A Colorful Universe Revealed in the Earth Motion


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When we first look at the night sky, we can not distinguish the color of each star with our own eyes, although, if we record in a long exposure sequence the path of each star, due to the motion of rotating Earth and consequently the rotation of celestial sphere, a starry trailed sky can reveal much of our colorful Universe. In the image above, captured in Noudar Park, Alqueva Dark Sky® Reserve, Portugal, we can see a lot of different color paths, like: blue, white, yellow or even orange. Each color is directly related to the type and temperature of each star. The hottest stars, are showing a blue color, but in opposition the coolest, can reveal an orange-reddish color.

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with absorption lines. Each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere. The spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere’s temperature and density.

Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type – blue stars) to the coolest (M type – red stars). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and class C for carbon stars.

In the MK system, a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star’s spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ stars for hypergiants, class I stars for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, and class D for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K. Source and more info: click here.

PT: Quando de noite olhamos para o céu não conseguimos percepcionar com os nossos próprios olhos a cor de cada estrela, no entanto, se registarmos numa sequência de longa exposição o rasto de cada estrela, devido ao movimento de rotação da Terra e à consequente rotação da esfera celeste, uma imagem startrail poderá revelar um Universo colorido e rico em informação. Na imagem acima captada no Parque de Natureza de Noudar, Reserva Dark Sky® Alqueva, em Barrancos, podemos ver uma grande diversidade de cores desde o azul, ao amarelo, branco ou laranja. Cada cor está directamente relacionada com o tipo e temperatura de cada estrela. Algumas delas são mais quentes e por isso apresentam uma cor mais azulada, outras por sua vez são mais frias apresentando uma cor mais alaranjada.

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