April 21
The Heart of Charles
Sun Position
The Sun is in Taurus near +20° declination. Northern hemisphere spring is at its warmest, with late sunsets; southern hemisphere observers have clean, dark autumn skies by 8 PM.
Sky Highlight
The Lyrids are now two days from peak. Lyra is well clear of the northeast horizon by 11 PM from northern mid-latitudes, and the shower is producing 10 to 15 meteors per hour on the nights approaching the maximum. The radiant is near Vega; meteors radiate outward from that point but can appear anywhere in the sky.
Deep Sky Object
M51, the Whirlpool Galaxy (NGC 5194), interacting spiral in Canes Venatici, about 23 million light-years. M51 is the textbook interacting galaxy pair, with a clear tidal bridge connecting the main spiral to its smaller companion NGC 5195; the interaction has triggered widespread star formation in the primary's arms. Best from northern mid-latitudes.
Featured Star
Cor Caroli (α² CVn) lies 110 light-years away, a chemically peculiar A0p main-sequence star, its spectrum shows unusually strong lines of silicon, europium, and mercury, the result of a powerful magnetic field that causes heavier elements to settle and concentrate in patches on its surface. It is the prototype of the α² Canum Venaticorum class of magnetic, spectroscopically variable stars.
Around This Date
- April 21, 1972Apollo 16 astronauts John Young and Charles Duke conducted their second moonwalk at the Descartes Highlands, collecting highland geological samples and deploying the Far Ultraviolet Camera/Spectrograph, the first astronomical observatory operated on the Moon.
- April 23, 1858Max Planck was born in Kiel, Germany; his 1900 quantum hypothesis explaining blackbody radiation became the theoretical foundation for understanding how stars radiate energy and enabled the modern science of stellar spectral classification.
Cor Caroli's magnetic field has organized heavy elements into patches on its surface, a star with a complicated interior life, wearing its chemistry on the outside.