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Siddhesh Durgude recently graduated with a bachelor’s degree in physics from Fergusson College (Autonomous), Pune, India. Siddhesh worked under the guidance of Mr. Aniruddha Deshpande, Vice-President of Jyotirvidya Parisanstha (JVP), India’s oldest amateur astronomers association. He presented this work through a poster, which received a second prize at the 17th Frontiers in Physics (FIP), a national-level physics seminar at Fergusson College (Autonomous).
Variable stars are stars whose brightness changes with time. Studying these entities gives us insights into the stellar processes in the star’s core, which further reveal crucial insights into the universe’s workings. The cause of the variation in the star’s brightness may be due to various intrinsic or extrinsic factors. If factors like the periodic pulsations of the star’s layers or violent processes like flares in their atmospheres are at play, then the star’s total energy output (luminosity) changes, and so does its brightness. Variables which affect the total energy output are called intrinsic variables. Extrinsic factors like eclipsing binary star systems and rotating stars with star spots contribute to the change in observed brightness and not their total energy output. Such variables are extrinsic variables.
But still, the question arises: why observe and study variable stars?
Let me answer this by giving a few examples: a class of variable stars known as Cepheid variables bear a period-luminosity relation that has been instrumental in gauging distances to distant galaxies and determining the universe’s age. Supernovae fall under the class of eruptive variables, and the discovery of the universe’s expansion stems from their observations. The observed dip in the brightness of a star when a planet passes in front of it is a smoking gun for an exoplanet discovery. Due to these reasons, it is imperative to document variable behaviour across all observables.
My colleagues and I at Jyotirvidya Parisanstha (JVP) association, under the guidance of the vice-president Mr Aniruddha Deshpande, chose RR Geminorum, an RR Lyrae class of pulsating variables, for our study because of its availability for observations over the whole night during our observation period. Another reason behind choosing this particular class of stars is the peculiar behaviour of variations it shows. We took overnight observations for around three months, from January to March 2024, using a 12-inch Newtonian reflector telescope mounted on an equatorial mount with a charged couple device (CCD) as the imaging sensor. The technique used for the study is photometry, which means measurement of the light intensity of an object.
How do we arrive at the results and what are its interpretations?
For the analysis, we first calibrated the photometric data using calibration frames which help reduce the noise and irregularities. After calibration, we plotted light curves of the brightness over time.
The results show an asymmetry in the light curves, meaning the shape of the curve is not uniform on either side of the maximum point of brightness. This behaviour is typical of RR Lyrae variables, as opposed to the upside-down bell-shaped curve, which is symmetrically distributed about its peak, as demonstrated in the case of other variable types. But what’s not generally shown by these stars is an additional secondary variation in the period of maximum brightness on top of its primary brightness variations. The peaks in the light curve also vary for different night observations. All these variations in the star RR Geminorum arise from the Blazkho effect (discovered in 1907), which has yet to be entirely understood. The Blazhko effect in RR Lyrae stars consists of a slow cyclical modulation of the light curve’s period, amplitude, and shape. Typically, RR Lyrae stars pulsate with very regular periods, but those exhibiting the Blazhko effect show additional periodic variations in their brightness.
The next step in our study is to gather more data to plot light curves and observe the behaviour of the additional variations to help understand the nature of RR Geminorum and primarily to have an archive of data to uncover the mystery of the Blazhko effect in the future. It is necessary to obtain large volumes of data because the Blazhko effect can exhibit variations with additional irregularities. Hence, more data will help identify these complexities and provide a more complete picture.
Featured Image Credit: Siddhesh Durgude
Astrobite edited by: Emma Clarke
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