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.

Most stars shine steadily year after year; century after century. But some do not - they brighten and fade over relatively short periods. These are variable stars. They are many different types; some are predictable and relatively mild, while others are violently explosive. Studies of them are of immense importance in modern astronomy, and amateurs can play an important role.

Variable stars are stars that vary in magnitude over a period of time. Some stars vary by a hundredth of a 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 minimum, it falls to the 10th magnitude and is visible with the naked eye for only a few weeks every year. It was named Mira 'the wonderful' and has given its name to a whole 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 actually change in luminosity, i.e. the star expands and contracts periodically.
• Extrinsic variables do not actually change luminosity, but they appear to 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 there were 12 variables discovered, and by the dawn of the 20th century with the advent of photography the rate of discovery increased dramatically. In the latest NGC catalogue (2003) there are over 40,000 variable stars listed in our galaxy and a further 20,000 outside.

Variable stars are analysed using a few methods, including spectroscopy, photometry and spectrophotometry. By comparing a variable's brightness with a known fixed star, a light curve is established. 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 variables 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 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, which causes the degree of ionisation to also decrease. This makes the solar material more transparent, thus 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 once again be 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 take the form of an eclipsing binary, where two stars orbit each other and periodically they 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. There could also be other objects 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, a Cepheid star's period (how often it pulsates) is directly related to its luminosity or brightness.

Cepheid variables are extremely luminous and very 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 be used to 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. To make an observation you'll need a pen and paper, a red torch, a watch and a field chart showing variable stars and several comparison stars. Lots of websites provide these charts, so download and print the one you're interested in. Once you have the right chart, 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 repeatedly shift your glance. One should be slightly brighter than the variable and one 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 5 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) B(3). Finally, make a note of the field chart you use, the date, time and magnitudes of the stars and the sky conditions.

Examples of Variable Stars

Here is a personal list of my favourite northern hemisphere naked-eye variables. Many types of 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 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'