The University of Arizona
Mt. Lemmon SkyCenter

The Captivating Cosmos

March 24, 2015

Coma star cluster

Listed in P. J. Melotte's (1880-1961) catalog of deep sky objects as #111 is this open cluster in the constellation Coma Berenices, prosaically named the Coma star cluster. Noted by neither Messier nor the New General Catalog, this group of about 40 stars is reasonably close to us, galactically speaking, at only 288 light-years, and has only recently been verified as motion-related and therefore a genuine cluster. Originally thought of as "Leo's tail," the third century B. C. astronomer Ptolemy III renamed it in honor of the Egyptian queen Berenice II, and the legendary sacrifice of her hair.


Known popularly as The Whirlpool Galaxy, Messier 51 (NGC 5194) is, in fact, two galaxies interacting with each other. (Computer models suggest the smaller galaxy, NGC 5195, passed near M51 (NGC 5194) some 70 million years ago; eventually, in one to two billion years, the models suggest a final merger.) M51 was the first spiral structure ever observed, by Lord Rosse in 1845 through his 6-foot reflector in Parsonstown, Ireland. At the time, the true nature of galaxies was unknown but the recognizable spiral structure was indisputable. 37 million light-years distant, The Whirlpool is a showpiece of the night sky with two distinct arms, star-forming regions and intricate dust lanes. It is one of the major highlights in the little-known constellation of Canes Venatici, The Hunting Dogs.


Messier 81 (NGC 3031 or Bode's Galaxy) is a nearly face-on spiral galaxy located in the constellation of Ursa Major (the Big Dipper). M81 is one of the most impressive examples of a grand-design spiral galaxy, with near perfect arms spiraling into the very center. Because of its proximity to Earth (approximately 12 million light-years), its large size, and its active galactic center (which houses a supermassive black hole), M81 is a popular object of study to both professional and amateur astronomers alike. Interstellar dust lanes, found primarily within the galaxy's spiral arms, have been shown to be associated with star-formation regions. The general explanation is that the newly-formed, hot, short-lived blue stars that are found within such regions are very effective at heating the dust and hence enhancing its infrared emission.


Ursa Major, The Great Bear, is home to many deep sky objects, including Messier 82. Also known as The Cigar Galaxy, due to the lanes of dust and gas that appear to cross the center of the almost-edge-on spiral arms like a cigar band, this is a starburst galaxy -- the closest one to the Milky Way at 12 million light-years distance. It is called a starburst galaxy because it shows a rate of star formation 10 times greater than our own galaxy. 300 to 600 million years ago, M82 is thought to have gravitationally interacted with its neighbor, M81, triggering the energetic star formation we see today.

NGC 4565

Nestled among the many galaxies in Coma Berenices (Berenice's Hair), NGC 4565 is known as The Needle Galaxy and is considered the sky's finest edge-on spiral galaxy. 32.6 million light- years distant, the galaxy's size (more than 100,000 light-years across), number of stars (200 billion), number of globular clusters (200), and rotational velocity make it appear as if The Needle is very similar to our own Milky Way. Even with its dust lanes obscuring much of the brightness of the spiral arms, this is an object that has gained iconic status among astronomers, professional and amateur alike.

Binary star

Among the loveliest objects in the sky, a binary star system (two stars that are physically close and gravitationally bound to each other) with distinctly different colored stars, such as Iota Cancri (in the constellation Cancer) and Albireo (the "head" of Cygnus the Swan), can dazzle the eye with their beauty. Albireo's brighter amber/yellow star is actually a binary itself, while its blue/green companion seems to be solitary. It's not even known if the two major stars are gravitationally related to each other; if they are, a complete orbit would take over 100,000 years. Other yellow/blue binary systems often shown in the program include Izar (in Bootes) and Cor Caroli (Alpha Canum Venaticorum).

Blue Shadows

What color is your shadow? Generally on Earth they are slighly blue in color. You certainly block the light of the Sun, but you cannot block the entire sky. On Earth, our skies are Blue due to Rayleigh scattering. (Shorter (blue) wavelengths of light are scattered more effectively by the molecules in the air than longer (redder) wavelengths.) On Mars where the sky is a bit more pink your shadow would blush accordingly.

Daytime view of a star

Finding a star or planet through a telescope while it's still daylight used to be tricky and highly dependent on luck, but it has been rendered a "piece o' cake" with go-to telescopes and the software that directs them. Still, it is a thrill to see Sirius (in Canis Major), Regulus (in Leo), or Arcturus (in Bootes), or any of the other bright stars while our closest star (the Sun) is still flooding the sky with its light. Somehow one doesn't think of the stars being "there" during the daytime, but they certainly are, and a good telescope will show them to you: brilliant points of light against a dazzlingly blue sky.

Green Rim

The Earth's atmosphere distorts the image of the sun when it is near the horizon. This dispersion creates a spectrum of solar images in a rainbow of light. Since the images are refracted with a wavelength dependance, the bottom portion of the Sun's image is red in color and the top portion is expected to be blue. However the red portion sets below the horizon first and bluish light rarely makes it through the intervening thick air when looking towards the horizon. So the last bit of light on the top rim of the Sun is colored green. Further distortions of the green rim are called the "green flash."


The International Space Station (ISS) and Hubble Space Telescope (HST) orbit the Earth in precise paths that cross over parts of the U.S. and the rest of the world at predictable times. Although it's conceivable that you might be able to see one during daylight, if you knew exactly where and when to look, it is much more likely that they will be sighted at night, when sunlight reflects off their shiny surfaces and makes them appear as swiftly-moving stars. provides the information necessary to find out if you will be able to see these and other satellites from your location (a dark sky site is preferable as neither the ISS nor HST is particularly bright and would probably be lost in the glare of city light pollution.)

Scintillation of starlight

Most likely we showed you Sirius or Canopus, the brightest two stars in the sky, low on the horizon. As we peered through many miles of the Earth's atmosphere the starlight was distorted and broken into a dazzling display of colors and sparkles. This scintillation of starlight occurs for any star low in the sky. Look high overhead and the stars twinkle less since you are looking through less air. Sunlight reflected from the moon or the planets also scintillates in exactly the same way. However since they are close to use they appear as small disks on the sky and the average distortion of light across the disk makes it appear to be steadier than the distortion for stars which are points of light.


Few cosmic vistas excite the imagination like the Orion Nebula. Also known as M42, the nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1,500 light-years away.The Orion Nebula offers one of the best opportunities to study how stars are born partly because it is the nearest large star-forming region, but also because the nebula's energetic stars have blown away obscuring gas and dust clouds that would otherwise block our view - providing an intimate look at a range of ongoing stages of starbirth and evolution.


Messier 76, an irregular planetary nebula in the constellation Perseus, bears a resemblance to M27 (the Dumbbell Nebula) in shape, but since it is somewhat smaller in apparent size it is known as the Little Dumbbell Nebula (or the Cork or Butterfly or Barbell Nebula). From its evolved, complex structure, it probably began forming more than 10,000 years ago, and is a stunning example of a bipolar planetary nebula. The distance estimates to this object vary widely from 1,700 to 15,000 light-years, and it is one of the faintest objects in Messier's catalog.


The second planet out from the Sun, Venus is also the second brightest object in our night sky (only the Moon is brighter), but it has an atmosphere "gone bad." It is so hot, dense, caustic, and potentially destructive to anything we recognize as life, and the objects we humans create, that it defied all our attempts to learn about its surface until we could orbit a radar-mapping satellite, Magellan, in the early 1990s. Now we know its surface shows signs of volcanic activity (possibly recent, geologically speaking), and cratering due to impacts. Since it orbits the Sun closer than we do, its highest point in our evening or morning twilight is no more than 47 degrees; it never crosses the night sky.


Jupiter, a gas-giant planet fifth out from the Sun, is the largest in our solar system, and, it is now believed, one of the reasons why life has flourished on Earth. Had Jupiter not been around, sweeping, vacuuming, and ejecting objects from the orbits of the inner planets, Earth might never have had the relatively-unbombarded span of years necessary in which to evolve complex living organisms. It was toward Jupiter that Galileo Galilei turned his newly-constructed telescope in 1609, discovering a system of four moons orbiting the planet. This was contrary to "known" doctrine at the time and led to the science of modern astronomy, as well as to Galileo's imprisonment. Jupiter has a ring system, aurorae, massive lightning storms, and a multitude of moons. The Great Red Spot is a "storm" that has been visible and studied for over 300 years; if you're very lucky, it will be visible through the 24" on the night you observe the planet.


Our closest star, the Sun, along with every other visible object in the sky, was once thought to circle the Earth, since most observations supported this hypothesis. However, over centuries, scientists dissatisfied with discrepancies in the orbits of the known planets discovered step by step that the Earth was simply a member of the group of planets and other objects which orbit the Sun. At almost 93 million miles away (one Astronomical Unit, or AU), the Sun, a "main sequence" star, provides the energy, in the form of sunlight, which supports the majority of life on Earth. Through the process of nuclear fusion, the Sun constantly converts hydrogen to helium and other elements. It has been doing this for approximately 4.5 billion years and will continue to do so for another 5+ billion years.


Our closest non-human-made satellite, the Moon was almost certainly gouged out of the proto-Earth by a Mars-sized object which impacted our planet at a steep angle in the very distant past. Once much closer to us than it is today, it is slowly receding at 3.8 centimeters per year. The Moon rotates about its axis in approximately the same period of time it takes to orbit the Earth, which results in its showing us very nearly the same "face" at all times. Of course, as we orbit the Sun, sunlight and shadow create the phases we see.


One of several jewels in the constellation Canes Venatici, Messier 3 is a globular cluster containing about half a million stars located between 33,000 and 36,000 light-years away. It is one of the "halo" of globular clusters that surround our Milky Way galaxy. Having 50% of its total number of suns packed within the central 22 light-years, this is one of the densest clusters known, yet the 32-inch Schulman telescope is capable of resolving many of the central stars.


This open cluster in Gemini, the Twins, is in close proximity to the open cluster NGC 2158, but it's an optical illusion: Messier 35 is only 2,700 light-years away while NGC 2158 is over 15,000. Relatively young at perhaps only 70 million years, it contains many blue and white massive stars, scattered evenly over an area of sky the size of the full Moon. M35 hosts 120 stars brighter than 13th magnitude, and, under excellent seeing conditions, is visible to the naked eye near "the feet of the twins."


The brightest of the three open clusters in Auriga, Messier 37 was discovered by Giovanni Batista Hodierna before 1654, but was then lost until rediscovered and cataloged by Messier in 1764 (somehow Le Gentil missed it when he rediscovered what are now known as M36 and M38 in 1749). At a distance of between 3,600 and 4,700 light-years, its 150 stars brighter than 12.5 magnitude include a dozen red giants. The total number of stars in the cluster may number as many as 500.


M41, like many other open star clusters, is a beautiful collection of brilliant stellar diamonds. What makes this cluster so memorable is how easy it is to find in a pair of binoculars. Simply find the brightest star in the sky, Sirius, and move your field of view a bit to the south (4 degrees) and you will be aptly rewarded with the young stars of this cluster located more than 2,000 light years away. Each of the stars in this cluster is hundreds of times more luminous than our own Sun.


Messier 44: this swarm of stars, called the "Beehive" star cluster, buzzes at a safe distance of 500 light-years away. The cluster is comprised of 200-350 stars towards the constellation of Cancer. Like the Pleiades, this is another conspicuous object in the sky that was known to most ancient cultures. Galileo was the first to resolve the cluster into stars using his small telescope -- an instrument markedly inferior to the binoculars you used this evening. Before telescopic aid, ancient mariners would use the appearance of the Beehive as a way to monitor the weather at night. If the cluster could not be seen, there would be moisture or thin clouds on their way in.


Messier 45, the Pleiades, is a striking open star cluster which formed 75 to 150 million years ago. It is among the nearest star clusters to Earth and is visible in the evening during the late fall and winter months. The Pleiades are one of the most conspicuous clusters in the sky. Most people can see 6 or 7 members with their unaided eyes. However, a view through binoculars reveals the myriad beauty of this cluster of young stars, which actually contains over 1,000 confirmed members. The Pleiades also carry the name "Seven Sisters", which, according to Greek mythology, are seven daughters and their parents. Their Japanese name is "Subaru", which was taken to brand the car of same name. The Pleiades do not comprise the "Little Dipper" which is associated with the North Star, Polaris!

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Suggested books and websites:
Brian Greene, The Fabric of the Cosmos                   Satellites:
Lynch and Livingston, Color and Light in Nature          Weather imagery:
Peterson Field Guide to the Stars and Planets            Clear Sky Chart:
Michael Bakich:  The Cambridge Encyclopedia of Amateur Astronomy
J. Richard Gott and Robert Vanderbei:  National Geographic: Sizing up the UNIVERSE

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Steward Observatory Mirror Lab Biosphere 2 The Laboratory of Tree Ring Research

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