Magellanic Clouds

The Magellanic Clouds are two dwarf galaxies that orbit as companion galaxies to the Milky Way, the home galaxy of our solar system. The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are visible to stargazers in the Southern Hemisphere, where they look like patchy wisps of stars detached from the Milky Way. The LMC is home to a stellar nursery that is actively producing new stars and was the site of the last supernova explosion visible on Earth. Although they were noted by several ancient cultures and medieval astronomers, the clouds were named for Portuguese explorer Ferdinand Magellan who observed them on his attempt to sail around the world in the early sixteenth century. For years, astronomers believed that the Magellanic Clouds had formed along with the Milky Way and always orbited the galaxy; however, evidence now suggests that they may have been expelled from a nearby galaxy and may eventually merge with ours.

Background

According to the most accepted scientific theory, the universe formed about 13.8 billion years ago in a sudden expansion known as the Big Bang. This infant universe was so hot that electrons were unable to join with hydrogen and helium nuclei to form more complex atoms. It was not until 380,000 years after the big bang that the universe cooled enough to allow this process to occur. Astronomers believe that about two hundred million years after the big bang, the gas and dust of the early universe began to coalesce to form clouds. As the clouds became larger and larger, gravity caused them to begin spinning, forming the first galaxies. The matter within these galaxies also clumped together, eventually becoming so hot that they became stars.

Mergers among galaxies were fairly common in the early universe. Because the space between the stars in galaxies is so vast, these mergers were not violent collisions. Rather, gravity from larger galaxies slowly drew in smaller galaxies and absorbed them. Our galaxy, the Milky Way, is believed to have formed about thirteen billion years ago. Astronomers speculate that the Milky Way may have consumed several smaller galaxies over its lifetime. They believe that many of the brighter galaxies in the sky were formed from the merger of larger galaxies with smaller ones over the course of billions of years.

Overview

Astronomers believe that the Magellanic Clouds were formed about the same time as the Milky Way. This fact, along with their position as companion galaxies orbiting the Milky Way led astronomers to believe that they had formed alongside our galaxy. However, in 2006, observations from the orbiting Hubble Space Telescope showed that the clouds were moving too fast to have been in long-term orbits around the Milky Way. A later study suggested that they may have originally been part of the neighboring Andromeda Galaxy before being expelled more than eight billion years ago. They traveled through space before eventually being captured by the Milky Way’s gravity. The shapes of both clouds have been distorted by the gravitational pull of the larger Milky Way, likely altering the original spiral shape of the LMC. Astronomers further believe that the clouds may one day be completely consumed by the larger galaxy.

The Magellanic Clouds have been observed for thousands of years by the indigenous peoples of the Southern Hemisphere. The Polynesian peoples of the South Pacific used them as navigational guides for sea voyages while Australia’s aboriginal people believed the LMC was the campsite of an old man and the SMC was that of his wife. The Muslim astronomer Al Sufi recorded the clouds in his Book of Fixed Stars, which was written about 964 CE. Sufi called the LMC the “sheep of the southern Arabs.” During Ferdinand Magellan’s attempt to circumnavigate the globe from 1519–1522, the clouds were recorded as being faint star clusters in the southern sky. The clouds were originally referred to as nubecula major (big cloud) and nubecula minor (little cloud) before Magellan’s name was attached to them years later.

The LMC is about 163,000 light years from Earth, meaning light from the galaxy takes that long to reach our planet. It is about 14,000 light years across and contains an estimated ten billion stars—less than one-tenth the number in the Milky Way. The LMC appears as a faint luminous smudge in the night sky. It is located near the southern celestial pole, but parts of it can be seen low in the horizon from tropical latitudes in the Northern Hemisphere.

Both clouds contain large amounts of hydrogen and helium gas, the prime ingredients for creating new stars. One of the most active-known regions of stellar formation is the Tarantula Nebula, which is part of the LMC. Astronomers refer to a section of the nebula as a “starburst” region because of its propensity for producing massive new stars. As of 2020, they have discovered more than forty stars at least fifty times more massive than our sun in an area one light year across. In 1987, the nebula was the site of a supernova—a stellar explosion caused by the death of a massive star—that produced the power of more than one hundred million suns. It was the first supernova observed from Earth since 1604.

The SMC is farther away from the LMC at about 200,000 light years from Earth. The SMC has an estimated seven billion stars and is about half the size of its larger companion at 7,000 light years across. Astronomers speculate that the SMC may once have been a larger galaxy but was split apart by gravitational interaction with the LMC. In the early twentieth century, astronomer Henriette Swan Leavitt used her observations of variable stars in the SMC to achieve a scientific breakthrough. Variable stars are those whose brightness seems to fluctuate as seen from Earth. Leavitt used this variation in brightness to calculate the distance of the stars from Earth. Astronomers have since used her method to determine the distance of faraway galaxies.

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