Alan talks about the Mira type variable star R Hydrae and tells how to observe it

A finder chart for the variable star R Hydrae, prepared by Alan Plummer from a Sydney Observatory star map.

This is the second in a series of posts on variable star observing, following the recent one on Eta Carinae.

Throughout this 2010 autumn and winter a star will be slowly brightening from the darkness only to become unobservable in the evening spring twilight just as it gets to naked eye brightness. The star is R Hydrae, and I’ve marked its location on the finder chart above made from the Sydney Observatory Sky Map. The coincidence of being in conjunction with the Sun at maximum light is temporary; because this star pulsates with a period of around 388 days, the blind spot drifts across the star’s cycle as the years go by.

R Hydrae is a Mira type variable, which is defined as being a pulsating red giant changing in brightness by more than 2.5 magnitudes over a period longer than about 90 days. R Hya is on the AAVSO ‘Legacy’ list of objects, meaning that they have a century or more of observations in the International Data Base. See the light curve pictured below. Such a span of observations can tell a story; but this one starts even further back than 100 years…

Observations of R Hydrae, courtesy of the AAVSO, used with permission

The first and best of the ‘modern’ European star atlas’ was Beyer’s Uranometria, compiled in 1603. This did not mark R Hya, but it was later plotted on Hevelius’ version in 1690. In 1669 a man called Montanari, who had Beyer’s 1603 atlas, noticed the star, and thinking the 1603 atlas in error, marked it on his own copy. It was a bit of bad luck he didn’t suspect a variable, as Montanari himself had already discovered one variable star – Algol (beta Persei). The nature of the star still remained undiscovered. To quote from Kerri Malatesta of the AAVSO:

Montanari’s marked atlas later came into the hands of Giacomo Maraldi…whose curiosity about the addition brought him to the field of R Hya in 1702. Using Montanari’s positional reference, Maraldi tried, without success, to identify the star. Intrigued by the mystery, Maraldi continued to monitor the area until 1712, noting maxima of the star in 1704 and 1708 and hence its variable nature.

Now to the story such an observational history reveals: a study has found that the pulsation period has declined from 495 to 380 days between around 1700 and 1950, and the amplitude has changed considerably. Recorded maxima have been observed as bright as 3rd mag and as faint as 6th. And the minima have been seen from 9th to 11th mag.

The most likely explanation for the changing nature of this pulsating star is that it had a so-called thermal pulse just before 1700. This is when a shell of burning hydrogen near the core accumulates enough helium in a shell below it to ignite that helium, in a flash, and in so doing extinguish the hydrogen shell. All this is wrapped around an inert carbon oxygen core. The idea is that changes within the star effect changes in the visible envelope, and how that behaves.

Chart and sequence for R Hydrae, excerpt from VSS RASNZ chart 12B, by Mati Morel

So if you want to join in the study of this star you’ll need 7×50 binoculars and/or a 4 inch telescope, and use the chart and sequence above in the same way that was described in the blog post on Eta Carinae. Observe this star once a fortnight and keep your observations on file; my next blog post will be about the agencies that collect and distribute the observations, and how you can use can those agencies. Or you can go to the AAVSO website now and do that yourself!

Acknowledgement: This post has used Malatesta’s work R Hydrae, May 2002 Variable Star Of The Month, which is recommended for further reading.

Alan Plummer, Sydney City SkyWatchers