Physics Photo of the Week

Physics Photo of the Week

December 20, 2024 

Galaxy M33 - Color photo by Donald Collins, College View Observatory

Several weeks ago I published a monochrome image of the Galaxy M33.

The gray scale of astronomical photos are rather dull compared to the photos printed in color.  The color in astro photos also reveal much more about the physics of the astronomical objects - especially galaxies. 

In this deep sky image of the galaxy that is several million light years distant from our planet Earth, all the discrete stars that we see in the image are foreground stars that exist rather close to us within our own Milky Way galaxy.  The billions of stars in the distant galaxy are so far away and so numerous that they blend together to resemble clouds.  Notice that the clouds of stars in the outer spiral arms of Galaxy M33 appear bluish, while the star clouds near the core of the galaxy are more reddish.  These colors mainly refer to the average temperatures of the billions of stars in the clouds.  Blue stars are hotter than the red stars.  The blue stars are averaging about 10,000 degrees Kelvin (about 18,000 degrees Fahrenheit).  The red stars are about 3,000 degrees Kelvin (about 5,000 degrees Fahrenheit).  The Sun is intermediate temperature (5700 deg. K or 10,000 deg F).  The colors of the stars also indicate the ages of the stars.  Blue hot stars are more massive, have greater rates of thermonuclear reactions that convert hydrogen to helium.  Since the more massive stars are hotter, they use up their hydrogen faster than the cool stars, and transform to helium burning giants (like the red super giant star Betelgeuse in Orion).  Where the clouds of stars show blue, these clouds consist of young massive hot stars, but because these hot stars burn out more quickly, the blue clouds are very young - "recently" formed stars (less than about 10 million years old).  

The core of the galaxy, on the other hand, appears red, thus has less massive and longer lived stars that have been there a long time (more than about 5-6 billion years).  Due to the dichotomy between the old, red stars near the core of the galaxy, and the young, hot stars in the peripheral spiral arms of the galaxy, astronomers have concluded that star formation is still occurring in these spiral arms.  Typical galaxies are basically cosmic disks of gigantic dust clouds.  A density wave propagates around in the plane of the galactic disk and precipitates new stars (that are hot and blue) and form the spiral arms.  A place "nearby" in our own Milky Way galaxy, is the Great Nebula in Orion (see PPOW for March 27, 2024), which is called a "stellar nursery" where new stars are currently being born.

In summary, the color photos of galaxies show that they consist of hot young stars in the outer spiral arms and old predominately red old stars near the core of the galaxies.

In this color photo, the foreground stars do not show much color.  Because they appear much brighter than the much more distant stars of the far off galaxies, they are over-exposed and the colors are "bleached out".  All three colored pixels in the digital images show maximum values that the foreground stars all appear white.  To see the colors of the foreground stars, we would  need much shorter exposures, but then the much fainter galaxy clouds would not be visible.

The color astro photo of M33 in this week's photo was made with many long exposure photos of the galaxy through separate red, green, and blue filters and placed together with software to produce the composite colors. 

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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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