In simple terms, a variable star is a star whose luminosity changes or fluctuates as seen from Earth. The changes in luminosity can be caused by a wide range of possible causes, some of which involve the structure/composition of the star, while others may involve extrinsic factors such as a companion star that eclipses a normally relatively stable star periodically. In fact, there are so many possible reasons why some stars’ brightness may fluctuate that there are currently dozens of variable star classifications. However, listing them all here falls outside the scope of this article.
For this reason, this article will list a few general, but hopefully interesting facts about variable stars you may not have known, as well as some details about a few major variable star classifications or types. It is our hope that at least some of the information in this article will be new to you.
The first variable star was described thousands of years ago
While Johannes Holwarda was the first among modern astronomers to identify the star Mira (Omicron Ceti) as a variable star in 1683, the oldest known record of a variable star dates back about 3,200 years. This record of a now-famous variable star, the eclipsing binary star Algol in the constellation Perseus, is contained in an ancient calendar that lists auspicious and inauspicious days, or as we would view them today, lucky and unlucky days.
About two-thirds of variable stars pulsate
Studies over many years have shown that about 66% of all variable stars seem to be pulsating. It turns out that the mathematical equations developed by Arthur Eddington in the 1930’s that describe the interiors of stars also describe the instabilities that cause many stars to expand and contract. It also turns out that the most common reason why pulsating stars behave in this way involves the degree to which the outer convective layers of these stars are ionized.
Variable stars are classified as either intrinsic, or extrinsic
In intrinsic variable stars, the mechanisms that cause luminosity variations involve the physical properties of the stars. Intrinsic variables can be subdivided into three broad categories:
1) Pulsating variables that grow and shrink as a natural consequence of their natural evolutionary progression.
2) Eruptive variables that change in brightness due to eruptive events such as mass ejections or flares.
3) Cataclysmic or explosive variable stars that suffer a cataclysmic event such as destruction in a supernova explosion. In extrinsic variables, the variability involves external processes and mechanisms, and these stars can be subdivided into two broad categories:
– Eclipsing binary systems, in which one or both stars is eclipsed by the other star in the system. Seen from Earth, these stars dim significantly every time an eclipse occurs.
– Rotating variables, in which variability is caused by the star’s high rate of rotation. In these stars, extremely big sunspots can develop that swing in and out of view as the star rotates, which affects the star’s brightness when viewed from Earth.
NOTE: Below are some details of some of the main types of variable stars
Cepheids and Cepheid-like variables
Cepheids and Cepheid-like variables consist of a number of different types of pulsating stars that all inhabit the instability strip, a narrow region on the H-R diagram. All the members of this large category of stars expand and contract with an almost metronome-like regularity, and all display a distinct and fixed relationship not only between their luminosity and their absolute magnitudes, but also between their mean densities and periods of variability. In practice, the luminosity-period relationship serves as a means to calculate distances to galaxies both in the Local Group and beyond. Note however that Cepheids are so named only after the star Delta Cephei; the star Beta Cephei serves as the prototype for a completely different class of variable stars.
RR Lyrae variables
While RR Lyrae variable stars are somewhat similar in nature to Cepheids, RR Lyrae variables have much shorter periods, and are not as bright. In general, RR Lyrae variables are significantly older than Type I Cepheids, and are also less massive than Type II Cepheid variable stars. RR Lyrae variable stars are well represented in globular clusters, and they also have fixed period-luminosity relationships, which makes them useful as distance calculators. Typically, class-A RR Lyrae variable stars vary in brightness by between 20% and 500% over periods that vary from a few hours to about a day or so.
Delta Scuti variables
Once known as “dwarf Cepheids, Delta Scuti variable stars are also similar to Cepheids, but with very much shorter periods. Stars of this type often show several patterns of variability that are superimposed on one another, which means that these stars display extremely complex light curves. Typical Delta Scuti variable stars vary in luminosity by between 0.3% and 130%, over periods that range between 0.01 and 0.2 days, and have spectral classifications that fall between A0 and F5.
Mira variables
All Mira-type variable stars are red giants, and fall into the AGB (Asymptotic Giant Branch) part of the H-R diagram. Typical Mira variables can dim and brighten by as little as 6 times to as much as 30,000 times over periods of about 332 days on average. These large changes in brightness are due to the stars shifting between radiating in optical and (optically) invisible infrared frequencies as their temperatures change. In a few cases, Mira variables can show even greater variations than normal over periods of several decades, which is caused by thermal pulsing in the most evolved AGB stars.
RV Tauri variable
These are all yellow giant stars that can be describes as low-mass post-AGB stars that are in the most luminous phase of their evolutionary development. Typically, RV Tauri variable stars show very deep minima that are alternated by periods of shallower minima. This produces double-peaked light curves that have amplitudes of 3 to 4 magnitudes over periods of between 30 and 100 days. In practice, these stars go from being spectral types F or G at maximum brightness, to being spectral type K or M during periods of minimum brightness.
Luminous blue variables
Better known as S Doradus variables, this stellar class represents some of the most luminous stars known to exist, two examples being P Cygni, and Eta Carina, which is several million times brighter than the Sun. An interesting characteristic of S Doradus-class stars is that they lose their mass at prodigious rates, and that internal pulsations can cause them to exceed their Eddington Limit, which vastly increases the rate at which these stars blow off mass. However, while the visual brightness of these stars can and do vary greatly, their overall luminosity remains largely constant throughout their lives.
Our Sun is a variable star
Our Sun may appear to be stable but its energy output, and hence its luminosity, varies by about 0.1% over an 11-year period. While there is as yet no proven correlation between reduced sunspot activity and the climate on Earth, an extended period of reduced sunspot activity is thought to have caused the Little Ice Age in the 17th century, when Europe experienced unusually long and cold winters.
Nonetheless, our Sun is known to undergo cyclical variations in radiation, with known or recorded minima occurring in about 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC and 9170 BC.