What are Variable Stars and How to Observe Them

Most stars have a fixed and constant brightness but there are some stars that undergo dramatic changes. These are called Variable Stars.

Variable stars are stars that vary in magnitude over some time. Some stars vary by a hundredth magnitude, others by a factor of 15 or more. The time taken for each period can be seconds or years, with some variables having a periodic frequency while others are irregular.

The first variable star to be identified as such was seen in August 1596 by David Fabricius, a Dutch pastor and amateur observer. It was of the 3rd magnitude and seemed unremarkable. It lay in the constellation of Cetus, the Whale, south of the celestial equator. By October, it had disappeared, and Fabricius thought little more of it. Johann Bayer, the German amateur who drew up a famous star catalogue, saw the strange object again in 1603 and gave it the Greek letter Omicron (o). It was next recorded in 1638 by another Dutchman, Phocylides Holwarda, who followed its changes and found that it appeared and vanished regularly. At a minimum, it falls to the 10th magnitude and is visible with the naked eye for only a few weeks yearly. It was named Mira 'the wonderful' and has given its name to a class of variable stars.

Types of Variable Stars

There are two main types of variable stars: the intrinsic variable and the extrinsic variable.

• Intrinsic variables change in luminosity, i.e. the star expands and contracts periodically.
• Extrinsic variables do not change luminosity, but they do so from our viewpoint, usually because they are eclipsed.

Variable stars were first discovered in the mid-18th century when it was noticed that some stars periodically disappear. By the late 19th century, 12 variables were discovered, and by the dawn of the 20th century, with the advent of photography, the discovery rate increased dramatically. In the latest NGC catalogue (2003), over 40,000 variable stars are listed in our galaxy and 20,000 outside.

Variable stars are analysed using several methods, including spectroscopy, photometry and spectrophotometry. A light curve is established by comparing a variable's brightness with a known fixed star. The light curve can tell us if the variable is periodic or irregular, the period of fluctuation and the shape of the light curve.

By analysing the variable's spectrum, we can derive the star's temperature and luminosity class if it is a single or binary star, and if the spectral lines are shifted, it can indicate the star is expanding or contracting.

Intrinsic Variable Stars

About two-thirds of variable stars are Intrinsic variables, which vary in brightness due to physical changes in the star. In most cases, the layers in the star will expand and contract, increasing or decreasing the surface area. Since luminosity and brightness are linked, the change in the surface area affects the luminosity, hence its brightness.

As a star's outer layer expands, it cools down, decreasing the degree of ionisation. This makes the solar material more transparent, thus making it easier for the star to radiate its energy, which causes the star to contract. As the layers contract, the effect of ionisation increases, causing energy to be once again trapped, leading to another expansion cycle.

The most famous intrinsic variable stars are the Cepheid variable stars, which have a very predictable correlation between their period of variability and absolute luminosity. Because of this correlation, a Cepheid variable can be used as a standard candle to calculate the distance to its host cluster or galaxy.

Extrinsic variable stars

Extrinsic variables are caused by external properties acting on our apparent view of the star.

These could form an eclipsing binary, where two stars orbit each other and periodically eclipse each other as seen from Earth. They could be rotating variables, i.e. a large sunspot will affect the luminosity as the star rotates. Other objects could also be passing between the star and Earth, causing the magnitude to vary, i.e., a large orbiting planet or cosmic dust.

Cepheid Variable Stars

Cepheid variable stars are intrinsic variables which pulsate predictably. In addition, the period of a cepheid star (how often it pulsates) is directly related to its luminosity or brightness.

Cepheid variables are extremely luminous, and distant ones can be observed and measured. Once the period of a distant Cepheid has been measured, its luminosity can be determined from the known behaviour of Cepheid variables. Then, its absolute magnitude and apparent magnitude can be related by the distance modulus equation, and its distance can be determined. Cepheid variables can measure distances from about 1kpc to 50 Mpc.

How to View Variable Stars

You don't need expensive gear to observe variable stars. The brighter ones are naked-eye objects for their entire light cycle. But a pair of binoculars or a telescope will extend your range to thousands of stars. You'll need a pen and paper, a red torch, a watch and a field chart showing variable stars and several comparison stars to observe. Many websites provide these charts, so download and print the one you want once you have the right chart and head outside.

Choose two comparison stars of known magnitude on the chart. These should be as close to the variable star as possible to avoid having to shift your glance repeatedly. One should be slightly brighter than the variable, and one should be slightly dimmer. Try to select stars of similar colour to aid comparison. Subdivide the brightness difference between your comparison stars and record where the variable star sits on this scale. A moderate scale of about five should allow you to estimate within a 10th magnitude. For example, if variable (V) lies two sub-divisions from A and three from B, we write A(2) and B(3). Finally, note the field chart you use, the date, time, and magnitude of the stars, and the conditions of the sky.

Examples of Variable Stars

Here is a personal list of my favourite Northern Hemisphere naked-eye variables. Many variable stars go unrepresented in this sample since many are low-luminosity objects too faint to be visible without a telescope or vary too slightly for their changes to be visually noticeable. Small binoculars will help in observing the fainter phases of some of these stars, especially if you don't have dark skies.

U Orionis

A beautiful red star that can climb to naked eye visibility and never really becomes faint. It comes to a maximum of one week later each year, so there are periods of several years during which the maxima occur when the star is too close to the Sun to see.

Type: Mira
Period: 372 days
Mag Range: +4.8 to +12.6
RA: 05h 56.8m
Dec: +20&dec; 10`

Gamma γ Cassiopeiae

This is a shell star with a variable spectrum whose last major maximum occurred in 1936. Suitable comparison stars are beta and alpha Cassiopeiae.

Type: Prototype
Period: Irregular
Mag Range: +1.6 to +3.3
RA: 00h 56.7m
Dec: +60° 43'

Alpha α Orionis

Better known as Betelgeux, this is a semi-regular variable star with a long period. Betelgeuse typically shows only small brightness changes near magnitude +0.5, although, at its extremes, it can become as bright as magnitude 0.0 or as faint as magnitude +1.3.

Type: Pulsating red supergiant
Period: 2,110 days
Mag Range: +0.4 to +0.8
RA: 05h 55.2m
Dec: +07° 24'

Beta Persei

Beta Persei, also known as Algol, is an eclipsing binary which dips from magnitude 2.1 to 3.4 every 2.87 days. Each eclipse, including the partial phases, takes nearly 10 hours.

Type: Eclipsing binary
Period: 2.87 days
Mag Range: +2.1 to +3.4
RA: 03h 08m
Dec: 40° 57'

Mu μ Cephei

Mu Cephei is said to be a semi-regular star. William Herschel called it the Garnet Star because of its redness, and in binoculars, it is stunning. Mu is one of the largest stars known, and Nu Cephei makes a good comparison star.

Type: Mu Cephei
Period: 730 days
Mag Range: +3.4 to +5.7
RA: 21h 43.5m
Dec: +58° 47'