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Binocular Universe: Backyard Evolution

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Binocular Universe:
Backyard Evolution

January 2016


Phil Harrington


Each academic semester, I teach a different undergraduate astronomy course at Suffolk County Community College here on Long Island. Often, when I am teaching the Astronomy of Stars and Galaxies course, a student will come up to me before the first class and ask me "so, what's this class all about?" My answer is always the same: "Orion!"

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

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

Chart adapted from Touring the Universe through Binoculars Atlas (TUBA)
Click the chart to open a printable PDF version in a new window.

One of the key objectives in that course is for students to understand stellar evolution. I can think of no better way to illustrate the process from start to finish than with Orion, the Hunter. That entire topic can be condensed into that single constellation. There are bright stars and dim stars, hot stars and cool stars, and young stars and old stars. Orion is a "mine of wonders; this great constellation embraces almost every variety of interesting phenomena that the heavens contain," according to Garrett Serviss in his classic 1888 book Astronomy with an Opera Glass.  (Follow the link to view and download the full text from the U.S. Library of Congress.)

If you want to see star birth in action, look no further than the middle star in Orion's sword. That "star" is not a single sun at all, but rather a glowing cloud known as the Orion Nebula, M42, one of the sky's busiest delivery rooms. What may look like a dim, shapeless blur at first glance displays a wealth of subtle detail if you brace your binoculars against a rigid support to steady the view. The overall shape always reminds me of a cupped hand seen from the side that is seemingly grasping at two stars, Theta-1 and Theta-2 Orionis. 

Theta-1 is actually a family of four young suns neatly gathered in a trapezoid, called the Trapezium. Spotting all four stars through binoculars is a fun test.  But be forewarned, magnification and aperture are key here.  The Trapezium's stars are designated with letters (A, B, C, and D), ordered according to their location. The system's primary star, the brightest of the bunch at magnitude 5.1, is known as Theta-1C and marks the trapezoid's southern corner. The western star (Theta-1A) and the northern star (Theta-1B) are both known to be eclipsing binaries, with a smaller companion star alternately passing in front of and behind the larger primary star. These eclipses cause both stars to vary slightly in brightness, though they usually shine at magnitude 6.7 and 7.9, respectively. Theta-1D also shines at magnitude 6.7.

Also look for a dark, cigar-shaped cloud nicknamed the Fish's Mouth protruding against the brighter background clouds, just north of Theta-2 and east of Theta-1. There is also a little "bump" protruding off the north edge of the Orion Nebula. Although part of the same complex, Charles Messier cataloged it separately as M43.

M42 is just the tip of a huge nebulous "iceberg" known as the Orion Molecular Cloud that engulfs nearly the entire constellation. The cloud is between 1,500 and 1,600 light-years away, and is spread over hundreds of light-years. In addition to M42 and M43, the Orion Molecular Cloud also includes:

  • IC 434
  • Barnard 33 (Horsehead Nebula)
  • Barnard's Loop
  • M78
  • NGC 2024 (Flame Nebula)
  • Sh2-264 (Lambda Orionis molecular ring)
  • Orion OB1 Stellar Association, which can be further broken down into four parts:
    • Orion OB1a (the group of stars northwest of the Orion Belt stars, including 25 Orionis)
    • Orion OB1b (also known as Collinder 70, discussed below)
    • Orion OB1c (the stars in Orion's Sword)
    • Orion OB1d (the youngest stars in M42 and M43)

As this cloud wafts through the region from northwest to southeast, it leaves behind pockets of newly formed stars in its wake. The wave is currently cresting near the sword, but to the north, the clouds have parted to reveal a thriving open cluster of stars known as Collinder 70.  All three of Orion's belt stars, along with another hundred or so fainter suns, belong to that cluster. The Belt Cluster was not recognized as such until research conducted by the Swedish astronomer Per Collinder (1890-1974) showed that the stars were all moving in the same direction through our galaxy.

Most of the stars in the Orion Belt Cluster shine brighter than 9th magnitude, bringing them within range of 50mm binoculars from suburban skies. When you look their way, consider that those stars are probably less than 10 million years old. That's very young compared to our 4.5-billion-year old Sun, but much older than the stars in the Orion Nebula, which date back no more than 300,000 years.

Overall, Collinder 70 looks football-shaped, with the three Belt stars marking the ball's length. There is also a distinctive S-shaped chain of 11 faint stars that snakes from Mintaka, the Belt's western star, to Alnilam at its center.

Mintaka itself is a wide double star that, despite its large separation, can be a challenge to resolve with binoculars. That's because the bright component -- the star we see naked eye --shines at magnitude 2.2. But its companion is only magnitude 6.8, making it appear nearly 70 times fainter. So, despite the fact that they are separated by 53 arc-seconds, splitting the pair is tough due to glare. As a hint, look for the companion due north of the dominant star. Both appear pure white.

Mintaka, Alnilam, and many of the other stars within Collider 70 are either spectral class O or B stars. Astronomers use spectral classes (or spectral types, if you prefer) to sort stars according to temperature, size, and luminosity. Luminosity is a measure of how bright a star really is, while its temperature is a measure of the temperature of its visible surface. Knowing these two parameters, astronomers can classify stars according to spectral classifications. Stars are arranged from hottest to coolest using letters of the alphabet: O, B, A, F, G, K, and M.

Each class can be further subdivided into 10 sub-classes, numbered 0 through 9. By convention, the lower the number, the hotter the star in that particular grouping. For example, our Sun is classified as a G2 star. All class G stars share common spectral characteristics, yet a G1 star would a little hotter than our Sun, while a G3 (and G4, etc., all the way to G9) would be cooler.

The table here shows the spectral classes and visual magnitudes for the brightest stars in Orion.


Spectral class


Betelgeuse (Alpha)



Saiph (Kappa)



Bellatrix (Gamma)



Rigel (Beta)



Alnitak (Zeta)



Alnilam (Epsilon)



Mintaka (Delta)



Back in the early 1900's, Danish astronomer Enjar Hertzsprung and American astronomer Henry Russell independently began to look at these characteristics for a variety of stars across the sky. Specifically, they compared the stars' temperatures with their luminosities. Both astronomers plotted the star data on a graph. The graph's vertical axis (the Y-axis) was a measure of luminosity, while the horizontal axis (the X-axis) plotted the stars' spectral classes. Today, this graph is known as the Hertzsprung-Russell Diagram, or more simply, the H-R Diagram.

At first, they probably expected to find no correlation between a star's temperature and its luminosity, but in reality, there is a very distinct relationship. More than 90% of all stars lie along a curved line that stretches from the diagram's upper left corner to the lower right. This wide swath is called the "Main Sequence." Comparatively few stars are found in the other two corners of the H-R Diagram. Those that fall toward the upper right (that is, stars that have high luminous but are red in color, and therefore, quite cool) are called red giants or red supergiants.  In the opposite corner are white dwarfs, stars that are extremely hot, but are not very luminous because of their small size.

Lets follow the trail of star formation northward to the Hunter's tiny, triangular head. The triangle's top star, Meissa (Lambda Orionis), is a Class O8 blue-white giant star that would appear in the upper left corner of the H-R Diagram. Its surface temperature is estimated to be 35,000 K, making it one of the hottest stars in Orion visible to the naked eye. Meissa, along with several dozen fainter suns within about 1┬░, all belong to the star cluster Collinder 69. Most binoculars reveal between 15 and 20 stars ranging in brightness from 5th to 9th magnitude. Studies also show that Collinder 69 is probably no more than 5 million years old.

A third Collinder cluster, Collinder 65, is large enough that some of its stars cross the border into adjacent Taurus. By adding a few non-cluster stars to the east and north, I imagine this cluster as a spear that Orion is about to heave at the Bull. "Orion's Spear" measures about 8┬░ tip to tip, which makes it perfect for 7x and 8x binoculars.

So far, we have examined star birth and adolescence. What about the other end of the scale? For that, we need look no further than brilliant Betelgeuse, spectral class M2. While many of Orion's stars are quite young, happily fusing hydrogen into helium within their cores, Betelgeuse has been there, done that. The hydrogen supply in its core was exhausted long ago, causing the star to swell into an enormous red supergiant. Today, as heavier elements are undergoing fusion in its core, Betelgeuse is found in the upper right corner of the H-R Diagram, well off the Main Sequence. Eventually, that process will end and Betelgeuse will go out in an all consuming burst of glory as a type II supernova. For now, enjoy its brilliant ruby red color, which indicates a surface temperature of about 3,500 Kelvin.

Contrast that to Rigel, which is a Class B8 blue-white supergiant star. Rigel has exhausted all of the hydrogen fuel in its core, causing it to leave the Main Sequence. Rigel's surface temperature is estimated to be an incredible 12,000 Kelvin, which places it along the top of the HR Diagram. Rigel will continue to expand and brighten as it continues to slowly creep toward the upper right corner. Ultimately, it will also detonate as a type II supernova.

Indeed, Orion has it all. Your homework assignment this month is to view these objects on the next clear night.  If you are looking for some extra credit, here are many more targets in this month's Binocular Universe to ponder.

As you view these targets, consider what is going on behind the scenes of each.  And always remember that two eyes are better than one.  Class dismissed.

About the Author:

Phil Harrington is a contributing editor to Astronomy magazine and author of 9 books on astronomy.  Last month, his first book, Touring the Universe Through Binoculars, just marked 25 years in print last November.  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.



  • okiestarman56, ncdan3, NigelR and 1 other like this


Greetings yet again from sunny South Africa :-)


Just wanted to compliment you for ANOTHER very interesting read! Big advantage with object under discussion (Orion) is that your potential audience is not limited to the northern hemisphere <wink>. As an aside, last time I dropped you a note I mentioned having a couple of your books, good news is that I now have four of your works and would have no hesitation in recommending to anyone who is considering adding to their collection. 


Wishing you and your family everything of the best for 2016 and that clear skies be the norm!


Till the next time remember to KEEP SMILING :-)

    • John O'Hara likes this

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