What is radio astronomy? What is a pulsar? Apart from twinkling due to the effects of our atmosphere stars appear fixed and constant to the untrained eye.
Careful observations, some even done with the naked eye, show that some stars do in fact appear to change in brightness over time. Some exhibit periodic behaviour, brightening quickly then diminishing in brightness slowly only to repeat themselves. With some these changes take place over several days whilst others occur in a matter of hours or many months. Other stars exhibit a once-off dramatic change in brightness by orders of magnitude before fading away to obscurity.
All of these are examples of what are termed variable stars. A variable star is simply one whose brightness or other physical property such as radius or spectral type changes over time. At a fundamental level all stars are variable as they evolve and change over time from a main sequence to a red giant star as in the Sun's case for example. Furthermore we can infer that all stars are likely to vary their light output to some extent due to variations caused by phenomena such as sunspots.
In the section however, we focus on stars with a measurable change in brightness. In order to try and understand variable stars, astronomers have sought to classify them according to observable properties.
The diagram below the main types of variable stars. The first criteria for classification is whether a star in an intrinsic or an extrinsic variable.
Intrinsic variables are those in which the change in brightness is due to some change within the star itself such as in pulsating stars like the Cepheids. Extrinsic variables are those in which the light output changes due to some process external to the star itself.
The most common example of these are the eclipsing binaries. Brief details on the major classes are provided below whilst the pulsating variables are discussed in more detail on the next page. These are stars which vary their light output, hence their brightness, by some change within the star itself.
They are an extremely important and useful group of stars to astronomers as they provide a wealth of information about the internal structure of stars and models of stellar evolution. Perhaps their greatest value is the role of some types such as Cepheids and supernovae in distance determination. Intrinsic variables are further classified as to whether they exhibit periodic pulsations are more explosive or eruptive events as in cataclysmic variables. Pulsating variables periodically expand and contract their surface layers.
In the process they change their size, effective temperature and spectral properties. As they are a vital tool in galactic and extragalactic distance determination and have many types they are discussed in more detail on separate pages. Eruptive variables can exhibit significant and rapid changes in their luminosity due to violent outbursts caused by processes within the star. There is a wide variety of eruptive or cataclysmic variables.
Some event, as implied by the term cataclysmic result in the destruction of the star whilst others can reoccur one or more times. More details on the different types are provided below. Some are also discussed in more detail in the pages on stellar evolution. A supernova is a cataclysmic stage towards the end of a star's life that is characterised by a sudden and dramatic rise in brightness.
A typical supernova may see a star become brighter by up to 20 magnitudes to an absolute magnitude of about This means that a typical supernova may outshine the rest its galaxy for several days or a few weeks. Supernovae are caused by one of two main mechanisms. The first takes place when accreting material falling onto a white dwarf in a binary system takes it over the mass set by 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. The second process occurs in very massive stars once all the material in their core has been fused into iron. As fusion cannot occur in elements heavier than iron the drop in outwards radiation pressure means that gravitational collapse overwhelms the core which rapidly implodes. The core material gets crushed to form degenerate neutron-density material whilst the extreme temperature and pressure in the surrounding layers cause rapid R-process nuclear reactions that synthesise the heaviest elements.
A huge flux of neutrinos is thought to interact with the superdense material, ripping the star apart. Such core collapse supernovae may result in neutron stars and black holes forming from the remaining core material. More details are given in the later section on star death. Observationally, supernovae are classified according to their spectra. Type I supernova exhibit no hydrogen lines in spectra taken soon after the supernova event.
Those with silicon lines present are further classified as Type Ia and are thought to be due to thermonuclear explosions as in accreting white dwarfs. If no Si lines are present they are Type Ib or Ic depending on the high or low abundance of He lines respectively. These types occur due to core collapse following the outer layers being stripped away in Wolf-Rayet or binary stars. Type II supernovae show hydrogen lines in their early spectra. They are all examples of core collapse events with most arising due to a massive progenitor star exhausting its core fuel.
Perhaps the best known example of this was Supernova A. This was the first supernova visible to the naked-eye since Kepler's supernovae of It took place in the Large Magellanic Cloud, a satellite galaxy of our own about 50, pc distant. Although we expect two or three stars to go supernova in our galaxy each century, these may not be visible in optical wavebands due to absorption and scattering by the galaxy's dust lanes so the occurrence of a supernova in an nearby galaxy was a major boon for astronomers.
Observations of SN A continue today at many wavebands. A nova occurs in a close binary system and is characterised by a rapid and unpredictable rise in brightness of 7 - 16 magnitudes within 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. Our model for novae is that of an accreting white dwarf. It draws material off its close binary companion for about 10, to , years until there is sufficient material to trigger a thermonuclear explosion that then blasts the shell of material off into space. These are similar to novae with a change in magnitude of 7 - 16 and a period of outburst of up to about days.
They show two or more outburst over recorded observations. These are intrinsically faint stars that exhibit a sudden increase in brightness by 2 to 5 magnitudes over a few days with intervals of weeks or months between outbursts.
Note as with other types of variables, the class or type name is normally based on the first such type of that class discovered. The U Geminorum type is thus named after the star U Geminorum. As with other types of novae, dwarf novae are close binaries with a white dwarf as one of the component stars.
The most popular model explaining their outbursts is the disk instability model in which thermal instabilities in the accretion disk cause outbursts but no explosion. There is no significant ejection of material in these events. The first modern identified variable star was Omicron Ceti, later renamed Mira. It had been described as a nova in by David Fabricius. In , Johannes Holwards observed Omicron Ceti pulsating in a regular month cycle. This was an important discovery, as it helped verify that the stars were not eternal and invariable as ancient philosophers such as Aristotle had believed.
The discovery of variable stars, along with reports of supernovae , paved the way for development of the science of astronomy. Three of the four had been recorded as novae in early Chinese or Korean records. In , a second variable star was identified by Geminiano Monanari. It was an eclipsing variable called Algol , although its variability was not explained until more than a hundred years later by John Goodricke in The third variable star, Chi Cygni , was observed in and in Over the next 80 years seven more variable stars were identified.
Since numerous variable stars have been observed, a process aided by the development of photography. There are a number of reasons for variability. These include changes in star luminosity or in star mass, and obstructions in the amount of light that reaches Earth.
Pulsating variables swell and shrink. Eclipsing binaries get dimmer when a companion star moves in front, then brighten as the occulting star moves away. Some of the identified variable stars are actually two very close stars that exchange mass when one takes atmosphere from the other.
There are two different categories of variable stars. Intrinsic variables are stars whose luminosity physically changes due to pulsations, eruptions or through swelling and shrinking.
Extrinsic variables are stars that change in brightness because of being eclipsed by stellar rotation or by another star or planet. Cepheid Variables are very luminous stars, to , times greater than the sun, with short periods of change that range from 1 to days. They are pulsating variables that expand and shrink dramatically within a short period of time, following a specific pattern. Astronomers can make distance measurements to a Cepheid by measuring the variability of its luminosity, which makes them very valuable to the science.
Other pulsating variables include RR Lyrae stars, which are short period, older stars that are not as large as Cepheids; and RV Tauri stars, supergiants with greater light variations. Long-period pulsating variables include the Mira class, which are cool red supergiants with large pulsations; and Semiregular, which are red giants or supergiants with longer periods that can range from 30 to days. One of the best-known Semiregular Variables is Betelgeuse.
Irregular pulsating variables have also been identified. These are usually red supergiants, but very little study has been done on them. The important variable star allowed American astronomer Edwin Hubble to determine that the filmy nebula in which it lay was, in fact, another galaxy entirely, demonstrating that the Milky Way did not contain the entire universe.
Cataclysmic Variables also called Explosive Variables brighten because of sharp or violent outbursts caused by thermonuclear processes either on the surface or deep inside.
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