Blackpool & District Astronomical Society

The Great Globular Cluster in Hercules (also known as the Hercules Globular Cluster, Messier Object 13, Messier 13, M13, or NGC 6205) is a globular cluster in the Hercules constellation at right ascension 16^h 41.7^m and declination +36° 28'.

It was discovered by Edmond Halley in 1714, and catalogued by Charles Messier on June 1, 1764.

With an apparent magnitude of 5.8, it is barely visible with the naked eye on a very clear night. Its real diameter is about 145 light years, and is composed of several hundred thousand stars, the brightest of which is the variable star V11 with an apparent magnitude of 11.95. M13 is 25,100 light-years away from Earth.

Its diameter is about 23 arc minutes and is readily viewable in small telescopes. Nearby is NGC 6207, a 12th magnitude edge on galaxy that lies 28 arc minutes directly north east. The J2000 coordinates are RA: 16h 41m 41.5s and Dec: +36° 27' 37".

M92 is one of the brighter globular clusters in the northern hemisphere, but it is often overlooked by astronomers because of its proximity to the even more spectacular M13.

M57 is best seen through at least an 8-inch telescope, but even a 3-inch telescope will show the ring. Larger instruments will show a few darker zones on the eastern and western edges of the ring, and some faint nebulosity inside the disk.

M15 is at a distance of about 33,600 light-years from Earth. It has an absolute magnitude of -9.2 which translates to a total luminosity of 360,000 times that of the Sun. Messier 15 is one of the most densely packed globulars known in the Milky Way galaxy. The core of this cluster has undergone a contraction known as core collapse and it has a central density cusp (just left of centre in image), with an enormous number of stars surrounding what may be a central black hole.

Messier 15 contains a rather high number of variable stars; 112 of these are known to reside within its mass of stars. There have also been found to be at least 8 pulsars in M15 including one double neutron star system, M15 C. Moreover, M15 houses one of only four planetary nebulae known in a globular cluster, Pease 1, discovered in 1928.

To the amateur astronomer Messier 15 appears as a fuzzy star in even the smallest of telescopes. Mid to large size telescopes (at least 6 in./150 mm diameter) will start to reveal individual stars, the brightest of which are of magnitude +12.6.

Open Cluster M37 (also known as Messier Object 37, Messier 37, M37, or NGC 2099) is the richest open cluster in the Auriga constellation. It was discovered by Giovanni Batista Hodierna before 1654. With an average telescope about 150 stars can be identified in it.

M36

Open Cluster M36 (also known as Messier Object 36, Messier 36, M36, or NGC 1960) is an open cluster in the Auriga constellation. It was discovered by Giovanni Batista Hodierna before 1654. M36 is at a distance of about 4,100 light years away from Earth and is about 14 light years across. There are at least sixty members in the cluster. The cluster is very similar to the Pleiades cluster (M45), and if it were the same distance from Earth it would be of nearly similar magnitude.

The Pleiades (also known as M45 or the Seven Sisters) is an open cluster in the constellation of Taurus. It is among the nearest to the Earth of all open clusters, probably the best known and certainly the most striking to the naked eye.

Accurate knowledge of the distance to the cluster is very important in astronomy as it is a crucial first step on the cosmic distance ladder, the calibration of the distance scale of the whole universe. The Hipparcos satellite caused consternation when it measured a distance to the cluster which was 10% smaller than most previous measurements, but it was later found to have suffered from a systematic error when observing the Pleiades which led to the discrepancy. The cluster is now known to lie at a distance of about 135 parsecs (440 light years).

The cluster is dominated by hot blue stars, which have formed within the last 100 million years. Dust that forms faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster but is now known to be an unrelated dust cloud that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which time it will have dispersed due to gravitational interactions with the spiral arms of the galaxy and giant molecular clouds.

They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related ^[1]. When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something which seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list^[2]

The distance to the Pleiades is an important step in calibrating distance scales for the whole universe, and has been estimated by many methods. As the cluster is so close to the Earth, its distance is relatively easy to measure. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell Diagram for the cluster which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades.

Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Hipparcos caused consternation among astronomers by finding a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique which should yield the most direct and accurate results. Later work has consistently found that the Hipparcos distance measurement for the Pleiades was in error, but it is not yet known why the error occurred ^[3]. The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs ^[4]^, ^[5].

The cluster is about 12 light years in diameter and contains approximately 500 stars in total. It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses^[6].

The cluster contains many brown dwarfs — objects with less than about 8% of the Sun's mass, which are not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass ^[7]. Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Also present in the cluster are several white dwarfs. Given the young age of the cluster normal stars are not expected to have had time to evolve into white dwarfs, a process which normally takes several billion years. It is believed that, rather than being individual low- to intermediate-mass stars, the progenitors of the white dwarfs must have been high-mass stars in binary systems. Transfer of mass from the higher-mass star to its companion during its rapid evolution would result in a much quicker route to the formation of a white dwarf.

Ages for star clusters can be estimated by comparing the H-R diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher apparent ages.

Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to its very low ignition temperature of 2.5 million kelvins, the highest-mass brown dwarfs will burn lithium eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years^[8]^[9].

Like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of the galaxy also hastening its demise.

Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.

It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.

Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars^[10].

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The name of the cluster itself is of Greek origin, though of uncertain etymology. Suggested derivations include: from πλεîν plein, to sail, making the Pleiades the "sailing ones"; from pleos, full or many; or from peleiades, flock of doves. The following table gives details of the brightest stars in the cluster:

**Pleiades Bright Stars**
Name	Pronunciation (IPA & respelling)	Designation	Apparent magnitude	Stellar classification
Alcyone	/æl'saɪəni/, al-sye'-ə-nee	Eta (25) Tauri	2.86	B7IIIe
Atlas	/'ætləs/, at'-ləs	27 Tauri	3.62	B8III
Electra	/i'lɛktrə/, ee-lek'-trə	17 Tauri	3.70	B6IIIe
Maia	/'meɪə, 'maɪə/; may'-ə, mye'-ə	20 Tauri	3.86	B7III
Merope	/'mɛrəpi/, mair'-ə-pee	23 Tauri	4.17	B6IVev
Taygeta	/tei'ɪʤəti/, tay-ij'-ə-tee	19 Tauri	4.29	B6V
Pleione	/'plaɪəni/, plye'-ə-nee	28 (BU) Tauri	5.09 (var.)	B8IVep
Celaeno	/sə'lino/, sə-lee'-no	16 Tauri	5.44	B7IV
Asterope	/ə'stɛrəpi/, ə-stair'-ə-pee	21 and 22 Tauri	5.64;6.41	B8Ve/B9V
—	—	18 Tauri	5.65	B8V

Andromeda was believed to be the largest galaxy of the Local Group of galaxies, which consists of the Andromeda Galaxy, the Milky Way Galaxy, and the Triangulum Galaxy, and about 30 other smaller galaxies. Due to recent findings based on improved measurements and data, scientists now believe that the Milky Way contains more dark matter and may be the most massive in the grouping.^[5] However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (10¹²) stars, greatly exceeding the number of stars in our own galaxy.^[6]

The Andromeda Galaxy is easily visible to the naked eye in a moderately dark sky, though such a sky is available only in smaller towns and isolated areas reasonably far from population centers and sources of light pollution. It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon.

The structure and stellar content of M32 is difficult to explain by traditional galaxy formation models. Recent simulations suggest a new scenario in which the strong tidal field of M31 can transform a spiral galaxy into a compact elliptical. As a small spiral galaxy falls into the central parts of M31, most of the outer layers of the smaller spiral are stripped away. The central bulge of the galaxy is much less effected and retains its morphology. Tidal effects trigger a massive star burst in the core, resulting in the high density of M32 we observe today.

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula with a greenish hue and is situated below Orion's Belt. 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 about 1,500 light years away, and is the closest region of star formation to Earth. The M42 nebula is estimated to be 30 light years across.

The Orion Nebula is considered to be one of the most scrutinized and photographed objects in the night sky, and is among the most intensely-studied celestial features.^[6] 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 discs, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula.

General information

The Orion Nebula is in fact 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, and M78. M43 is also part of M42, as well as several nearby reflection nebulae noted in the New General Catalogue. Stars are forming throughout the Orion Nebula, and due to this heat-intensive process the region is particularly prominent in the infrared.

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 below Orion's Belt. The star appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through a pair of binoculars or a small telescope.

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 source of three runaway stars, AE Aurigae, 53 Arietis, and Mu Columbae, all of which are moving away from the nebula at velocities greater than 100 km/s.^[7]

Observers have long noted a distinctive greenish tint to the nebula, in addition to regions of red and areas of blue-violet. The red hue is well-understood to be caused by H^α 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 twentieth 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 "nebulum" was coined for this mysterious material. With better understanding of atomic physics, however, it was later determined that the green spectra 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.

The Maya of Central America have a folk tale that deals with the Orion constellation's part of the sky. Their traditional hearths include in their middle a smudge of glowing fire that corresponds with the Orion nebula. This is clear pre-telescope evidence that the Maya detected a diffuse area of the sky contrary to the pin points of stars.^[9]

This nebula is currently visible to the unaided eye, yet oddly there is no mention of the nebulosity in the written astronomical records prior to the seventeenth century. In particular, neither Ptolemy in the Almagest nor Al Sufi in his Book of Fixed Stars noted this nebula, even though they both listed patches of nebulosity elsewhere in the night sky. Curiously this nebula was also not mentioned by Galileo, even though he made telescope observations of this part of the Orion constellation in 1610 and 1617.^[10] This has led to some speculation that a flare up of the illuminating stars may have increased the brightness of the nebula.^[11]

The Orion Nebula is generally credited as being first discovered in 1610 by Nicolas-Claude Fabri de Peiresc as noted in Peiresc's own records. Johann Baptist Cysat, a Jesuit astronomer, was the first to publish note of it (albeit somewhat ambiguous) in a book on a bright comet in 1618. It was independently discovered by several prominent astronomers in the following years, including Christiaan Huygens in 1656 (who's 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. (The first detection of these three stars is now credited to Galileo in 1617, but he did not notice the surrounding nebula—possible due to the narrow field of vision of his early telescope.) Charles Messier published the first edition of his catalog of deep sky objects in 1774 (completed in 1771).^[12] As the Orion Nebula was the 42nd object in his list, it became identified as M42.

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.^[13]

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 further than the commonly-accepted distance estimate of the period but was much closer to the modern value. ^[14]

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 level of ultraviolet energy from the most massive stars has been studied.^[15]

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.^[16] 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 least luminous.^[17]

Structure

Optical images reveal clouds of gas and dust in the Orion Nebula; an infrared image (right) reveals the new stars shining within. Credit: C. R. O'Dell-Vanderbilt University, NASA, and ESA.

The nebula forms a roughly spherical cloud that peaks in density near the core.^[18] The cloud has a temperature ranging up to 10,000 K, but this temperature falls dramatically near the edge of the nebula.^[19] 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 higher.

The current astronomical model for the nebula consists of an ionized region roughly centered on θ¹ C Orionis, the star responsible for most of the ultraviolet ionizing radiation. (It emits 3-4 times as much photoionizing light as the next brightest star, θ² A Orionis.^[20]) 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".

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.^[22] 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 solar systems is fairly common in our universe.

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.^[23]

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.^[15] 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.^[24]

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.^[25] The wind forms shock waves 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.

There are three different kinds of shocks in the Orion Nebula. Many are featured in Herbig-Haro objects:^[26]

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 gasses.

Warped shocks appear bow-like to an observer. They are produced when a jet-driven shock encounters gas moving in a cross-current.

The dynamic gas motions in M42 are complex, but are trending out through the opening in the bay and toward the Earth.^[27] The large neutral area behind the ionized region is currently contracting under its own gravity.

The Beehive Cluster (also known as Praesepe (Latin for "manger"), M44 or NGC 2632) is an open cluster in the constellation Cancer. It looks like a nebulous object to the naked eye under dark skies, and thus has been known since ancient times--Ptolemy called it "The nebulous mass in the breast" of Cancer. It is also among the first objects Galileo studied with his telescope.

The 730-million-year old cluster is 577 light years away; its age and proper motion coincide with the Hyades open cluster, suggesting they were created in the same diffuse nebula. Both contain red giants and white dwarfs, but the brightest stars are class A, F, and G stars. It contains at least 200 stars, and perhaps as many as 350.

The Beehive is most easily observed in the spring, when Cancer is high in the sky. At 95 arc minutes across, it fits well in the field of view of binoculars or a telescope of low power.

Ancient Greeks and Romans saw this nebulous object as a manger with two donkeys, the stars Asellus Borealis and Asellus Australis, eating from it, specifically, the donkeys that Dionysos and Silenus rode into battle against the Titans.^[1]

Galileo was the first to observe the Beehive in a telescope, in 1609, and was able to resolve it into 40 stars. Charles Messier added it to his famous catalog in 1769 after precisely measuring its position in the sky. Along with the Orion Nebula and the Pleiades cluster, Messier's inclusion of the beehive has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list^[2]

The cluster is about 16 light years in diameter and contains at least 200 stars confirmed to be bound to the cluster, out of 350 total in the vicinity. It has a visual brightness of magnitude 3.7. Its brightest stars are blue-white and of magnitude 6 to 6.5. The Beehive is one of the older and larger open clusters known.

Deep Sky Objects

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General information

Structure