Small Magellanic Cloud

Last updated
SMC
Small Magellanic Cloud (Digitized Sky Survey 2).jpg
The Small Magellanic Cloud
(Source: Digitized Sky Survey 2)
Observation data (J2000 epoch)
Constellation Tucana and Hydrus
Right ascension 00h 52m 44.8s [1]
Declination −72° 49′ 43″ [1]
Redshift 0.000527 [1]
Distance 203.7 ± 1.5 kly (62.44 ± 0.47 kpc) [2]
Apparent magnitude  (V)2.7 [1]
Characteristics
Type SB(s)m pec [1]
Number of stars3 billion [3]
Size5.78 kiloparsecs (18,900 light-years) [1]
(diameter; 25.0 mag/arcsec2 B-band isophote) [4]
Apparent size  (V)5° 20′ × 3° 5′ [1]
Notable featuresCompanion dwarf to the
Milky Way
Other designations
SMC, [1] NGC 292, [1] PGC 3085, [1] Nubecula Minor [1]

The Small Magellanic Cloud (SMC) is a dwarf galaxy near the Milky Way. [5] Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about 5.78 kiloparsecs (18,900 light-years), [1] [4] and contains several hundred million stars. [5] It has a total mass of approximately 7 billion solar masses. [6] At a distance of about 200,000 light-years, the SMC is among the nearest intergalactic neighbors of the Milky Way and is one of the most distant objects visible to the naked eye.

Contents

The SMC is visible from the entire Southern Hemisphere and can be fully glimpsed low above the southern horizon from latitudes south of about 15° north. The galaxy is located across the constellation of Tucana and part of Hydrus, appearing as a faint hazy patch resembling a detached piece of the Milky Way. The SMC has an average apparent diameter of about 4.2° (8 times the Moon's) and thus covers an area of about 14 square degrees (70 times the Moon's). Since its surface brightness is very low, this deep-sky object is best seen on clear moonless nights and away from city lights. The SMC forms a pair with the Large Magellanic Cloud (LMC), which lies 20° to the east, and like the LMC, it is a member of the Local Group. It is currently a satellite of the Milky Way, but is likely a former satellite of the LMC.

Observation history

Panoramic Large and Small Magellanic Clouds as seen from ESO's VLT observation site. The galaxies are on the left side of the image. Panoramic Large and Small Magellanic Clouds.jpg
Panoramic Large and Small Magellanic Clouds as seen from ESO's VLT observation site. The galaxies are on the left side of the image.
Constellation of Tucana: the SMC is the green shape at the south (bottom) of picture Tucana IAU.svg
Constellation of Tucana: the SMC is the green shape at the south (bottom) of picture

In the southern hemisphere, the Magellanic clouds have long been included in the lore of native inhabitants, including south sea islanders and indigenous Australians. Persian astronomer Al Sufi mentions them in his Book of Fixed Stars , repeating a quote by the polymath Ibn Qutaybah, but had not observed them himself. European sailors may have first noticed the clouds during the Middle Ages when they were used for navigation. Portuguese and Dutch sailors called them the Cape Clouds, a name that was retained for several centuries. During the circumnavigation of the Earth by Ferdinand Magellan in 1519–1522, they were described by Antonio Pigafetta as dim clusters of stars. [7] In Johann Bayer's celestial atlas Uranometria , published in 1603, he named the smaller cloud, Nubecula Minor. [8] In Latin, Nubecula means a little cloud. [9]

Small Magellanic Cloud as photographed by an amateur astronomer. Unrelated stars have been edited out. Small Magellanic Cloud.png
Small Magellanic Cloud as photographed by an amateur astronomer. Unrelated stars have been edited out.

Between 1834 and 1838, John Frederick William Herschel made observations of the southern skies with his 14-inch (36 cm) reflector from the Royal Observatory. While observing the Nubecula Minor, he described it as a cloudy mass of light with an oval shape and a bright center. Within the area of this cloud he catalogued a concentration of 37 nebulae and clusters. [10]

In 1891, Harvard College Observatory opened an observing station at Arequipa in Peru. Between 1893 and 1906, under the direction of Solon Bailey, the 24-inch (610 mm) telescope at this site was used to survey photographically both the Large and Small Magellanic Clouds. [11] Henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, used the plates from Arequipa to study the variations in relative luminosity of stars in the SMC. In 1908, the results of her study were published, which showed that a type of variable star called a "cluster variable", later called a Cepheid variable after the prototype star Delta Cephei, showed a definite relationship between the variability period and the star's apparent brightness. Leavitt realized that since all the stars in the SMC are roughly the same distance from Earth, this result implied that there is similar relationship between period and absolute brightness. [12] This important period-luminosity relation allowed the distance to any other cepheid variable to be estimated in terms of the distance to the SMC. [13] She hoped a few Cepheid variables could be found close enough to Earth so that their parallax, and hence distance from Earth, could be measured. This soon happened, allowing Cepheid variables to be used as standard candles, facilitating many astronomical discoveries. [14]

Using this period-luminosity relation, in 1913 the distance to the SMC was first estimated by Ejnar Hertzsprung. First he measured thirteen nearby cepheid variables to find the absolute magnitude of a variable with a period of one day. By comparing this to the periodicity of the variables as measured by Leavitt, he was able to estimate a distance of 10,000 parsecs (30,000 light years) between the Sun and the SMC. [15] This later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique. [16]

Announced in 2006, measurements with the Hubble Space Telescope suggest the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way. [17]

Features

VISTA's view of the Small Magellanic Cloud. 47 Tucanae (NGC 104) is visible to the right of the Small Magellanic Cloud. VISTA's view of the Small Magellanic Cloud.jpg
VISTA's view of the Small Magellanic Cloud. 47 Tucanae (NGC 104) is visible to the right of the Small Magellanic Cloud.

The SMC contains a central bar structure, and astronomers speculate that it was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. [18]

There is a bridge of gas connecting the Small Magellanic Cloud with the Large Magellanic Cloud (LMC), which is evidence of tidal interaction between the galaxies. [19] This bridge of gas is a star-forming site. [20] The Magellanic Clouds have a common envelope of neutral hydrogen, indicating they have been gravitationally bound for a long time.

In 2017, using the Dark Energy Survey plus MagLiteS data, a stellar over-density associated with the Small Magellanic Cloud was discovered, which is probably the result of interactions between the SMC and LMC. [21]

The Small Magellanic Cloud pictured by the Hubble Space Telescope Head in the Clouds (potw2249a).tiff
The Small Magellanic Cloud pictured by the Hubble Space Telescope

X-ray sources

The Small Magellanic Cloud contains a large and active population of X-ray binaries. Recent star formation has led to a large population of massive stars and high-mass X-ray binaries (HMXBs) which are the relics of the short-lived upper end of the initial mass function. The young stellar population and the majority of the known X-ray binaries are concentrated in the SMC's Bar. HMXB pulsars are rotating neutron stars in binary systems with Be-type (spectral type 09-B2, luminosity classes V–III) or supergiant stellar companions. Most HMXBs are of the Be type which account for 70% in the Milky Way and 98% in the SMC. [23] The Be-star equatorial disk provides a reservoir of matter that can be accreted onto the neutron star during periastron passage (most known systems have large orbital eccentricity) or during large-scale disk ejection episodes. This scenario leads to strings of X-ray outbursts with typical X-ray luminosities Lx = 1036–1037  erg/s, spaced at the orbital period, plus infrequent giant outbursts of greater duration and luminosity. [24]

Monitoring surveys of the SMC performed with NASA's Rossi X-ray Timing Explorer (RXTE) [25] see X-ray pulsars in outburst at more than 1036 erg/s and have counted 50 by the end of 2008. The ROSAT and ASCA missions detected many faint X-ray point sources, [26] but the typical positional uncertainties frequently made positive identification difficult. Recent studies using XMM-Newton [27] and Chandra [28] have now cataloged several hundred X-ray sources in the direction of the SMC, of which perhaps half are considered likely HMXBs, and the remainder a mix of foreground stars, and background AGN.

No X-rays above background were observed from the Magellanic Clouds during the September 20, 1966, Nike-Tomahawk flight. [29] Balloon observation from Mildura, Australia, on October 24, 1967, of the SMC set an upper limit of X-ray detection. [30] An X-ray astronomy instrument was carried aboard a Thor missile launched from Johnston Atoll on September 24, 1970, at 12:54 UTC for altitudes above 300 km, to search for the Small Magellanic Cloud. [31] The SMC was detected with an X-ray luminosity of 5×1038 erg/s in the range 1.5–12 keV, and 2.5×1039 erg/s in the range 5–50 keV for an apparently extended source. [31]

The fourth Uhuru catalog lists an early X-ray source within the constellation Tucana: 4U 0115-73 (3U 0115-73, 2A 0116-737, SMC X-1). [32] Uhuru observed the SMC on January 1, 12, 13, 16, and 17, 1971, and detected one source located at 01149-7342, which was then designated SMC X-1. [33] Some X-ray counts were also received on January 14, 15, 18, and 19, 1971. [34] The third Ariel 5 catalog (3A) also contains this early X-ray source within Tucana: 3A 0116-736 (2A 0116-737, SMC X-1). [35] The SMC X-1, a HMXRB, is at J2000 right ascension (RA) 01h 15m 14s declination (Dec) 73° 42′ 22″.

Two additional sources detected and listed in 3A include SMC X-2 at 3A 0042-738 and SMC X-3 at 3A 0049-726. [35]

Mini Magellanic Cloud (MMC)

It has been proposed by astrophysicists D. S. Mathewson, V. L. Ford and N. Visvanathan that the SMC may in fact be split in two, with a smaller section of this galaxy behind the main part of the SMC (as seen from Earth perspective), and separated by about 30,000 ly. They suggest the reason for this is due to a past interaction with the LMC that split the SMC, and that the two sections are still moving apart. They dubbed this smaller remnant the Mini Magellanic Cloud. [36] [37]

In 2023, it was reported that the SMC is indeed two separate structures with distinct stellar and gaseous chemical compositions, separated by around 5 kiloparsecs. [38]

See also

Related Research Articles

<span class="mw-page-title-main">Star cluster</span> Group of stars

Star clusters are large groups of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered groups of stars, generally containing fewer than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

<span class="mw-page-title-main">Large Magellanic Cloud</span> Satellite galaxy of the Milky Way

The Large Magellanic Cloud (LMC) is a spiral satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (c. 16 kiloparsecs (52,000 light-years) away) and the possible dwarf irregular galaxy called the Canis Major Overdensity. Based on the D25 isophote at the B-band (445 nm wavelength of light), the Large Magellanic Cloud is about 9.86 kiloparsecs (32,200 light-years) across. It is roughly one-hundredth the mass of the Milky Way and is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Triangulum Galaxy (M33).

<span class="mw-page-title-main">Cepheid variable</span> Type of variable star that pulsates radially

A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.

<span class="mw-page-title-main">Magellanic Clouds</span> Two dwarf galaxies orbiting the Milky Way

The Magellanic Clouds are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both show signs of a bar structure, they are often reclassified as Magellanic spiral galaxies.

<span class="mw-page-title-main">Cosmic distance ladder</span> Succession of methods by which astronomers determine the distances to celestial objects

The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

<span class="mw-page-title-main">S Doradus</span> Star in the Large Magellanic Cloud

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<span class="mw-page-title-main">Extinction (astronomy)</span> Interstellar absorption and scattering of light

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<span class="mw-page-title-main">Magellanic Bridge</span> Stream of neutral hydrogen gas linking the two Magellanic clouds

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<span class="mw-page-title-main">AB7</span> Binary star in the Small Magellanic Cloud in the constellation Tucana

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<span class="mw-page-title-main">Type II Cepheid</span>

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<span class="mw-page-title-main">Classical Cepheid variable</span>

Classical Cepheids are a type of Cepheid variable star. They are young, population I variable stars that exhibit regular radial pulsations with periods of a few days to a few weeks and visual amplitudes ranging from a few tenths of a magnitude up to about 2 magnitudes. Classical Cepheids are also known as Population I Cepheids, Type I Cepheids, and Delta Cepheid variables.

<span class="mw-page-title-main">Jeremy Mould</span> Australian astronomer

Jeremy Richard Mould is an Australian astronomer currently at the Centre for Astrophysics and Supercomputing at Swinburne University of Technology. Mould was previously Director of the Research School of Astronomy and Astrophysics at the Australian National University and the American National Optical Astronomy Observatory. He is an Honorary Professorial Fellow, at the University of Melbourne.

<span class="mw-page-title-main">NGC 5204</span> Galaxy in the constellation Ursa Major

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<span class="mw-page-title-main">S Vulpeculae</span> Variable star in the constellation Vulpecula

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<span class="mw-page-title-main">HV 2112</span> Small Magellanic Cloud star in the constellation Tucana

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<span class="mw-page-title-main">NGC 121</span> Globular cluster in the constellation Tucana

NGC 121 is a globular cluster of stars in the southern constellation of Tucana. It is the oldest globular cluster in the Small Magellanic Cloud (SMC), which is a dwarf satellite galaxy of the Milky Way. This cluster was first discovered by English astronomer John Herschel on September 20, 1835. The compiler of the New General Catalogue, Danish astronomer John Louis Emil Dreyer, described this object as "pretty bright, pretty small, little extended, very gradually brighter middle". The cluster is located at a distance of around 200,000 light-years (60 kpc) from the Sun.

<span class="mw-page-title-main">NGC 1466</span> Globular star cluster in the constrellation Hydrus

NGC 1466 is the New General Catalogue designation for a globular cluster in the deep southern constellation of Hydrus. It is located in the outskirts of the Large Magellanic Cloud, which is a satellite galaxy of the Milky Way. The object was discovered November 26, 1834 by English astronomer John Herschel. John Dreyer described it as "pF, pS, iR, glbM, *7 f", meaning "pretty faint, pretty small, irregular round, gradually a little brighter middle, with a 7th magnitude star nearby". When using a small telescope, this is a "faint, small, unresolved and difficult" target with an angular size of 1.9 arc minutes. It has an integrated visual magnitude of 11.4.

<span class="mw-page-title-main">Period-luminosity relation</span> Astronomical principle

In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables, sometimes called the Leavitt Law. Discovered in 1908 by Henrietta Swan Leavitt, the relation established Cepheids as foundational indicators of cosmic benchmarks for scaling galactic and extragalactic distances. The physical model explaining the Leavitt's law for classical cepheids is called kappa mechanism.

References

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