Mira's 400th anniversary.
Mira is located in a dim region about 10 [degrees] southwest of Cetus's head and 30 [degrees] southwest of the Pleiades and Hyades. It's plotted on the all-sky naked-eye map in the center of this issue; look about halfway from the southwest horizon to the zenith. The chart at right shows more detail and gives the magnitudes of comparison stars for judging the variable's changing brightness.
Mira's coming maximum is special for two reasons. This is our first chance in several years to see the star bright in the evening sky. And 1996 is the 400th anniversary of its official discovery.
A WONDERFUL ENIGMA
Mira's brightness and long history have earned it a unique place in variable-star astronomy. In fact it had the field all to itself for nearly a century. Mira was the first variable star known, aside from a few novae. Credit for its discovery is traditionally given to David Fabricius (1564-1617), a Dutch clergyman and amateur astronomer. On August 13, 1596, he noticed a "new" star in the neck of Cetus brighter than Alpha Arietis (which is magnitude 2.0). The intruder was missing from every star catalog, atlas, and globe that Fabricius checked. He saw it again at the beginning of September and watched it fade below naked-eye visibility around mid-October.
There is, however, a good chance that Fabricius's observations were not the first of Mira on record. Just four years earlier, Korean and Chinese astronomers had noted a "guest star" in Cetus on November 28, 1592, that remained visible for 15 months (presumably not all the time). This could have been Mira seen at separate maxima four and three cycles earlier than Fabricius's sighting.
A follower of Tycho Brahe, Fabricius assumed that his star was a nova like Tycho's of 1572 in Cassiopeia (which is today classed as a supernova). Maybe this is why Fabricius failed to check for Mira's return; a reappearing nova was unheard of. Not until 1609 did he notice that the star had rebrightened.
Fabricius later became a pioneer in the use of the telescope, invented just a decade after he discovered Mira. He used the new instrument especially for solar observations, as did his son Johann, an independent discoverer of sunspots. Fabricius might have achieved a greater place in astronomical history had he lived longer into the telescopic age, but in 1617 he was murdered by a member of the church in which he was minister. As the story goes, he had just announced from the pulpit that he knew who had stolen one of his geese!
Meanwhile, Johann Bayer had included Mira in his 1603 star atlas Uranometria as an ordinary 4th-magnitude star labeled Omicron. Another Dutch astronomer, John Phocylides Holwarda, found the star to be bright once again in the winter of 1638-39 and realized that its appearances were likely to repeat. Never again would it be ignored; every maximum since 1638 has been observed at least in part. Mira is too close to the Sun to be seen from April through mid-June each year, so the actual peaks are sometimes necessarily missed.
Ismael Boulliau (1605-94) announced in 1667 that Mira's variations are periodic, with the star brightening almost like clockwork every 333 days - a value very close to the currently accepted average of 331.96 days listed in the General Catalogue of Variable Stars. Johannes Hewelcke (better known by his Latinized name Hevelius) began observing Mira regularly in 1648, and in 1662 he published a pamphlet about the star entitled Historiola Mirae Stellae, "Brief History of the Wonderful Star." This is widely given as the source of the star's popular name, though apparently Fabricius was first to call it Mira, "The Wonderful."
Mira typically ranges from magnitude 9.3 to 3.4, but these are only averages. In some cycles Mira brightens to an eye-catching 2nd magnitude; in others it barely reaches 5th. Fabricius's maximum in 1596 seems to have been a bright one. The brightest on record is that of November 1779, when William Herschel found that Mira "excelled Alpha Arietis so far as almost to rival Aldebaran [magnitude 0.9]; and continued in that state a full month." Maxima observed since 1920 by members of the American Association of Variable Star Observers (AAVSO) have ranged from 2.4 to 4.9. Minima have been more consistent, ranging only from 8.4 to 9.7 according to AAVSO data. The period is variable too; maxima can arrive up to three weeks before or after they're expected.
These various irregularities are not completely random. For example, bright and faint maxima tend to alternate, but this is such a weak tendency that it has little predictive value. Thus the bright maximum of 1992, magnitude 2.4, was followed not by a faint maximum but by one of average brightness.
Attempts to explain Mira's behavior began as soon as its periodic nature was recognized. Boulliau thought the star was a rotating globe, almost uniformly dim except for one very bright spot. Pierre de Maupertuis (1698-1759) suggested that Mira was a flattened object resembling a millstone or Saturn's rings, seen in different orientations at different times. Edward Pigott (1753-1825) thought the star was eclipsed periodically by an opaque satellite. Rudolf Wolf (1816-93) proposed an analogy with the sunspot cycle. Most of these early ideas have turned out to be the correct explanations for other classes of variable stars, but not for Mira.
The correct explanation was finally provided by Sir Arthur Eddington, who demonstrated in 1926 that Mira-type stars are pulsating much the way Cepheid variables do, but with longer periods because of their more distended physical size and hence lower surface gravity.
Mira Ceti remained unique until 1686, when a second Mira-type variable was found: Chi ([Chi]) Cygni. Modern catalogs list about 6,000 Mira variables, most of them single stars. Mira, on the other hand, is a double. It has a 10th-magnitude companion, a hot dwarf called VZ Ceti a fraction of an arcsecond away. The companion was first seen by Robert G. Aitken in 1923 using the 36-inch Lick refractor, but it had already been detected in Mira's spectrum five years earlier by Alfred H. Joy at Mount Wilson.
At maximum light Mira's spectrum shows several very bright. hydrogen lines. These originate in the bloated atmosphere of the primary star, a red giant of spectral class M6 at that phase with a surface temperature of 2,500 [degrees] Kelvin. Its spectrum also shows strong dark bands of titanium oxide. Along with the bright lines, these can easily be seen with a hand-held spectroscope at the eyepiece of a small telescope when Mira is near maximum light.
When Mira fades, its color deepens to a stronger red as its surface cools to 1,900 [degrees] K. Its spectrum shifts to class M9. The bright emission lines fade out, but at minimum light a new set of bright lines appears. Joy correctly surmised that these originate in a hot, faint companion. The companion is itself thought to vary between magnitude 9.5 and 12. A period of 13 years has been suggested, and more rapid variations have been reported over hours or even minutes, including rare flares. This star may be a white dwarf surrounded by an accretion disk sweeping up material from the primary's stellar wind. Mira can thus be classed among the symbiotic stars, with the unusual property that its hot component is distant enough to be seen separately. The orbit is uncertain; a solution by Paul Baize in 1980 gives a period of 400 years. The current separation is just a few tenths of an arcsecond.
Can Mira be resolved in amateur instruments? A 16-inch telescope of top optical quality might be needed, not to mention excellent atmospheric seeing.
The best time to try would be when the M star is near minimum. A blue filter should help. Mira B should not be confused with two much wider, optical companions that are merely field stars.
Mira stars are red giants of roughly the Sun's mass that are in their unstable old age, soon to shed their outer layers entirely to expose the white dwarf growing in their cores. Recent estimates of Mira's distance range from about 100 to nearly 600 light-years. The giant's angular diameter has been measured by interferometry as 0.06[inches] (S&T: January 1992, page 29). Depending on what distance is assumed, this means Mira's diameter is between 200 and 1,200 times the Sun's, big enough to include the orbit of Earth and possibly of Jupiter. Speckle interferometry with the 4-meter telescope on Kitt Peak indicates that Mira actually has a rather oblong shape (S&T: February 1992, page 130).
The irregularities in Mira's variations are obvious in the AAVSO light curve above, which is based on more than 17,000 amateur observations over the past 20 years. Most Mira stars are not tracked so thoroughly! The irregularities can be accounted for if Mira is what physicists call a chaotic oscillator. Statistical conclusions such as this require very long runs of data such as those collected by the AAVSO. Amateur records of the light changes of these stars are also used to help professional astronomers schedule and interpret their own observations.
Try to follow Mira until the latest possible date this season as it gets low in the southwest after sunset; some of its decline needs to be recorded in order to estimate accurately the actual date of peak brightness. When you observe Mira, you will be watching it rise to its 440th maximum since Fabricius first spotted the star - if he was first!
JOHN E. ISLES 1016 Westfield Dr. Jackson, MI 49203
An observer of variables since 1963, Isles was director of the British Astronomical Association's Variable Star Section for 10 years and now serves on the council of the AAVSO.
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|Title Annotation:||variable star|
|Author:||Isles, John E.|
|Publication:||Sky & Telescope|
|Date:||Feb 1, 1996|
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