March 25
The Chemically Odd One
Sun Position
The Sun is in Aries at roughly +3° declination; northern spring is firmly underway, and the Southern Hemisphere is settling into autumn with evenings shortening by about two minutes per day.
Sky Highlight
The Big Dipper is nearly at its highest point in the March evening sky from northern mid-latitudes, and Alioth (the brightest of its seven stars) is easily the most prominent Dipper star visible from both hemispheres, sitting at the handle's base where it connects to the bowl.
Deep Sky Object
M51 (Whirlpool Galaxy), a grand-design spiral galaxy about 23 million light-years away in Canes Venatici, interacting gravitationally with the smaller companion galaxy NGC 5195; even small telescopes show the core and a hint of structure. Best placed from the Northern Hemisphere; accessible from northern portions of the Southern Hemisphere.
Featured Star
Alioth (ε UMa) is the brightest star in the Big Dipper at 81 light-years away, spectral class A0pCr, the 'p' denoting a chemically peculiar star with unusually strong chromium and other heavy-element lines in its spectrum. It is an Alpha-2 Canum Venaticorum variable: its magnetic field is tilted relative to its rotation axis, and as the star rotates, different chemical patches rotate in and out of view, causing subtle brightness and spectral changes.
Around This Date
- March 25, 1655Christiaan Huygens discovered Titan, Saturn's largest moon, using a telescope he had designed and built himself, the first moon of Saturn to be found.
- March 25, 1807Heinrich Olbers discovered the asteroid Vesta, the second-most-massive body in the asteroid belt and one of the largest objects visited by a spacecraft (Dawn, 2011–2012).
The brightest Dipper star is also the strangest: chromium patches rotating in and out of view as Alioth turns, a subtlety visible only in the spectrum.