Large Magellanic Cloud: Nearby Satellite Dwarf Galaxy

The Large Magellanic Cloud (LMC) is a satellite dwarf galaxy of the Milky Way that is among the closest galaxies to Earth. At about 163,000 light-years from Earth, the dwarf galaxy looks like a faint cloud in Southern Hemisphere skies. It lies on the border of the constellations Dorado and Mensa.

Both the LMC and its companion, the Small Magellanic Cloud (SMC), are named after the explorer Ferdinand Magellan. While astronomers in the Southern Hemisphere saw these clouds before Magellan's round-the-world voyage in 1519, the explorer and his crew were the first to bring that knowledge to the Western world.

Magellan died in the Philippines during that voyage, but his crew provided documentation of the discovery upon their return to Europe.

LMC location

Magellan's discovery of the LMC and SMC predated telescopes, but even after the instruments allowed Galileo and astronomers in the 17th century to get a closer look, it was still several hundred years before scientists could accurately calculate the distance to the LMC, the SMC and other nearby galaxies.

Scientists came to better understand cosmic distances using tools such as "standard candles" (objects, such as certain types of variable stars, that have known luminosities). From then on, the LMC was considered the closest galactic object to Earth until 1994, when astronomers found the Sagittarius dwarf elliptical galaxy, according to NASA. Another discovery in 2003, the Canis Major dwarf galaxy, turned out to be even closer.

The LMC is part of a collection of dozens of galaxies known as the Local Group, so named because they are fairly close to our own Milky Way galaxy. The most prominent member is the Andromeda galaxy, a Northern Hemisphere object visible with the naked eye just north of the constellation of the same name. The Andromeda galaxy is 2.5 million light-years away and is moving closer to our galaxy for an eventual collision.

The giant star-forming region in the Large Magellanic Cloud, the Tarantula Nebula.
(Image: © ESO)

Star-birth hotspot

Other than its proximity to Earth, the LMC is also known for being a site where stars form. Within the LMC's borders, several observatories from NASA and other space agencies have witnessed vast amounts of gas coming together to create young stars.

A 2012 composite image of the Tarantula Nebula — a region of the LMC more properly known as 30 Doradus — revealed violence and radiation through the lenses of the Hubble, Chandra and Spitzer space telescopes. "At the center of 30 Doradus, thousands of massive stars are blowing off material and producing intense radiation along with powerful winds," NASA wrote at the time. [Images: 50 Fabulous Deep-Space Nebula Photos]

Another, smaller star-forming region within the LMC is in a spot known as LHA 120-N 11. Images captured by the Hubble Space Telescope show that this region consists of several pockets of gas and numerous brilliant new stars.

In general, the LMC is an excellent spot to look if you want to see stars being born, NASA said in a statement.

"It lies in a fortuitous location in the sky, far enough from the plane of the Milky Way that it is neither outshone by too many nearby stars, nor obscured by the dust in the Milky Way's center," NASA stated. It is also close enough to study in detail (less than a 10th of the distance to the Andromeda galaxy, the closest spiral galaxy) and lies almost face-on to us, giving us a bird's-eye view."

Star-date rotation

The LMC's relatively close location to Earth also affords astronomers the chance to study it in more detail, with the aim of extrapolating information that can help explain how other galaxies behave. One example of this kind of research is the study of the LMC's rotation, which was picked up by the Hubble Space Telescope and published in February 2014.

"Studying this nearby galaxy by tracking the stars' movements gives us a better understanding of the internal structure of disk galaxies," Nitya Kallivayalil, a researcher at the University of Virginia who participated in the research, said in a statement. "Knowing a galaxy's rotation rate offers insight into how a galaxy formed, and it can be used to calculate its mass."

The researchers discovered that the LMC makes a rotation every 250 million years. They found this by using Hubble to track the motion of stars in the galaxies sideways with respect to the plane of the sky. While this technique has been used for more-nearby objects before, this effort represented the first time the method was used for a galaxy.

The team next plans to turn its attention to the SMC to do the same sort of analysis. Since the SMC and the LMC are also close enough to gravitationally interact with each other, looking at how they move in relation to each other may reveal information about the movements of other galaxies in the Local Group, the researchers said.

NASA's Transiting Exoplanet Survey Satellite (TESS) took this snapshot of the Large Magellanic Cloud (right) and the bright star R Doradus (left) with just a single detector of one of its cameras on Aug. 7, 2018. The frame is part of a swath of the southern sky TESS captured in its "first light" science image as part of its initial round of data collection.
(Image: © NASA/MIT/TESS)

First time around

Until recently, the LMC and the SMC were thought to have made multiple trips around the Milky Way. Researchers said the gravitational pull of the Milky Way is what caused the tail of gas and dust known as the Magellanic Stream that was ripped from the SMC. In the last few years, however, scientists have realized that the pair of clouds is actually just making its very first trip around the Milky Way.

By pointing NASA's Hubble Space Telescope toward the two clouds, scientists began to catch a glimpse of the objects' histories. "Hubble's biggest contribution is enabling us to clock how fast the Magellanic clouds are moving," said Gurtina Besla, a researcher at the University of Arizona who studies dwarf galaxies. In 2007, Besla overturned conventional wisdom when she suggested that the LMC and SMC were making their first orbit of our galaxy.

"They're moving too fast to have been long-term companions of the Milky Way," Besla said.

She used data from the European Space Agency's Gaia spacecraft to clock smaller, satellite galaxies orbiting the LMC, as well. And, understanding how these galaxies move has helped researchers better calculate the mass of the LMC. Current estimates put the LMC at about 100 billion times as massive as Earth's sun, or a quarter the mass of the Milky Way. Besla said this size means the LMC is about 10 times heavier than previously calculated.

As researchers continue to make more detailed observations of the dwarf galaxies, they hope to learn more about the Milky Way's enigmatic neighbors. These measurements may help reveal more about our own galaxy, too.

The LMC carries a significant amount of stars and gas with it as it approaches our galaxy. While that can be helpful, according to Besla, the additional mass makes it more difficult to calculate the movement of other objects whose interactions help scientists determine the Milky Way’s mass.

"The LMC being there is both helpful and a bit of a hindrance to understanding the total mass of the Milky Way," Besla said.

This article was updated on Dec. 4, 2018 by Contributor, Nola Taylor Redd.

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