An Etymological Dictionary of Astronomy and Astrophysics
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A transitional class of object between the classic spiral galaxies and true irregular systems. The → Large Magellanic Cloud, the nearest and best studied example of the class, is, contrary to popular opinion, not an irregular galaxy. The LMC and other members of the SBm class have definite structural signatures. They are generally dominated by a pronounced asymmetric bar -- one that is offset from the optical center of the galaxy -- with a nascent spiral arm emanating from one end. As is the case with irregular galaxies, the optical centers of SBm type systems are not particularly special places. Disk systems later than Sc characteristically lack a central stellar concentration in addition to having weak spiral structure; this is true of SBm-type galaxies. SBm galaxies are typically very active in their star formation activity, often containing a large star-forming complex situated at one end of the bar. Beyond these general trends there is a tremendous amount of dispersion in physical properties within the SBm class, particularly in the strength of the spiral structure. At one extreme are the "one-armed" spirals such as NGC 3664 and NGC 4027 which are dominated by single, looping spiral arm. On the other hand NGC 4861 shows little evidence of spiral structure and it is dominated by a large star-forming complex at one end of its bar. The class smoothly leads to the Barred Magellanic irregulars (IBm) which show no indication of spiral structure (Wilcots et al. 1996, AJ 111, 1575).

→ Magellanic; → spiral; → galaxy.

The larger of the two Magellanic Cloud galaxies visible in the southern hemisphere at about 22 degrees from the South Celestial Pole. It is approximately on the border between the constellations → Dorado and → Mensa in a region of faint stars. The center of the LMC is approximately RA: 5h 23m 35s, dec: -69° 45' 22''. The LMC shines with a total → apparent visual magnitude of approximately zero. It spans an area of the sky about 9 by 11 degrees, corresponding to about 30,000 → light-years across in the longest dimension, for a distance of some 162,000 light-years. It has a visible mass of about one-tenth that of our own Galaxy (10¹⁰ Msun). The LMC and its twin, the → Small Magellanic Cloud, are two of our most prominent Galactic neighbors. The LMC is classified as a disrupted → barred spiral galaxy of type SBm, the prototype of a class of → Magellanic spirals. The galaxy is characterized by a prominent offset → stellar bar located near its center with the dominant → spiral arm to the north with two "embryonic" arms situated to the south. The → metallicity in the LMC is known to be lower than in the solar neighborhood by a factor 2 or more. Based on 20 → eclipsing binary systems, the distance to the LMC is measured to one percent precision to be 49.59±0.09 (statistical) ±0.54 (systematic) kpc (Pietrzynski et al., 2019, Nature 567, 200).

→ large; → Magellanic; → cloud.

1) Of, relating to, or named from, Ferdinand Magellan (see below).
2) Of or pertaining to characteristic of the → Magellanic Clouds.

Named in honor of Ferdinand Magellan (c. 1480-1521), the Portuguese navigator, who undertook the first voyage around the world. The two Clouds were first described by Magellan's chronicler Pigafetta, after leaving the Strait of Magellan in 1520; → -ic.

A filament of → neutral hydrogen which connects the → Small Magellanic Cloud and → Large Magellanic Cloud. The Magellanic Bridge appears to result from a → close encounter between these two galaxies some 200 million years ago.

→ Magellanic; → bridge.

Two irregular satellite galaxies of our own Galaxy which are visible from the Southern Hemisphere as misty patches in the night sky. → Large Magellanic Cloud; → Small Magellanic Cloud.

→ Magellanic; → cloud.

A class of low-mass galaxies with relatively rare features. In particular, these galaxies are characterized by a → stellar bar whose center is displaced from that of the disk and a one-armed spiral. The → Large Magellanic Cloud (LMC) is considered the prototype of this class of objects. However, despite a wealth of data, there is still a good deal of uncertainty concerning the nature of the LMC's bar. The majority of the observed Magellanic spirals in the nearby Universe share the LMC's structure, in particular the evidence of an offset bar and a one-armed spiral structure. A good example of these systems is NGC 3906, which shows evidence of the bar offset from the photometric center of the galaxy by 1.2 kpc (Pardy et al., 2016, ApJ 827, 149).

→ Magellanic; → spiral; → galaxy.

A thin trail of gas stretching from the → Magellanic System toward our own Galaxy over about 150° on the sky, corresponding to hundreds of thousands of light-years. This gas consists primarily of → neutral hydrogen and is thought to have originated from the Large and Small Magellanic Clouds as a result of tidal interactions with the Milky Way. See, e.g., Fox et al. 2013, arxiv/1304.4240, and references therein.

→ Magellanic; → stream.

A system consisting of the → Magellanic Clouds, the → Magellanic Bridge, and the → Magellanic stream.

→ Magellanic Clouds; → system.

A → metal-poor, → irregular galaxy like the → Large Magellanic Cloud or the → Small Magellanic Cloud. Other examples are: NGC 4449, NGC 4214, and NGC 3109.

→ Magellanic; → type; → galaxy.

An irregular galaxy, the smaller of the two → Magellanic Clouds that are satellites of our own Galaxy, lying in the southern constellation → Tucana about 20 degrees from the → south celestial pole. The SMC covers an area roughly 3 by 5 degrees in dimension and has an overall → visual magnitude about +2.7. The SMC is about 10,000 → light-years in diameter and some 210,000 light-years (61 → kpc) away. It has a visible mass of about 1/50-th that of our Galaxy and 1/10-th of that of the → Large Magellanic Cloud (LMC). Its → heavy element content is about a factor 5 smaller than that of the Galaxy. The SMC is the third-nearest external galaxy after the → Sagittarius Dwarf Elliptical Galaxy and the LMC.

→ small; → Magellanic; → cloud.