Astronomy Picture of the Day

Explanation: Spiral galaxy M66 is largest galaxy in the a group known as the Leo Triplet. M66 is somewhat peculiar because of its asymmetric spiral arms. Usually dense waves of gas, dust, and newly formed stars - called spiral density waves - circle a galactic centre and create a symmetric galaxy. Gravity from nearby Leo Triplet neighbour M65, however, has probably distorted this galaxy. In M66, intricate long dust lanes are seen intertwined with the bright stars that light up the spiral arms. Recent research indicates that M66 is unusual in that older stars are thought to heat up the dust in the galaxy's central bulge - a job attributed to young and hot stars in many other galaxies. M66 is famous for a powerful "Type Ia" supernova that was observed in 1989. Stellar explosions like this are thought nearly identical and so by noting how bright they appear, astronomers can estimate their true distance - and therefore calibrate the scale of the universe!