Introduction

One of the gems of the night time sky is situated in the rather large and obscure constellation of Hydra. This gem is the variable star R Hydrae, a bright and easily found Mira type variable. One of the remarkable aspects of R Hydrae is that its period of brightness fluctuation has changed considerably over the years.

R Hydrae was first observed in 1662 by Johannes Hevelius but its true variable nature was not identified until 1704 by Italian astronomer Giacomo Maraldi. At that time the period was 495 days and the star varied between magnitude 4.5 at maximum and magnitude 9.5 at minimum with some minor variations in amplitude. After its identification as a variable star it became apparent that the period was declining. By 1800 the period had shortened to 480 days and by 1850 it had shortened further to 450 days. Since 1937 the period has been constant at around 389 days. The change in its cycle has also induced changes in the amplitude with the maximum reported to be as bright as 3rd magnitude and the minimum as faint as 11th magnitude.

The cause of this behaviour is believed to be connected with the sudden ignition of a layer of helium that builds up against the star’s core as a result of hydrogen shell burning over a period of 1,000 to 10,000 years. This sudden ignition or flash commences once the helium reaches critical mass in the shell surrounding the core. The helium shell flash temporarily turns off the hydrogen burning in the overlying shell for a short period (1 – 2 centuries). Hydrogen burning recommences in the overlying shell once the helium is converted into carbon and oxygen. Several helium shell flashes are expected to occur in the life of a red iant. The helium shell flash temporarily induces an increase in the luminosity and diameter of the star and consequently an increase in its period of oscillation. Once the hellium flash ends and the hydrogen shell burning recommences the star contracts to its diameter prior to the helium shell flash thereby producing the observed period reduction.

How To Find R Hydrae 

R  Hydrae's position (equinox 2000) is 13h 29.7m RA and -23° 17.1' Dec. It is located approximately 13° east of Beta Corvi and 12° south of Alpha Virginis (Spica). It can be easily found by star hopping along a line extended east from Epsilon Corvi through Beta Corvi to Psi Hydrae to Gamma Hydrae to R Hydrae. The adjacent finder charts are provided to assist you to locate R Hydrae. Click on the thumbnails to view the full resolution finder charts. 

The photograph below provides a similar field of view to the above right finder chart. The photograph was taken on 14 May 2005 (JD 2453504.89). North is to the top and east is to the left. Hold your mouse over the photograph to identify R, Gamma and Psi Hydrae. The magnitudes of some of the comparison stars are shown in orange (the decimal point has been omitted).

Variable Star Charts

The AAVSO A and B variable star charts for R Hydrae are shown opposite. Click on the thumbnail to view the full resolution variable star charts. These charts are reproduced by permission of the AAVSO.

As the variable star charts are frequently updated please regularly check the charts page of the AAVSO website to ensure you are using the most recent chart. 

Observations

AAQ members are encouraged to observe R Hydrae and to submit their observations to the AAVSO.

Instructions on how to submit your observations to the AAVSO can be found here. The preferred method is to submit your data online using WebObs.

Please also provide a copy of your observations to the AAQ Variable Star Section Director. The AAQ member’s observations will be collated and used to regularly update our light curve for this star (see below).

Light Curve

The light curve (a plot of the visual magnitude estimate versus time in Julian Date) of the AAQ observations of R Hydrae is provided below. Each dot represents a member’s observation. The light curve covers a 2200 day period (~ 6 years) from 21 January 1999 (JD 2451200) to the 29 January 2005 (JD 2453400). R Hydrae has gone through 5 cycles during that period with its brightness ranging from approximately magnitude 4.5 at maximum to approximately magnitude 9.5 at minimum. The lack of observations whilst the star is dimming from maximum to minimum is caused by the seasonal gap as the star field passes through conjunction with the Sun and becomes an early morning object. There is a suggestion of a pause in brightening from minimum to maximum as it passes through the magnitude 7.0 to 6.0 band. This behavioural feature is present in other Mira stars such as R Carinae.

Further Information

For further information on this fascinating star visit the excellent Variable Star Of The Season article on R Hydrae by the AAVSO.