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58-465: This characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of variable stars in the Magellanic Clouds . The discovery establishes the true luminosity of a Cepheid by observing its pulsation period. This in turn gives the distance to the star by comparing its known luminosity to its observed brightness, calibrated by directly observing

116-420: A likely valve for the engine. Cepheid variables are divided into two subclasses which exhibit markedly different masses, ages, and evolutionary histories: classical Cepheids and type II Cepheids . Delta Scuti variables are A-type stars on or near the main sequence at the lower end of the instability strip and were originally referred to as dwarf Cepheids. RR Lyrae variables have short periods and lie on

174-463: A 2005 biography, Miss Leavitt's Stars , which showcases the triumphs of women's progress in science through the story of Leavitt. Robert Burleigh wrote the 2013 biography Look Up!: Henrietta Leavitt, Pioneering Woman Astronomer for a younger audience. It is written for four- to eight-year-olds. Lauren Gunderson wrote a 2015 play, Silent Sky , which followed Leavitt's journey from her acceptance at Harvard to her death. Theo Strassell wrote

232-550: A Cepheid, partly because it is a member of a star cluster and the availability of precise parallaxes observed by the Hubble , Hipparcos , and Gaia space telescopes. The accuracy of parallax distance measurements to Cepheid variables and other bodies within 7,500 light-years is vastly improved by comparing images from Hubble taken six months apart, from opposite points in the Earth's orbit. (Between two such observations 2 AU apart,

290-550: A German astronomer who worked in the United States from 1931 to 1959. Baade was born the son of a teacher in North Rhine-Westphalia , Germany . He finished school in 1912. He then studied maths, physics and astronomy at the universities of Münster and Göttingen . He received his PhD in 1919. Baade worked at Hamburg Observatory at Bergedorf from 1919 to 1931. In 1920 he discovered 944 Hidalgo ,

348-524: A course in astronomy, in which she earned an A−. Leavitt also began working as volunteer assistant, one of the "computers" at the Harvard College Observatory . In 1902, she was hired by the director of the observatory, Edward Charles Pickering , to measure and catalog the brightness of stars as they appeared in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes, but

406-627: A graph of magnitude versus logarithm of period and determined that, in her own words, A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the Cepheid variables and their periods. Leavitt then used the simplifying assumption that all of the Cepheids within the Small Magellanic Cloud were at approximately

464-639: A measuring stick with vastly greater reach. After Leavitt's death, Edwin Hubble found Cepheids in several nebulae, including the Andromeda Nebula , and, using Leavitt's Law, calculated that their distance was far too great to be part of the Milky Way and were separate galaxies in their own right. This settled astronomy's Great Debate over the size of the universe. Hubble later used Leavitt's Law, together with galactic redshifts , to establish that

522-507: A play, The Troubling Things We Do , an absurdist piece that details the life of Henrietta Leavitt, among other scientists from her era. Dava Sobel's book The Glass Universe chronicles the work of the women analyzing images taken of the stars at the Harvard College Observatory. The BBC included Leavitt in their Missed Genius series designed to celebrate individuals from diverse backgrounds who have had

580-602: A profound effect on our world. Central Square Theater commissioned a play, The Women Who Mapped The Stars , by Joyce Van Dyke , as part of the Brit D'Arbeloff Women in Science Production Series, staged by the Nora Theatre Company. The play features Leavitt's story, among others. Wilhelm Heinrich Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was

638-433: A result, the outer layer of the star cycles between being compressed, which heats the helium until it becomes doubly ionized and (due to opacity) absorbs enough heat to expand; and expanded, which cools the helium until it becomes singly ionized and (due to transparency) cools and collapses again. Cepheid variables become dimmest during the part of the cycle when the helium is doubly ionized. The term Cepheid originates from

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696-525: A star at a distance of 7500 light-years = 2300 parsecs would appear to move an angle of / 2300 arc-seconds = 2 x 10 degrees, the resolution limit of the available telescopes.) The accepted explanation for the pulsation of Cepheids is called the Eddington valve, or " κ-mechanism ", where the Greek letter κ (kappa) is the usual symbol for the gas opacity. Helium is the gas thought to be most active in

754-468: A twofold increase in the distance to M31, and the extragalactic distance scale. RR Lyrae stars, then known as Cluster Variables, were recognized fairly early as being a separate class of variable, due in part to their short periods. The mechanics of stellar pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium as

812-399: Is a constant, called the pulsation constant. Henrietta Swan Leavitt Henrietta Swan Leavitt ( / ˈ l ɛ v ɪ t / ; July 4, 1868 – December 12, 1921 ) was an American astronomer . Her discovery of how to effectively measure vast distances to remote galaxies led to a shift in the scale and understanding of the scale and the nature of the universe. Nomination of Leavitt for

870-403: Is mounted on one side of the monument. Nearby are the graves of Henry and William James . Anna Von Mertens designed a book-based work of art, Attention Is Discovery: The Life and Legacy of Henrietta Leavitt . The pages weave Von Merton's artistic interpretations of Leavitt's work with photos and descriptions of the work of Leavitt and her fellow Harvard Computers. George Johnson wrote

928-426: Is not awarded posthumously.) Cepheid variables allow astronomers to measure distances up to about 60 million light years. Even greater distances can now be measured by using the theoretical maximum mass of white dwarfs calculated by Subrahmanyan Chandrasekhar . Leavitt's scientific work at Harvard was frequently interrupted by illness and family obligations. Her early death at the age of 53, from stomach cancer,

986-542: Is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant. Uncertainties have diminished over the years, due in part to discoveries such as RS Puppis . Delta Cephei is also of particular importance as a calibrator of the Cepheid period-luminosity relation since its distance is among the most precisely established for

1044-553: The American Association for the Advancement of Science , and an honorary member of the American Association of Variable Star Observers . In 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry . By the end of that year she had died from cancer. According to science writer Jeremy Bernstein , "variable stars had been of interest for years, but when she

1102-560: The Andromeda Galaxy , until then known as the "Andromeda Nebula " and showed that those 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 merely one of many 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

1160-641: The Annals of the Astronomical Observatory of Harvard College , noting that the brighter variables had the longer period. In a 1912 paper, Leavitt examined the relationship between the periods and the brightness of a sample of 25 of the Cepheids variables in the Small Magellanic Cloud . The paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt". Leavitt made

1218-477: The Magellanic Clouds . She published it in 1912 with further evidence. Cepheid variables were found to show radial velocity variation with the same period as the luminosity variation, and initially this was interpreted as evidence that these stars were part of a binary system . However, in 1914, Harlow Shapley demonstrated that this idea should be abandoned. Two years later, Shapley and others had discovered that Cepheid variables changed their spectral types over

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1276-590: The inverse-square law , she calculated that the Small Magellanic cloud was 100 times as far away as Delta Cepheus, thus having discovered a way to calculate the distance to another galaxy. Leavitt also developed, and continued to refine, the Harvard Standard for photographic measurements, a logarithmic scale that orders stars by brightness greater than 17 magnitudes. She initially analyzed 299 plates from 13 telescopes to construct her scale, which

1334-526: The logarithm of the period is linearly related to the logarithm of the star's average intrinsic optical luminosity (the amount of power radiated by the star in the visible spectrum ). Henrietta found that Delta Cepheus was the " standard candle " that had long been sought by astronomers. A similar five-day cepheid variable in the Small Magellanic cloud she found to be about one ten-thousandth as bright as our five-day Delta Cepheus. Using

1392-413: The parallax distance to the closest Cepheids such as RS Puppis and Polaris . Cepheids change brightness due to the κ–mechanism , which occurs when opacity in a star increases with temperature rather than decreasing. The main gas involved is thought to be helium . The cycle is driven by the fact doubly ionized helium, the form adopted at high temperatures, is more opaque than singly ionized helium. As

1450-616: The 1952 meeting of the International Astronomical Union in Rome. Together with Fritz Zwicky , he identified supernovae as a new category of astronomical objects. Zwicky and he also proposed the existence of neutron stars , and suggested supernovae might create them. Beginning in 1952, he and Rudolph Minkowski identified the optical counterparts of various radio sources, including Cygnus A . He discovered 10 asteroids , including 944 Hidalgo , which has

1508-601: The American astronomer who established that the universe is expanding, also were made possible by Leavitt's groundbreaking research. Hubble often said that Leavitt deserved the Nobel Prize for her work. Mathematician Gösta Mittag-Leffler , a member of the Swedish Academy of Sciences , tried to nominate her for that prize in 1925, only to learn that she had died of cancer three years earlier. (The Nobel Prize

1566-439: The Harvard College Observatory in 1903. Because Leavitt was financially independent, Pickering initially did not have to pay her. Later, she received $ 0.30 (equivalent to $ 10.17 in 2023) an hour for her work, and only $ 10.50 (equivalent to $ 356.07 in 2023) per week. She was reportedly "hard-working, serious-minded …, little given to frivolous pursuits and selflessly devoted to her family, her church, and her career." At

1624-733: The Harvard Observatory, Leavitt worked alongside Annie Jump Cannon , who also was deaf. Pickering assigned Leavitt to study variable stars of the Small and Large Magellanic Clouds , as recorded on photographic plates taken with the Bruce Astrograph of the Boyden Station of the Harvard Observatory in Arequipa , Peru . She identified 1,777 variable stars. In 1908, Leavitt published the results of her studies in

1682-568: The Leavitt family plot at Cambridge Cemetery in Cambridge, Massachusetts ."Sitting at the top of a gentle hill", writes George Johnson in his biography of Leavitt, "the spot is marked by a tall hexagonal monument, on top of which sits a globe cradled on a draped marble pedestal. Her uncle Erasmus Darwin Leavitt and his family also are buried there, along with other Leavitts." A plaque memorializing Henrietta and her two siblings, Mira and Roswell,

1740-646: The Milky Way 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 often separated out as Small Amplitude Cepheids or s-Cepheids, many of them pulsating in the first overtone. Type II Cepheids (also termed Population II Cepheids) are population II variable stars which pulsate with periods typically between 1 and 50 days. Type II Cepheids are typically metal -poor, old (~10 Gyr), low mass objects (~half

1798-412: The Nobel Prize had to be halted because of her death. A graduate of Radcliffe College , she worked at the Harvard College Observatory as a human computer , tasked with measuring photographic plates to catalog the positions and brightness of stars. This work led her to discover the relation between the luminosity and the period of Cepheid variables . Leavitt's discovery provided astronomers with

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1856-549: The Sun, and 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 (~25% for the longer-period I Carinae ) millions of kilometers during a pulsation cycle. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established. Classical Cepheids have also been used to clarify many characteristics of

1914-771: The adiabatic radial pulsation period for a homogeneous sphere is related to its surface gravity and radius through the relation: T = k R g {\displaystyle T=k\,{\sqrt {\frac {R}{g}}}} where k is a proportionality constant. Now, since the surface gravity is related to the sphere mass and radius through the relation: g = k ′ M R 2 = k ′ R M R 3 = k ′ R ρ {\displaystyle g=k'{\frac {M}{R^{2}}}=k'{\frac {RM}{R^{3}}}=k'R\rho } one finally obtains: T ρ = Q {\displaystyle T{\sqrt {\rho }}=Q} where Q

1972-460: The center of the Andromeda Galaxy for the first time. These observations led him to define distinct " populations " for stars ( Population I and Population II ). The same observations led him to discover that there are two types of Cepheid variable stars. Using this discovery he recalculated the size of the known universe, doubling the previous calculation made by Edwin Hubble in 1929. He announced this finding to considerable astonishment at

2030-466: The course of a cycle. In 1913, Ejnar Hertzsprung attempted to find distances to 13 Cepheids using their motion through the sky. (His results would later require revision.) In 1918, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way and of the placement of the Sun within it. In 1924, Edwin Hubble established the distance to classical Cepheid variables in

2088-451: The distance of several Cepheids in the Milky Way ; with this calibration, the distance to any Cepheid could be determined accurately. Cepheids were soon detected in other galaxies, such as Andromeda (notably by Edwin Hubble in 1923–24), and they became an important part of the evidence that " spiral nebulae " are independent galaxies located far outside of the Milky Way . Thus, Leavitt's discovery would forever change humanity's picture of

2146-402: The effects of photometric contamination (blending with other stars) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature. These unresolved matters have resulted in cited values for the Hubble constant (established from Classical Cepheids) ranging between 60 km/s/Mpc and 80 km/s/Mpc. Resolving this discrepancy

2204-481: The end of the 19th century, and they were referred to as a class as Cepheids. Most of the Cepheids were known from the distinctive light curve shapes with the rapid increase in brightness and a hump, but some with more symmetrical light curves were known as Geminids after the prototype ζ Geminorum . 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

2262-703: The family name was spelled "Levett".) Henrietta Leavitt remained deeply religious and committed to her church throughout her life. Leavitt attended Oberlin College for two years before transferring to Harvard University 's Society for the Collegiate Instruction of Women (later Radcliffe College), where she received a bachelor degree in 1892. At Oberlin and Harvard, Leavitt studied a broad curriculum that included Latin and classical Greek, fine arts, philosophy, analytic geometry, and calculus. It wasn't until her fourth year of college that Leavitt took

2320-451: The first standard candle with which to measure the distance to other galaxies . Before Leavitt discovered the period-luminosity relationship for Cepheid variables (sometimes referred to as Leavitt's Law), the only techniques available to astronomers for measuring the distance to a star were based on stellar parallax . Such techniques can only be used for measuring distances out to several hundred light years . Leavitt's great insight

2378-579: The first of a class of minor planets now called Centaurs which cross the orbits of giant planets. From 1931 to 1958, he worked at Mount Wilson Observatory In 1937, the University of Hamburg wanted Baade as successor of Richard Schorr for the Hamburg Observatory, but he refused. At Mount Wilson Observatory, during World War II , he took advantage of wartime blackout conditions (which reduced light pollution ), to resolve stars in

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2436-473: 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, and the 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

2494-519: The instability strip where it crosses the horizontal branch . Delta Scuti variables and RR Lyrae variables are not generally treated with Cepheid variables although their pulsations originate with the same helium ionisation kappa mechanism . Classical Cepheids (also known as Population I Cepheids, type I Cepheids, or Delta Cepheid variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are Population I variable stars which are 4–20 times more massive than

2552-650: The mass of the Sun). 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 , and 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 , and galaxies . A group of pulsating stars on

2610-416: The process. Doubly ionized helium (helium whose atoms are missing both electrons) is more opaque than singly ionized helium. As helium is heated, its temperature rises until it reaches the point at which double ionisation spontaneously occurs and is sustained throughout the layer in much the same way a fluorescent tube 'strikes'. At the dimmest part of a Cepheid's cycle, this ionized gas in the outer layers of

2668-505: The same distance, so that their intrinsic brightness could be deduced from their apparent brightness as registered in the photographic plates, up to a scale factor, since the distance to the Magellanic Clouds were as yet unknown. She expressed the hope that parallaxes to some Cepheids would be measured. This soon occurred, allowing her period-luminosity scale to be calibrated. This reasoning allowed Leavitt to establish that

2726-447: The same time, usually the fundamental and first overtone, occasionally the second overtone. A very small number pulsate in three modes, or an unusual combination of modes including higher overtones. Chief among the uncertainties tied to the classical and type II Cepheid distance scale are: the nature of the period-luminosity relation in various passbands , the impact of metallicity on both the zero-point and slope of those relations, and

2784-640: The scientific data were on the photographic plates.) In 1893, Leavitt obtained credits toward a graduate degree in astronomy for her work at the Harvard College Observatory, but due to chronic illness, she never completed that degree. In 1898, she became a member of the Harvard staff. Leavitt left the observatory to make two trips to Europe and completed a stint as an art assistant at Beloit College in Wisconsin. At this time, she contracted an illness that led to progressive hearing loss . Leavitt returned to

2842-831: The speed at which those galaxies recede from us. They discovered that the Universe is expanding , confirming the theories of Georges Lemaître . In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with very different properties. In the 1940s, Walter Baade recognized two separate populations of Cepheids (classical and type II). Classical Cepheids are younger and more massive population I stars, whereas type II Cepheids are older, fainter Population II stars. Classical Cepheids and type II Cepheids follow different period-luminosity relationships. The luminosity of type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes (but still brighter than RR Lyrae stars). Baade's seminal discovery led to

2900-441: The star Delta Cephei in the constellation Cepheus , which was one of the early discoveries. On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae , the first known representative of the class of classical Cepheid variables. The eponymous star for classical Cepheids, Delta Cephei , was discovered to be variable by John Goodricke a few months later. The number of similar variables grew to several dozen by

2958-403: The star is relatively opaque, and so is heated by the star's radiation, and due to the increasing temperature, begins to expand. As it expands, it cools, but remains ionised until another threshold is reached at which point double ionization cannot be sustained and the layer becomes singly ionized hence more transparent, which allows radiation to escape. The expansion then stops, and reverses due to

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3016-416: The star's gravitational attraction. The star's states are held to be either expanding or contracting by the hysterisis generated by the doubly ionized helium and indefinitely flip-flops between the two states reversing every time the upper or lower threshold is crossed. This process is rather analogous to the relaxation oscillator found in electronics. In 1879, August Ritter (1826–1908) demonstrated that

3074-539: The universe is expanding (see Hubble's law ). Henrietta Swan Leavitt was born in Lancaster, Massachusetts , the daughter of Henrietta Swan Kendrick and Congregational church minister George Roswell Leavitt. She was a descendant of Deacon John Leavitt , an English Puritan tailor, who settled in the Massachusetts Bay Colony in the early seventeenth century. (In the early Massachusetts records

3132-421: The universe, as it prompted Harlow Shapley to move the Sun from the center of the galaxy in the " Great Debate " and Edwin Hubble to move the Milky Way galaxy from the center of the universe. Leavitt's discovery of an accurate way to measure distances on an inter-galactic scale, paved the way for modern astronomy's understanding of the structure and scale of the universe. The accomplishments of Edwin Hubble ,

3190-593: Was accepted by the International Committee of Photographic Magnitudes in 1913. In 1913, Leavitt discovered T Pyxidis , a recurrent nova in the constellation Pyxis , and one of the most frequent recurrent novae in the sky, with eruptions observed in 1890, 1902, 1920, 1944, 1967, and 2011. Leavitt was a member of Phi Beta Kappa , the American Association of University Women , the American Astronomical and Astrophysical Society ,

3248-400: Was seen as a tragedy by her colleagues for reasons that went beyond her scientific achievements. Her colleague Solon I. Bailey wrote in his obituary for Leavitt that "she had the happy, joyful, faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning." She was buried in

3306-468: Was studying those plates, I doubt Pickering thought she would make a significant discovery—one that would eventually change astronomy." The period–luminosity relationship for Cepheids, now known as "Leavitt's law", made the stars the first " standard candle " in astronomy, allowing scientists to compute the distances to stars too remote for stellar parallax observations to be useful. One year after Leavitt reported her results, Ejnar Hertzsprung determined

3364-433: Was that while no one knew the distance to the Small Magellanic Cloud , all its stars must be roughly the same distance from Earth. Therefore, a relationship she discovered in it, between the period of certain variable stars (Cepheids) and their apparent brightness, reflected a relationship in their absolute brightness. Once calibrated by measuring the distance to a nearby star of the same type via parallax, her discovery became

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