Twinkle, twinkle, magnified star

Two ‘twinkling’ stars in galaxies billions of light years away have been spotted thanks to the gravity-based magnifying power of the galactic clusters between the stars and us, report two papers published online this week in Nature Astronomy. One of the stars - which is 14 billion light years away - was magnified over 2,000 times, an unprecedented observation that simultaneously reveals information on the dark matter hidden in the magnifying cluster in question.

Stars can change brightness abruptly when undergoing explosive events such as a nova or supernova explosion. Although the brightest supernovas are visible up to 10 billion light years away, most stars beyond our galactic neighbourhood are too faint to detect individually.

Using the Hubble Space Telescope, the two teams of authors repeatedly observed parts of the sky that contain massive clusters of galaxies. The immense gravitational field generated by these clusters acts as a lens, magnifying the light coming from the stars behind them - enough that otherwise-too-distant stars can be observed from Earth.

The ‘twinkling’ observed by Steven Rodney and colleagues is due to repeating explosions on the star’s surface. On the other hand, the star found by Patrick Kelly and colleagues is twinkling because of the relative motion between the lensed star and the lensing cluster.

Studying these twinkles, the authors can reveal not only the physical properties of the stars themselves, but more importantly the distribution of the dark matter - the material that cannot be directly observed but whose additional mass is thought to explain the observed internal motions of galaxies - in the lensing galaxy clusters.