Variable Stars Now You See Me Now You Dont

Our lives on Earth are dominated by the presence of a star that shines with a light that never changes. The Sun is today what it was yesterday and will be precisely the same tomorrow. Not all stars are so consistent though. Many will vary considerably in luminosity from year to year or month to month or even day to day. Some will do so on a very precise schedule and others will do so on no schedule at all. And they will do so for a wide variety of reasons. Variables can be categorized into two types, intrinsic and extrinsic. Extrinsic variables brighten and dim because of a factor external to the star such as the presence of a fainter companion that regularly eclipses its brighter counterpart. Intrinsic variables change brightness due to an internal factor. Intrinsic variables either actually physically pulse in their cores causing their luminosity to vary on a regular scale or for some reason erupt violently, sometimes on a recurring basis and sometimes they are single events. Some intrinsic variables not only vary in luminosity, but in temperature and color as well. Unlike double stars, which require a telescope to see in most cases, many variables can be easily observed, enjoyed and tracked with no optical aid whatsoever.

In the constellation Perseus, the star Algol (Beta Persei) is the prototype for a type of extrinsic variable star called eclipsing binaries. The name Algol is derived from Arabic for "the demon," and though its nature was not scientifically documented until 1667, observers clearly knew about its regular fading for many hundreds of years before then. Algol is located 93 light years from Earth and has two components separated by less than five million miles. The system consists of a bright spectral type B8 bluish white star that shines at a temperature of 12,500K and with 100 times the luminosity of the Sun. The star is a hydrogen-burning main-sequence dwarf with about 3.8 times the mass of the Sun. The star itself is as regular as our Sun is but exactly every 2.867 days Algol does something amazing. Its light dims from magnitude +2.1 to magnitude +3.4 or only one-third its normal luminance. The reason why is because the primary star's companion passes in front of the star at that time creating a deep partial eclipse. The secondary star is yellow-orange K type star, though the uncertainty of its exact spectral class is considerable varying from G5 to K2. It has only 2.5% of the luminosity of the primary so when it blocks the primary star's light, there is a dramatic fading in the combined luminosity of the system. The eclipse lasts for about five hours from first fading to full recovery. Algol-class eclipsing binaries are very important to astronomers because they provide a powerful tool to measuring stellar masses and sized. Thousands of these stars have been catalogued over the years.

Algol is equally famous to astronomers for another reason, which astronomers call the Algol Paradox. Spectroscopic observations tell us that the two stars that make up the Algol system are of about the same age. The primary "B" star is a main sequence dwarf with 3.8 solar masses while the secondary "K" star has only 0.81 solar masses and yet it is the K star that has evolved into a dying giant while the more massive blue star is still on the main sequence. The reverse should be true as more massive stars burn through their hydrogen fuel at an exponentially greater rate. The reason why is that the primary star is stripping material from the secondary at an amazing rate almost exposing its core. In looking at Algol you are observing a pair of stars in the dramatic act of one star literally stealing the life of the other in an interstellar tragedy of Shakespearean proportions.

In Cetus the Whale, each eleven months a new star appears amid the faint patterns of the mighty celestial cetacean. Dubbed "Mira" (derived from the root word for "miracle") and designated Omicron Ceti. Mira is the best known of these intrinsic long-period pulsating variables and the first to be discovered. A long-period pulsating variable or "Mira-type" variable changes magnitude by more than 2.5 and has a clearly defined period of somewhere between 100 and 1,000 days. David Fabricius, a disciple of Tycho Brahe, discovered Mira in 1596. Mira resides 420 light years from Earth so at its peak it is some 1,500 times more luminous than the Sun in visible light reaching an apparent magnitude of about +3.0. Normally though Mira is invisible being near magnitude +10 and about as luminous as the Sun. Mira is a spectral type M5 red giant that is deep into the dying stages of its life. Both hydrogen fusion and helium fusion have long since ceased within the core. Carbon and oxygen are being fused into silicon and sulfur in the core. Mira has become hugely distended in its death throes and is extremely unstable. Observations with the Hubble telescope show that the star is so unstable that it is visibly no longer round anymore. Pulsations in the slowly collapsing core cause the brightness outbursts at regular intervals. This in turn creates powerful stellar winds that are driving Mira's outer layers out into space. These in turn will eventually create a beautiful planetary nebula very close to Earth over the next few million years.

Each year as Mira builds to its peak brightness, it gradually brightens to magnitude +3.0 before fading back although at times it has become brighter. Twice during the past century, Mira peaked brighter than magnitude +2.0. William Her-schel observed Mira to be "almost equal to Aldebaran" (+0.85) in November, 1779. There have also been apparitions where Mira failed to brighten beyond magnitude +5. Mira's period also varies slightly, by about a day or so from the accepted value of 332 days. The Hubble Space Telescope also discovered that Mira has a companion that orbits it once each 400 years. This discovery will now allow for precise mass determinations to be made of this most wondrous of stars.

In the polar north, in the constellation Cepheus the King is another very famous variable. Delta Cephei is at first glance a nondescript star in a nondescript constellation. This star is the prototype for perhaps the most important of intrinsic variable stars, the Cepheid variable. Delta Cephei is a yellow-white spectral type F5 supergiant. "Supergiant" is something of an understatement for this star, which has 40 times the diameter of the Sun and pours out the energy of 2,000 Suns. Delta Cephei is a high-mass star with about five solar masses. But pinning down Delta Cephei's spectral type is a difficult thing because unlike with the Mira-types, Cepheids change color and temperature as they oscillate. Delta Cephei varies between F and cooler G with its light variations. The star's surface temperature varies between 6,800 K and 5,500 K. Delta Cephei and stars of its type are helium-burners that pulsate at a high frequency. In the case of Delta Cephei, that period is always exactly 5 days 8 hours 47 minutes 32 seconds during which time the star varies between magnitude +3.5 to +4.3.

Cepheid-type variables have several traits in common. They are very massive, vary in spectral type as they oscillate, are powerfully luminous and are very regular. Delta Cephei and stars of its class are dying stars whose helium-burning cores cannot find the equilibrium between gravitational collapse and the countering force of energy released through fusion that most stars have. So gravity tries to collapse the core, creating more gravitational energy, which in turn causes more fusion causing the core to grow again, reducing gravitational energy and resultant fusion energy thus causing the core to shrink again. The star can never find balance. This pulsation of the core is as precise as the ticking of the finest watch. What makes Cepheids so special is that the rate of pulsation (which ranges from anywhere between one and fifty days) is directly related to their luminosity. Once we know how luminous the star is with great precision by measuring the period, we then can determine from apparent magnitude the exact distance to the star. Because Cepheids are so luminous, they can even be detected in distant galaxies. It was the discovery of a Cepheid variable within the Andromeda galaxy by Edwin Hubble that led to the first reliable measurement of the distance to Andromeda and later other galaxies. The finding of Cepheid variables in distant galaxies is one of the key projects of the mighty orbiting telescope that now bears Edwin Hubble's name.

Delta Cephei also has some naked-eye cousins including Mekbuda (Zeta Gemi-norum) and Eta Aquilae, both of which are actually slightly brighter than is Delta Cephei. The pulsations of Delta Cephei are easy to monitor because Delta Cephei is also a double star. The companion is a B8 located 41 arc seconds away and shines at sixth magnitude.

There are many different types of these pulsating variables that can be categorized based upon frequency of the peaks, amplitude of magnitude change and spectral type. Each type is named for the first variable of that type to be discovered. RR Lyrae types are very short period (less than a day usually), less than two magnitudes change in brightness and are always spectral type A. RV Tauri types have longer periods, between 30 and 150 days, vary by a maximum of three magnitudes and are K or G spectral type yellow supergiants. Semiregulars have periods of anywhere between 30 and 1,000 days and exhibit both regular periods and show periods of irregular behavior. Small amplitude pulsating red giants (SAPRG) are M type giants that vary between 5 and 100 days with an amplitude of one magnitude or less. These stars tend to vary because of the natural instability inherent in helium-burning giants. The effect is caused by variations in convection within the star.

Eruptive variables also come in several types. These variables are very irregular and in fact can often be one-time-only events. R Coronae Borealis types are very low-luminosity stars that are spectral type M or below. They maintain their normal brightness for long periods of time, then suddenly fade by several magnitudes and need months to recover. They do so on no regular schedule. The stars are hydrogen-poor, carbon-rich red dwarfs. Since they are red dwarfs and so poor in hydrogen,these stars must be immensely old. UV Ceti types are also known as flare stars. These stars will erupt on an irregular and unpredictable basis from localized spots on the surface. The star can brighten by more than two magnitudes in just a few seconds and then fade back over the next twenty minutes. Two local examples of UV Ceti types are Wolf 359 and Proxima Centauri. Novae are stars that show a massive eruptive increase in brightness and always occur in binary systems. Usually one star is a Sun-type star and the other is a white dwarf orbiting in close proximity. The gravity of the white dwarf draws material away from the main sequence star and begins to accumulate in a disk around the white dwarf. Eventually the material builds to sufficient mass to initiate hydrogen fusion and the mass explodes creating a massive increase in brightness that may last several months before the star fades back to normal magnitude. Often times these are single events, but in recurrent novae they occur repeatedly. The most amazing type of variable is the supernova, the catastrophic destruction of a high-mass star when its iron core collapses. The star will exhibit a sudden, dramatic (and final) brightening of as much as twenty magnitudes. They become so bright that they can be viewed from clear across the universe and become more luminous than the entire galaxy in which they reside. When they occur locally, a supernovae can become as bright in the sky as a full moon and be visible in broad daylight for weeks. There has not been a bright supernova found in the Milky Way for several hundred years, still the Milky Way is so dusty that no matter how bright a supernova might become, galactic dust clouds could block our view of it. The search of brighter galaxies for supernovae is carried out by amateur astronomers as vigorously as is the search for comets or asteroids. Some dedicated observers have found supernovae by the dozens and one has even found a hundred of them. They can occur at any time and in any galaxy. Many will peak at values bright enough to become easily visible in telescopes as small as 6 inches, while the host galaxy may in fact not even be visible. Finding supernovae may in fact be the simplest way to get your name in the astronomy magazines.

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