Jump to content


- - - - -

Binocular Universe: The Wonderful Universe of Color

Discuss this article in our forums

Binocular Universe:
The Wonderful Universe of Color

May 2016


Phil Harrington

One look at the beautiful astrophotos posted here on Cloudynights and its quite evident that we live in a vibrant and colorful universe. But when we swing our binoculars (and telescopes) skyward, most of what we see visually are varying shades of gray.  What's up with that?

The problem is not with our universe, but rather with our eyes. Our eye
s image detector, the retina, is composed of ten layers of nerve cells, including photo-sensitive receptors called rods and cones. Cones are concerned with brightly lit scenes, color vision, and resolution. Rods are low-level light receptors, but cannot distinguish color.

Under dim light conditions, the eye's rods are in their element, increasing in sensitivity. But the cones seem to say
okay, you take it from here, as our color perception drops dramatically. We become nearly color blind.

Above: Spring star map from Star Watch by Phil Harrington.
Click the chart to open a printable PDF version in a new window.

This month, we are going to prove that wrong, that the universe is indeed colorful through binoculars. We are going to ferret out some crimson carbon stars. Most carbon stars are red giants, having evolving off the main sequence and into the upper right realm of the Hertzsprung-Russell (HR) Diagram. For more on the HR Diagram, revisit my January 2016 Binocular Universe.

Red giant stars continue to generate energy by nuclear fusion. They do so, however, not by the hydrogen-into-helium process that defines Main Sequence stars like our Sun. Been there, done that. Red giants have exhausted their supply of fusible hydrogen in their cores. Instead, they are fusing helium into carbon. This is referred to as the triple-alpha process, where three Helium-4 nuclei (also known as alpha particles) are transformed into carbon. The diagram below shows the two-step process.

Above: The Triple-Alpha Process.

Source: https://commons.wikimedia.org/wiki/File:Triple-Alpha_Process.png

In some cases, strong internal convection currents carry some of that carbon byproduct up toward the star
s surface, where it forms a cloud of soot. The soot scatters the stars light at the blue end of the spectrum, but passes the oranges and reds. The end result is a vibrant carbon star. (Incidentally, this scattering effect is behind the deep red sunsets and sunrises we witness here on Earth after a major volcanic eruption scatters carbon-rich volcanic ash into our planets atmosphere.)

Not surprisingly, all carbon stars fluctuate in brightness as their degree of sootiness comes and goes over time. As a result, most carry designations as variable stars. Traditionally, variable stars are grouped together by their "home" constellation using capital Roman letters, beginning with "R", in the order in which they were discovered. For example, R Coronae Borealis was the first variable found in Coronae Borealis, while S Scuti was the second discovered in Scutum, and so on. Once the ninth variable was catalogued in a particular constellation (denoted by "Z"), the next was identified by "RR", followed by "RS" to "RZ", then "SS" to "SZ", and continuing in this fashion until "ZZ" was discovered. After this, "AA" to "AZ" were assigned, then "BB" to "BZ", etc. The system continues, except for "JJ" to "JZ", until "QZ" is used, allowing 334 variable stars to be identified and labeled in a single constellation.  While this would certainly seem adequate to cover all variables in a constellation, many, especially those along the main stream of the Milky Way, contain more than this system permits. In these cases, the 335th variable identified in a particular constellation is listed as "V355" (V for variable), with each subsequent discovery placed in numerical order.

There are several carbon stars in this month
s evening sky that are within reach of most common binoculars. Once aimed their way, it will be readily which is the carbon star thanks to their distinctive red color. 

Of course, one trip to the paint store and you
ll immediately realized that there is a lot more to the color red than just, well, red. Benjamin Moore alone lists 502 variants of red and another 435 of orange!  The same can be said for carbon stars. Not all red stars are just red. Some are red and some are RED! Have you ever seen a "Vermilion" star?  Or would you say that star is more of a "Tawny Day Lily?"

Since descriptions can be so subjective, astronomers use photometry to objectively rate the color of stars. To do this, photometers are used to measure a stars magnitudes. Thats right, magnitudes: the blue (or photographic) magnitude and the visual magnitude. The blue value is then subtracted from the visual value to determine the B- V color index of the star. For blue stars, the B-V value will be negative, while for red stars, it will be positive. The greater the positive value, the redder the star. Betelgeuse, which we all know is a red supergiant, has a B-V value of +1.9, while orangy Arcturus B-V index value is +1.23.

There are many carbon stars scattered across the sky, but here are six that should stand out nicely through your binoculars tonight, if it
s clear. The table here lists them by name, celestial coordinates, their B-V index value, magnitude range, current magnitude (end of April 2016), and their period of variability.


B-V Index

Magnitude range


current magnitude


UU Aurigae


5.1 6.6


234 days

X Cancri


5.6 7.5


195 days

T Cancri


7.6 10.5


482 days

U Hydrae


4.5 6.2


450 days

V Hydrae


6.0 12.3


531 days

Y Canum Venaticorum


4.9 5.9


268 days


UU Aurigae

T and X Cancri

U and V Hydrae

Y Canum Venaticorum  

Above: Finder charts for this month's Binocular Universe.

Chart adapted from Touring the Universe Through Binoculars Atlas (TUBA) by Phil Harrington.
Click on each chart to open a printable PDF version in a new window.

Hopefully the finder charts here, each scaled to 15° across, will help you locate each of these beauties. All should be obvious because of their color, although bear in mind that if they are near minimum, they may be too much of a challenge right now.  And be sure to post your observations of these, and any other carbon stars you may have crossed paths with, in this months discussion forum.

While not quite as colorful as the stars above, we have another variable star of interest this month. R Boötis is a long-period variable star lying just west of Izar (Epsilon Boötis), one of the primary stars in the constellations familiar kite-like shape. Like most long-period variables, R Boo is a pulsating red giant. Often referred to as Mira-type stars for Mira in Cetus, the first long-period variable discovered, these stellar oddities are actually experiencing physical expansion and contraction as we watch them alternately fade and brighten. Our timing is great, as R was predicted to reach maximum brightness around magnitude 6.5 to 7 in late April. That means its still bright enough to be visible through our binoculars this month and well into the summer. Use the chart here to monitor R Boos behavior over the next several weeks as it begins its descent toward its 13th-magnitude minimum. 

Above: Finder chart for R Boötis.

Chart adapted from Touring the Universe Through Binoculars Atlas (TUBA) by Phil Harrington using magnitude data from the American Association of Variable Star Observers.
Click on each chart to open a printable PDF version in a new window.

I hope you
ve enjoyed this quick look at the colorful spring sky. If youre interested in learning more about carbon stars, as well as find a list of 100 visible through binoculars and small telescopes, pay a visit to the Astronomical Leagues Carbon Star Observing Program home page.  The AL's program lists 100 carbon stars for us to enjoy.

Meanwhile, join me next month as we continue our journey through the binocular universe. Till then, remember that two eyes are better than one.

About the Author:

Phil Harrington is a contributing editor to Astronomy magazine and author of 9 books on astronomy, including Touring the Universe Through Binoculars.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Binocular Universe is copyright 2016 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.



  • Man in a Tub and davejlec like this


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics