The Pinwheel Galaxy is nearly three times as wide as the Milky Way
Seen face-on from 21 million light-years away, Messier 101 spans roughly 252,000 light-years and holds around a trillion stars. Its arms are lopsided because neighbouring galaxies tug on it, and it has produced six recorded stellar explosions since 1909, one of them bright enough to be the standout supernova of 2011.
Pierre Méchain spotted the galaxy in Ursa Major in 1781 and passed it to Charles Messier, who checked its position and slotted it in near the end of his catalogue. Méchain described a faint, fairly large nebula without stars, hard to make out once his instrument's wires were lit. In 1784 William Herschel saw it as mottled and resolvable, guessing that a better telescope would show the individual stars. In the second half of the 19th century Lord Rosse, using a 72-inch reflector, became the first to record its spiral form in detail, making several sketches.
For comparison, the Milky Way is about 87,400 light-years across. M101's disk contains on the order of 100 billion solar masses, with a small central bulge of around 3 billion. Its spiral arms are packed with H II regions, clouds of hydrogen ionised by hot young stars; a 1990 survey counted 1,264 of them, and three are prominent enough to carry their own NGC numbers. Gravitational interactions with six companions, among them NGC 5474, compress its gas and fuel intense star formation, which may also explain its asymmetry and grand spiral pattern.
The galaxy harbours an oddity. In 2001 the Chandra X-ray Observatory identified a source called M101 ULX-1 as ultra-luminous, brighter than any single star though weaker than a whole galaxy. Later observations suggested it is an X-ray binary with a black hole of only 20 to 30 solar masses that feeds faster than theory predicts.
Supernova hunters love M101. SN 2011fe, found by the Palomar Transient Factory, peaked at magnitude 9.9, the brightest of that year, and Kōichi Itagaki discovered SN 2023ixf in May 2023. In 2006 NASA and the European Space Agency released a Hubble portrait assembled from 51 exposures, then the largest and most detailed image of any galaxy taken by that telescope. Binoculars can pick M101 up, but seeing the spiral by eye needs a big telescope and very dark skies.
Source: Pinwheel Galaxy
