Why does a cepheid star vary in luminosity

This in turn allows us to determine the distance to such stars and is discussed in more detail on the next page. As with non-pulsating variables, there are several types of pulsating stars and some of the key types are described briefly below. The same star was noted to vary in brightness during by another Dutch observer and became known as Mira the "Wonderful" due to its behaviour.

It was eventually found to have a period of about days and was the first pulsating variable discovered. Its light curve was different to that of Algol which was correctly inferred to be an eclipsing binary by the brilliant young English astronomer John Goodricke in Cepheids are very luminous, massive variables with periods of 1 days. Cepheid light curves are distinctive and show a rapid rise in brightness followed by a more gradual decline, shaped like a shark fin. Their amplitude range is typically 0.

The spectral class of a Cepheid actually changes as it pulsates, being about an F at maximum luminosity and down to a G or K at minimum. Most have a period of between 5 days and an amplitude range of 0. The variations are less pronounced at infrared wavebands.

They are 1. Classical Cepheids follow a well-defined period-luminosity relationship. This means that the longer the period of the Cepheid, the more intrinsically luminous it is.

This has important implications as it allows Cepheids to be used as standard candles for distance determination and is discussed in detail on the next page. Their pulsation mechanism is discussed in more detail below. It has a period of W Virginis -type Cepheids are intrinsically less luminous by 1. As they are older stars than Type Is their spectra are characterised by having lower metallicities.

Type II light curves show a characteristic bump on the decline side and they have an amplitude range of 0. As with the Type I Cepheids they also display a similar well-defined period-luminosity relationship and can be used for distance determination. These old population II giant stars are mostly found in globular clusters.

They are characterised by their short periods, usually about 1. Spectral classes range from A7 to F5. They are thus useful in determining distances to the globular clusters within which they are commonly found to a distance of about kiloparsecs. Sub-types are classified according to the shape of their light curves. RV Tauri variables are yellow supergiants, mostly G and K-class stars. Their distinctive light curves show alternating deep and shallow minima with the period equal to the time between two successive deep minima.

They compare the Cepheid variable's apparent brightness with its intrinsic brightness. The difference between observed and actual brightness yields the distance. This introduction leads us to the subscriber's question.

One would think so, actually. This "Cloud" is a satellite galaxy to the Milky Way at an estimated distance of , light years from Earth. Even though the SMC has an diameter of 7, light years, we can assume that all the stars are at more or less the same distance from us. Similarly, we can assume that every citizen of Los Angeles County is at the same distance from Portland, Maine.

Professor Leavitt surveyed the Cepheid variables within the SMC and noticed that the brightest Cepheids exhibited the longest variability periods. Through careful observations, she determined a correlation between the Cepheid variable's brightness and variability period. These observations served to established a relationship only.

From these numbers one can extract the distance to the stars. This method works up to 13 million light-years when Earth-bound telescopes are used; for larger distances these stars become too dim to be observed. Recently, space-based telescopes such as the Hubble Telescope, have used these stars to much farther distances. Looking at a galaxy in the Virgo cluster called M, astronomers used the Cepheid variables observed there to determine its distance - 56 million light-years.

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