Having "lived" in the Milky Way for so long, do you know who our "neighbors" are?

Author | Zhou Yihao

Audit | Huang Jian

Editor | Zhao Jingyuan

The Large and Small Magellanic Clouds are two irregular dwarf galaxies that are relatively close to our Milky Way galaxy and can only be seen south of 20 degrees north latitude. They are bright deep sky objects in the southern hemisphere sky, with apparent magnitudes of 0.4 and 2.2 respectively, visible to the naked eye, so the indigenous people of the southern hemisphere knew about the Large and Small Magellanic Clouds a long time ago, but it was not until the early 16th century that the Italian explorer Amerigo Vespucci left a clear record of these two deep sky objects (although the Persian astronomer Abd al-Rahman al-Sufi may have recorded them around 964 AD, but this view is controversial).

The two deep sky objects were observed by an expedition led by Portuguese explorer Ferdinand Magellan during his circumnavigation of the globe between 1519 and 1522. Although Magellan himself may not have observed and recorded them personally, his crew, including Italian explorer Antonio Pigafetta, did observe and record the two deep sky objects. As a result of Magellan's voyage, the two deep sky objects came to be known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC).

Large Magellanic Cloud (Image courtesy of ESO)

The Large Magellanic Cloud is located at the junction of the constellations Dorado and Montenegro in the sky, about 160,000 light-years away from the Earth, with a diameter of about 30,000 light-years and a mass equivalent to 10 billion suns. It is one of the brightest satellite galaxies of the Milky Way. Near the center of the lower half of the above picture, there is a particularly eye-catching feature, the Tarantula Nebula in the Large Magellanic Cloud, a huge star-forming region about 2,000 light-years wide.

The Tarantula Nebula in the Large Magellanic Cloud (Image courtesy of ESO)

The Small Magellanic Cloud is located in the direction of Tucana, and its projection on the celestial sphere is close to the globular cluster 47 Tucanae in the Milky Way. The Small Magellanic Cloud is about 200,000 light-years away from the Earth, with a diameter of about 20,000 light-years and a total mass of about 7 billion times that of the Sun. Like the Large Magellanic Cloud, it is one of the closest extragalactic galaxies to us.

Small Magellanic Cloud and globular cluster 47 Tucanae (Image courtesy of ESA)

If we have the opportunity to travel to the Southern Hemisphere, we must not forget to observe these two deep sky objects in a clear night sky. The Large Magellanic Cloud and the Small Magellanic Cloud are about 20 degrees away from the South Celestial Pole. Due to their brighter apparent magnitude, they are very easy to identify in the night sky and are important observation targets for astronomy enthusiasts in the Southern Hemisphere. In areas south of 20 degrees south latitude, they are located in the permanent visible circle, always on the horizon, and visible almost all year round, especially in the spring and summer of the Southern Hemisphere. The observation conditions are best (it should be noted that the seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere. The spring and summer in the Southern Hemisphere are approximately from October each year to March of the following year).

The Big and Small Magellanic Clouds over the Paranal Observatory (Image courtesy of ESO)

The Small and Large Magellanic Clouds also hold an important place in the history of science. In 1912, American astronomer Henrietta Leavitt studied the light variation period and brightness of 25 Cepheid variables in the Small Magellanic Cloud and confirmed that there was a certain relationship between the two (called the period-luminosity relation), which can be used to measure distance. This is a very important discovery. Later in 1913, Danish astronomer Ejnar Hertzsprung used this relationship to estimate the distance of the Small Magellanic Cloud for the first time, and the result was about 30,000 light years. Although the distance he measured now seems to be too small, it also proves the practical value of this distance measurement method. In addition, the supernova SN 1987A discovered in the Large Magellanic Cloud in 1987 was the first supernova visible to the naked eye since 1604 (although the maximum brightness of the supernova SN 1885A that exploded in the Andromeda Galaxy reached 5.8 magnitude, there was no definite record of naked-eye observation). It provided very important information for stellar evolution and opened up the new field of neutrino astronomy.

SN 1987A (brightest star in the picture) near the Tarantula Nebula (Image courtesy of ESO)

The Large and Small Magellanic Clouds have a large number of stars, as well as many structures such as star clusters and nebulae, which provide excellent opportunities for studying galaxy interactions and evolution, and help answer important scientific questions such as star and galaxy formation, and Milky Way accretion. For example, in 2019, astronomers used the 2.5-meter du Pont Telescope at Las Campanas Observatory to discover that the star formation rate in the Large and Small Magellanic Clouds is only 1/50 of that in the Milky Way, but the star formation rate in the Large Magellanic Cloud increased six times about 2 billion years ago, which may be caused by its interaction with the Small Magellanic Cloud. This discovery helps astronomers understand the history of star formation and galaxy interactions.

As the study of the Large and Small Magellanic Clouds becomes more and more in-depth, astronomers are gradually unveiling the mystery of these two neighbors of the Milky Way. More and more discoveries have not only enriched our understanding of the universe, but also provided us with valuable data and theoretical basis to help people build more accurate cosmic models. In the future, with the continuous advancement of observation technology and a deeper understanding of the universe, the Large and Small Magellanic Clouds will continue to serve as a treasure trove of astronomical research, leading us to explore more mysteries of the universe.

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