The University of Arizona
Mt. Lemmon SkyCenter

The Captivating Cosmos

March 23, 2012

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.


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.


Barred spiral galaxy M95 is about 75,000 light-years across, comparable in size to our own Milky Way and one of the larger galaxies of the Leo I galaxy group. In fact, it is part of a not quite so famous trio of Leo galaxies with neighbors M96 and M105, about 38 million light-years distant. In this sharp and colorful cosmic portrait, a bright, compact ring of star formation surrounds the galaxy's core. Surrounding the prominent yellowish bar are tightly wound spiral arms traced by dust lanes, young blue star clusters, and telltale pinkish star forming regions. As a bonus, follow along the spiral arm unwinding down and to the right and you'll soon get to M95's latest supernova SN 2012aw, discovered on March 16 2012 and now identified as the explosion of a massive star.

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."


A meteor is the bright trail of light created by a piece of space debris, generally no larger than a grain of sand. It enters Earth's atmosphere and is heated to white-hot temperatures by friction with air molecules. The meteor usually burns up before it impacts the surface of our planet. (Popular myth has it that, if the head or tail of the meteor burns green, the object will strike the ground.) If the meteoroid is of sufficient size to make it through the atmosphere and does land, it is called a meteorite. Brilliant meteors are popularly known as "shooting stars" or "falling stars." Meteor showers, such as the well-known Leonids and Perseids (the radiant appears centered in the constellation of Leo or Perseus respectively) are caused when the Earth moves through the stream of dust left in a comet's wake. These "showers" occur each year when the Earth runs through the stream, often covering several nights, and if they occur during the dark of the Moon, can be spectacular!


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.)


Known as the Crab Nebula in the constellation Taurus the Bull, Messier 1 is one of the most interesting and most studied objects in the sky. In the year 1054, Chinese observers noted the sudden appearance of a "guest star" which was four times brighter than Venus and visible in broad daylight for 23 days. The first item in Charles Messier's catalog, M1 is now known to be the remnant of a supergiant star which exploded approximately 6,300 years prior to 1054, the light of its death having taken that long to reach us. A large-aperture telescope reveals complex interwoven veins, ridges, and tendrils of gas and dust -- the ever-expanding and interacting remains of the rapidly rotating (30 times per second) pulsar at its center. The stellar explosion which produced these delicately formed and colored tendrils of gas contained all of the elements heavier than hydrogen and helium. Stars are the crucibles in which these heavy elements are formed, and it is only through their deaths that these elements are scattered throughout our galaxy to perhaps coalesce into planets and people. We are the ashes of stars -- we are star stuff.


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.

NGC 2392

Seeming to show a "face" surrounded by a circular fur hood, this planetary nebula, NGC 2392 in the constellation Gemini, the Twins, is known as the Eskimo or Clownface Nebula. It is a bipolar double-shell planetary nebula discovered by William Herschel in 1787. The Hubble Space Telescope has taken images of this object which show gas clouds, ripples, shells, knots, filaments, and cometary heads that are so complex they are not fully understood. At a distance of 2,900 light-years, astronomers will continue to study this evolving planetary closely.


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.


Mars is the next planet out from the Sun past Earth. Its obvious surface features and seasonal changes captured the imagination of Percival Lowell, who built a large (for the time) telescope on the outskirts of Flagstaff, Arizona, for the express purpose of studying Mars. He is famous for depicting Mars as having an extensive system of canals; while few astronomers accepted this idea, it caught the imagination of the public to the extent that it remained a controversy until Mariner 4 finally shattered the idea with fly-by images in 1965. Our knowledge of Mars is much more extensive today, with several active orbiters and a pair of active robots on the surface returning new images every day that you can see on the Internet. We now know that Mars was once a warm ocean planet much like Earth, but its small size and lack of a stabilizing moon doomed it to an early death.


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.


The sixth planet, counted outward from the Sun, in our solar system, Saturn is fondly known as The Lord of the Rings. Its ring system is wider, finer and more complex than we ever imagined until Voyager I & II provided their stunning views, updated constantly now by the highly successful Cassini mission. Saturn is a gas giant planet which shows us only its cloud tops of muted ochres, buffs, light oranges and yellows, with only a rare storm system to interrupt its bands. With the number of moons orbiting Saturn standing at 61, so far, many of them herding entire rings or arcs of rings, Saturn continues to captivate and humble us.


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.


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!


M46 and M47 appear side-by-side in the night sky. However even a casual look through binoculars reveals that M47 is more easily resolved than M46 which looks like a fuzzy ball of light. This is because M46, at a distance of 5,000 light years away, is more than twice as far as M47. Even though it is dimmer due to its distance, M46 is typically the more observed cluster due to NGC 2438- a nebula formed by a dying star that happens to be in the foreground and thereby enhances the view!

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Other programs:

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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

Other UA College of Science Outreach Facilities
Steward Observatory Mirror Lab Biosphere 2 The Laboratory of Tree Ring Research

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