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M65 M66

M66 & M65 Galaxies in Leo

The Messier galaxies M66 and M65 are part of the Leo Triplet. The third galaxy (NGC 3628) is outside the field of view to the north (top). This is a galaxy group, not a chance alignment. The distance to these three galaxies is about 35 million light years.

M66 Sb Spiral Galaxy
This galaxy has a visual magnitude of 8.9 and has an apparent size of 8x2.5 arc minutes. 

The spiral arms are clearly not symmetrical. They have been distorted by a close encounter with one of its neighbors. There are significant kinks in the spiral arm on the right hand side. The first just above the nucleus and the other half way down the arm on the right hand side. 

Prominent dust lanes are visible along with pink emission nebula within the spiral arms (just like the orion nebula). But why are the spiral arms blue? And why is the nucleus red? 

Stars are born with a range of masses. The vast majority will be low mass stars with masses ranging from 0.1 solar masses (red dwarfs) up to the mass of the sun. These low mass stars appear red due to their low surface temperature. The rest of the stars will have masses up to 100 times greater than the sun. These stars have a high surface temperature during their hydrogen 'burning' phase (this phase is often referred to as the 'main sequence'). This high temperature makes them appear blue.

Stars spend the vast majority of their life burning hydrogen (i.e. on the main sequence). During this phase their luminosity is proportional to their mass to the power 4. For example, a 0.1 solar mass red dwarf will have a light output  0.0001 times that of the sun. A 100 solar mass star will be 100,000,000 times the luminosity of the sun.

Burning so very brightly comes at a cost - the star's hydrogen fuel can run out very quickly. The length of the hydrogen burning phase is inversely proportional to the cube of the mass. The 0.1 solar mass red dwarf lives one thousand times longer than the sun (many times longer than the current age of the universe). But the super luminous 100 solar mass blue star will consume all its hydrogen one million times quicker than the sun. So the low mass red dwarf lives one thousand million times longer than the high mass blue star!

In a young stellar population, the high mass blue stars are so luminous they dominate the combined light output. Hence a young population looks blue. So we know that the blue spiral arms in this galaxy must be full of young stars. As a stellar population starts to age, all the super massive stars soon die by exploding (supernova). The lower mass red stars survive, so the combined light gradually turns red over time. The nucleus is red because it is full of ancient stars, many of which will be almost as old as the universe. Of course when the galaxy was young, the nucleus would have been blue! The nucleus is partially saturated in this photo which makes it appear less red than it really is.

M65 Sa Spiral Galaxy
This galaxy is fainter than M66 with a visual magnitude of 9.3. It has an apparent size of 8x1.5 arc minutes.

The spiral arms in M65 is dominated by medium aged (old population) stars and therefore has a redder appearance. The nucleus is partially saturated in this photo which makes it appear less red than it really is. It should appear significantly more red than the spiral arms. This galaxy appears to have a low rate of star birth for a spiral galaxy. A very prominent dust lane is visible on the leading edge.

 Date14th March 2007
 Exposure13x300s Clear, 8x120s R, 8x59s G, 8x82s B
 CCD CameraST2000
 Telescope250mm F4.8 Newtonian Reflector with MPCC coma corrector
 MountLosmandy G11 Gemini
 SoftwareCCDSoft, IRIS