Orion Nebula

From Wikipedia, the free encyclopedia

Orion Nebula

diffuse nebula
The entire Orion Nebula in visible light.
Observation data: J2000 epoch
Subtype	Reflection/Emission[2]
Right ascension	05h 35m 17.3s[1]
Declination	−05° 23′ 28″[1]
Distance	1,344±20 ly (412 pc)[3] ly
Apparent magnitude (V)	+4.0[4]
Apparent dimensions (V)	65×60 arcmins[5]
Constellation	Orion
Physical characteristics
Radius	12 ly[a] ly
Absolute magnitude (V)	—
Notable features	Trapezium cluster
Designations	NGC 1976, M42, LBN 974, Sharpless 281
See also: Lists of nebulae

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way south^[b] of Orion's Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of 1,344 ± 20 light years^[3][6] and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.^[7]

The Orion Nebula is one of the most scrutinized and photographed objects in the night sky, and is among the most intensely studied celestial features.^[8] The nebula has revealed much about the process of how stars and planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula.

Physical characteristics

Play media

Discussing the location of the Orion Nebula, what is seen within the star-formation region, and the effects of interstellar winds in shaping the nebula.

Amateur image of the Orion Nebula taken with a DSLR camera.

The constellation of Orion with the Orion Nebula (lower middle).

The nebula is visible with the naked eye even from areas affected by some light pollution. It is seen as the middle "star" in the sword of Orion, which are the three stars located south of Orion's Belt. The star appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through binoculars or a small telescope. The peak surface brightness of the central region is about 17 Mag/arcsec² (about 14 millinits) and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec² (about 0.27 millinits).^[9] (In the photos shown here the brightness, or luminance, is enhanced by a large factor.)

The Orion Nebula contains a very young open cluster, known as the Trapezium due to the asterism of its primary four stars. Two of these can be resolved into their component binary systems on nights with good seeing, giving a total of six stars. The stars of the Trapezium, along with many other stars, are still in their early years. The Trapezium may be a component of the much larger Orion Nebula Cluster, an association of about 2,000 stars within a diameter of 20 light years. Two million years ago this cluster may have been the home of the runaway stars AE Aurigae, 53 Arietis, and Mu Columbae, which are currently moving away from the nebula at velocities greater than 100 km/s.^[10]

Coloration

Observers have long noted a distinctive greenish tint to the nebula, in addition to regions of red and of blue-violet. The red hue is a result of the Hα recombination line radiation at a wavelength of 656.3 nm. The blue-violet coloration is the reflected radiation from the massive O-class stars at the core of the nebula.

The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known spectral lines at that time could explain it. There was some speculation that the lines were caused by a new element, and the name nebulium was coined for this mysterious material. With better understanding of atomic physics, however, it was later determined that the green spectrum was caused by a low-probability electron transition in doubly ionized oxygen, a so-called "forbidden transition". This radiation was all but impossible to reproduce in the laboratory because it depended on the quiescent and nearly collision-free environment found in deep space.^[11]

History

Messier's drawing of the Orion Nebula in his 1771 memoir, Mémoires de l'Académie Royale

There has been speculation that the Mayans of Central America may have described the nebula within their "Three Hearthstones" creation myth; if so, the three would correspond to two stars at the base of Orion, Rigel and Saiph, and another, Alnitak at the tip of the "belt" of the imagined hunter, the vertices of a nearly perfect equilateral triangle^[vague] with Orion's Sword (including the Orion Nebula) in the middle of the triangle^[vague] seen as the smudge of smoke from copal incense in a modern myth, or, in (the translation it suggests of) an ancient one, the literal or figurative embers of a fiery creation.^[12][13]

Neither Ptolemy's Almagest nor Al Sufi's Book of Fixed Stars noted this nebula, even though they both listed patches of nebulosity elsewhere in the night sky; nor did Galileo mention it, even though he also made telescopic observations surrounding it in 1610 and 1617.^[14] This has led to some speculation that a flare-up of the illuminating stars may have increased the brightness of the nebula.^[15]

The first discovery of the diffuse nebulous nature of the Orion Nebula is generally credited to French astronomer Nicolas-Claude Fabri de Peiresc, on 26 November 1610 when he made a record of observing it with a refracting telescope purchased by his patron Guillaume du Vair.^[14]

The first published observation of the nebula was by the Jesuit mathematician and astronomer Johann Baptist Cysat of Lucerne in his 1619 monograph on the comets (describing observations of the nebula that may date back to 1611).^[16] He made comparisons between it and a bright comet seen in 1618 and described how the nebula appeared through his telescope as:

"one sees how in like manner some stars are compressed into a very narrow space and how round about and between the stars a white light like that of a white cloud is poured out"^[17]

His description of the center stars as different from a comet's head in that they were a "rectangle" may have been an early description of the Trapezium Cluster^[14][17][18] (The first detection of three of the four stars of this cluster is credited to Galileo Galilei in a February 4, 1617 although he did not notice the surrounding nebula — possibly due to the narrow field of vision of his early telescope.^[19])

The nebula was independently discovered by several other prominent astronomers in the following years, including, in 1656, Christiaan Huygens (whose sketch was the first published, in 1659).

Charles Messier first noted the nebula on March 4, 1769, and he also noted three of the stars in Trapezium. Messier published the first edition of his catalog of deep sky objects in 1774 (completed in 1771).^[20] As the Orion Nebula was the 42nd object in his list, it became identified as M42.

Henry Draper's 1880 photograph of the Orion Nebula, the first ever taken.

One of Andrew Ainslie Common's 1883 photograph of the Orion Nebula, the first to show that a long exposure could record new stars and nebulae invisible to the human eye.

In 1865 English amateur astronomer William Huggins used his visual spectroscopy method to examine the nebula showing it, like other nebulae he had examined, was made up of "luminous gas".^[21] On September 30, 1880 Henry Draper used the new dry plate photographic process with an 11-inch (28 cm) refracting telescope to make a 51-minute exposure of the Orion Nebula, the first instance of astrophotography of a nebula in history. Another set of photographs of the nebula in 1883 saw breakthrough in astronomical photography when amateur astronomer Andrew Ainslie Common used the dry plate process to record several images in exposures up to 60 minutes with a 36-inch (91 cm) reflecting telescope that he constructed in the backyard of his home in Ealing, outside London.
These images for the first time showed stars and nebula detail too faint to be seen by the human eye.^[22]
In 1902, Vogel and Eberhard discovered differing velocities within the nebula and by 1914 astronomers at Marseilles had used the interferometer to detect rotation and irregular motions. Campbell and Moore confirmed these results using the spectrograph, demonstrating turbulence within the nebula.^[23]

In 1931, Robert J. Trumpler noted that the fainter stars near the Trapezium formed a cluster, and he was the first to name them the Trapezium cluster. Based on their magnitudes and spectral types, he derived a distance estimate of 1,800 light years. This was three times farther than the commonly accepted distance estimate of the period but was much closer to the modern value.^[24]

In 1993, the Hubble Space Telescope first observed the Orion Nebula. Since then, the nebula has been a frequent target for HST studies. The images have been used to build a detailed model of the nebula in three dimensions. Protoplanetary disks have been observed around most of the newly formed stars in the nebula, and the destructive effects of high levels of ultraviolet energy from the most massive stars have been studied.^[25]

In 2005, the Advanced Camera for Surveys instrument of the Hubble Space Telescope finished capturing the most detailed image of the nebula yet taken. The image was taken through 104 orbits of the telescope, capturing over 3,000 stars down to the 23rd magnitude, including infant brown dwarfs and possible brown dwarf binary stars.^[26] A year later, scientists working with the HST announced the first ever masses of a pair of eclipsing binary brown dwarfs, 2MASS J05352184–0546085. The pair are located in the Orion Nebula and have approximate masses of 0.054 M_☉ and 0.034 M_☉ respectively, with an orbital period of 9.8 days. Surprisingly, the more massive of the two also turned out to be the less luminous.^[27]

Structure

Optical images reveal clouds of gas and dust in the Orion Nebula; an infrared image (right) reveals the new stars shining within.

The entirety of the Orion Nebula extends across a 1° region of the sky, and includes neutral clouds of gas and dust, associations of stars, ionized volumes of gas, and reflection nebulae.

The Nebula is part of a much larger nebula that is known as the Orion Molecular Cloud Complex. The Orion Molecular Cloud Complex extends throughout the constellation of Orion and includes Barnard's Loop, the Horsehead Nebula, M43, M78, and the Flame Nebula. Stars are forming throughout the Orion Nebula, and due to this heat-intensive process the region is particularly prominent in the infrared.

The nebula forms a roughly spherical cloud that peaks in density near the core.^[28] The cloud has a temperature ranging up to 10,000 K, but this temperature falls dramatically near the edge of the nebula.^[28] Unlike the density distribution, the cloud displays a range of velocities and turbulence, particularly around the core region. Relative movements are up to 10 km/s (22,000 mi/h), with local variations of up to 50 km/s and possibly more.

The current astronomical model for the nebula consists of an ionized region roughly centered on Theta¹ Orionis C, the star responsible for most of the ultraviolet ionizing radiation. (It emits 3-4 times as much photoionizing light as the next brightest star, Theta² Orionis A.)^[29] This is surrounded by an irregular, concave bay of more neutral, high-density cloud, with clumps of neutral gas lying outside the bay area. This in turn lies on the perimeter of the Orion Molecular Cloud.

Observers have given names to various features in the Orion Nebula. The dark lane that extends from the north toward the bright region is called the "Fish's Mouth". The illuminated regions to both sides are called the "Wings". Other features include "The Sword", "The Thrust", and "The Sail".^[30]

Star formation

View of several proplyds within the Orion Nebula taken by the Hubble Space Telescope.

Star Formation Fireworks in Orion.

The Orion Nebula is an example of a stellar nursery where new stars are being born. Observations of the nebula have revealed approximately 700 stars in various stages of formation within the nebula.

Recent observations with the Hubble Space Telescope have yielded the major discovery of protoplanetary disks within the Orion Nebula, which have been dubbed proplyds.^[31] HST has revealed more than 150 of these within the nebula, and they are considered to be systems in the earliest stages of solar system formation. The sheer numbers of them have been used as evidence that the formation of star systems is fairly common in our universe.

Stars form when clumps of hydrogen and other gases in an H II region contract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extreme temperatures by converting gravitational potential energy to thermal energy. If the temperature gets high enough, nuclear fusion will ignite and form a protostar. The protostar is 'born' when it begins to emit enough radiative energy to balance out its gravity and halt gravitational collapse.

Typically, a cloud of material remains a substantial distance from the star before the fusion reaction ignites. This remnant cloud is the protostar's protoplanetary disk, where planets may form. Recent infrared observations show that dust grains in these protoplanetary disks are growing, beginning on the path towards forming planetesimals.^[32]

Once the protostar enters into its main sequence phase, it is classified as a star. Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster.^[25] Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.^[c]

Stellar wind and effects

Once formed, the stars within the nebula emit a stream of charged particles known as a stellar wind. Massive stars and young stars have much stronger stellar winds than the Sun.^[33] The wind forms shock waves or hydrodynamical instabilities when it encounters the gas in the nebula, which then shapes the gas clouds. The shock waves from stellar wind also play a large part in stellar formation by compacting the gas clouds, creating density inhomogeneities that lead to gravitational collapse of the cloud.

Herbig–Haro 47 seen with a bow shock and a series of jet-driven shocks.^[34]

View of the ripples (Kelvin–Helmholtz instability) formed by the action of stellar winds on the cloud.

There are three different kinds of shocks in the Orion Nebula. Many are featured in Herbig–Haro objects:^[35]

• Bow shocks are stationary and are formed when two particle streams collide with each other. They are present near the hottest stars in the nebula where the stellar wind speed is estimated to be thousands of kilometers per second and in the outer parts of the nebula where the speeds are tens of kilometers per second. Bow shocks can also form at the front end of stellar jets when the jet hits interstellar particles.
• Jet-driven shocks are formed from jets of material sprouting off newborn T Tauri stars. These narrow streams are traveling at hundreds of kilometers per second, and become shocks when they encounter relatively stationary gases.
• Warped shocks appear bow-like to an observer. They are produced when a jet-driven shock encounters gas moving in a cross-current.
• The interaction of the stellar wind with the surrounding cloud also forms "waves" which are believed to be due to the hydrodynamical Kelvin-Helmholtz instability.^[36]

The dynamic gas motions in M42 are complex, but are trending out through the opening in the bay and toward the Earth.^[28] The large neutral area behind the ionized region is currently contracting under its own gravity.
There are also supersonic "bullets" of gas piercing the hydrogen clouds of the Orion Nebula. Each bullet is ten times the diameter of Pluto's orbit and tipped with iron atoms glowing bright blue. They were probably formed one thousand years ago from an unknown violent event.^[37]

Evolution

Panoramic image of the center of the nebula, taken by the Hubble Telescope. This view is about 2.5 light years across. The Trapezium is at center left.

Interstellar clouds like the Orion Nebula are found throughout galaxies such as the Milky Way. They begin as gravitationally bound blobs of cold, neutral hydrogen, intermixed with traces of other elements. The cloud can contain hundreds of thousands of solar masses and extend for hundreds of light years. The tiny force of gravity that could compel the cloud to collapse is counterbalanced by the very faint pressure of the gas in the cloud.

Whether due to collisions with a spiral arm, or through the shock wave emitted from supernovae, the atoms are precipitated into heavier molecules and the result is a molecular cloud. This presages the formation of stars within the cloud, usually thought to be within a period of 10-30 million years, as regions pass the Jeans mass and the destabilized volumes collapse into disks. The disk concentrates at the core to form a star, which may be surrounded by a protoplanetary disk. This is the current stage of evolution of the nebula, with additional stars still forming from the collapsing molecular cloud. The youngest and brightest stars we now see in the Orion Nebula are thought to be less than 300,000 years old,^[38] and the brightest may be only 10,000 years in age.

Some of these collapsing stars can be particularly massive, and can emit large quantities of ionizing ultraviolet radiation. An example of this is seen with the Trapezium cluster. Over time the ultraviolet light from the massive stars at the center of the nebula will push away the surrounding gas and dust in a process called photo evaporation. This process is responsible for creating the interior cavity of the nebula, allowing the stars at the core to be viewed from Earth.^[8] The largest of these stars have short life spans and will evolve to become supernovae.

Within about 100,000 years, most of the gas and dust will be ejected. The remains will form a young open cluster, a cluster of bright, young stars surrounded by wispy filaments from the former cloud.^[39] The Pleiades is a famous example of such a cluster.

Gallery

• 

  An APEX view of star formation in the Orion Nebula.
• 

  Orion Nebula was captured using the Wide Field Imager camera on the MPG/ESO 2.2-metre telescope.
• 

  Orion Nebula Complex including M42, M43, Running Man Nebula (NGC 1973, 1975, and 1977) and much of the surrounding nebulosity.
• 

  Panoramic image of the Orion Nebula, taken by Ioannidis Panos with an 8 Inch Newtonian telescope and a Nikon D70 camera.
• 

  Infant stars, image from NASA's Spitzer Space Telescope.
• 

  The Orion Nebula imaged with the 2.2m ESO/MPG telescope.
• 

  The central part of the Orion Nebula.
• 

  This wide-field view of the Orion Nebula (Messier 42), was taken with the VISTA infrared survey telescope at ESO's Paranal Observatory in Chile.
• 

  Orion by Spitzer.
• 

  The Orion Nebula's biggest stars.
• 

  An infrared image showing fledgling stars located in the Orion Nebula.
• 

  Core detail of the nebula with all the stars identified.
• 

  A shot of the core details of the Orion Nebula.

Monarch butterfly

From Wikipedia, the free encyclopedia

Monarch butterfly
Female
Male
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Lepidoptera
Family:	Nymphalidae
Tribe:	Danaini
Genus:	Danaus Kluk, 1802
Species:	D. plexippus
Binomial name
Danaus plexippus (Linnaeus, 1758)
Synonyms
Papilio plexippus Linnaeus, 1758 Danaus archippus (Fabricius, 1793)[1] Danaus menippe (Hübner, 1816)[2] Anosia plexippus Dyar, 1903

The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the most familiar North American butterfly. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in)^[3] (the viceroy butterfly is similar in color and pattern, but is markedly smaller and has an extra black stripe across the hind wing).

The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the United States and southern Canada to Mexico, covering thousands of miles, with a corresponding multi-generational return North. The western North American population of monarchs west of the Rocky Mountains most often migrate to sites in California but have been found in overwintering Mexico sites.^[4][5] Monarchs were transported to the International Space Station and were bred there.^[6]

Description

The monarch’s wingspan ranges from 8.9 to 10.2 centimetres (3.5–4.0 in).^[3] The upper side of the wings is tawny-orange, the veins and margins are black, and in the margins are two series of small white spots. The forewings also have a few orange spots near the tip. The underside is similar, but the tip of the forewing and hindwing are yellow-brown instead of tawny-orange and the white spots are larger.^[7] The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants.^[8] Wings size and shape differ between migratory and non-migratory monarchs. Monarchs from the eastern population of North America have larger and more angular forewings than those in the western population.^[6]

Its flight has been described as "slow and sailing".^[9]

Adults exhibit sexual dimorphism. The male has a black patch or spot of androconial scales on either hindwing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs), and the black veins on its wing are lighter and narrower than those on the female’s.^[10] The male is also slightly larger.^[6][7] One variation has been observed in Australia, New Zealand, Indonesia and the United States termed nivosus by lepidopterists. It is grayish-white in all areas of the wings that are normally orange and is only about 1% or less of all monarchs, but populations as high as 10% exist on Oahu in Hawaii.^[11]

Like all insects, the monarch has six legs, but uses the four hindlegs as it carries its two front legs against its body.^[12]

Range

The range of the western and eastern populations of the monarch butterfly expands and contracts dependent upon the season. The range differs between breeding areas, migration routes and winter roosts.^[6]:(p18)

In North America, the monarch ranges from southern Canada to northern South America. It has also been found in Bermuda, Cook Islands,^[13] Hawaii, Cuba^[14] and other Caribbean islands^[6]:(p18) the Solomons, New Caledonia, New Zealand,^[15] Papua New Guinea,^[16] Australia, New Guinea, Sri Lanka, India, Nepal, the Azores, the Canary Islands Philippines, North Africa^[17] and Honolulu.^[18][19] It appears in the UK in some years as an acccidental.^[20] No genetic differences between monarch populations exist.^[21] Reproductive isolation has had no effect in creating sub species.^[6]:(p19)

Status

The monarch butterfly is not currently listed under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) or protected specifically under U.S. domestic laws.^[22] On 14 August 2014, the Center for Biological Diversity and the Center for Food Safety filed a legal petition requesting Endangered Species Act protection for the monarch and its habitat.^[6]

Habitat

Overwintering populations of D. plexippus are found in Mexico, California, along the Gulf coast, year-round in Florida, and in Arizona where the habitat provides the specific conditions necessary for their survival.^[23][24] The overwintering habitat typically provides access to streams, plenty of sunlight (for body temperatures that allow flight), appropriate vegetation on which to roost, and is relatively free of predators. Overwintering, roosting butterflies have been seen on sumacs, locusts, basswood elm, oak, osage orange, mulberry, pecan, willow, cottonwood, and mesquite.^[25] While breeding, its habitat can be found in agricultural fields, pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides – anywhere where there is access to larval host plants.^[26] Habitat restoration is a primary goal in monarch conservation efforts. Habitat requirements change during migration. During the fall migration, butterflies must have access to nectar-producing plants. During the spring migration, butterflies must have access to larval food plants and nectar plants.

Life cycle

The monarch undergoes the four stages of complete metamorphosis:

Eggs

The eggs are derived from materials ingested as a larvae and from the spermataphores received from males during mating.^[27] Eggs are laid singly on the underside of a young leaf of a milkweed plant during the spring and summer months.^[28] The eggs are cream-colored or light green, ovate to conical in shape, and about 1.2 x 0.9 mm in size.
The eggs weigh less than 0.5 mg each and have raised ridges that form longitudinally from the point to apex to the base. Though each egg is 1/1000th the mass of the female, she may lay up to her own mass in eggs. Females lay smaller eggs as they age. Larger Females lay larger eggs.^[27] The number of eggs laid by a female, who may mate several times, is from 290-1179 eggs. ^[29] Females lay their eggs on milkweed that make their offspring less sick.^[30][31] Eggs take 3 to 8 days to develop and hatch into larva or caterpillars.^[6]:(p21)Monarchs will lay eggs along the southern migration route.^[32]

Larvae

The caterpillar goes through five major, distinct stages of growth and after each one, it molts. Each caterpillar, or instar, that molts is larger than the previous as it eats and store energy in the form of fat and nutrients to carry it through the nonfeeding pupal stage.

5th instar with the white spots visible on the prolegs.

The first instar caterpillar that emerges out of the egg is pale green and translucent. It lacks banding coloration or tentacles. The larvae or caterpillar eats its egg case and begins to feed on milkweed. It is during this stage of growth that the caterpillar begins to sequester cardenolides. The circular motion a caterpillar uses while eating milkweed prevents the flow of latex that could entrap it.

The second instar larva develops a characteristic pattern of white, yellow and black transverse bands. It is no longer translucent but is covered in short setae. Pairs of black tentacles begin to grow. One pair grows on the thorax and another pair on the abdomen.

The third instar larva has more distinct bands and the two pairs of tentacles become longer. Legs on the thorax differentiate into a smaller pair near the head and larger pairs further back. These third stage caterpillars began to eat along the leaf edges.

The fourth instar has a different banding pattern. It develops white spots on the prolegs near the back of the caterpillar.

The fifth instar larva has a more complex banding pattern and white dots on the prolegs, with front legs that are small and very close to the head.

At this stage of development, it is relatively large compared to the earlier instars. The caterpillar completes its growth. At this point, it is 25 to 45 mm long and 5 to 8 mm wide. This can be compared to the first instar which was 2 to 6 mm long and 0.5 to 1.5 mm wide. Fifth instar larvae increase 2000 times from first instars. Fifth-stage instar larva chew through the petiole or mid-rev of milkweed leaves and stop the flow of latex. After this, they eat more leaf tissue. Before pupation, larva must consume milkweed to increase their mass prior to pupation. Larva stop feeding and search for a pupation site. The caterpillar attaches itself securely to a horizontal surface, using a silk pad. At this point, it latches on with its hind legs and hangs down. It then molts into an opaque, blue-green chrysalis with small gold dots. At normal summer temperatures, it matures in a few weeks. The cuticle of the chrysalis becomes transparent and the monarch's characteristic orange and black wings become visible. At the end of metamorphosis, the adult emerges from the chrysalis, expands and dries its wings and flies away. Monarch metamorphosis from egg to adult occurs during the warm summer temperatures in as little as 25 days, extending to as many as seven weeks during cool spring conditions. During the development, both larva and their milkweed hosts are vulnerable to weather extremes, predators, parasites and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive.^{[6]:(pp21-22)}

Pupa

In the pupa or chrysalis stage, the caterpillar spins a silk pad on to a horizontal substrate. It then hangs from the pad by the last pair of prolegs upside down, resembling the letter 'J'. It sheds its skin, leaving itself encased in an articulated green exoskeleton. During this pupal stage, the adult butterfly forms inside. The exoskeleton becomes transparent before it ecloses (emerges), and its adult colors can finally be seen.

Adult

The adult butterfly emerges after about two weeks, and hangs until its wings are dry. Fluids are pumped into wings and they expand and stiffen. The monarch expands and retracts its wings, and once conditions allow it then flies to feed on a variety of nectar plants. During the breeding season adults reach sexual maturity in four or five days, however, the migrating generation will not reach maturity until overwintering is complete.^[33] Monarchs typically live two to five weeks during the breeding season.^{[6]:(pp22-23)} Larvae growing in high densities are smaller, have lower survival, and weigh less as adults compared to lower densities.^[34]

Reproduction

Play media

Monarch butterfly mating

Males that are fit are more likely to mate. Females and males typically mate more than once. Females that mate several times lay more eggs.^[35] Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus.^[36]

Courtship occurs in two phases. During the aerial phase, the male pursues and often forces the female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes.^[37] Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating, though some have more success than others.^[38][39] During copulation, the male transfers the spermatophore to the female. Along with sperm, the spermatophore provides the female with nutrition to aid her in egg-laying. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more females' eggs.^[40]

Pictorial lifecycle

• 

  Mating
• 

  Laying eggs
• 

  egg
• 

  caterpillar
• 

  beginning pupation
• 

  chrysalis
• 

  emerging from chrysalis

Taxonomy

White morph of the monarch in Hawaii called White Monarch

The name "monarch" may be in honor of King William III of England.^[41] The monarch was originally described by Linnaeus in his Systema Naturae of 1758 and placed in the genus Papilio.^[42] In 1780, Jan Krzysztof Kluk used the monarch as the type species for a new genus Danaus.

There are three species of Monarch butterflies:

• D. plexippus, described by Linnaeus in 1758, is the species known most commonly as the monarch butterfly of North America. Its range actually extends worldwide and can be found in Hawaii, Australia, New Zealand, Spain and on Oceanic Islands.

• D. erippus, the southern monarch, was described by Cramer in 1775. This species is found in tropical and subtropical latitudes of South America, mainly in Brazil, Uruguay, Paraguay, Argentina, Bolivia, Chile and southern Peru. The south American monarch and the North American monarch may have been one species at one time. Some researchers believe the southern monarch separated from the monarch's population some 2 mya, at the end of the Pliocene. Sea levels were higher, and the entire Amazonas lowland was a vast expanse of brackish swamp that offered limited butterfly habitat.^[43]

• D. cleophile Jamaican monarch (Godart in 1819) – ranges from Jamaica to Hispaniola.^[44]

Six subspecies and two color morphs of D. plexippus have been identified:^[45]

• D. p. plexippus – nominate subspecies, described by Linnaeus in 1758, is the migratory subspecies known from most of North America.
  • D. p. p. form nivosus, the white monarch commonly found on Oahu, Hawaii and rarely in other locations.^[11]
  • D. p. p. (as yet unnamed) – a color morph lacking some wing vein markings.^[46]

• D. p. nigrippus (Richard Haensch, 1909) – as forma: Danais [sic] archippus f. nigrippus. Hay-Roe et al. in 2007 identified this taxon as a subspecies:^[47]
• D. p. megalippe (Jacob Hübner, [1826]) – nonmigratory subspecies, and is found from Florida and Georgia southwards, throughout the Caribbean and Central America to the Amazon River.
• D. p. leucogyne (Arthur G. Butler, 1884) − St. Thomas.
• D. p. portoricensis A. H. Clark, 1941 − (Puerto Rico).
• D. p. tobagi A. H. Clark, 1941 − (Tobago).

The percentage of the white morph in Oahu is nearing 10%. On other Hawaiian islands, the white morph occurs at a relatively low frequency. White Monarchs (nivosus) have been found throughout the world, including Australia, New Zealand, Indonesia, and the United States.^[11]

Some taxonomists disagree on these classifications.^[43][47]

Monarchs were classified under the family Danaidae, but have been re-classified under Nymphalidae since at least 1958.^[48]

Larvae host plants

The host plants used by the monarch caterpillar include:

Swamp milkweed, one of many species of Asclepias milkweeds used by the monarch

North America

• Asclepias nivea – Caribbean Milkweed
• Asclepias syriaca – Common Milkweed
• Asclepias incarnata – Swamp Milkweed
• Asclepias verticillata – Whorled Milkweed
• Asclepias tuberosa – Butterfly Weed
• Asclepias exaltata – Poke Milkweed
• Asclepias viridis – Green Antelopehorn Milkweed
• Asclepias asperula – Antelopehorns Milkweed
• Asclepias oenotheroide – Zizotes Milkweed
• Asclepias variegata – White Milkweed
• Asclepias perennis – Aquatic Milkweed
• Asclepias humistrata – Sandhill/Pinewoods Milkweed
• Asclepias fascicularis – Mexican Whorled Milkweed
• Asclepias speciosa – Showy Milkweed
• Asclepias subulata – Rush Milkweed
• Asclepias erosa – Desert Milkweed
• Asclepias californica – California Milkweed
• Asclepias cordifolia – Heartleaf Milkweed
• Asclepias vestita – Woolly Milkweed
• Asclepias eriocarpa – Woolly Pod Milkweed^[49]
• Sarcostemma clausa – white vine^[18][50]
• Cynanchum laeve – Sand vine milkweed^[51]

Asclepias curassavica has been planted as an ornamental and naturalized. Its distribution is probably worldwide. Year-round plantings may be the cause of new overwintering sites along the Gulf coast and in Spain.

Adult food sources

Nectaring on purple coneflower Echinacea purpurea.

Although larvae eat only milkweed, adult monarchs feed on the nectar of many plants including:

• Apocynum cannabinum – Indian hemp
• Asclepias sp. – milkweeds
• Aster sp. – asters
• Cirsium sp. – thistles
• Daucus carota – wild carrot
• Dipsacus sylvestris – teasel
• Echinacea sp. – coneflowers
• Erigeron canadensis – horseweed
• Eupatorium maculatum – spotted joe-pye weed
• Eupatorium perfoliatum – common boneset
• Hesperis matronalis – dame's rocket
• Medicago sativa – alfalfa
• Solidago sp. – goldenrod
• Syringa vulgaris – lilac
• Trifolium pratense – red clover
• Vernonia altissima – tall ironweed^[24]

Monarchs obtain moisture and minerals from damp soil and wet gravel, a behavior known as mud-puddling. The monarch has also been noticed puddling at an oil stain on pavement.^[24]

Origin of name

Danaus (Greek Δαναός), a great-grandson of Zeus, was a mythical king in Egypt or Libya, who founded Argos; Plexippus was one of the 50 sons of Aegyptus, the twin brother of Danaus.

In Homeric Greek plexippos (πληξιππος) means "one who urges on horses", i.e. "rider or charioteer". In the 10th edition of Systema Naturae, at the bottom of page 467,^[52] Linnaeus wrote that the names of the Danai festivi, the division of the genus to which Papilio plexippus belonged, were derived from the sons of Aegyptus. Linnaeus divided his large genus Papilio, containing all known butterfly species, into what we would now call subgenera. The Danai festivi formed one of the 'subgenera', containing colourful species, as opposed to the Danai candidi, containing species with bright white wings. Linnaeus wrote: "Danaorum Candidorum nomina a filiabus Danai Aegypti, Festivorum a filiis mutuatus sunt." (= The names of the Danai candidi have been derived from the daughters of Danaus, those of the Danai festivi from the sons of Aegyptus).

Robert Michael Pyle suggested Danaus is a masculinised version of Danaë (Greek Δανάη), Danaus’s great-great-granddaughter, to whom Zeus came as a shower of gold, which seemed to him a more appropriate source for the name of this butterfly.^[53] He masculinized the genus name because it had to agree in gender with the species name. If the species-group name is not a noun in apposition, Pyle could have been right and the genus name and specific epithet have to agree in gender, but in that case it is the specific epithet and not the genus name, that is to be altered. In the case of Danaus plexippus, however, the specific epithet is a noun in apposition, formed from a personal name in the nominative case, which should not be altered (see ICZN art. 31.1 and art. 32.3). If, instead of Danaus, Danaë had been intended, the name would simply have been Danae plexippus. Moreover, in Systema Naturae, there is a very strong connection of the names with Danaus, and not a single one with Danaë.

Migration

The eastern population migrates both north and south on an annual basis. The population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in Mexico. The western population overwinters in various coastal sites in central and southern California. The overwintered population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. The second, third and fourth generations return to their northern locations in the United States and Canada in the spring.^[54] 

Commercially bred monarchs migrate to overwintering sites in Mexico adding to already existing data of migratory behavior. Not all monarchs in the eastern population migrate to Mexico.^[55]

Defense against predators

In both caterpillar and butterfly form, monarchs are aposematic—warding off predators with a bright display of contrasting colors to warn potential predators of their undesirable taste and poisonous characteristics.

Large larvae are able to avoid wasp predation by dropping from the plant or by jerking their bodies.^[56]

Aposematism

Chemical structure of oleandrin, one of the cardiac glycosides

Monarchs are foul-tasting and poisonous due to the presence of cardenolide aglycones in their bodies, which the caterpillars ingest as they feed on milkweed.^[36] By ingesting a large amount of plants in the genus Asclepias, primarily milkweed, monarch caterpillars are able to sequester cardiac glycosides, or more specifically cardenolides, which are steroids that act in heart-arresting ways similar to digitalis.^[57] It has been found that monarchs are able to sequester cardenolides most effectively from plants of intermediate cardenolide content rather than those of high or low content.^[58]

Additional studies have shown that different species of milkweed have differing effects on growth, virulence, and transmission of parasites.^[59] One species, Asclepias curassavica, appears to reduce the proportion of monarchs infected by parasites. There are two possible explanations for the positive role of A. curassavica on the monarch caterpillar: that it promotes overall monarch health to boost the monarch's immune system; or that chemicals from the plant have a direct negative effect on the parasites.^[59]

After the caterpillar becomes a butterfly, the toxin shift to different parts of the body. Since many birds attack the wings of the butterfly, having three times the cardiac glycosides in the wings leaves predators with a very foul taste and may prevent them from ever ingesting the body of the butterfly.^[57] In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.^[60]

Mimicry

Monarch (left) and viceroy (right) butterflies exhibiting Müllerian mimicry

Monarchs share the defense of noxious taste with the similar-appearing viceroy butterfly in what is perhaps one of the most well-known examples of mimicry. Though long purported to be an example of Batesian mimicry, the viceroy is actually reportedly more unpalatable than the monarch, making this a case of Müllerian mimicry.^[61]

Human interaction

The monarch is the state insect of Alabama,^[62] Idaho,^[63] Illinois,^[64] Minnesota,^[65] Texas,^[66] Vermont,^[67] and West Virginia.^[68] It was nominated in 1990 as the national insect of the United States,^[69] but the legislation did not pass.^[70]

Monarchs can be attracted by cultivating a butterfly garden with specific milkweed species and nectar plants. Efforts are underway to establish these Monarch Waystations. Monarchs are raised as a hobby and for educational purposes.^[71]

An IMAX film Flight of the Butterflies describes the story of the Urquharts, Brugger and Trail to then unknown migration to Mexican overwintering areas.^[72]

Sanctuaries and reserves have been created at over-wintering locations in Mexico and California to limit habitat destruction. These sites can generate significant tourism revenue.^[73]

Organizations and individuals participate in tagging programs. Tagging information is used to study migration patterns.^[74]

Monarchs are bred and used in schools, hospices, memorial services and weddings.^[75] Memorial services for 911 include the release of captive bred monarchs.^[76][77][78] Monarchs are used in schools and nature centers for educational purposes.^[79]

Threats

There is increasing concern related to the ongoing decline of monarchs; based on a 2014 twenty-year comparison, the population west of the Rocky Mountains has dropped more than 50 percent since 1997 and the numbers east of the Rockies have declined by more than 90 percent since 1995.^[6]

Predators

Larva feed exclusively on milkweed and consume protective cardiac glycosides. Toxin levels in Asclepias sp.vary. Not all monarchs are unpalatable, but exhibit Batesian or automimics. Cardiac glycosides levels are higher in the abdomen and wings. Some predators can differentiate between these parts and consume the most palatable ones.^[80]
Bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, pinyon jays,^[80]Black-headed Grosbeak, and orioles.^[18]

Some mice are able to withstand large doses of the toxin. Overwintering adults become less toxic over time making them more vulnerable to predators. In Mexico, about 14% of the overwintering monarchs are eaten by birds and mice.^[23]

In North America, eggs and first instar larvae of the monarch are eaten by larvae and adults of the introduced Asian lady beetle (Harmonia axyridis).^[81] The Chinese mantis ("Tenodera sinensis") will consume the larvae once the gut is removed thus avoiding cardenolides.^[82] Wasps commonly consume larvae.^[83]

Several birds have also adapted various methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides. The oriole is able to eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them.^[84] The grosbeak, on the other hand, has adapted the ability an insensitivity to secondary plant poisons which allows it to ingest monarchs without vomiting. As a result, orioles and grosbeaks will periodically have high levels of cardenolides in their bodies, and they will be forced to go on periods of reduced monarch consumption. This cycle of predation effectively reduces the potential predation of monarchs by 50 percent and indicates that monarch aposematism has a legitimate purpose.^[84]

On Oahu, a white morph of the monarch has emerged. This is because of the introduction, in 1965 and 1966, of two bulbul species, Pycnonotus cafer and Pycnonotus jocosus. They are now the most common insectivore birds, and probably the only ones preying on insects as large as the monarch. Monarchs in Hawaii are known to have low cardiac glycoside levels, but the birds may also be tolerant of the chemical. The two species hunt the larvae and some pupae from the branches and undersides of leaves in milkweed bushes. The bulbuls also eat resting and ovipositing adults, but rarely flying ones. Because of its colour, the white morph has a higher survival rate than the orange one. This is either because of apostatic selection (i.e. the birds have learned the orange monarchs can be eaten), because of camouflage (the white morph matches the white pubescence of milkweed or the patches of light shining through foliage), or because the white morph does not fit the bird's search image of a typical monarch, so is thus avoided.^[85]

Parasites

Parasites include the tachinid flies Sturmia convergens^[86] and Lespesia archippivora. Lesperia-parasitized butterfly larvae complete the formation of their crysalid but die before they emerge as an adult. Before pupation is complete, one white maggot comes out of the chrysalid. The maggot forms a brown pupa on the ground then emerges as an adult.^[87]

The bacterium Micrococcus flacidifex danai also infects larvae. Just before pupation, the larvae migrate to a horizontal surface and die a few hours later, attached only by one pair of prolegs, with the thorax and abdomen hanging limp. The body turns black shortly after. The bacterium Pseudomonas aeruginosa has no invasive powers, but causes secondary infections in weakened insects. It is a common cause of death in laboratory-reared insects.^[87]

The protozoan Ophryocystis elektroscirrha is another parasite of the monarch. It infects the subcutaneous tissues and propagates by spores formed during the pupal stage. The spores are found over all of the body of infected butterflies, with the greatest number on the abdomen. These spores are passed, from female to caterpillar, when spores rub off during egg-laying and are then ingested by caterpillars. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels variy in populations. This is not the case in laboratory rearing, where after a few generations, all individuals can be infected.^[88] Infection with this parasite creates an effect known as culling whereby migrating monarchs that are infected are less likely to complete the migration. This results in overwintering populations with lower parasite loads.^[89]

Confusion of host plants

The black swallow-wort (Cynanchum louiseae) and pale swallow-wort (Cynanchum rossicum) plants are problematic for monarchs in North America. Monarchs lay their eggs on these relatives of native vining milkweed (Cynanchum laeve) because they produce stimuli similar to milkweed. Once the eggs hatch, the caterpillars are poisoned by the toxicity of this invasive plant from Europe.^[90]

Habitat loss due to herbicide use

Conservationists attribute the disappearance of mikweed speices to agricultural practices in the Midwest, where genetically modified seeds are bred to resist herbicides that eliminate milkweed nearby. Growers eliminate milkweed that previously grew between the rows of food crops. Corn and soybeans are resistant to the effect of the herbicide glyphosate. The increased use of these crop strains is correlated with the decline in Monarch populations between 1999 and 2010.^[91][92] Chip Taylor, director of Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone" with 120–150 million acres lost.^[93][94] To help fight this problem, Monarch Watch encourages the planting of “Monarch Waystations”.^[71]

Loss of overwintering habitat

The area of forest occupied has been declining and reached its lowest level in two decades in 2013. The decline is continuing but is expected to increase during the 2013–2014 season. Mexican environmental authorities continue to monitor illegal logging of the oyamel trees. The Oyamel is a major species of evergreen on which the overwintering butterflies spend a significant time during their winter diapause, or suspended development.^[95]

A 2014 study acknowledged that while “the protection of overwintering habitat has no doubt gone a long way towards conserving monarchs that breed throughout eastern North America", their research indicates that habitat loss on breeding grounds in the United States is the main cause of both recent and projected population declines.^[96]

Climate

Climate variations during the fall and summer affect butterfly reproduction. Rainfall, and freezing temperatures affect milkweed growth. Omar Vidal, director general of WWF-Mexico, said "The monarch’s lifecycle depends on the climatic conditions in the places where they breed. Eggs, larvae and pupae develop more quickly in milder conditions. Temperatures above 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate." ^[97]

Genome

The monarch was the first butterfly to have its genome sequenced.^[6]:(p12) The 273-million base pair draft sequence includes a set of 16,866 protein-coding genes. The genome provides researchers insights into migratory behavior, the circadian clock, juvenile hormone pathways and microRNAs that are differentially expressed between summer and migratory monarchs.^{[98][99][100]} More recently, the genetic basis of monarch migration and warning coloration has been described.^[101]

There is no genetic differentiation between the migratory populations of eastern and western North America.^[6]:(p16) Recent research has identified the specific areas in the genome of the monarch that regulate migration. There appears to be no genetic difference between a migrating and nonmigrating monarch but the gene is expressed in migrating monarchs but not expressed in nonmigrating monarchs.^[21]

Conservation

The Center for Biological Diversity, The Center for Food Safety, The Xerces Society and Lincoln Brower have filed a petition to the United States Department of the Interior to protect the monarch by having it declared an endangered species.^[6]
Conservationists are lobbying transportation departments and utilities to reduce their use of herbicides and specifically encourage milkweed to grow along roadways and power lines. Reducing roadside mowing and application of herbicides during the butterfly breeding season will encourage milkweed growth. Conservationists lobby agriculture companies to set aside areas that remain unsprayed to allow the butterflies to breed.^[92] Butterfly gardening is thought to increase the populations of butterflies.^[102]