Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8; the temperature range of supergiant stars spans from about 3,450 K to over 20,000 K. The title supergiant, as applied to a star, does not have a single concrete definition; the term giant star was first coined by Hertzsprung when it became apparent that the majority of stars fell into two distinct regions of the Hertzsprung–Russell diagram. One region contained larger and more luminous stars of spectral types A to M and received the name giant. Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were larger and more luminous than the bulk, the term super-giant arose adopted as supergiant. Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity. In 1897, Antonia C. Maury had divided stars based on the widths of their spectral lines, with her class "c" identifying stars with the narrowest lines.
Although it was not known at the time, these were the most luminous stars. In 1943 Morgan and Keenan formalised the definition of spectral luminosity classes, with class I referring to supergiant stars; the same system of MK luminosity classes is still used today, with refinements based on the increased resolution of modern spectra. Supergiants occur in every spectral class from young blue class O supergiants to evolved red class M supergiants; because they are enlarged compared to main-sequence and giant stars of the same spectral type, they have lower surface gravities, changes can be observed in their line profiles. Supergiants are evolved stars with higher levels of heavy elements than main-sequence stars; this is the basis of the MK luminosity system which assigns stars to luminosity classes purely from observing their spectra. In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials which can produce emission lines, P Cygni profiles, or forbidden lines.
The MK system assigns stars to luminosity classes: Ib for supergiants. In reality there is much more of a continuum than well defined bands for these classifications, classifications such as Iab are used for intermediate luminosity supergiants. Supergiant spectra are annotated to indicate spectral peculiarities, for example B2 Iae or F5 Ipec. Supergiants can be defined as a specific phase in the evolutionary history of certain stars. Stars with initial masses above 8-10 M☉ and smoothly initiate helium core fusion after they have exhausted their hydrogen, continue fusing heavier elements after helium exhaustion until they develop an iron core, at which point the core collapses to produce a Type 2 supernova. Once these massive stars leave the main sequence, their atmospheres inflate, they are described as supergiants. Stars under 10 M☉ will never form an iron core and in evolutionary terms do not become supergiants, although they can reach luminosities thousands of times the sun's, they cannot fuse carbon and heavier elements after the helium is exhausted, so they just lose their outer layers, leaving the core of a white dwarf.
The phase where these stars have both hydrogen and helium burning shells is referred to as the asymptotic giant branch, as stars become more and more luminous class M stars. Stars of 8-10 M☉ may fuse sufficient carbon on the AGB to produce an oxygen-neon core and an electron-capture supernova, but astrophysicists categorise these as super-AGB stars rather than supergiants. There are several categories of evolved stars which are not supergiants in evolutionary terms but may show supergiant spectral features or have luminosities comparable to supergiants. Asymptotic-giant-branch and post-AGB stars are evolved lower-mass red giants with luminosities that can be comparable to more massive red supergiants, but because of their low mass, being in a different stage of development, their lives ending in a different way, astrophysicists prefer to keep them separate; the dividing line becomes blurred at around 7–10 M☉ where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars refer to them as super AGB stars, since they have many properties in common with AGB such as thermal pulsing.
Others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae. Many post-AGB stars receive spectral types with supergiant luminosity classes. For example, RV Tauri has an Ia luminosity class despite being less massive than the sun; some AGB stars receive a supergiant luminosity class, most notably W Virginis variables such as W Virginis itself, stars that are executing a blue loop triggered by thermal pulsing. A small number of Mira variables and other late AGB stars have supergiant luminosity classes, for example α Herculis. Classical Cepheid variables have supergiant luminosity classes, although only the most luminous and massive will go on to develop an iron core; the majority of them are intermediate mass stars fusing helium in their cores and will transition to the asymptotic giant branch. Δ Cephei itself is an example with a luminosity of 2,000 L☉ and a mass of 4.5 M☉. Wolf–Rayet stars are high-mass luminous evolved stars, hotter than most supergiants and smaller, visually less
An exoplanet or extrasolar planet is a planet outside the Solar System. The first evidence of an exoplanet was not recognized as such; the first scientific detection of an exoplanet was in 1988. The first confirmed detection occurred in 1992; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included; the least massive planet known is Draugr, about twice the mass of the Moon.
The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit; some are so far out. All of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist; the nearest exoplanet is Proxima Centauri b, located 4.2 light-years from Earth and orbiting Proxima Centauri, the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface.
The study of planetary habitability considers a wide range of other factors in determining the suitability of a planet for hosting life. Besides exoplanets, there are rogue planets, which do not orbit any star; these tend to be considered as a separate category if they are gas giants, in which case they are counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way number in the billions; the convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union. For exoplanets orbiting a single star, the designation is formed by taking the name or, more designation of its parent star and adding a lower case letter; the first planet discovered in a system is given the designation "b" and planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.
A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist. For centuries scientists and science fiction writers suspected that extrasolar planets existed, but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers; the first evidence of an exoplanet was not recognized as such. The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12; the first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method.
In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not bound to stars, so they're wandering through space or loosely orbiting between stars. We can estimate. In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets. In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."In 1952, more than 40 years before the first hot Jupiter was discovere
A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
The Messier objects are a set of 110 astronomical objects cataloged by the French astronomer Charles Messier in his Catalogue des Nébuleuses et des Amas d'Étoiles. Because Messier was interested in finding only comets, he created a list of non-comet objects that frustrated his hunt for them; the compilation of this list, in collaboration with his assistant Pierre Méchain, is known as the Messier catalogue. This catalogue of objects is one of the most famous lists of astronomical objects, many Messier objects are still referenced by their Messier number; the catalogue includes some astronomical objects that can be observed from Earth's Northern Hemisphere such as deep-sky objects, a characteristic which makes the Messier objects popular targets for amateur astronomers. A preliminary version first appeared in the Memoirs of the French Academy of Sciences in 1771, the last item was added in 1966 by Kenneth Glyn Jones, based on Messier's observations; the first version of Messier's catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris.
In addition to his own discoveries, this version included objects observed by other astronomers, with only 17 of the 45 objects being Messier's. By 1780 the catalogue had increased to 80 objects; the final version of the catalogue containing 103 objects was published in 1781 in the Connaissance des Temps for the year 1784. However, due to what was thought for a long time to be the incorrect addition of Messier 102, the total number remained 102. Other astronomers, using side notes in Messier's texts filled out the list up to 110 objects; the catalogue consists of a diverse range of astronomical objects, ranging from star clusters and nebulae to galaxies. For example, Messier 1 is a supernova remnant, known as the Crab Nebula, the great spiral Andromeda Galaxy is M31. Many further inclusions followed in the next century when the first addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding Messier's side note in his 1781 edition exemplar of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, M110 by Kenneth Glyn Jones in 1967.
The first edition of 1771 covered 45 objects numbered M1 to M45. The total list published by Messier in 1781 contained 103 objects, but the list was expanded through successive additions by other astronomers, motivated by notes in Messier's and Méchain's texts indicating that at least one of them knew of the additional objects; the first such addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding a note Messier made in a copy of the 1781 edition of the catalog. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, M110 by Kenneth Glyn Jones in 1967. M102 was observed by Méchain. Méchain concluded that this object was a re-observation of M101, though some sources suggest that the object Méchain observed was the galaxy NGC 5866 and identify that as M102. Messier's final catalogue was included in the Connaissance des Temps for 1784, the French official yearly publication of astronomical ephemerides; these objects are still known by their "Messier number" from this list.
Messier did his astronomical work at the Hôtel de Cluny, in Paris, France. The list he compiled contains only objects found in the sky area he could observe: from the north celestial pole to a celestial latitude of about −35.7°. He did not observe or list objects visible only from farther south, such as the Large and Small Magellanic Clouds; the Messier catalogue comprises nearly all the most spectacular examples of the five types of deep-sky object – diffuse nebulae, planetary nebulae, open clusters, globular clusters, galaxies – visible from European latitudes. Furthermore all of the Messier objects are among the closest to Earth in their respective classes, which makes them studied with professional class instruments that today can resolve small and visually spectacular details in them. A summary of the astrophysics of each Messier object can be found in the Concise Catalog of Deep-sky Objects. Since these objects could be observed visually with the small-aperture refracting telescope used by Messier to study the sky, they are among the brightest and thus most attractive astronomical objects observable from Earth, are popular targets for visual study and astrophotography available to modern amateur astronomers using larger aperture equipment.
In early spring, astronomers sometimes gather for "Messier marathons", when all of the objects can be viewed over a single night. Lists of astronomical objects List of Messier objects Caldwell catalogue Deep-sky object Herschel 400 Catalogue New General Catalogue SEDS Messier Database Charles Messier Charles Messier's Catalog of Nebulae and Star Clusters History of the Messier Catalog Interactive Messier Catalog Greenhawk Observatory Listing of Copyright-free Images of all Messier Objects CCD Images of Messier Objects 12 Dimensional String Messier Gallery The Messier Catalogue Merrifield, Mike. "Messier Objects". Deep Sky Videos. Brady Haran. Messier Objects at Constellation Guide
The South Pole known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of Earth and lies on the opposite side of Earth from the North Pole. Situated on the continent of Antarctica, it is the site of the United States Amundsen–Scott South Pole Station, established in 1956 and has been permanently staffed since that year; the Geographic South Pole is distinct from the South Magnetic Pole, the position of, defined based on Earth's magnetic field. The South Pole is at the center of the Southern Hemisphere. For most purposes, the Geographic South Pole is defined as the southern point of the two points where Earth's axis of rotation intersects its surface. However, Earth's axis of rotation is subject to small "wobbles", so this definition is not adequate for precise work; the geographic coordinates of the South Pole are given as 90°S, since its longitude is geometrically undefined and irrelevant.
When a longitude is desired, it may be given as 0°. At the South Pole, all directions face north. For this reason, directions at the Pole are given relative to "grid north", which points northwards along the prime meridian. Along tight latitude circles, clockwise is east, counterclockwise is west, opposite to the North Pole; the Geographic South Pole is located on the continent of Antarctica. It sits atop a featureless, barren and icy plateau at an altitude of 2,835 metres above sea level, is located about 1,300 km from the nearest open sea at Bay of Whales; the ice is estimated to be about 2,700 metres thick at the Pole, so the land surface under the ice sheet is near sea level. The polar ice sheet is moving at a rate of 10 metres per year in a direction between 37° and 40° west of grid north, down towards the Weddell Sea. Therefore, the position of the station and other artificial features relative to the geographic pole shift over time; the Geographic South Pole is marked by a stake in the ice alongside a small sign.
The sign records the respective dates that Roald Amundsen and Robert F. Scott reached the Pole, followed by a short quotation from each man, gives the elevation as "9,301 FT.". A new marker stake is fabricated each year by staff at the site; the Ceremonial South Pole is an area set aside for photo opportunities at the South Pole Station. It is located some meters from the Geographic South Pole, consists of a metallic sphere on a short bamboo pole, surrounded by the flags of the original Antarctic Treaty signatory states. Amundsen's Tent: The tent was erected by the Norwegian expedition led by Roald Amundsen on its arrival on 14 December 1911, it is buried beneath the snow and ice in the vicinity of the Pole. It has been designated a Historic Site or Monument, following a proposal by Norway to the Antarctic Treaty Consultative Meeting; the precise location of the tent is unknown, but based on calculations of the rate of movement of the ice and the accumulation of snow, it is believed, as of 2010, to lie between 1.8 and 2.5 km from the Pole at a depth of 17 m below the present surface.
Argentine Flagpole: A flagpole erected at the South Geographical Pole in December 1965 by the First Argentine Overland Polar Expedition has been designated a Historic Site or Monument following a proposal by Argentina to the Antarctic Treaty Consultative Meeting. In 1820, several expeditions claimed to have been the first to have sighted Antarctica, with the first being the Russian expedition led by Fabian Gottlieb von Bellingshausen and Mikhail Lazarev; the first landing was just over a year when American Captain John Davis, a sealer, set foot on the ice. The basic geography of the Antarctic coastline was not understood until the mid-to-late 19th century. American naval officer Charles Wilkes claimed that Antarctica was a new continent, basing the claim on his exploration in 1839–40, while James Clark Ross, in his expedition of 1839–43, hoped that he might be able to sail all the way to the South Pole. British explorer Robert Falcon Scott on the Discovery Expedition of 1901–04 was the first to attempt to find a route from the Antarctic coastline to the South Pole.
Scott, accompanied by Ernest Shackleton and Edward Wilson, set out with the aim of travelling as far south as possible, on 31 December 1902, reached 82°16′ S. Shackleton returned to Antarctica as leader of the British Antarctic Expedition in a bid to reach the Pole. On 9 January 1909, with three companions, he reached 88°23' S – 112 miles from the Pole – before being forced to turn back; the first men to reach the Geographic South Pole were the Norwegian Roald Amundsen and his party on December 14, 1911. Amundsen named his camp Polheim and the entire plateau surrounding the Pole King Haakon VII Vidde in honour of King Haakon VII of Norway. Robert Falcon Scott returned to Antarctica with his second expedition, the Terra Nova Expedition unaware of Amundsen's secretive expedition. Scott and four other men reached the South Pole on January 17, 1912, thirty-four days after Amundsen. On the return trip and his four companions all died of starvation and extreme cold. In 1914 Ernest Shackleton's Imperial Trans-Antarctic Expedition set out with the goal of crossing Antarctica via the South Pole, but his ship, the Endurance, was frozen in pack ice and sank 1
A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances; this robust characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of variable stars in the Magellanic Clouds. This discovery allows one to know the true luminosity of a Cepheid by observing its pulsation period; this in turn allows one to determine the distance to the star, by comparing its known luminosity to its observed brightness. The term Cepheid originates from Delta Cephei in the constellation Cepheus, identified by John Goodricke in 1784, the first of its type to be so identified. Cepheid variables are divided into two subclasses which exhibit markedly different masses and evolutionary histories: classical Cepheids and type II Cepheids.
Delta Scuti variables are A class stars on or near the main sequence at the lower end of the instability strip and were referred to as dwarf Cepheids. RR Lyrae variables have short periods and lie on the instability strip where it crosses the horizontal branch. Delta Scuti variables and RR Lyrae variables are not treated with Cepheid variables although their pulsations originate with the same helium ionisation kappa mechanism. Classical Cepheids undergo pulsations with regular periods on the order of days to months. Classical Cepheids are Population I variable stars which are 4–20 times more massive than the Sun, up to 100,000 times more luminous; these Cepheids are yellow bright giants and supergiants of spectral class F6 – K2 and their radii change by millions of kilometers during a pulsation cycle. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, are a means by which the Hubble constant can be established. Classical Cepheids have been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.
A group of classical Cepheids with small amplitudes and sinusoidal light curves are separated out as Small Amplitude Cepheids or s-Cepheids, many of them pulsating in the first overtone. Type II Cepheids are population II variable stars which pulsate with periods between 1 and 50 days. Type II Cepheids are metal-poor, low mass objects. Type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10–20 days belong to the W Virginis subclass, stars with periods greater than 20 days belong to the RV Tauri subclass. Type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, galaxies. A group of pulsating stars on the instability strip have periods of less than 2 days, similar to RR Lyrae variables but with higher luminosities. Anomalous Cepheid variables have masses higher than type II Cepheids, RR Lyrae variables, our sun, it is unclear whether they are young stars on a "turned-back" horizontal branch, blue stragglers formed through mass transfer in binary systems, or a mix of both.
A small proportion of Cepheid variables have been observed to pulsate in two modes at the same time the fundamental and first overtone the second overtone. A small number pulsate in three modes, or an unusual combination of modes including higher overtones. On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of classical Cepheid variables. However, the eponymous star for classical Cepheids is Delta Cephei, discovered to be variable by John Goodricke a few months later. A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds, she published it in 1912 with further evidence. In 1913, Ejnar Hertzsprung attempted to find distances to 13 Cepheids using the motion through the sky, his research would require revision, however. In 1915, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way, of the placement of our Sun within it.
In 1924, Edwin Hubble established the distance to classical Cepheid variables in the Andromeda Galaxy, until known as the Andromeda Nebula, showed that the variables were not members of the Milky Way. Hubble's finding settled the question raised in the "Great Debate" of whether the Milky Way represented the entire Universe or was one of numerous galaxies in the Universe. In 1929, Hubble and Milton L. Humason formulated what is now known as Hubble's Law by combining Cepheid distances to several galaxies with Vesto Slipher's measurements of the speed at which those galaxies recede from us, they discovered. However, the expansion of the Universe was posited several years before by Georges Lemaître. In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with different properties. In the 1940s, Walter Baade recognized two separate populations of Cepheids. Classical Cepheids are younger and more massive population I stars, whereas type II Cepheids are older fainter Population II stars.
Classical Cepheids and type
The Homunculus Nebula is a bipolar emission and reflection nebula surrounding the massive star system Eta Carinae, about 7,500 light-years from Earth. The nebula is embedded within a large star-forming H ii region. From the Latin meaning Little Man, the Homunculus is a small H ii region, with gas shocked into ionised and excited states, it absorbs much of the light from the luminous central stellar system and re-radiates it as infra-red. It is the brightest object in the sky at mid-IR wavelengths. Within the Homunculus is a smaller Little Homunculus, within that a shell of shocked material from stellar winds, called Baby Homunculus. In 1914, Eta Carinae was reported to have a faint companion and to be non-stellar: "fuzzy". Observations in 1944 and 1945 showed a somewhat elongated nebulosity around 5" 10" long, it was recognised to be expanding consistent with having originated in an explosion in the mid 19th century, measured at 3".2 - 7".5 per century. At that time the shape of the nebula showed a central bulge with a single large lump to the NW and two smaller extensions to the SE, described as a Homunculus.
Other observations at around the same time described a orange central region in a larger fainter green nebulosity. The phrase "red spade-beard" was applied; the Homunculus consists of two lobes, referred to as NW and SE based on their orientation as seen from Earth, each 7" wide by 5" long. There is a ragged equatorial skirt of material which can be seen faintly in deep images at certain wavelengths; the lobes are hollow with the material concentrated towards the poles. The equatorial skirt appears to contain material of younger than the lobes, it contains a much smaller mass of material than the lobes, shining by reflected light which escapes most at equatorial latitudes. There is less dust and little molecular hydrogen compared to the lobes; the bipolar nebula is angled. The whole nebula is expanding so that the SE lobe is blue-shifted and the NW lobe is red-shifted, relative to the central source; the lobes contain the majority of the material in the Homunculus Nebula, in thin shells concentrated towards the poles.
The shells consist of an inner warm region and a more massive outer cool skin. The shells are smooth and thin suggesting they were ejected in as little as five years, but there are streaks of thicker dust detectable within the shells; each lobe has polar "hole" although it isn't clear if it is an actual gap in the shell of the lobe or just a deep indentation. Surrounding each polar hole is a "trench"; the trenches are visible as approximate half-circles centred on the axis of the lobes but may form complete circles. There are other smaller irregular indentations and protrusions to the lobes symmetrical with the same features appearing on each lobe; the most striking are flattened protrusions at about 10°latitude, one on each lobe, but there are other smaller protrusions near the equatorial skirt. The mass of the nebula cannot be determined directly; however the amount of dust can be measured accurately and estimates of the gas:dust ratio used to calculate the total mass. The total dust mass is calculated at 0.4 M☉, leading to estimates that up to 40 M☉ of gas are contained in the Homunculus itself.
Nearly as much material is detected within outer ejecta, but still formed within the last thousand years. Older calculations had produce consensus estimates of 10-15 M☉ Early speckle interferometry showed that the central region of the Homunculus contains four point-like sources tagged A1, A2, A3, A4; the four speckle objects were referred to as A, B, C, D. Higher resolution studies showed that only the brightest source A was stellar, the other three were small nebular condensations; the three Weigelt Blobs are visible in light directly reflected from the Eta Carinae stars. The blobs are believed to lie near the equatorial plane of the stellar system, but their origin is unclear, their speed has been measured, but within uncertainties they could have been emitted in the 1890 outburst or a 1941 event. The situation is complicated further by the acceleration of their slow movement due to the intense stellar winds; the spectrum of the Homunculus is complex, consisting of reflected and emission components at wavelengths across the electromagnetic spectrum.
The dominant feature is blackbody radiation from dust heated by the stars within. Overlaid on this is some light from the stars themselves reflected from dense features within the nebulosity, showing strong visual and UV spectral lines in emission. There are "nebular" emission lines from ionised gas where it collides with slower moving material or is excited by high energy electromagnetic radiation from the stars; the ionisation emission is similar to a planetary nebula but at lower levels of ionisation due to the lower temperatures of the central stars. The strongest lines are and, similar to those from the stellar winds of the stars themselves, but with narrower profiles. Shockwaves at the outer edge of the ejecta are heated to millions of kelvin and emit x-ray radiation; the lobes of the Homunculus emit copious radio waves, most the 21cm line of hydrogen. The reflected spectrum of the Homunculus lobes varies with position, due to the central star emitting different radiation at different latitudes.
This is the only star. The Homunculus was ejected in an enormous outburst from Eta Car