Galaxy M101, visible in small telescopes near the "handle" of the Big Dipper, is stunning by its spiral appearance. It is thus called the "Pinwheel Galaxy". At about 21 million light years distant this galaxy contains about 100 billion stars and is about 1 1/2 times larger in diameter than the Milky Way galaxy. We see no individual stars of the galaxy in this image. All the bright stars are "foresky" stars relatively close in our own Milky Way galaxy. The distant galaxy stars are so numerous that they blend together in "clouds" of stars.
Spiral galaxies are obviously dynamic systems and rotate
slowly, but the inner stars and clouds rotate faster than
the outer parts. As a result of the differential
rotation, clouds build up along spiral paths, these clouds
have enough self gravity that they eventually condense
gravitationally and form stars making the spiral arms
visible. The spiral arms are thus young stars, much
younger than the galaxy itself.
Galaxies are also huge systems. The width of this
photograph spans a distance of 50,000 light years based on
the galaxy's distance of 21 million light years. At
the typical rotation rate of about 80 km/sec about 1/3 the
distance from the center to the fringe, a star would require
about 1 billion years to orbit the galaxy. The Solar
System in the Milky Way galaxy rotates considerably faster
requiring only about 100 million years.
Most of the mass and luminous stars of a spiral galaxy
are in the central bulge of the galaxy called the
nucleus. The stars of the nucleus are much older than
the stars in the spiral arms. The nucleus of a spiral
galaxy resembles an ellipsoidal galaxy.
Three years ago there was a very bright supernova in
this galaxy featured in PPOW
for September 16, 2011. The 2011 image of the
galaxy was taken with a smaller focal length telescope
resulting in a larger field of view. Unfortunately,
the location of the supernova is not available in this
week's image. However, three years later, the
bright supernova of 2011 would have faded to invisibility
for all but very large telescopes.
This is the third galaxy photograph featured in 2014 in
Physics Photo of the Week - all images made at the College
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to email@example.com.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.