Profile of Variable Stars

So what are variable stars?

Very simply stated, variable stars are stars that vary in brightness. This variability can occur over the span of seconds as in the case of ZZ Ceti stars, it can take years as with Mira-type variable stars or it can happen within several hours and just once as in the case of a supernova.

Each of the variable stars just mentioned vary in brightness for different reasons. The ZZ Ceti stars are pulsating white dwarfs that change their brightness with periods that range from approximately 100 to 1000 seconds. Occasionally, flares occur and will result in the star doubling its brightness.

The Mira-type variable stars are supergiant or giant stars that also pulsate but instead of within seconds, the measured period of their pulsation lasts hundreds or even thousands of days. These stars are of interest to astrophysicists because they are representatives of a very short-lived phase of stellar evolution. Mira stars are ending their lives and some will soon become planetary nebulae. Because they are so large and their atmospheres so far removed from the core, their extended atmosphere is extremely rarefied and would be considered an excellent vacuum here on Earth.

The rare supernova dramatically alters the structure of a star in such a way that the star is irrevocably changed. The shell of the star is violently expelled and eventually interacts with the interstellar medium, forming a supernova remnant. It is possible to observe many supernova remnants in the night sky and they serve as a reminder that the Universe is constantly changing. Supernovae could be thought of as the recycling facilities of the cosmos. Almost everything comes from supernovae and almost everything will go back into producing new supernovae.

These examples are just a few types of variable stars and many other types of variable stars exist. Currently, astronomers recognize over 80 types of variable stars, an additional five types for eclipsing binary variables according to the companion's physical characteristics, nine types based on the degree of filling of inner Roche lobes5, and ten types of optically variable close binary sources of strong, variable X-ray radiation. The number of possible combinations is staggering and as a result, variable-star observers usually specialize in just a handful of types. Some observers spend a few years observing a half-dozen classes of variable stars, then move on to observe different types. Other variable-star observers observe only a few classes of stars, never moving to other types. As a result, they are very knowledgeable after having spent years or even decades specializing on their selected stars.

Finding a variable star is not too difficult to do. Because all stars oscillate to some degree, they are all to a greater or lesser degree variable, and these oscillations give rise to variations in luminosity. However, the amplitude of this oscillation for any given star is usually very small, so that the associated variations in luminosity are tiny. Not very exciting since these micro-variations are invisible to the naked eye.

For example, solar-type stars vary in luminosity by an order of micro-magnitudes, far too faint to be observed with the naked eye. The good news is that you can study these micro-oscillations with instruments such as stellar photometers and charge-coupled devices that are available to amateur astronomers. The study of small-amplitude stellar oscillations has become an important branch of astronomy because the oscillations involve the entire star, so from them one can glean information about the deep stellar interior. An excellent example of this type of effort is the study of Delta (8) Scuti stars.

Some stars vary in brightness because of intrinsic properties but others vary in brightness as a result of external characteristics that have nothing to do with the actual make-up of the star. For example, two stars that would not be considered variable individually may be classed as variable stars if they orbit each other in such a way that one moves in front of another and causes an eclipse. In some cases, this eclipse will result in a reduction of luminosity when viewed from Earth. In effect, the two stars vary in brightness. This variability is not a result of the inherent properties of either individual star but rather is a result of their interaction with each other. Now, imagine two different classes of variable stars that are each variable for intrinsic reasons, perhaps one is pulsating while the other

5 Roche lobes will be explained in Chapter 5, Cataclysmic Variables.

experiences flare activity, orbiting each other in such a way as to cause an eclipse when observed from Earth. Understanding this complex type of configuration can be difficult at best but the labor required to understand this type of situation is just part of the challenge of variable-star observing. Six major classes of variable stars will be described to show the diverse mechanisms for producing variability.

Eruptive variable stars are defined as stars that show a sudden, large outburst of energy causing their visual brightness to increase by 200 times or more in a few days. These outbursts of energy are caused by violent processes, such as flares, that occur within the visible edge of the star. In some cases, stellar material is being blown away from the star and interacts with the surrounding interstellar medium, causing changes in visual brightness.

Pulsating variable stars show a periodic expansion and contraction of their surface layers, as if breathing. In some cases, the expansion occurs uniformly throughout the star. In other cases, the star quivers during unequal expansions that occur within the various layers of the star.

Cataclysmic variables show outbursts caused by thermonuclear processes on or within their surface layers or deep within their interiors. Novae and dwarf novae are members of this much-observed class of variable star and an important property that is shared by these stars is that they are all extremely close binary systems, in most cases possessing orbital periods of less than half a day. Supernovae are recognized as cataclysmic variable stars too. The first supernova to be observed using modern techniques was the variable star S Andromedae in the Great Andromeda galaxy, also known as M31. When it was first observed in 1885, it was believed to be a normal nova, with a relatively modest brightness. This led to estimates of the distance to M31 that misled astronomers about the size of galaxies. In 1924, Edwin Hubble discovered a type of variable star known as a Cepheid in M31 and was able to make an independent estimate of the distance using the period-luminosity relationship. It then became clear that S Andromedae was more than ten thousand times brighter than an ordinary nova. Supernovae are rare events, with a typical galaxy producing no more than about two or three per century.

Rotating variable stars possess irregular surface brightness and/or elliptical shapes. Their variability is caused by axial rotation with respect to the observer. The irregular surface brightness may be caused by the presence of spots or by some thermal or chemical variations of the atmosphere caused by magnetic fields. Our Sun displays sunspots that are the size of the Earth. Imagine a star with sunspots that are the size of our Sun!

Eclipsing binary stars are multiple star systems that have the orbital plane of the orbiting stars oriented approximately along the line of sight of the observer so that one star may periodically pass in front of the other thereby blocking the light of the eclipsed star. The study of these light curves not only reveal the presence of two stars but can also provide information about relative temperatures and radii of each component from the amount of light decrease and the length of the eclipse. Recently, amateur astronomers have detected the possible transit of extra-solar planets across distant stars.

Optically variable X-ray sources are a somewhat ambiguous class of variable stars. Some astronomers consider X-ray binaries to be any kind of interacting close binary with a compact degenerate object, such as a white dwarf, a neutron star, or a black hole. The definition that we will use is that X-ray binaries are only those interacting close binary systems that contain a neutron star or black hole. The main difference between the cataclysmic variables and the X-ray binaries is the X-ray luminosity. Many X-ray binaries produce optically variable light phenomena that can be observed by amateur astronomers. Another type of high-energy variable star is the gamma ray burst, also known as a GRB. These enigmatic objects have just recently come to the attention of amateur astronomers. Presumably stars, these objects are so far distant that we have found no evidence of their existence on any deep image taken before their outburst. They eventually fade away and again nothing is seen where the GRB appeared. Today, amateur astronomers can be notified through a network of interested astronomers when satellites have detected a GRB. If quick enough, amateur astronomers have been able to catch a glimpse of these interesting objects.

These six classes of variable stars; eruptive, pulsating, cataclysmic, rotating, eclipsing, and variable X-ray sources will be characterized in the subsequent chapters of this book. Suggestions of how best to observe each class of star and the basic methods for recording and analyzing your observations will be given so that you may preserve and later examine the fruits of your labor. A description of the basic telescope and binocular types that variable star observers use, as well as the various eyepieces, mounts and other accessories that you will routinely use to observe variable stars will also be explained. Along with visual observation techniques, the use of semiconductor light detectors (known as CCDs), photoelectric (PEP) methods and the use of science filters will be briefly investigated.

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