Physics Photo of the Week

May 17, 2013

The Whirlpool Nebula
The Whirlpool Nebula, also known as Messier 51 (M51) is an external galaxy.  It is near the "tail" of the Big Dipper, but it requires a telescope to see and long exposure photography to record.  This galaxy is approximately 25 million light years from Earth, is very popular for amateur astronomers to photograph because it is rather "close" compared to many galaxies that are hundreds of millions of light years distant.

The distinct spiral structure is characteristic of many distant galaxies including the Milky Way galaxy of which the Sun and the Solar System are located.  Galaxies such as this are huge - about 100,000 light years across and contain about 100 billion (1 x 1011) stars - most of the stars are located in the "bright" cores of galaxies.  No individual stars are resolved in the photograph.  The stars scattered in the photo are "foreground" stars in the Milky Way.  The interesting spiral structure of galaxies consists of clouds of "young" stars (stars younger than about 20 million years).  Much of the mass of galaxies is dust and cold gas.  As the galaxy slowly rotates, the differential rotation causes the mass to bunch up into shock fronts.  The extra density in the shock wave and the increased self gravity trigger star formation.  From studying the rotation rates of galaxies, astronomers can determine the total mass of galaxies.  The total masses are about 20 times that which is accounted for from the visible mass (visible light-emitting stars as well as dust and gas clouds).  The missing dark mass - called "Dark Matter" - is an important cosmological problem of 21st century astronomy.  Dark Matter has no interactions with ordinary matter except for gravity which is very weak compared with the other major forces in physics.  At the centers of all galaxies lies a super-massive black hole - whose mass is millions of times larger than the mass of the Sun.  The black hole's presence is determined from studying rotation rates of stars near the galaxy centers - measured with dust-penetrating infrared radiation observed by satellites for closer galaxies including the Milky Way.

A final feature of the Whirlpool Galaxy is that it consists of two obvious galaxies.  The smaller galaxy has passed rather close to the big spiral and has distorted the shape of the spiral arm that is closest to the smaller galaxy.  These galaxies may be involved in a cosmic collision - a very slow process in terms of human lifetimes because of the huge physical size of galaxies.

The photograph was made at the newly completed College View Observatory using the 14 inch telescope donated to Warren Wilson College by Gary Starkweather, WWC class of 1978.  The camera is my own digital camera mounted at the focal point of the telescope and recorded this past Wednesday.  I intend to obtain an astronomical grade CCD camera which is considerably more sensitive and can produce better photos.

This is the last Physics Photo of the Week for the college academic year due to summer vacation.  Physics Photo of the Week will return when classes resume in late August.

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

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: