Erwin van Ballegoij (BVE)
(Adapted from AAVSO's Eyepiece Views, June, 2005)
Nowadays many observers focus their observing efforts on cataclysmic variables and a few other oddballs. They use every clear night to observe as many CV's as possible, hoping to catch one in outburst. It can be very rewarding to be the first to observe an outburst, especially of the little studied or newly discovered systems. It can also be very rewarding to observe Miras. There are many underobserved Miras, waiting for someone to study them. Often a big part of the whole light curve can be observed. And even some very well observed stars may deliver a surprise or two. In this short article you will find some theory concerning Miras and some interesting examples are discussed.
Mira variables pulsate with a period between 80 and 1000 days. In visual light, the amplitude of the light change is in general between 2.5 and 10 magnitudes. In infrared the amplitude is significantly smaller. Miras are of spectral type M, S or C, dependent on the ratio between the amount of carbon and oxygen in their photosphere. M-type Mira stars have less carbon than oxygen, S-types contain roughly an equal amount of both elements, while C-types are over abundant in carbon. M, S and C type stars have a noticeable orange color, especially around maximum. This color points to a relatively
low surface temperature of 3000K. Special care has to be taken while estimating Miras near maximum. The orange color makes it difficult to make a reliable estimate of its brightness. This explains the large scatter in brightness estimates of Miras near maximum. US observers mostly use the "quick glance method", while European observers generally use the "out-of-focus" method. It is probably best to observe maxima of Miras with an as small as possible instrument, or to use a diaphragm to reduce the aperture of the telescope. The mass of Mira variables is comparable to the Sun, but with a diameter 200 to 300 times as big. This large diameter causes the big luminosity. Miras radiate 3000 to 4000 times as much light as the Sun.
As a Mira star expands, the diameter increases greatly. Therefore, the escape velocity in the outer parts of the photosphere will be very small. A part of the hot plasma (ionized gas) at the edge of the star will move fast enough (10 to 20 kilometers per second) to escape to interstellar space. This outflow of plasma is connected to shockwaves moving through the star and convection in the outer layers of the star. The out flowing gasses will condense at a distance of 2 to 6 AU from the star. Here they react with the dust present. Infrared satellites have proven the existence of this dust. The dust particles measure about one micron and are mainly composed of silicon dioxide. This makes them grains of sand, "polluted" with iron, magnesium and aluminum compounds. Radio telescopes have shown the presence of hydroxyl (OH) around Miras. This way Miras loose about one millionth of a solar mass a year. When you realize that Miras have a mass of about one solar mass, then it becomes obvious that the Mira stage is only a short period in the evolution of these stars. Mira variables play an important role enriching interstellar space with heavy elements. Because of their continuous mass loss the are predecessors of planetary nebula and as such also of white dwarf stars.
Evolution of Miras
Stars spend most of their lives on the main sequence of the HR diagram. In this phase they convert hydrogen to helium. When the nuclear fuel runs out, the star expands and gets brighter. The star changes into a giant. At a certain stage the radiation pressure of the star is not sufficient to overcome gravity any more. The star compresses and the brightness decreases. The density and the temperature in the nucleus increase. At this higher temperature helium can be converted into carbon. The star reaches the horizontal branch above the main branch of the HR diagram. When the helium in the nucleus is depleted, the star turns into a giant again. The nucleus contains carbon and hydrogen, surrounded by two nuclear fusion zones, one in which helium is converted to carbon, surrounded by a layer where hydrogen is fused to helium. The star reaches a second horizontal branch in the HR diagram. This Asymptotic Giant Branch (AGB) lies above and parallel to the first giant branch. The star becomes bigger and more luminous than the first time. In the AGB you can find the Mira stars and the SRa and SRb stars as well.
Some Miras do not have a constant period. Some show an increasing period and others a decreasing period, sometimes after a prolonged time with a constant period. Astronomers suspect that the change in period is connected to a thermal pulse. During a thermal pulse there is a short period with an enhanced fusion in the helium layer. This releases extra energy. When the star processes this extra energy, the carbon/oxygen nucleus and the helium and the hydrogen layers mix a little bit, enriching the convection layer of the star with carbon and oxygen. This change in interior structure influences the pulsation period of the star. It could even be that a Mira star stops its pulsations, to become a Mira variable again later on.
Of the about 6000 Mira variables in the General Catalogue of Variable Stars (GCVS), less than 1000 are regularly observed. Of these, only a few handfuls show a clear period change. The most illustrative examples are discussed below.
T Ursae Minoris
Mrs. L. Ceraski discovered T UMi on February 13, 1902. Until 1968 the period stayed almost constant at about 315 days. Starting in 1968, the period of this star started to decrease. Nowadays the period is roughly 240 days and the decrease hasn't stopped yet. T UMi can reach magnitude + 7.8, and can get as faint as +15.2. The average magnitude range lies between +9.2 and +14.0. The maxima are easy to observe in a small scope, but for a faint minimum you need at least a 30 cm (12") telescope. For a big part of the northern hemisphere this object is circumpolar. T UMi is easy to find starting from beta UMi.
Maraldi discovered the variability of R Hya in 1704. Until 1770, the period was nearly constant at 495 days. From 1770 to 1950 the period decreased to 395 days and has remained constant since that date. R Hya can be as bright as + 3.7 and as faint as + 10.3. On average the brightness lies between +4.5 and +9.5. Although this star has a southern declination, it is also very easy to observe from a big part of the northern hemisphere. At maximum it is a naked eye or a binocular object, at minimum only a small telescope is needed R Hya is very easy to find starting from gamma Hya.
This star near delta Aql was discovered by astronomers in Bonn, Germany in 1856. Since its discovery the period decreased to about 270 days nowadays. The star varies between magnitude +6.1 and +11.5. Although this star has a northern declination, it can be observed from all major landmasses from the southern hemisphere. At maximum binoculars are sufficient, but at minimum at least a 11 cm (4.5") telescope is needed.
This star can reach a maximum of +9.0, but the minimum could be as low as +14.8. On average the brightness lies between +9.8 and +13.9. Around 1925 the period was about 500 days, but it has decreased to 450 days at the moment. Z Tau can be observed from both hemispheres, as it lies near the border between Taurus and Orion.
The period of R Cen remained virtually constant at around 550 days until 1950. Nowadays it has decreased to 510 days. R Cen is a so-called "double-peaked" Mira, whose light curve shows two maxima per cycle. Besides the period, also the amplitude of this star has decreased. The amplitude is nowadays one third of what it was in the beginning of the twentieth century. R Cen can only be observed from the equatorial region and the southern hemisphere. It is located near beta Cen. This star seems an easy target, but there are many faint stars out there to confuse you. Take care that you identify R Cen properly. This star is observable in small telescopes.
There are also Mira's that increase their period. In 1968 the period of LX Cyg was around 480 days, but it has increased to 580 days since then. LX Cyg is only visible from the northern hemisphere, and is located near the open cluster NGC7209. The star is located near the Cygnus / Lacerta border, in a region of the sky devoid of bright stars. This makes LX Cyg a difficult target for beginners. At maximum a small telescope is sufficient, but to follow this star as it goes to its minimum you need a 25 cm (10") telescope.
Also BH Cru is a variable that has increased in period. In 1979 it had a period of 490 days, nowadays the period has increased to 530 days. BH Cru can only be observed from equatorial regions and the southern hemisphere. The star is easy to locate near gamma and delta Cru. At maximum a binocular is sufficient, around minimum a small telescope will do. Unfortunately, there is no AAVSO chart available for this star. However, there is a good chart available from Sebastian Otero's website:
The last star in this discussion is W Dra. This star also increased in period, from 255 days in the beginning of the twentieth century to 280 days now. W Dra has a declination of +66 degrees, and is therefore only observable form the northern hemisphere. Unfortunately, this star lies in a region with no bright stars and is for beginners difficult to locate. At maximum a small scope will do, but at minimum you will need a 25 cm (10") telescope at least.
Above a number of stars are discussed that show a period change. They need continuous monitoring, to see how their periods evolve. But do not focus your observing efforts to the mentioned stars alone. There might be many other Miras with this kind of behavior, but they are simply not studied long or frequent enough to draw any conclusions yet. To draw a conclusion, we need a long time base of decades to centuries. Furthermore, realize that some stars that have a 'constant' period nowadays may show period change in the future. T UMi is a clear example. It had a constant period, but this started to decrease in 1968. This period decrease has not stopped yet. Just observe and let the stars surprise you. Clear Skies.
1. Secular Evolution in Mira Variable Pulsations, Templeton M.R., Mattei J.A. & Willson, L.A. AJ, in press
2. Post-AGB Stars, Van Winckel H., ARAA 41 (2003), p. 391-427