Gnomon's Guide

Dorado is a small southern constellation, invented by the Dutch explorers Pieter Dirkszoon Keyser and Frederick de Houtman in the late 16th Century. Its southerly position means that it is not fully visible at latitudes further north than 20°N, so it is best observed from the southern hemisphere. Although small, Dorado is rather long, so it borders many other constellations: Mensa, Hydrus, Reticulum, Horologium, Caelum, Pictor, and Volans. The constellation is rather faint, the brightest star being 3rd magnitude and with no other stars brighter than 4th magnitude. It has no Messier objects, but there is one major item of interest, the Large Magellanic Cloud. This is a complete galaxy, although a small one, and has all the things you would expect in a galaxy, including nebulas, clusters and even a supernova. Part of the Cloud is in the neighbouring constellation of Mensa, but most of it is in Dorado.

History

Dorado is not visible from Europe or the Middle East due to its southerly location, so it was never seen by the ancient Greeks and Babylonians, who devised the ancient constellations. As a result, it has no mythological associations.

The Dutch navigators, Keyser and de Houtman, devised 12 new constellations to cover the region of the southern sky not visible by the ancients. They named their new constellations after the exotic creatures they had seen on their travels, such as the Toucan and the Chameleon. The Dorado is a type of fish with a blue-green back and golden sides.

The main feature of the constellation is the long line of stars: gamma, alpha, zeta, beta and delta, heading towards the southeast. At the end of the line lies the Large Magellanic Cloud.

Stars

There are a number of different systems used to name the stars. The brightest and most interesting ones have proper names given to them by various cultures over the centuries, such as Aldebaran, Sirius and Vega. None of the stars in Dorado have such names, as the constellation was unknown to the ancient people. In the Bayer system, devised by Johann Bayer, each star is identified by a Greek letter followed by the genitive form of the constellation name. The letters are normally assigned roughly in order of brightness, so that alpha is the brightest star in the constellation¹.

Beta Doradus is a type of pulsating star called a Cepheid variable (named after the prototype delta Cephei). It varies in brightness from +3.5 to +4.1 every 9.8 days.

The star R Doradus is a red giant. Its diameter is 370 times that of the Sun. If it were at the centre of our solar system, its surface would be beyond the orbit of Mars. This is not the biggest star in the sky by any means, but because it is only 200 light years away, it is the star with the biggest apparent diameter, that is, the biggest as seen from Earth (excluding the Sun). The star is rather faint, varying between +4.8 and +6.6, but in infra-red frequencies it is one of the brightest stars in the sky (again excluding the Sun).

New General Catalogue (NGC)

The NGC catalogue is a list of interesting deep-space objects (that is, objects outside our solar system). It was compiled by Dreyer at Armagh Observatory based on the observations of Sir William Herschel. Dorado has many objects in the NGC catalogue and most of them are in the treasure chest that is the Large Magellanic Cloud. The following table lists a few of the NGC objects which are not in the Cloud. The Cloud objects will be dealt with in a separate section.

These galaxies are all part of the Dorado Group of galaxies, one of the richest in the southern hemisphere. NGC 1566 is a Seyfert galaxy - a type of galaxy in which the central section seems to give off much more radiation than a normal spiral, indicating a possible black hole at the core.

The Large Magellanic Cloud

Our galaxy is known as the Milky Way, and is one of a group of about 30 galaxies of all shapes and sizes known as the Local³ Group. The Milky Way is the second largest galaxy in the group, first place going to the Andromeda Galaxy (M31). Third in the list is another spiral galaxy, the Triangulum Galaxy (M33). Fourth largest in the local group is the irregular shaped blob known as the Large Magellanic Cloud. Most of the Cloud lies within the bounds of Dorado, although the southern end strays into the constellation of Mensa. It is the closest significant galaxy to our own and is easily visible to the naked eye, looking like a detached portion of the Milky Way.

The Large Magellanic Cloud (LMC) is a dwarf galaxy, about one tenth the size of the Milky Way, and is believed to be in orbit around the Milky Way. This is not certain, though, as recent reports have suggested that it may be just passing. Although irregular, it displays some evidence of a barred structure, so it is considered to be an intermediate form between a true irregular and a barred spiral galaxy. The Cloud was recorded as early as 964 AD by Arabic astronomer Al Sufi, and may have been mentioned by early seafarer Amerigo Vespucci, but only really came to the attention of the Western World when Ferdinand Magellan came back from his travels in 1519, so it is named after him, along with its brother, the Small Magellanic Cloud.

The Cloud, when examined closely, appears to contain all the same sort of things that you get in our own galaxy, including about 400 planetary nebulas, more than 700 clusters and even the only supernova near to us since the invention of the telescope! Examination of the supernova SN1987A in the LMC in 1987 as it occurred provided an accurate distance to the Cloud of 168,000 light years.

Interesting Objects in the Large Magellanic Cloud

Some of the objects in the Cloud have NGC numbers, others use a special series of N number. Because the cloud is far away, all the objects in it are effectively at the same distance from us.

• About 800 light years in diameter, the Tarantula Nebula, also known as NGC 2070 or 30 Doradus, is considered to be the biggest and most active star-forming region in the local group of galaxies. If it were the same distance away from us as the Great Nebula in Orion, it would take up half the sky. The Tarantula Nebula is at the leading edge of the LMC as it moves through intergalactic space. The galaxy compresses the intergalactic hydrogen at its front edge, causing much greater concentrations there than anywhere else in the galaxy, and hydrogen is the basic building block of nebulas and stars, so star-formation is more active there than anywhere else known in the Local Group. The stars formed are much bigger, and as a result have much shorter lives, ending in violent explosions which redistribute their contents in the nebula, causing more stars to form. This makes the whole nebula a fascinating place for anybody studying the life cycle of stars. See below for a fuller description of a supernova.

  • At the centre of the Tarantula Nebula lies R136, a group of stars described by astronomers as 'the largest, hottest, most massive stars known.' They are young enough that none of them has yet exploded as a supernova, and they all emit very high levels of stellar wind, which sculpts the nebula around them.

  • Hodge 301 is another cluster of stars within the Tarantula Nebula. It is estimated to be about 25 million years old, and at least 40 stars in this cluster have exploded as supernovas.

• The Ghost Head Nebula, NGC 2080, is a star-forming region about 50 light years across. There are two bright spots, with the appearance of eyes in the 'head'. The stars in these regions are reckoned to be less than 10,000 years old, as they haven't yet blown away the dust around them. Man had already begun to look at the stars at the time when the first light from these stars arrived on Earth.

• N44 is another giant emission nebula, about 1,000 light years across. It glows red because the hydrogen gas is heated by the embedded stars. Various parts of it have been studied in detail by the Hubble Space Telescope (HST):

  • N44C is a part of N44 which shows beautiful wispy filaments. This portion of the nebula glows with colours which have not been fully explained.

  • N44F is a part of N44 which is illuminated by a central star. The star has blown the dust around it outwards, leaving a glowing hole in the nebula.

• N49, N63A and N132D are all remnants of supernovas.

• NGC 1850 is a young globular cluster surrounded by nebulosity. Such clusters are of great interest because they don't occur in our own galaxy. All the globular clusters in the Milky Way are very old and as a result have quite different characteristics.

• N30B is a curious peanut-shaped nebula within a larger nebula. It appears to be caused by two stars, each one pushing the interstellar medium back in a sphere around itself.

Supernova 1987A

Most of the stars we can see in the sky, including our sun, burn steadily in a predictable pattern, going through a number of different phases and eventually burning out when they completely run out of fuel. If the star is really big to begin with, however, it has a different fate: it will suddenly collapse inwards, then erupt outwards. The entire star will be destroyed in a giant explosion known as a supernova. The matter of the star will be splattered across the galaxy - most of the heavier elements on earth, such as gold, are thought to have originated in nearby supernovas long ago. There is nothing left behind except a superdense star remnant, which may be a neutron star or a black hole, depending on the mass of the original star.

Based on observations of very distant galaxies, we know that such explosions occur reasonably frequently, and there should be about two each century in our own galaxy. Unfortunately, the large amount of dust in our galaxy obscures them from our view. Only seven supernovas have been observed within our galaxy in recorded history, and the last one was in 1604 before the invention of the telescope. So astronomers would love to see a supernova close up⁴. Their wish was almost granted in 1987 when a star did explode, not in our galaxy but in one of the nearest ones to it, the Large Magellanic Cloud. Labelled SN 1987A (because it was the first supernova in 1987), this was of immense interest to all astronomers. Within minutes of it being detected, all available telescopes were directed towards it and the whole course of the explosion was studied in detail over a period of months. Because this area of the had been studied in detail before the explosion, astronomers were able to identify the star which had exploded; it had been a blue giant star.

Supernovas recorded in our own galaxy in the past were so bright they could be seen during the day. SN 1987A, because it is in a different galaxy, was not so impressive, reaching a modest magnitude of +3 before fading away after about 10 months. Telescopically, the views were stunning. The expanding shell of debris could be seen changing week by week. Detailed measurements of the speed at which light emanating from the explosion illuminated the dust clouds around the explosion were used to provide an accurate distance measurement to the Cloud.

Most interesting of all, the predicted subatomic particles known as neutrinos were detected - in fact before the astronomers were even aware of the supernova. Neutrinos are bizarre particles with no mass and no charge, so they react very little with ordinary matter. It has been said that a neutrino has a 50% chance of passing straight through a sheet of lead a light year thick without being affected. As a result, neutrino observatories only detect a tiny proportion of the neutrinos that pass through them, perhaps one in a billion. In order to protect the observatories from other types of particle, they are placed deep underground in abandoned mines. The rock above them filters out other types of particle, but leaves the neutrinos unchanged.

Three neutrino observatories observed neutrinos at 07:35 UTC on 23 February, 1987. The supernova was detected visually on 24 February, but the time on the previous day is considered the time when the light first reached the Earth. Current theories of supernovas predict huge amounts of neutrinos, and the number observed, while small, was in keeping with the theory.

Observations of the remains of the supernova have so far failed to show any evidence of a neutron star or black hole, but astronomers are not unduly worried yet. There are two strange debris rings, which no-one has yet explained.

Extrasolar Planets

Extrasolar planets are ones outside our solar system, orbiting other stars. They are a hot topic for astronomers, with new ones being discovered all the time. Because the distances involved are so huge, the planets have to be detected by indirect means, such as by analysing the effect their gravity has on their parent star.

Astronomers would like to find an earth-like planet in the 'habitable zone', at the right distance from the parent star for earth-like life to be possible⁵. Planet HD 30177 b is much bigger than the earth, making it likely to be a gas giant like Jupiter and as a result not a suitable place for life as we know it.

Meteor Showers

The space debris which creates a meteor shower usually comes from the tail of a comet, as the Earth crosses the point where the comet has passed previously on its own orbit. Imagine a trail of breadcrumbs; now imagine breadcrumbs travelling at kilometre per second speeds and burning up in the Earth's atmosphere.

The meteor shower connected with this constellation is called the Gamma Doradids. The radiant, that is, the point in the sky from which the meteors appear to be coming, is actually across the border in the adjacent constellation of Horologium, but the nearest bright star to it is gamma Doradus, hence the name. The shower is a minor one, and extends over the period 19 August to 6 September each year, with a peak of activity on 28 August. The peak hourly rate is 5 meteors per hour.

Dorado Trivia

• The word Dorado is Spanish, but it is treated by the astronomers as if it were Greek, so the genitive is Doradus. It literally means 'golden'.

• The Dorado, Coryphaena hippurus, is found throughout the world in tropical seas. Also known as Dolphinfish or Mahimahi, it has a blue-green back and golden sides. It is a large fish, growing to about 13kg in size, although specimens as large as 18kg have been caught. Dorados like to swim just below the surface of the ocean. They are fast: they can reach speeds of 90 km/h. The male Dorado has a curious protruding forehead, like a cartoon picture of a brainy scientist.

• The search for gold drove the bloody conquest⁶ of Central and South America by the Spanish conquistadores in the 16th Century. Following Hernando Cortes' slaughter of the Aztecs in Mexico, news came to them of a story which filled them with greed. The king of a distant land, on one day a year, would strip naked, cover his entire body with gold dust, and dive into a sacred lake to honour the gods. The man became known in Spanish as El Dorado, the golden one. The legend grew in the telling, until eventually El Dorado was a city of gold where the metal was so common that people used it for everyday articles such as plates and cups. It led others, notably Francisco Pizarro, to pursue a similar campaign against the Incas in Peru. The golden city was believed to be situated near the town of Bogota, yet when the Spanish eventually reached the highlands in 1538, no trace of it was found. Explorers from many nations continued to search throughout the South American continent for El Dorado until well into the next century.

• Dorado is a holiday resort on the north coast of Puerto Rico, about 17 miles to the west of San Juan. Its official name is San Antonio del Dorado, and its patron saint is Saint Anthony of Padua.