Variable Stars Novae And Supernovae

Variable Stars, Novae, and Supernovae Some stars appear to change in brightness over time. Some of these variable stars exhibit periodic behaviour, changing their brightness in a repeating pattern. Other stars vary in an irregular, unpredictable fashion. Still others exhibit a one-time dramatic change in brightness by orders of magnitude before fading away to obscurity. Astronomers classify variable stars according to their observable properties. The first criteria for classification is whether a variable star in an intrinsic or an extrinsic variable. Intrinsic variables are those whose change in brightness is due to some physical change in the star itself. Extrinsic variables are those in which the light output changes due to some process external to the star itself. The main types of variable stars. A graph showing the variation of brightness with respect to time is called a light curve. Light curves are used to distinguish and classify variable stars.

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Eclipsing Binaries

These variables are binary star systems. When one star goes behind the other, there is a reduction of their combined luminosity. Since the orbital motion of the stars is periodic, the light curve is also periodic. However, not all binary systems are eclipsing - it depends on the inclination angle of the orbital plane of the system to our line of sight.

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Rotating Variables

Our Sun has sunspots on its visible surface, which are cooler regions that appear darker than the surrounding areas. A side of the Sun with a lot of sunspots would have a fractionally lower light output than a side with fewer spots. This principle applies to other stars, some of which are have much more “starspot” activity. Starspots can be either dimmer or brighter than surrounding regions. As a star with starspots rotates, its brightness changes slightly. Stars exhibiting such behaviour are called rotating variables.

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Pulsating variables

Pulsating variables periodically expand and contract their surface layers. In the process they change their size, effective temperature and spectral properties. Unlike the eclipsing and rotating variables whose brightness changes are due to geometry, these stars are variables because of their intrinsic structure. Cepheid variables are named after Delta Cephei (δ Cep), whose variability was discovered by John Goodricke in 1784. Cepheids have light curves similar to the one shown above. Their periods have a range of a few days to a few months, and show a definite relation to their luminosities. The longer the period, the brighter the star is. Thus, by measuring the period of a Cepheid variable, we know its absolute magnitude; from that, we can tell how far the star is by comparing its apparent magnitude. Cepheid variables, therefore, are a vital tool in galactic and extragalactic distance determination. Light curve of Delta Cephei, the prototypical Cepheid variable.Delta Cephei has a period of 5.37 days and a magnitude rang of just under 1. Long Period Variables have periods of months to years. They are further classified according to whether they exhibit regular periodicity, or more irregular behavior. They are cool red giants or supergiants whose luminosities can range from 10 to 10,000 times the Sun’s. The first long-period variable discovered was Mira or Omicron Ceti (ο Cet), established as a variable star in 1638. Mira has a period of 331 days, and varies in brightness by almost 6 magnitudes over the course of one cycle. Its radius varies by 20 percent, peaking at 330 times that of our Sun. As a red giant, its surface temperature ranges from 1900 K to 2600 K. Semiregular Variables show some periodicity, but also exhibit irregularities where they appear to be stable. They are giant and supergiant stars, with periods ranging from a few days to several years. Their change in brightness is typically less than two magnitudes. The light curves of semiregulars have a variety of shapes. Prominent examples of this type include Antares or Alpha Scorpii (α Sco), and Betelgeuse or Alpha Orionis (α Ori).

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Eruptive Variables

Pulsating variables vary periodically, but some stars exhibit sudden changes in their brightness due to violent outbursts caused by processes within the star. Such kinds of stars are called eruptive variables. They include novae and supernovae. A nova is characterised by a rapid and unpredictable rise in brightness of 7 - 16 magnitudes over a few days. The eruptive event is followed by a steady decline back to the pre-nova magnitude over a few months. This suggests that the event causing the nova does not destroy the original star. Novae are usually close binary stars where one component is a white dwarf that draws material off its companion. Material is transferred from one component to another until there is sufficient matter accumulated to trigger a thermonuclear reaction that then blasts the shell of material off into space. A nova occurrs in a binary system where one component is a white dwarf which draws material from the other. A supernova is a cataclysmic event characterised by a sudden and dramatic rise in brightness. In a supernova, a star become brighter by up to 20 magnitudes, to an absolute magnitude of about -15. This means that a typical supernova may outshine its entire galaxy for a few days or weeks. Supernovae are caused by one of two main mechanisms: Type I supernovae take place when accreting material falling onto a white dwarf in a binary system takes its mass over the Chandrasekhar limit. The resulting instability triggers a runaway thermonuclear explosion that destroys the star and releases large amounts of radioactive and heavy elements into space. Type II supernovae occur in very massive stars once all the material in their core has been fused into iron. Since fusion in elements heavier than iron consumes more energy than it produces, gravitation overwhelms the core, which rapidly implodes. The core material gets crushed to form degenerate neutron-density material, while the extreme temperature and pressure in the surrounding layers cause rapid nuclear reactions that synthesise the heaviest elements, ripping the star apart. Such core collapse supernovae result in neutron stars and/or black holes. Supernova 1987A (right) and its progentor star, Sanduleak -69 202 (arrow, left) in the Large Magellanic Cloud.Images copyrighted by the Anglo-American Observatory. Although we expect two or three supernovae in our galaxy each century, they may not all be visible due to galactic dust. The most recent supernova visible to the naked eye was SN1987A. This was a core-collapse (Type II) event that took place in the Large Magellanic Cloud, a satellite galaxy of our own about 50,000 parsecs distant.