Messier 101 is a classic type Sc, face-on, grand-design spiral galaxy in Ursa Major. It is located 25 million light-years away. It is sometimes called the Pinwheel Galaxy.

Numerous HII emission nebulae and star-forming regions in M101 are recorded in this image. Hold your mouse cursor over the image to see their identifications. PGC 49919 is a small background galaxy near the top of the frame.

An asymmetry can be seen in the spiral arms of M101. It is believed to be caused by a near collision with another galaxy. The interaction caused density waves in the spiral arms to be amplified, compressing the hydrogen gas, leading to bursts of star formation. Bright pink knots in the spiral arms are gigantic clouds of glowing hydrogen gas ionized by hot young stars in clusters that formed out of the gas.

M101 is twice the size of the Milky Way at 170,000 light-years in diameter, with a mass of 100 billion suns. It was discovered by Pierre Méchain on March 27, 1781, and Charles Messier added it as one of the last objects in his list of deep-sky objects.

In 1851, William Parsons, the third Earl of Rosse, identified M101 as one of the first "spiral nebula". He speculated that it might be comprised of a rotating mass of stars, but it was not until the 20th century that spiral nebula were proven to be galaxies external to our own Milky Way Galaxy.

Parsons, a wealthy amateur astronomer, observed M101 with a 72-inch speculum-metal reflector telescope. The f/10 scope had a focal length of 60 feet and its mirror weighed 3 and 1/2 tons. It was called "the Leviathan of Parsontown".

North is to the left in the above image.

Messier 101 Catalogs: M101, NGC 5457 Common Name: Pinwheel Galaxy Object Type: SAB(rs)cd Galaxy Magnitude: 7.5v Size: 28.8' x 26.9' Constellation: Ursa Major Image Field Centered At: RA: 14h 03m 03s Dec: +54° 21' 29"