Physics Photo of the Week

Physics Photo of the Week

March 7, 2025

Galaxy M81 - Bode's Galaxy - photos by Donald Collins

This galaxy, near the Big Dipper asterism in the sky, part of Ursa Major constellation, is one of my favorite galaxies to photograph at the College View Observatory.  Not only does it illustrate the definite spiral structure, but it also shows the predominant blue colors in the spiral arms. The "bluish" arms contrast with the bright reddish color of the broad nucleus or core of the galaxy.

Galaxies have been around in the Universe since very close to the creation of the Universe according to the Big Bang theory.  The James Webb telescope has imaged countless galaxies at red shifts very close to the speed of light. This large redshift indicates that these most distant galaxies are 12 billion light years distant.  That is about 90 percent of the believed age of the Universe.   Will there be an ultimate limit to the distance of galaxies?

This galaxy, Messier 81 in the famous catalog of deep sky objects published by the Frenchman Charles Messier in 1771. It is relatively close to our own Milky Way galaxy - about 12 million light years - only about 1/1000 the distance to the furthest known galaxies in the Universe (12 billion light years).  This galaxy is also about 6 times further than the closest spiral galaxy - Andromeda (Messier 31) pictured at right in a very low quality photo.  The Andromeda Galaxy looks much smaller than the Bode's Galaxy because the Andromeda Galaxy is photographed by an regular camera lens rather than the big 14 inch diameter telescope.  If the Andromeda Galaxy were photographed at optical system (14 inch telescope) as Bode's Galaxy, it (Andromeda) would appear about 6 to 10 times larger than Bode's Galaxy.   

Back to Bode's Galaxy.  The blueness of the stars in the spiral arms indicates that many of the stars in the spiral arms are very young.  The apparent disk of a galaxy represents lots of dust - like the dust clouds that formed the horsehead nebula (Feb 21, 2025 PPOW).  Stars are formed in the dust as variations in the dust density causes extensive star formation.  Because most of the light in the dust lanes comes from the brightest stars, and the brightest stars are also the hottest stars, the hot blue color associated with extremely hot objects lights up the rest of the dust.

 

The image at right is an infrared image taken by the Spitzer Space Telescope - released by NASA to the public domain - copied from the Wikepedia article - shows the dust as the strong reddish glow indicating the dust lanes.  The core of the galaxy is much brighter than the spiral arms. 

 

 

 

 

 

 

For the visible light images of M81 (left), the spiral arms had to be enhanced many times and the core intensity reduced in order to make both the core and the spiral arms visible.

 

 

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. 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 dcollins@warren-wilson.edu.

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.

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