The globular cluster Messier 5, one of the oldest belonging to the Milky Way. Credit: NASA/ESA/HST
In the late 18th century, Charles Messier was busy hunting for comets in the night sky, and noticed several “nebulous” objects. After initially mistaking them for the comets he was seeking, he began to compile a list of these objects so other astronomers would not make the same mistake. Known as the Messier Catalog, this list consists of 100 objects, consisting of distant galaxies, nebulae, and star clusters.
Among the many famous objects in this catalog is the M5 globular star cluster (aka. NGC 5904). Located in the galactic halo within the Serpens Constellation, this cluster of stars is almost as old as the Universe itself (13 billion years)! Though very distant from Earth and hard to spot, it is a favorite amongst amateur astronomers who swear by its beauty.
The constellation Apus, located in the Southern Hemisphere. Credit: astropixels.com
Welcome back to Constellation Friday! Today, we will be dealing with the beautiful bird-of-paradise itself, the Apus constellation!
The Southern Hemisphere is replete with beautiful stars and constellations, enough to keep a stargazing enthusiast busy for a lifetime. For countless centuries, the indigenous peoples of South America, South Africa, Australia and the South Pacific have looked up at these stars and drawn inspiration. However, to European astronomers, they remained uncharted and unknown until the 16th century.
It was during this time that Flemish astronomer Petrus Plancius designated twelve constellations, using asterisms found in the southern skies. One such constellation was Apus, a faint constellation in the southern sky that is named for the bird-of-paradise – a beautiful bird that is indigenous to the South Pacific. Today, it is one of the 88 constellations defined by the International Astronomic Union (IAU).
Name and Meaning:
The name Apus is derived from Greek word apous, which literally means “no feet”. The name applies to a species of bird that is indigenous to Indonesia, Papua New Guinea, and Eastern Australia (which was believed at one time to have no feet). Its original name on Plancius’ charts was “Apis Indica” – the Latin term for “Indian Bee” (presumably an error for “avis”, which means bird).
This faint southern constellation of Apus was one of the original twelve created by Plancius, based on observations provided by Pieter Dirkszoon Keyser and Frederick de Houtman – two Dutch explorers/navigators who mapped the southern sky around Australia between 1595 and 1597.
It was included on a celestial globe published in 1597 or 1598 in Amsterdam by Plancius and his associate, Flemish cartographer and engraver Jodocus Hondius. After it’s introduction on Plancius’ globe, it also appeared in Uranometria, a star atlas published by Johann Bayer – a German celestial catrographer – in 1603.
Here, it appeared under the name “Apis Indica”. It also grouped with the other members of the “Johann Bayer family” of constellations, all of which appeared in Uranometria. These include Chamaeleon, Dorado, Grus, Hydrus, Indus, Musca, Pavo, Phoenix, Tucana, and Volans. The constellation also appears as part of the Chinese constellations, where it is known as the “Little Wonder Bird”.
In the 17th century, Ming Dynasty astronomer Xu Guangqi adapted the European southern hemisphere constellations when producing The Southern Asterisms. Combining Apus with some of the stars in Octans, he designated the stars in this area of the night sky into the constellation known as Yìquè (“Exotic Bird”). In 1922, Apus was included by the International Astronomical Union in the list of 88 constellations.
The southern constellation Apus and neighboring Deep Sky Objects. Credit: absoluteaxarquia.com
Notable Features:
Within the Apus constellation, there are 39 stars that are brighter than or equal to apparent magnitude 6.5. The most notable of these is Alpha Apodis. an orange giant star with a magnitude of 3.8, located roughly 411 light years away from Earth. Beta Apodis is also an orange giant, with a magnitude of 4.2. and located 158 light years from Earth. And Gamma Apodis , another orange giant, has a magnitude of 3.9 and is located 160 light years away.
Delta Apodis is a binary star system consisting of a red giant and an orange giant. Delta¹ has a magnitude of 4.7 and is located 765 light years away, while Delta² has a magnitude of 5.3 and is located 663 light years away. Then there is Theta Apodis, a variable red giant star with a maximum magnitude of 4.8 and a minimum of 6.1 that is located 328 light years away.
NO Apodis is a red giant that varies between magnitudes 5.71 and 5.95 and is located around 883 light-years away from Earth. This star shines with a luminosity that is approximately 2059 times greater than our Sun’s and has a surface temperature of 3568 K.
Apus is also home to a few Deep Sky Objects. These include the IC 4499 loose globular cluster (shown below), which is located in the medium-far galactic halo and has an apparent magnitude of 10.6. This object is rather unique in that its metallicity readings indicate that it is younger than most other globular clusters in the region.
NASA/ESA Hubble Space Telescope image of the globular cluster IC 4499, located in the Apus constellation. Credit: ESA/NASA/HST
Then there’s NGC 6101, a 14th mangitude globular cluster located seven degree north of Gamma Apodis. Last, there is the spiral galaxy IC 4633, which is very faint due to its location well within the Milky Way’s nebulous disc.
Finding Apus:
For binoculars, take a look at Alpha Apodis. This 3.8 magnitude star is located 411 light years away from Earth. Now move on to Delta. It is a wide double star which is two orange 5th-magnitude members separated by 103 arc seconds and an easy split. Or try observing Theta – its a variable star whose brightness ranges from magnitude 4.8 to 6.1 in a period of 109 days.
For telescopes, take a look at more difficult binary star Kappa-1 Apodis. The brightest component of this disparate pair has a magnitude of 5.4 and the companion is 12th magnitude, 27 arcseconds away. Need more? Then turn your gaze towards Kappa-2 only 0.63 degrees from Kappa-1. Kappa-1 Apodis is a binary star approximately 1020 light years from Earth. The primary component, Kappa-1 Apodis A, is a blue-white B-type subgiant with a mean apparent magnitude of +5.40. It is classified as a Gamma Cassiopeiae type variable star and its brightness varies from magnitude +5.43 to +5.61. The companion star, Kappa-1 Apodis B, is a 12th magnitude orange K-type subgiant. It is 27 arc seconds from the primary.
For larger telescopes, wander off and look at NGC 6101 located about seven degrees north of Gamma. Here we have a small, 14th magnitude globular cluster! If you’re really good you can try for spiral galaxy IC 4633. It’s so faint it doesn’t even have a magnitude listing!
This M4 globular cluster, as imaged by the Wide Field Imager at ESO’s La Silla Observatory. Credit: ESO
During the late 18th century, Charles Messier began to notice that a series of “nebulous” objects in the night sky that he originally mistook for comets. In time, he would notice that they were in fact something significantly different. With the hope of preventing other astronomers from making the same mistake, he began compiling a list of these in what would come to be known as the Messier Catalog.
Consisting of 100 objects, the catalog became an important milestone in both astronomy and the research of Deep Sky objects. Among the many famous objects in this catalog is the M4 loose globular cluster (aka. NGC 6121). Located in the Scorpius (Scorpio) Constellation, this great cluster of ancient stars is one of the closest Messier Objects of its kind to Earth.
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come. Thanks to the development of modern telescopes and astronomy, this list was amended by the early 20th century to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.
One such constellation is Antlia, who’s name means “the Pump”. Discovered by French astronomer Nicolas Louis de Lacaille in the mid-eighteenth century, Antlia is located in a rather remote and open section of the southern skies, and was used to chart the southern hemisphere. Today, it is one of the 88 constellations recognized by the IAU.
The eight planets of our Solar System vary widely, not only in terms of size, but also in terms of mass and density (i.e. its mass per unit of volume). For instance, the 4 inner planets – those that are closest to the Sun – are all terrestrial planets, meaning they are composed primarily of silicate rocks or metals and have a solid surface. On these planets, density varies the farther one ventures from the surface towards the core, but not considerably.
By contrast, the 4 outer planets are designated as gas giants (and/or ice giants) which are composed primarily of of hydrogen, helium, and water existing in various physical states. While these planets are greater in size and mass, their overall density is much lower. In addition, their density varies considerably between the outer and inner layers, ranging from a liquid state to materials so dense that they become rock-solid.
Messier Object 3, as imaged by Adam Block at Mount Lemmon SkyCenter, University of Arizona. Credit: caelumobservatory.com
During the late 18th century, Charles Messier began to notice a series of “nebulous” objects in the night sky which he originally mistook for comets. With the hope of preventing other astronomers from making the same mistake, he began compiling a list of these in what would come to be known as the Messier Catalog. Consisting of 100 objects, the catalog became an important milestone in the discovery and research of Deep Sky objects.
One such object is Messier Object 3 (aka. M3 or NGC 5272) globular cluster that is located in the northern constellation of Canes Venatici. Since it was first observed, this globular star cluster has gone on to become one of the best-studied objects in the night sky, and is considered by many amateur astronomers to be one of the finest visible clusters.
A photo of the constellation Andromeda with all Bayer-designated stars marked and the IAU figure drawn in. Credit: Roberto Mura/Wikipedia Commons
In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of the then-known 48 constellations. His treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come. Thanks to the development of modern telescopes and astronomy, this list was amended by the early 20th century to include the 88 constellation that are recognized by the International Astronomical Union (IAU) today.
Of these, Andromeda is one of the oldest and most widely recognized. Located north of the celestial equator, this constellation is part of the family of Perseus, Cassiopeia, and Cepheus. Like many constellation that have come down to us from classical antiquity, the Andromeda constellation has deep roots, which may go all the way back to ancient Babylonian astronomy.
The compact globular cluster known as Messier Object 2. Credit: NASA/STScI/WikiSky
In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.
In addition to cataloging some of the most beautiful objects in the night sky, this list would come to be an important milestone in the discovery of Deep Sky Objects. The second object to make the list is known as Messier Object 2 (aka. M2 or NGC 7089), one of the largest globular cluster in the Milky Way, and which is located in the constellation Aquarius.
Description:
As one of the largest known globular clusters, Messier 2 is a rich, round concentration of gravitationally bound stars that orbits the galactic core. Located about 33,000 light years (10,000 parsecs) from our Solar System, this cluster measures some 175 light-years in diameter and is believed to contain about 150,000 stellar members – including 21 known variable stars. Its brightest stars are red and yellow giant stars.
Because its members are so tightly packed together, it has a density classification of II – which is reserved for clusters that are particularly rich and compact. And like most globular clusters, M2’s central region is highly compressed, measuring just 3.7 light years in diameter. It’s tidal influence, on the other hand, has a radius of 233 light years, beyond which members stars would escape due to the influence of the Milky Way’s tidal forces.
Positioned well beyond the galactic center, M2 is also noted for its elliptical shape, and is believed to be as much as 13 billion years old.
The constellation Aquarius. M2 is visible right of center, above Beta Aquirii. Credit: iau.org
History of Observation:
M2 was first discovered by Jean-Dominique Maraldi in 1746 while observing a comet with Jacques Cassini. According to Cassini’s notes, which detail the discovery, the two believed it to be a “nebulous star” at the time:
“On September 11 I have observed another one [nebulous star] for which the right ascension is 320d 7′ 19″ [21h 20m 29s], and the declination 1d 55′ 38″ south, very near to the parallel where the Comet should be. This one is round, well terminated and brighter in the center, about 4′ or 5′ in extent and not a single star around it to a pretty large distance; none can be seen in the whole field of the telescope. This appears very singular to me, for most of the stars one calls nebulous are surrounded by many stars, making one think that the whiteness found there is an effect of the light of a mass of stars too small to be seen in the largest telescopes. I took, at first, this nebula for the comet.”
The object was independently recovered by Charles Messier in 1769, though he too mistook it for something else. In his notes, which were also taken on September 11th (fourteen years later), he described the object as a nebula:
“On September 11, 1760, I discovered in the head of Aquarius a beautiful nebula which doesn’t contain any star; I examined it with a good Gregorian telescope of 30 pouces focal length, which magnified hundred four [104] times; the center is brilliant, and the nebulosity which surrounds it is round; it resembles quite well the beautiful nebula which is located between the head and the bow of Sagittarius: It extends 4 minutes of arc in diameter; one can see it quite well in an ordinary telescope [refractor] of 2 feet [focal length]: I compared its passage of the meridian with that of Alpha Aquarii which is situated on the same parallel; its right ascension was derived at 320d 17′, and its declination at 1d 47′ south. In the night of June 26 and 27, 1764, I reviewed this nebula for a second time; it was the same, with the same appearances. This nebula can be found placed in the chart of the famous Comet of Halley, which I observed at its return in 1759 (b).”
Charles Messier, French astronomer, at the age of 40, by Nicolas Ansiaume. Credit: Public Domain.
Ultimately, it was William Herschel who finally resolved Messier 2 into the object we recognize today. This took in 1783, where – according to his notes – he was able to resolve individual stars:
“The scattered stars were brought to a good, well determined focus, from which it appears that the central condensed light is owing to a multitude of stars that appeared at various distances behind and near each other. I could actually see and distinguish the stars even in the central mass. The Rev. Mr. Vince, Plumian Professor of Astronomy at Cambridge, saw it in the same telescope as described.”
Locating Messier 2:
Messier 2 is located approximately 5 degrees (about 3 finger widths) north of Beta Aquarii, on the same declination as Alpha Aquarii. M2 is sufficiently bright enough to be seen in urban settings where light pollution is a factor, and can alternately be found by looking about 10 degrees (a fist width) south/southwest of Epsilon Pegasi (Enif).
Using binoculars, it will appear as a large, fuzzy ball with little or no resolution. To amateur astronomers using small telescopes, individual stars will be visible around the outer edges, with resolution improving significantly with aperture size of 6” or more. Those with large telescopes, and who are looking for a challenge, should look for a dark dust lane which crosses the north-east edge of this globular cluster.
An all-sky map showing the distribution of Messier objects. Credit: Jim Cornmell/Wikimedia Commons
Of course, John Herschel saw it as “It is like a heap of fine sand!” which is perhaps as apt an description as can be rendered. Through a large telescope, the globular cluster does resemble a glittering mass of sparkling granules.
And for your convenience, here are the vital statistics of this globular cluster:
Object Name: Messier 2 Alternative Designations: NGC 7089, GC 4678, Bode 70 Object Type: Class II Globular Cluster Constellation: Aquarius Right Ascension: 21 : 33.5 (h:m) Declination: -00 : 49 (deg:m) Distance: 33 (kly) Visual Brightness: 6.5 (mag) Apparent Dimension: 16.0 (arc min)
Good luck searching for this and other Deep Sky Objects!
