News

Kepler satellite discovers variability in the Seven Sisters



6 September 2017


The Seven Sisters, as they were known to the ancient Greeks, are now known to modern astronomers as the Pleiades star cluster - a set of stars which are visible to the naked eye and have been studied for thousands of years by cultures all over the world. Australian Aboriginal artistic depictions of the Seven Sisters are currently the subject of a major exhibition at the National Museum of Australia.


Now Sydney University PhD alumnus, Dr Tim White (currently of the Stellar Astrophysics Centre at Aarhus University) and his team of Danish, Australian and international astronomers have demonstrated a powerful new technique for observing stars within this cluster, ordinarily far too bright to look at with high-performance telescopes. Their work is published in the Monthly Notices of the Royal Astronomical Society.


To enhance observations from the Kepler Space Telescope in its K2 Mission , the team has developed a new and novel algorithm to perform the most detailed study yet of variable stars in the Pleiades star cluster.

The Kepler mission was designed to look at thousands of faint stars at a time, to search for planets orbiting distant stars, and, also, to do asteroseismology for studying the structure and evolution of stars. However, some of the brightest stars are actually too bright to be observed: a beam of light from a bright star at a point on a camera detector will cause the central pixels of the star's image to be saturated, causing a significant loss of precision in the measurement of the total brightness of the star.

This image from NASA's Kepler spacecraft shows members of the Pleiades star cluster taken during Campaign 4 of the K2 Mission. The cluster stretches across two of the 42 charge-coupled devices (CCDs) that make up Kepler's 95 megapixel camera. The brightest stars in the cluster - Alcyone, Atlas, Electra, Maia, Merope, Taygeta, and Pleione - are visible to the naked eye. Kepler was not designed to look at stars this bright; they cause the camera to saturate, leading to long spikes and other artefacts in the image. Despite this serious image degradation, the new technique has allowed astronomers to carefully measure changes in brightness of these stars as the Kepler telescope observed them for almost three months.Credit: NASA / Aarhus University / T. White.
This image from NASA's Kepler spacecraft shows members of the Pleiades star cluster taken during Campaign 4 of the K2 Mission. The cluster stretches across two of the 42 charge-coupled devices (CCDs) that make up Kepler's 95 megapixel camera. The brightest stars in the cluster - Alcyone, Atlas, Electra, Maia, Merope, Taygeta, and Pleione - are visible to the naked eye. Kepler was not designed to look at stars this bright; they cause the camera to saturate, leading to long spikes and other artefacts in the image. Despite this serious image degradation, the new technique has allowed astronomers to carefully measure changes in brightness of these stars as the Kepler telescope observed them for almost three months.Credit: NASA / Aarhus University / T. White.

The solution to observing bright stars with Kepler turned out to be rather simple said lead author Dr Tim White. We're chiefly concerned about relative, rather than absolute, changes in brightness. We can just measure these changes from nearby unsaturated pixels, and ignore the saturated areas altogether.


The authors developed a new method, named halo photometry, weight the contribution of each pixel to reveal the true stellar variability and remove the effect of satellite's motion and detector's imperfection. The simple and fast algorithm has been released as free open-source software.


With this method, the authors discovered that most of the stars in the cluster are slowly-pulsating B stars, a class of variable star in which the star's brightness changes with day-long periods. The study of the frequencies of these stars can shield light on some of the poorly understood processes in the core of these stars.


No signs of exoplanetary transits were detected in this study, but the authors show that their new algorithm can attain the precision that will be needed for Kepler and future space telescopes such as theTransiting Exoplanet Survey Satellite(TESS) to detect planets transiting stars as bright as our neighbouring star Alpha Centauri. Second authorBenjamin Pope (SIfA, University of Sydney) notes that these nearby bright stars are the best targets for future missions and facilities such as the Hubble Space Telescope's successor, theJames Webb Space Telescope, which is due to launch in late 2018. Using our radical new approach, we will be able to combine K2 and TESS observations and complete the first all-sky survey for these golden worlds for exoplanet exploration.


Prof. Tim Bedding Head of the School of Physics at University of Sydney was a coauthor on the study and said: I'm very pleased to be working with Sydney graduate Tim White on this project. It's fascinating to be able to measure brightness variations in the stars that appear in the logo on every Subaru vehicle!


The paper is available on arXiv