M51 Whirlpool Galaxy
Messier 51 was discovered by Messier in October, 1773 while hunting for objects that could confuse comet hunters. Its “whirlpool” spiral pattern was not seen, however, until 1845 when Lord Rosse discerned it in his 6-foot reflector in Ireland. These "spiral nebulae" were not recognized as galaxies until 1923 when Edwin Hubble with the 100 inch Hooker telescope observed Cepheid variables in some of these spiral nebulae, showing that they were external to our Milky Way and much more remote than previously expected. M51 is comparable to the Andromeda Galaxy (M31) and our Milky Way Galaxy in size, mass, luminosity, and the presence of a major satellite galaxy (NGC 5195).
The very pronounced spiral structure of the Whirlpool Galaxy is believed to occur from close interaction between it and its companion galaxy NGC 5195, specifically, its passing through the main disk of M51 about 500 to 600 million years ago. In this model,[ NGC 519 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 million years ago until it is where we observe it to be now, slightly behind M51.
Hydrogen gas is the most common component of the interstellar medium and forms huge clouds throughout the entire galaxy. When large sources of gravitational pull pass nearby, such as other galaxies, gravitational interactions produce compression (density) waves that sweep through these hydrogen clouds. This causes some regions of the previously diffuse gas to compress into tight pockets of opaque and dense gas; these are dust lanes one so often sees in the spiral arms. In regions where the concentration and density of gas reaches a critical value, further gravitational collapse occurs, and stars are born at the center of the collapse, where the gas is compressed so strongly that fusion initiates.
- Right Ascension: 13h 29m 52.7s, Declination: +47deg 11m 43s.
- Constellation: Canes Venatici
- Distance: 23 Mly
- Apparent magnitude: 8.4
- Apparent Size: 11.2 X 6.9 arc min
- Size: Approx 60,000 ly in diameter
- Date: April – July 2012
- Exposure: Lum: 1.94 hrs, Red: 1.94 hrs, Green: 1.67 hrs, Blue 1.67 hrs. (Total: 7.2 hrs)
- Instrument: RCOS-20 inch at f8.1 (fl=4116 mm), w/SBIG ST8-XME/AO-7 (1 pixel = 0.458 arc-sec)
- Processing: MaxIm DL and Photoshop