Henrietta Swan Leavitt
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1912: Henrietta Leavitt descubre la clave de la distancia.

El método de paralaje utilizado para medir las distancias a las estrellas cercanas, iniciado por Bessel y otros, solo se podía usar en estrellas más próximas de 100 años luz de distancia. Pero la mayoría de las estrellas y otras galaxias están mucho más allá de esa distancia. La clave para encontrar la distancia a estrellas mucho más alejadas fue descubierta por Henrietta Swan Leavitt, quien trabajó en el Observatorio de la Universidad de Harvard como una "computadora", una de muchas mujeres que cobraban de 25 a 30 centavos por hora para extraer datos de miles de placas fotográficas.

Por desgracia, las estrellas no tienen el mismo brillo intrínseco (o luminosidad), así es imposible decir si una estrella se muestra mortecina porque está muy lejos , o porque no emite mucha luz. Sólo si fuera posible determinar la luminosidad real de una estrella, su brillo aparente podría utilizarse entonces para determinar su distancia hasta nosotros.

La tarea de Leavitt era identificar estrellas variables que son estrellas que cambian de brillo en pocas horas, días o semanas. Para hacer ésto, ella comparaba dos fotos de un campo estelar tomadas en pocos días o semanas de intervalo. Utilizaba un instrumento llamado comparador de parpadeo que pasa rápidamente de una imagen a otra, de forma que una estrella variable se muestra como un punto que parpadea. Con este método encontró más de 2400 estrellas variables.

Leavitt sintió curiosidad sobre si puede existir una relación entre el brillo de una estrella variable y la duración de su periodo (cuánto tiempo le cuesta a la estrella hacerse más brillante, atenuarse y luego volver a brillar más de nuevo). Eso resultaba difícil porque no conocía el brillo intrínseco de una variable dada. Resolvió el problema restringiendo su búsqueda a una clase particular de estrella variable conocida como Cefeida, variables que residen en la Pequeña Nube de Magallanes, un cúmulo de estrellas lejano. Argumentó que todas las estrellas del cúmulo debían de estar aproximadamente a la misma distancia de la Tierra.

Su presentimiento dio fruto. Leavitt descubrió 25 variables Cefeo en el cúmulo y creó un gráfico mostrando el máximo brillo de cada estrella y la duración de su período. Como sospechaba, existía una clara relación. Las estrellas más brillantes tenían períodos más largos. Todo lo que necesitaba para encontrar las distancias reales era encontrar precisamente la distancia a una variable Cefeida cercana. Pocos años más tarde, un equipo de astrónomos hizo exactamente eso, haciendo posible la medida de la distancia a una cefeida en solo tres pasos.
a) Medida del período de la estrella.
b) Utilizar el gráfico de Leavitt para determinar realmente su brillo.
c) Medida del brillo que aparece y determinar su distancia.

Al descubrir la llave de la distancia, Henrietta Swan Leavitt hizo posibles todos los posteriores descubrimientos en astronomía de los siglos XIX y XX.

https://cosmology.carnegiescience.edu/timeline/1912
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1912: Henrietta Leavitt Discovers the Distance Key.
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But most stars and other galaxies are far beyond that distance.
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To do this she would compare two photos of a star field taken a few days or weeks apart.
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With this method she found more than 2,400 variable stars.
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That was difficult because she did not know the intrinsic brightness of any given variable.
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She reasoned that all stars in the cluster must be approximately the same distance from Earth.
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Her hunch paid off.
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As she suspected, there was a clear relationship.
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Brighter stars had longer periods.
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a) Measure the period of the star.
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b) Use Leavitt’s graph to determine how bright it really is.
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c) Measure how bright it appears and determine its distance.
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https://cosmology.carnegiescience.edu/timeline/1912
1 Translations, 3 Upvotes, Last Activity 7 years, 5 months ago

1912: Henrietta Leavitt Discovers the Distance Key.

The parallax method used to measure the distances to nearby stars, pioneered by Bessel and others could only be used on stars closer than 100 light years away. But most stars and other galaxies are far beyond that distance. The key for finding the distance to stars much further away was discovered by Henrietta Swan Leavitt who worked at Harvard College Observatory as a “computer,” one of several women paid 25 to 30 cents per hour to extract data from thousands of photographic plates.

Unfortunately stars are not the same intrinsic brightness (orluminosity), so it is impossible to tell if a star appears dim because it’s far away, or because it doesn’t put out much light. If only it were possible to determine the actual luminosity of a star, then its apparent brightness could be used to determine its distance from us.

Leavitt’s assignment was to identify variable stars, which are stars that change in brightness over a few hours, days, or weeks. To do this she would compare two photos of a star field taken a few days or weeks apart. She used an instrument called a blink comparator that flips back and forth quickly between the two images so that a variable star shows up as a flashing spot. With this method she found more than 2,400 variable stars.

Leavitt became curious about whether there might be a relationship between the brightness of a variable star and the length of its period (how long it takes for the star to get brighter, dimmer, then brighter again). That was difficult because she did not know the intrinsic brightness of any given variable. She solved the problem by restricting her search to a particular kind of variable star known as Cepheid variables that reside in the Small Magellanic Cloud—a distant star cluster. She reasoned that all stars in the cluster must be approximately the same distance from Earth.

Her hunch paid off. Leavitt discovered 25 Cepheid variables in the cluster and created a graph showing the maximum brightness of each star and the length of its period. As she suspected, there was a clear relationship. Brighter stars had longer periods. All that was needed to find actual distances was to find the distance to just one nearby Cepheid variable. A few years later a team of astronomers did just that, making it possible to measure the distance to any Cepheid in just three steps.
a) Measure the period of the star.
b) Use Leavitt’s graph to determine how bright it really is.
c) Measure how bright it appears and determine its distance.

By discovering the distance key, Henrietta Swan Leavitt made possible all of the subsequent discoveries in astronomy of the 19th and 20th centuries.

https://cosmology.carnegiescience.edu/timeline/1912