Most stars shine with almost constant brightness – the Sun’s brightness, for example, varies by only 0.1% over a period of 11 years. Yet some stars vary significantly in brightness and over much shorter time periods. Some of them shrink and expand as their internal structure changes because they are exhausting the fuel in their interior, a type of variation called pulsation.
.One of the most common and best studied types of pulsating star is the Cepheid variable. These are older stars, usually yellow giants, that vary quite regularly over a period of 1 to 50 days. They brighten and dim as they physically expand and contract, acting like a spring. The gas of the star’s envelope compresses due to gravity, causing it to heat and brighten. Eventually pressure overcomes gravity, the envelope of gas rebounds, and the temperature drops causing the brightness to decrease. Then, the pressure decreases, gravity takes over, and the cycle begins again. One might expect
that the pulsations would eventually stop because of energy losses in each cycle, but in fact, a complex phenomenon due to the ionization of helium in the star’s envelope sustains the pulsation.
The first variable star of this type was discovered in 1784 by John Goodrick, a Dutch amateur astronomer who paid for the discovery with his life: he caught pneumonia while making his long nocturnal observations – he was not yet 22 when he died. The object of his interest was the star Cephei in the constellation Cepheus, which gave its name to this class of variable star.Cepheids play a crucial role in astronomy because, as demonstrated by the American astronomer Henrietta Swan Leavitt in 1912, the period of a Cepheid’s variation is related to the intrinsic luminosity of the star, and can thus serve as a “standard candle” to measure large distances. Cepheids are very bright, with up to 10 000 times the luminosity of the Sun, making it possible to detect them even in relatively distant galaxies. Once a Cepheid is found and monitored, its pulsation period supplies its intrinsic luminosity. The comparison of the intrinsic luminosity to the apparent brightness provides the distance to the star’s home galaxy. It was this method that Edwin Hubble used in 1929 to demonstrate the expansion of the Universe, and was again used recently with the Hubble Space Telescope in order to determine its rate of expansion with greater precision.
Pic Source: Wikipedia
.One of the most common and best studied types of pulsating star is the Cepheid variable. These are older stars, usually yellow giants, that vary quite regularly over a period of 1 to 50 days. They brighten and dim as they physically expand and contract, acting like a spring. The gas of the star’s envelope compresses due to gravity, causing it to heat and brighten. Eventually pressure overcomes gravity, the envelope of gas rebounds, and the temperature drops causing the brightness to decrease. Then, the pressure decreases, gravity takes over, and the cycle begins again. One might expect
that the pulsations would eventually stop because of energy losses in each cycle, but in fact, a complex phenomenon due to the ionization of helium in the star’s envelope sustains the pulsation.
The first variable star of this type was discovered in 1784 by John Goodrick, a Dutch amateur astronomer who paid for the discovery with his life: he caught pneumonia while making his long nocturnal observations – he was not yet 22 when he died. The object of his interest was the star Cephei in the constellation Cepheus, which gave its name to this class of variable star.Cepheids play a crucial role in astronomy because, as demonstrated by the American astronomer Henrietta Swan Leavitt in 1912, the period of a Cepheid’s variation is related to the intrinsic luminosity of the star, and can thus serve as a “standard candle” to measure large distances. Cepheids are very bright, with up to 10 000 times the luminosity of the Sun, making it possible to detect them even in relatively distant galaxies. Once a Cepheid is found and monitored, its pulsation period supplies its intrinsic luminosity. The comparison of the intrinsic luminosity to the apparent brightness provides the distance to the star’s home galaxy. It was this method that Edwin Hubble used in 1929 to demonstrate the expansion of the Universe, and was again used recently with the Hubble Space Telescope in order to determine its rate of expansion with greater precision.Pic Source: Wikipedia
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