September 11
The Variable Corner
Sun Position
The Sun is in Virgo near +1° declination, just a step north of the celestial equator. Day and night are very nearly equal everywhere; the equinox arrives in about ten days.
Sky Highlight
The September epsilon Perseids minor shower is active this week, with occasional bright meteors from a radiant in Perseus. The shower is best observed in the Northern Hemisphere after midnight, with a typical hourly rate of 5 to 10.
Deep Sky Object
M15 (NGC 7078), globular Cluster, Pegasus. About 33,600 light-years away, M15 is notable for having a collapsed core, its central density is extreme even among globular clusters, and it contains the only confirmed globular-cluster planetary nebula (Pease 1). Northern Hemisphere observers find it well-placed; visible but low for southern observers.
Featured Star
Scheat (β Peg) is a slow semi-regular pulsating variable (spectral type M2.5II-III) about 196 light-years away, one of the cooler and more dramatic red giants within naked-eye range. Its brightness wanders over roughly half a magnitude on a ~38-day cycle, driven by oscillations in its extended, tenuous outer envelope.
Around This Date
- September 11, 2008The Large Hadron Collider at CERN circulated its first proton beam, beginning the experimental program that would, four years later, confirm the existence of the Higgs boson.
- September 14, 2015LIGO made its first confirmed detection of gravitational waves (from the merger of two black holes) an event designated GW150914, though the announcement was not made public until February 2016.
A star that breathes is not a metaphor. It is an outer envelope contracting and expanding under its own gravity, slowly, over weeks.