Three Easy Variable Stars to Follow in Carina

By Andrew Wood

Variable stars are stars whose brightness changes over time, either because the star itself is changing or because something, such as a companion star, temporarily blocks part of its light. These changes may follow a regular cycle, vary only slightly, or involve dramatic brightening and fading over weeks, months, or years. By recording these changes and plotting them as light curves, astronomers can learn about a star’s size, temperature, structure, and evolution. Variable stars are especially rewarding for amateur astronomers because careful observations, even with modest equipment, can contribute useful data to professional studies of stellar behaviour.

To estimate the brightness (magnitude) of a variable star, compare it with nearby comparison stars whose magnitudes are already known. Choose one comparison star that looks slightly brighter than the variable and another that looks slightly fainter, then judge where the variable falls between them. For example, if the variable appears halfway between a magnitude 5.0 star and a magnitude 5.6 star, you will estimate it as about magnitude 5.3. It is best to use a star chart with labelled comparison magnitudes, avoid looking at the stars for too long, and record the time, date, instrument, sky conditions, and your estimate so that the observation can be useful later.

Variable-star charts can be generated from websites such as the American Association of Variable Star Observers (AAVSO). The AAVSO also allows amateur astronomers to submit their brightness estimates to its variable-star database.

Carina is a prominent constellation of the southern sky, best known as the Keel of the old ship constellation Argo Navis. It lies in a rich part of the Milky Way and is especially well placed for observers in Australia, where it can climb high in the sky during the warmer months. Carina contains Canopus, the second-brightest star in the night sky, as well as the remarkable Eta Carinae region and the vast Carina Nebula. With its bright stars, star clusters, glowing nebulae, and several interesting variable stars, Carina is one of the most rewarding constellations for binocular and small-telescope observing.

Within Carina are three variable stars that are easy to locate and follow:

• R Carinae is a Mira-type variable star and a cool red giant nearing the later stages of its life. Its brightness rises and falls from magnitude 3.9 to 10.5 over a cycle of about 308 days.

  
  

  • Mira-type variable stars are pulsating red giants in a late stage of stellar evolution. Their outer layers expand and contract over long periods, usually more than 100 days, causing large changes in brightness. Many Miras become bright enough to see with the unaided eye near maximum light but fade to binocular or telescope range near minimum. Because their cycles are slow, dramatic, and often fairly regular, they are excellent targets for amateur observers who want to follow a star’s changing light over many months.

• l Carinae is a bright classical Cepheid variable and a yellow supergiant. Its brightness changes from about magnitude 3.4 to 4.1 over a period of roughly 35.5 days, making it easy to see with the unaided eye from southern skies.

  
  

  • Cepheid variable stars are pulsating giant or supergiant stars whose brightness changes in a regular, repeating cycle. As they expand and contract, their size, temperature, and light output change. Cepheids are especially important because their pulsation period is closely related to their true brightness: longer-period Cepheids are intrinsically more luminous. This period–luminosity relationship allows astronomers to estimate distances to stars and galaxies.

• S Carinae is another Mira-type variable and an M-type red giant in the later stages of stellar evolution. Its brightness changes from about magnitude 4.5 to 10.0 over a period of roughly 149.5 days, so it can be seen with the unaided eye near maximum but needs binoculars or a telescope when faint. Its large, regular variation and relatively short Mira period make it a useful target for observers who want to follow a complete brightness cycle over several months.

Figure 1 indicates the position of these three variable stars. They are located south of the False Cross asterism, the four brightest stars at the top of the image. Looking south in mid-March, the False Cross lies above the Southern Cross.

Figure 1: The positions of the variable stars S, I and R Carinae as seen about 60 degrees above the southern horizon at 9PM mid-March. The four bright stars at top right make up the False Cross asterism. (The actual Southern Cross would be seen closer to the horizon.) The coloured area at lower left is the eta-Carinae Nebula. [Generated by Stellarium© software].

I have always been interested in getting into a Variable Star observing program and have made some intermittent observations in previous years. Earlier this year, on three occasions I have made estimates of the magnitudes of the three stars described above. Since R and S Carinae were both at the brighter part of their cycles, and I Carinae is always bright, all I needed was a pair of basic 10x42 binoculars.

The results are summarized below:

As can be seen from these results, R Carinae was fading, S Carinae was brightening, and the much shorter period l Carinae went down then up in magnitude over the 55-day period of these observations. When I checked the AAVSO database, my magnitude estimates were in close agreement with those other observers had submitted.

Figure 2 is an old finder chart from the Variable Star Section of the Royal Astronomical Society of New Zealand, which I used to make these measurements. I would advise now going to the AAVSO website to learn more about variable star observing and to generate finder charts. (Figure 2 shows another variable star, IW Carinae, for which I have not as yet made any observation).

Star charts generally indicate a variable star as a solid black dot surrounded by a circle. Fixed magnitude comparison stars are represented by a 2-digit number without the decimal point. For example, a star of magnitude 5.9 is represented as 59.

Figure 2: A comparison chart chart for R and l Carinae. S Carinae can also be seen in the finder chart inset. (Royal Astronomical Society of NZ).

Both AAVSO, and the website of our parent organisation, the BAA, have extensive information about variable stars. Observing them, you can see that the universe is not static but ever changing.