You may think of stars as being fixed points of light in the night sky. But many of them change their brightness over time. They are known as variable stars. Some change brightness in a regular way, and others do so erratically.
Many thousands of variable stars are known, and it is fascinating to follow their changes. Careful monitoring can add to our general understanding of the universe. Sheer numbers of stars make this impossible for professional scientists to do, without relying on the automatic sky surveys that are coming online.
But fortunately the work is well-suited to amateur astronomers, and variable star observing has become a popular branch of the hobby. This article will help you learn how to observe variable stars for yourself.
No special equipment is needed to observe the brightest, which are visible to the unaided eye, but binoculars or a telescope will bring many more into view.
You might imagine that you would need an instrument to measure the brightness of a star. In fact most observers use their eyes alone, estimating a star’s brightness simply by comparing it with other stars whose magnitudes are constant and already known.
Types of variable star
Variable stars are divided into three main classes: pulsating, eruptive and eclipsing – with many subdivisions.
Pulsating variable stars
Cepheid variables represent a temporary evolutionary stage and are yellow supergiants undergoing pulsations lasting between 1 – 7O days, with brightness variations of up to two magnitudes. They take their name from Delta Cephei (3.4 – 4.3 magnitude, period 5.37 days). Other bright examples include Eta Aquilae (3.5 – 4.3, period 7.18d) and Zeta Geminorum (3.7 – 4.2, period 10.15d).
Perhaps the most interesting naked-eye variable stars are red giants and supergiants nearing the final stage of their development:
Irregular variables are stars with little or no trace of a period, varying slowly from a few tenths of a magnitude upwards. Beta Pegasi (2.3 – 2.8) is of this type.
Semiregular variables show clear periods of between 30 – 1,OOO days or more, with amplitudes of one or two magnitudes accompanied or disturbed by irregular activity. Two of the four sub-types are on the vss programme : SRb – giants with poorly expressed periods such as Eta Geminorum (3.3 – 3.9) and Rho Persei (3.3 – 4.0) ; SRC – semiregular super- giants, e.g. Mu Cephei (3.6 – 5.1), Alpha Herculis (3.0 – 4.0) and Alpha Orionis, or Betelgeuse (0.4 _ 1.3) seem to have simultaneous periods superimposed on each other.
Long Period variables, or Mira stars, are red supergiants with more violent periods between 80 – 1,OOO days and ranges of more than 2. 5 magnitudes. Mira itself (Omicron Ceti) can become particularly bright and easy to spot (1.7 – 9.3).
Eruptive variable stars
Nova-like variables often bear only slight resemblance to true novae. Gamma Cassiopeiae (1.6 – 3.0) is unstable because of its rapid rotation – in 1936 it shed a shell and brightened by a magnitude. It is fair to say that not much variation has been seen since, but the star is worth keeping an eye on!
Novae are hot dwarf stars which suddenly brighten by 7 – 16 magnitudes and then fade irregularly over several years.
Eclipsing binaries
These are binary systems in which one star passes in front of the other at regular intervals causing a drop in brightness.
Algol variables are represented by Algol itself (Beta Persei 2.1 – 3.4, period 2.87d and Lambda Tauri (3.5 – 4.0, period 3.95d). Eclipses last lO hours and 14 hours respectively. They are subject to occasional small period changes due to mass being exchanged between the stars. Eta Geminorum, the SRb star mentioned earlier as a semiregular variable is also an eclipsing variable with a period of over 8 years.
Beta Lyrae variables. With Algol-stars the light-curve outside eclipse is almost flat. The light-curves of stars like the prototype Beta Lyrae (3.3 – 4.2, period l2.93d) are constantly changing because the components are egg-shaped and almost in contact.
How to observe variable stars
It is important to keep records of your observations in a notebook, including the date and time, estimated magnitude, comparison stars used, and sky conditions. If you need to use a torch, make sure it has a red filter to help preserve your eyes’ adaption to the darkness. White light will remove your “dark adaption” for several minutes.
If you’re starting out by following the brighter variable stars, including some visible with binoculars, you will find useful guides to many of them, including finder charts, on the website of the Society for Popular Astronomy’s Variable Star Section.
Beginners may like to try the following simple method of estimating a variable’s magnitude. Having located the variable star, the observer selects two comparisons: (A) which is slightly brighter than the variable, and (B) which is a little fainter. The magnitude of the variable is then estimated by observing how much fainter it is than A and how much brighter than B.
E.g. An observer is about to estimate the brightness of the irregular variable star Mu Cephei. He chooses Nu Cephei (mag 4.29) as the brighter comparison and Beta Lac (mag 4.44) as the fainter. The magnitude of Mu is estimated to be only slightly above that of Beta Lac but fainter by a rather larger margin than Nu Cep. The mag is then recorded as 4.40. If the variable appears equal in brightness to a comparison, then obviously its magnitude is the same as that of the comparison.
Thus “A+2” means the variable is estimated to be two steps (i.e. O.2m) brighter than comparison star A, while “B-3” means three steps fainter than B, and so on. Work out the variable star’s magnitude from each of your step estimates and take the average. Do not expect your step estimates always to agree closely. Disagreements of a few tenths are quite normal.
The main advantage of the Step Method over other methods of brightness estimation is that only one comparison star need be used. This is an advantage when cloud obscures much of the sky or when the variable is low down and only one comparison may be conveniently used.
When the variable is low in the sky, it will appear fainter owing to the greater depth of atmosphere through which its light must travel to reach the eye. In this case, care should be taken to select comparisons at nearly the same altitude so that the effect – known as extinction – is compensated for. If this is not possible, correction factors will have to be applied. However, these are extremely unreliable and entirely useless when mist or haze is present.
For a short while each year, most variables will be practically unobservable owing to their proximity to the Sun in the sky, but it is important that this period is kept as short as possible if you want to monitor a star’s changes closely. This will involve making estimates shortly after sunset or before dawn under difficult circumstances, (bright skies and the star’s low altitude), but doubtful estimates are usually better than none.
If you have a digital camera, capable of taking time exposures, then you may like to try making photographic observations of variable stars. You will need to use a tripod. Pick a high “film speed”, or ISO, and keep your exposures to just a few seconds so that the stars resemble points of light and don’t trail as the Earth turns. A cable release helps avoid camera shake, or you set the camera’s timer delay – usually between 2 and 10 seconds.
