Draco is a constellation in the far northern sky. Its name is Latin for dragon, it was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, remains one of the 88 modern constellations today. The north pole of the ecliptic is in Draco. Draco is circumpolar, can be seen all year from northern latitudes. Thuban was the northern pole star from 3942 BC, when it moved farther north than Theta Boötis, until 1793 BC; the Egyptian Pyramids were designed to have one side facing north, with an entrance passage geometrically aligned so that Thuban would be visible at night. Due to the effects of precession, it will again be the pole star around the year AD 21000, it is a blue-white giant star of 309 light-years from Earth. The traditional name of Alpha Draconis, means "head of the serpent". There are three stars under magnitude 3 in Draco; the brighter of the three, the brightest star in Draco, is Gamma Draconis, traditionally called Etamin or Eltanin. It is an orange giant star of 148 light-years from Earth.
The aberration of starlight was discovered in 1728. Nearby Beta Draconis, traditionally called Rastaban, is a yellow giant star of magnitude 2.8, 362 light-years from Earth. Its name shares a meaning with Thuban, "head of the serpent". Draco is home to binary stars. Eta Draconis is a double star with a yellow-hued primary of magnitude 2.8 and a white-hued secondary of magnitude 8.2 located south of the primary. The two are separated by 4.8 arcseconds. Mu Draconis, traditionally called Alrakis, is a binary star with two white components. Magnitude 5.6 and 5.7, the two components orbit each other every 670 years. The Alrakis system is 88 light-years from Earth. Nu Draconis is a similar binary star with 100 light-years from Earth. Both components are of magnitude 4.9 and can be distinguished in a small amateur telescope or a pair of binoculars. Omicron Draconis is a double star divisible in small telescopes; the primary is an orange giant of 322 light-years from Earth. The secondary is of magnitude 7.8.
Psi Draconis is a binary star divisible in binoculars and small amateur telescopes, 72 light-years from Earth. The primary is a yellow-white star of magnitude 4.6 and the secondary is a yellow star of magnitude 5.8. 16 Draconis and 17 Draconis are part of a triple star 400 light-years from Earth, divisible in medium-sized amateur telescopes. The primary, a blue-white star of magnitude 5.1, is itself a binary with components of magnitude 5.4 and 6.5. The secondary is of magnitude 5.5 and the system is 400 light-years away. 20 Draconis is a binary star with a white-hued primary of magnitude 7.1 and a yellow-hued secondary of magnitude 7.3 located east-northeast of the primary. The two have an orbital period of 420 years; as of 2012, the two components are approaching their maximum separation. 39 Draconis is a triple star 188 light-years from Earth, divisible in small amateur telescopes. The primary is a blue star of magnitude 5.0, the secondary is a yellow star of magnitude 7.4, the tertiary is a star of magnitude 8.0.
40 Draconis and 41 Draconis are a binary star divisible in small telescopes. The two orange dwarf stars are 170 light-years from Earth and are of magnitude 5.7 and 6.1. R Draconis is a red Mira-type variable star with a period of about 8 months, its average minimum magnitude is 12.4, its average maximum magnitude is 7.6. It was discovered to be a variable star by Hans Geelmuyden in 1876; the constellation contains the star named Kepler-10, confirmed to be orbited by Kepler-10b, the smallest rocky Earth-sized planet yet detected outside of the Solar System. One of deep-sky objects in Draco is the Cat's Eye Nebula, a planetary nebula 3,000 light-years away, discovered by English astronomer William Herschel in 1786, it is 9th magnitude and was named for its appearance in the Hubble Space Telescope, though it appears as a fuzzy blue-green disk in an amateur telescope. NGC 6543 has a complex shape due to gravitational interactions between the components of the multiple star at its center, the progenitor of the nebula 1,000 years ago.
It is located 9.6 arcminutes away from the north ecliptic pole to the west-northwest. It is related to IC 4677, a nebula that appears as a bar 1.8 arcminutes to the west of the Cat's Eye nebula. In long-term exposures, IC 4677 appears as a portion of a ring surrounding the planetary nebula. There are several faint galaxies in Draco, one of, the lenticular galaxy NGC 5866 that bears its name to a small group that includes the spiral galaxies NGC 5879 and NGC 5907. Another is the Draco Dwarf Galaxy, one of the least luminous galaxies with an absolute magnitude of −8.6 and a diameter of only about 3,500 light years, discovered by Albert G. Wilson of Lowell Observatory in 1954. Another dwarf galaxy found in this constellation is PGC 39058. Draco features several interacting galaxies and galaxy clusters. One such massive cluster is Abell 2218, located at a distance of 3 billion light-years, it acts as a gravitational lens for more distant background galaxies, allowing astronomers to study those galaxies as well as Abell 2218 itself.
One of the most well-known interacting galaxies is Arp 188 called the "Tadpole Galaxy". Named for its appearance, which features a "
Galaxy morphological classification
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and expanded by Gérard de Vaucouleurs and Allan Sandage; the Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubble's scheme divides galaxies into three broad classes based on their visual appearance: Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images, they are denoted by the letter "E", followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a flattened disk, with stars forming a spiral structure, a central concentration of stars known as the bulge, similar in appearance to an elliptical galaxy.
They are given the symbol "S". Half of all spirals are observed to have a bar-like structure, extending from the central bulge; these barred spirals are given the symbol "SB". Lenticular galaxies consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not forming stars in any significant quantity; these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left and the barred and unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”. To this day, the Hubble sequence is the most used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.
The de Vaucouleurs system for classifying galaxies is a used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies—based on the tightness of the spiral arms and the presence or absence of a bar—did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses are important structural components of spiral galaxies; the de Vaucouleurs system retains Hubble's basic division of galaxies into ellipticals, lenticulars and irregulars. To complement Hubble's scheme, de Vaucouleurs introduced a more elaborate classification system for spiral galaxies, based on three morphological characteristics: The different elements of the classification scheme are combined — in the order in which they are listed — to give the complete classification of a galaxy. For example, a weakly barred spiral galaxy with loosely wound arms and a ring is denoted SABc.
Visually, the de Vaucouleurs system can be represented as a three-dimensional version of Hubble's tuning fork, with stage on the x-axis, family on the y-axis, variety on the z-axis. De Vaucouleurs assigned numerical values to each class of galaxy in his scheme. Values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three'stages': compact ellipticals, normal ellipticals and late types. Lenticulars are subdivided into early and late types. Irregular galaxies can be of type magellanic irregulars or'compact'; the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan. Together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra; the Yerkes scheme uses the spectra of stars in the galaxy. Thus, for example, the Andromeda Galaxy is classified as kS5.
Morphological Catalogue of Galaxies Galaxy color–magnitude diagram Galaxy Zoo William Wilson Morgan Fritz Zwicky Galaxies and the Universe - an introduction to galaxy classification Near-Infrared Galaxy Morphology Atlas, T. H. Jarrett The Spitzer Infrared Nearby Galaxies Survey Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project Go to GalaxyZoo.org to try your hand at classifying galaxies as part of an Oxford University open community project
New General Catalogue
The New General Catalogue of Nebulae and Clusters of Stars is a catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. It expands upon the cataloguing work of William and Caroline Herschel, John Herschel's General Catalogue of Nebulae and Clusters of Stars; the NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of deep space objects, including galaxies, star clusters, emission nebulae and absorption nebulae. Dreyer published two supplements to the NGC in 1895 and 1908, known as the Index Catalogues, describing a further 5,386 astronomical objects. Objects in the sky of the southern hemisphere are catalogued somewhat less but many were observed by John Herschel or James Dunlop; the NGC had many errors, but an attempt to eliminate them was initiated by the NGC/IC Project in 1993, after partial attempts with the Revised New General Catalogue by Jack W. Sulentic and William G. Tifft in 1973, NGC2000.0 by Roger W. Sinnott in 1988.
The Revised New General Catalogue and Index Catalogue was compiled in 2009 by Wolfgang Steinicke. The original New General Catalogue was compiled during the 1880s by John Louis Emil Dreyer using observations from William Herschel and his son John, among others. Dreyer had published a supplement to Herschel's General Catalogue of Nebulae and Clusters, containing about 1,000 new objects. In 1886, he suggested building a second supplement to the General Catalogue, but the Royal Astronomical Society asked Dreyer to compile a new version instead; this led to the publication of the New General Catalogue in the Memoirs of the Royal Astronomical Society in 1888. Assembling the NGC was a challenge, as Dreyer had to deal with many contradicting and unclear reports, made with a variety of telescopes with apertures ranging from 2 to 72 inches. While he did check some himself, the sheer number of objects meant Dreyer had to accept them as published by others for the purpose of his compilation; the catalogue contained several errors relating to position and descriptions, but Dreyer referenced the catalogue, which allowed astronomers to review the original references and publish corrections to the original NGC.
The first major update to the NGC is the Index Catalogue of Nebulae and Clusters of Stars, published in two parts by Dreyer in 1895 and 1908. It serves as a supplement to the NGC, contains an additional 5,386 objects, collectively known as the IC objects, it summarizes the discoveries of galaxies and nebulae between 1888 and 1907, most of them made possible by photography. A list of corrections to the IC was published in 1912; the Revised New Catalogue of Nonstellar Astronomical Objects was compiled by Jack W. Sulentic and William G. Tifft in the early 1970s, was published in 1973, as an update to the NGC; the work did not incorporate several previously-published corrections to the NGC data, introduced some new errors. Nearly 800 objects are listed as "non-existent" in the RNGC; the designation is applied to objects which are duplicate catalogue entries, those which were not detected in subsequent observations, a number of objects catalogued as star clusters which in subsequent studies were regarded as coincidental groupings.
A 1993 monograph considered the 229 star clusters called non-existent in the RNGC. They had been "misidentified or have not been located since their discovery in the 18th and 19th centuries", it found that one of the 229—NGC 1498—was not in the sky. Five others were duplicates of other entries, 99 existed "in some form", the other 124 required additional research to resolve; as another example, reflection nebula NGC 2163 in Orion was classified "non-existent" due to a transcription error by Dreyer. Dreyer corrected his own mistake in the Index Catalogues, but the RNGC preserved the original error, additionally reversed the sign of the declination, resulting in NGC 2163 being classified as non-existent. NGC 2000.0 is a 1988 compilation of the NGC and IC made by Roger W. Sinnott, using the J2000.0 coordinates. It incorporates several errata made by astronomers over the years; the NGC/IC Project is a collaboration formed in 1993. It aims to identify all NGC and IC objects, collect images and basic astronomical data on them.
The Revised New General Catalogue and Index Catalogue is a compilation made by Wolfgang Steinicke in 2009. It is a authoritative treatment of the NGC and IC catalogues. Messier object Catalogue of Nebulae and Clusters of Stars Astronomical catalogue List of astronomical catalogues List of NGC objects The Interactive NGC Catalog Online Adventures in Deep Space: Challenging Observing Projects for Amateur Astronomers. Revised New General Catalogue
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in two thirds of all spiral galaxies. Bars affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well; the Milky Way Galaxy, where our own Solar System is located, is classified as a barred spiral galaxy. Edwin Hubble classified spiral galaxies of this type as "SB" in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy. A new type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Clouds, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy.
Barred galaxies are predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers; the bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy; the creation of the bar is thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure. Bars are thought to be temporary phenomena in the lives of spiral galaxies. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have stubby bars.
Since so many spiral galaxies have bar structures, it is that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years. Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts; the general classification is "SB". The sub-categories are based on how tight the arms of the spiral are. SBa types feature bound arms. SBc types are at the other extreme. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Firehose instability Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity."
SPACE.com 16 August 2005. An article about the Spitzer Space Telescope's Milky Way discovery Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." 16 August 2005. The original press release regarding the article above, from the Univ. of Wisconsin'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March 2001. Hastings and Jane Hastings. Classifying Galaxies: Barred Spirals, 1995. "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15, 2000. A press release concerning NGC 1326 Barred spirals come and go Sky & Telescope April 2002. "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." November 29, 2001. A press release from the European Southern Observatory. Horton, Adam. "Spitzer NGC 1291 barred spiral galaxy seen in infrared." 22 October 2014
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth; the techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, an astronomical object that has a known luminosity; the ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, so on; each rung of the ladder provides information that can be used to determine the distances at the next higher rung. At the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question.
The precise measurement of stellar positions is part of the discipline of astrometry. Direct distance measurements are based upon the astronomical unit, the distance between the Earth and the Sun. Kepler's laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, but provides no measurement of the overall scale of the orbit system. Radar is used to measure the distance of a second body. From that measurement and the ratio of the two orbit sizes, the size of Earth's orbit is calculated; the Earth's orbit is known with an absolute precision of a few meters and a relative precision of a few 1×10−11. Observations of transits of Venus were crucial in determining the AU. Presently the orbit of Earth is determined with high precision using radar measurements of distances to Venus and other nearby planets and asteroids, by tracking interplanetary spacecraft in their orbits around the Sun through the Solar System; the most important fundamental distance measurements come from trigonometric parallax.
As the Earth orbits the Sun, the position of nearby stars will appear to shift against the more distant background. These shifts are angles in an isosceles triangle, with 2 AU making the base leg of the triangle and the distance to the star being the long equal length legs; the amount of shift is quite small, measuring 1 arcsecond for an object at 1 parsec's distance of the nearest stars, thereafter decreasing in angular amount as the distance increases. Astronomers express distances in units of parsecs; because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars which are near enough to have a parallax larger than a few times the precision of the measurement. Parallax measurements have an accuracy measured in milliarcseconds. In the 1990s, for example, the Hipparcos mission obtained parallaxes for over a hundred thousand stars with a precision of about a milliarcsecond, providing useful distances for stars out to a few hundred parsecs; the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars.
In 2018, Data Release 2 from the Gaia space mission provides accurate distances to most stars brighter than 15th magnitude. Stars have a velocity relative to the Sun that causes radial velocity; the former is determined by plotting the changing position of the stars over many years, while the latter comes from measuring the Doppler shift of the star's spectrum caused by motion along the line of sight. For a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities; this statistical parallax method is useful for measuring the distances of bright stars beyond 50 parsecs and giant variable stars, including Cepheids and the RR Lyrae variables. The motion of the Sun through space provides a longer baseline that will increase the accuracy of parallax measurements, known as secular parallax. For stars in the Milky Way disk, this corresponds to a mean baseline of 4 AU per year, while for halo stars the baseline is 40 AU per year.
After several decades, the baseline can be orders of magnitude greater than the Earth–Sun baseline used for traditional parallax. However, secular parallax introduces a higher level of uncertainty because the relative velocity of observed stars is an additional unknown; when applied to samples of multiple stars, the uncertainty can be reduced. Moving cluster parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster. Only open clusters are near enough for this technique to be useful. In particular the distance obtained for the Hyades has been an important step in the distance ladder. Other individual objects can have fundamental distance estimates made for them under special circumstances. If the expansion of a gas cloud, like a supernova remnant or planetary nebula, can be observed over time an expansion parallax distance to that cloud can be estimated; those measurements however suf
Lewis A. Swift
Lewis A. Swift was an American astronomer who discovered 13 comets and 1,248 uncatalogued nebulae. Only William Herschel discovered more nebulae visually. Swift discovered or co-discovered a number of comets, including periodic comets 11P/Tempel-Swift-LINEAR, 64P/Swift-Gehrels, 109P/Swift-Tuttle, he discovered comets C/1877 G2, C/1878 N1, C/1879 M1, C/1881 J1, C/1881 W1, C/1892 E1, D/1895 Q1, C/1896 G1 and C/1899 E1, co-discovered C/1883 D1. Note, comet 54P/de Vico-Swift-NEAT was discovered by his son Edward D. Swift rather than by him, he discovered his last comet at the age of 79. He was one of the few people to see Comet Halley at two of its appearances, 76 years apart. In 1878 he believed he had observed two Vulcan-type planets. Apart from comets, he discovered hundreds of nebulae, such as IC 289, galaxies, such as NGC 6, NGC 19 and NGC 27, he independently observed NGC 17, leading to its separate listing in the New General Catalogue as NGC 34. According to Swift, he first became interested in astronomy as young boy after observing the Great Comet of 1843 while on his way to school in Clarkson, New York.
His teacher dismissed his observation, but three days the'discovery' of the comet was announced. Swift conducted his early observations in Rochester, NY,'lain out in the snow' in an alley on Ambrose Street or on the roof of Duffy's Cider Mill, he gained a patron in the Rochester patent medicine businessman Hulbert Harrington Warner, who financed the building of an observatory for Swift. A fund of $13,000 was raised to purchase a 16-inch telescope for Swift. Warner went bankrupt in the Panic of 1893, which ended his financial support, Swift went to California to become director of Mount Lowe Observatory, taking the 16 inch telescope with him, he was married twice, first to Lucretia Hunt in 1850 and to Carrie D. Topping in 1864. Edward D. Swift was his son by the latter wife. Swift was awarded the degree of Ph. D. from the University of Rochester. Swift received more medals than any other astronomer of his time including three made of gold from the Austrian Academy of Sciences in Vienna, four made of bronze from the Astronomical Society of the Pacific, the Laplace Medal from the Société astronomique de France.
In 1897 he was the first person awarded the Jackson-Gwilt Medal of the Royal Astronomical Society. The asteroid 5035 Swift is named in his honour. Works by or about Lewis A. Swift at Internet Archive The Story of Lewis Swift Lewis Swift L. Swift @ Astrophysics Data System