SMC). Indeed, interactions, mergers, and catastrophic collisions are common among galaxies, and a new study shows that our own Milky Way Galaxy is on a tragic collision course with the LMC that will occur in about two billion years. If this potentially fatal collision occurs, it could hurl our entire Solar System screaming into space.

This horrific collision would take place much sooner than the predicted smash-up between our Milky Way and another neighboring galactic member of the Local Group, the large spiral named Andromeda. Astronomers predict the collision between our Galaxy and Andromeda will occur in approximately eight billion years. The Local Group of galaxies hosts about 54 known members, of which the Milky Way and Andromeda are its largest constituents.

The fatal smash-up between our Galaxy and the LMC could awaken the sleeping beast that haunts the center of our Milky Way–a dormant supermassive black hole, which would then emerge from its long slumber and begin to voraciously devour ambient gas. As a result, this gravitational beast would balloon up to ten times its current already enormous size. As our Galaxy’s resident black hole dines greedily on its surroundings, it would hurl out high-energy radiation.

Supermassive black holes probably reside in the dark hearts of almost every large galaxy in the observable Universe, and they weigh-in at millions to billions of times solar-mass. Our Galaxy’s own supermassive dark heart is a relative light-weight, weighing in at millions–as opposed to billions–of times solar-mass. It is named Sagittarius A*– Sgr A*, for short (pronounced saj-a-star)– and it is a peaceful elderly black hole now, dozing quitely in its old age. Sgr A* currently wakes up only now and then when a luckless star or floating cloud of gas travels too close to its waiting gravitational maw. At that dreadful point of no return, Sgr A* wakes up and dines greedily– just like it did long ago when both it and our Galaxy were young inhabitants of the ancient Universe. But this latter-day blaze of glory will only last for one brief shining moment.

The good news is that this cosmic fire-storm–predicted to occur when the LMC crashes into our Milky Way–is unlikely to influence life on Earth. In fact, researchers believe there is really only a small chance that the initial smash-up could launch our Solar System into space.

The Magellanic Clouds

Both the LMC and SMC were named after the Portuguese explorer Ferdinand Magellan (1480-1521) who mistakenly believed they were clouds. While the LMC is only approximately 163,000 light years from us, the SMC is not much farther at about 199,000 light-years. In order to envision this, our entire Milky Way Galaxy is 100,000 light-years across, and it is approximately 3,000,000 light-years away from the approaching Andromeda Galaxy (M31).

Over twenty small satellite galaxies circle our own, but only the duo of Magellanic Clouds are bright enough to be viewed from Earth with the unaided human eye. Both the LMC and SMC are brimming with gas, in dramatic contrast to our Milky Way’s other orbiting galactic satellites, and gas is the stuff that stars are made of. For this reason, both Magellanic Clouds are sufficiently rich in gas to create brilliant new fiery baby stars.

The people of several ancient cultures knew of the existence of the Magellanic Clouds. The most ancient continuous extant references to the pair of “clouds” were likely conducted by ancient sky-watchers from the Khoisan culture located in Southern Africa. The ancestors of these people are thought to have lived in isolation from other living human cultures for thousands of years.

Another substantially long history of cultural association may have re-emerged with the migration of ancient humans south from their region of origin in the Middle East. These migrating people are thought to have reached Australia approximately 50 to 60 thousand years ago, and they were the ancient forebears of modern Aborigines, whose several cultures have created a variety of interesting and colorful myths, legends, and folktales centered around this mesmerizing duo of starlit satellite galaxies.

The ancient people of Polynesia were also aware of the existence of this bewitching pair of “clouds”, and they used them as navigation markers. Taking both “clouds” together, the duo were also known to the Maori of New Zealand as Nga Patori-Kaihau or as Te Reporepo. The ancient Maori used the pair of “clouds” as predictors of winds.

Even though both “clouds” have been readily visible to the prying eyes of southern nighttime sky-watchers well back into prehistoric times, the first known written reference to the LMC was made by the Persian astronomer ‘Abd al-Rahman al-Sufi Shirazi (303 AD-986 AD)–later known in Europe as “Azophi”–in his Book of Fixed Stars (~964 A.D.).

The second recorded observation of the Magellanic Clouds was made in 1503-4 by the Italian explorer Amerigo Vespucci ((1454-1512) in his letter describing his third voyage. In this letter, Vespucci mentions “three Canope [sic], two bright and one obscure”; “bright” here refers to the LMC and SMC, while “obscure” refers to the Coalsack Nebula, a cloud composed of gas and dust, that is the most prominent dark nebula in the skies.

Measurements conducted with the Hubble Space Telescope (HST), that were announced back in 2006, indicate that both Magellanic Clouds are actually traveling too fast to be orbiting our Milky Way. For some time, many astronomers proposed that both Magellanic Clouds had been in orbit around our Milky Way at approximately their current distances for eons. However, more recent observations indicate that it is rare for the duo to be located as close to our Galaxy as they are now. Both theory and observation indicate that the “clouds” have both been greatly distorted by tidal interactions with our own considerably larger Galaxy as they travel ever closer and closer to it.

The distance to the LMC has been determined by astronomers using several different methods that depend on “standard candles”, with Cepheid variables being one of the most favored “candles”. “Standard candles” are astrophysical objects such as supernovae or variable stars, which have known luminosity because of some characteristic quality possessed by its entire class of objects. Cepheids are intrinsic variable stars that pulsate in a predictable way, and have long been known to display a relationship between their absolute luminosity and the period over which their brightness changes. However, Cepheids are often considered to be inadequate because they suffer from a “metallicity effect”. This means that Cepheids of differing metallcities show different period-luminosity relations. Alas, the Cepheids that flicker brightly in our Milky Way, that are usually used to calibrate the period-luminosity relation, are more rich in metals than those found dwelling within the LMC.

In astronomy, the term metal differs from the same term as it is used by chemists. The Big Bang birth of the Universe, thought to have occurred about 14 billion years ago, produced only the lightest of atomic elements: hydrogen, helium, and trace amounts of lithium. All of the atomic elements heavier than helium are called metals by astronomers. Metals, in the terminology used by astronomers, originate in the hot nuclear-fusing cores of the Universe’s myriad stars–or, alternatively, in the case of the heaviest atomic elements (such as gold and uranium), in the supernovae explosions that mark the end of a massive star’s life on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution.

Currently used 8-meter-class telescopes have found eclipsing binaries throughout the entire Local Group, and these systems have proven useful for distance measurements. This is because the parameters of such systems–composed of two eclipsing stars–can be determined without mass or compositional assumptions. Also, the light echoes emanating from supernova 1987A provide useful geometric measurements, without any stellar models or assumptions.

By cross-correlating differing methods of measurement, an astronomer can determine distance; the residual errors are now less than the estimated size parameters of the LMC. The results of a study using late-type eclipsing binaries, to calculate the distance more accurately, was published in the March 2013 edition of the journal Nature. This method was used to determine that the LMC’s distance is about 163,000 light-years with an accuracy of 2.2%.

The LMC is classified as an irregular galaxy and, like others of its kind, it is richly endowed with star-birthing gas and dust. For this reason, the LMC is currently in the midst of vigorous star-forming activity. It also plays host to the Tarantula Nebula, which is the most active star-birthing regions in the entire Local Group.

The LMC houses a vast range of galactic objects and phenomena that have made it famous for being an “astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars,” as described by the American astronomer Robert Burnham Jr. (1931-1993).

A Terrible Beauty Is Born

The LMC wandered into our Milky Way’s neighborhood approximately 1.5 billion years ago, and now it circles our Galaxy beautifully–and ominously. Until recently, astronomers thought that the LMC would either orbit our Galaxy for many billions of years, or, because it is zipping along at such a great speed, tear itself from our Milky Way’s gravitational ties that bind, and escape from its powerful and relentless pull.

However, more recent measurements suggest an entirely different fate, both for our Galaxy and the LMC! The new calculations indicate that the LMC possesses almost 50% more dark matter that previously thought. Dark matter is a much more abundant form of matter than the “ordinary” atomic matter that we are most familiar with, and it is believed to be composed of exotic non-atomic particles that do not dance with light or any other form of electromagnetic radiation. For this reason, the mysterious dark stuff is transparent–and it haunts the Cosmos like an invisible ghost, revealing its secretive presence only by way of its gravitational influence on objects that can be seen, such as starlit galaxies.

The team of astronomers propose that since it possesses a greater than predicted mass, the LMC is quickly losing energy and is doomed to blast into our our Milky Way–which may have fatal consequences for our Solar System.

Lead scientist Dr. Marius Cautun, a postdoctoral fellow in Durham University’s (UK) Institute for Computational Cosmology, explained in a January 4, 2019 Durham University Press Release that “There is a small chance that we might not escape unscathed from the collision between the two galaxies, which could knock us out of the Milky Way and into space.”