Showpieces of the Sky - M42 - Diffuse Nebula in Orion

Jack Kramer

The following is some information about those deep sky objects that we always show off at public star parties during different seasons of the year. You might want to check this list prior to our public events in order to dazzle visitors with your knowldege of things astronomical! Telling guests something about what they're seeing in the telescope makes a star party more interesting for them. Even in light-polluted skies, our instruments provide some inkling of the grandeur that these objects possess. Imagine the payoff to observers who linger with them in a dark sky! And the nice thing about these objects is that they're attainable in virtually any amateur-sized telescope. The list is roughly arranged beginning with winter objects and it ends with those especially well-placed in the autumn. However, many are accessible over a period of several months, so don't let this sequence be a hindrance when checking to see what's up.

M42 - Diffuse Nebula in Orion

A mass of gas that is caused to glow by ultraviolet radiation from the stars embedded in it, like a fluorescent light bulb. Photographs show the color as distinctly red, due to the hydrogen, and green, due to oxygen. There is enough material in the nebula to form about 10,000 stars like our Sun. Stars are actually being formed within the nebula, which is referred-to as a "stellar nursery". The Trapezium in the heart of the gas cloud is a good example of a cluster of new stars. There are four bright stars in the Trapezium, plus two fainter ones. The diameter of the nebula is roughly 30 light years, or more than 20,000 times the diameter of our solar system. It lies at a distance of between 1600 and 1900 light years away from us.

M35 - Open Cluster in Gemini

Located at a distance of about 2200 light years and 30 light years in diameter. Although most open (galactic) clusters contain young stars, M35 has several middle-aged members, including some orange-colored giants. About a half degree southwest of M35 is another open cluster - NGC 2158 - which appears as a fuzzy patch. It has no physical relationship with M35, since its distance is 16,000 light years, or six times as far away as M35. NGC 2158 is one of the most remote open clusters.

Perseus Double Cluster (NGC 869 & NGC 884)

These are two open clusters that lie near each other as we see them in the sky, but NGC 844 is about 1000 light years farther away than 869, at a total distance of about 7500 light years. NGC 869 contains about 400 stars and NGC 884 contains 300 stars, but the stars in 869 are some of the youngest known, while 884's stars are older, and they include three red supergiant stars.

M44 - The Beehive Cluster in Cancer

This is one of the nearest and brightest open clusters, at 525 light years from Earth. It spans about 40 light years. This was one of the first objects observed by Galileo in 1610; he was amazed to find that it's composed of a myriad of bright stars. The brightest member of this cluster has a luminosity 70 times that of our Sun.

M81 & M82 - Galaxies in Ursa Major

These two galaxies are the brightest members of a small group of galaxies that is located at about 7 million light years - just beyond our own "Local Group". M81 is a spiral galaxy similar to M31 in Andromeda and is about 36,000 light years across. M82 is a spindle-shaped galaxy with dark lanes visible in most telescopes. Strong radio emissions and polarization of its light indicate that M82 is an extremely active galaxy and suggest that a violent outburst occurred in the central region of the galaxy about 1.5 million years ago. M82 has a diameter of 16,000 light years and contains only one-fifth the mass of its neighbor, M81.

The Stars Mizar and Alcor in Ursa Major

Mizar and Alcor are double stars visible to the naked eye in the handle of the Big Dipper (Ursa Major). There is a fainter companion to Mizar ("Mizar B") that's visible only with optical aid. In addition, both Mizar A and Mizar B are themselves spectroscopic binaries - double stars detectable only with spectroscopes. More info about this pair:
  1. The ability to separate Mizar and Alcor with the naked eye was once considered a test of good eyesight, but since it's now quite easy to spot Alcor, the dimmer of the two, it's believed that this star has brightened over several hundred years.
  2. Mizar A is of magnitude 2.4 at a distance of 88 light years. Inherently, it is 70 times brighter than our Sun. Alcor is of magnitude 4.0 and lies about 1/4 light year from Mizar.
  3. Alcor also is itself a double star; however, its companion lies too close to spot even through a telescope - it's a spectroscopic binary.
  4. This "double-double-double" system is moving toward Earth at the rate of 5.5 miles per second.
  5. Mizar and Alcor have the distinction of being the first double star system to be discovered and the first to be photographed.

M65 & M66 - Galaxies in Leo

Both these galaxies are spirals located at a distance of about 29 million light years. The galaxies are separated from each other by 180,000 light years. M65 is the more edge-on galaxy and is 60,000 light years in diameter. M66 is about 50,000 light years across. About a half degree north of M66 lies another edge-on galaxy - NGC 3628 - that has a fine dust lane running across the major axis.

M57 - The Ring Nebula in Lyra

This is an expanding bubble of gas blown off when the central star exploded 20,000 years ago. The gases (primarily hydrogen) are being caused to fluoresce (ionize) due to ultraviolet radiation from the small, hot central star, which has a surface temperature of about 100,000oK. The Ring Nebula lies about 1400 light years from Earth and is about a half light year in diameter. The central star is too faint to be detected in most telescopes under 12" in diameter. The Ring is referred-to as a "planetary" nebula, but that's a misnomer left over from a couple of hundred years ago when early observers saw this type of nebula as a little round smudge that looked to them like a faint planet.

M13 - Globular Cluster in Hercules

This is a swarm of stars estimated to number about 500,000; it lies about 25,000 light years away and has a diameter of 160 light years. The density of the cluster is probably one star per cubic light year. The stars are quite advanced in age - about 10 billion years old - and are relatively cool. Globulars are found quite far from the centers of galaxies and some are believed to drift away from their galaxies and wander inter-galactic space. The manner in which globular clusters came to be formed is not well understood.

The Star Albireo in Cygnus

The star Albireo, also known as Beta Cygni, is considered one of the most beautiful double stars in the sky. Due to its position at the tip of the constellation Cygnus, this double system is also nicknamed "The Eye of the Swan". This is a "visual" double, because the two stars are not believed to be physically related to each other since no orbital motion has been detected. They lie about 410 light years away. The brighter star is of magnitude 3.1 and is golden yellow in color; the fainter companion is of magnitude 5.1 and is generally described as bluish-green in color. Different observers tend to describe the colors differently.

The Lagoon Nebula in Sagittarius

This is a glowing cloud of hydrogen gas that is known as a "stellar nursery", since stars are actually being formed here. Gravity pulls the gas together, then the tremendous pressures cause the hydrogen to ignite, and a star is born. Due to its low position in the sky, light pollution and haze often interefere with our ability to get a good view of this nebula. However, it still ranks just behind M42 among the diffuse nebulae as one of the most inspiring sights.

M31 - Spiral Galaxy in Andromeda

This is the nearest galaxy to the Milky Way (other than the Magellanic Clouds, visible only from the southern hemisphere). It lies at a distance of 2.2 million light years from us, has a diameter of 180,000 light years, and is estimated to contain about 300 billion stars. It's one of the largest galaxies known. By comparison, our Milky Way is thought to be about 100,000 light years in diameter. When we look through a telescope, the brightest part we see is the central hub that is about 12,000 light years across, but longer observation shows the spiral arms which contain many hot blue stars and dust lanes. Near the southern tip of the arms is a large star cloud that bears its own identifier - NGC 206. M31 is part of the "Local Group" of galaxies, and is drawing closer to the Milky Way at the rate of 300 kilometers per second - in two billion years, we should collide. There are two small companion eliptical galaxies - M32 and M110 - that lie on opposite sides of M31.

M1 - The Crab Nebula in Taurus

This is a planetary nebula that is the result of a supernova explosion recorded by Chinese astronomers and perhaps by American Indians on July 4, 1054 AD. The name "planetary" comes from its circular shape that to early astronomers looked like a planet. The Crab Nebula is a rapidly-expanding shell of ionized gas and is one of the strongest sources of radio energy in the sky. It's believed that a compressed white dwarf star - a "pulsar" - lies at the core of the Crab. The nebula generates about 100 times more X-ray energy than visible light energy. It lies at a distance of 6300 light years.

Published in the March 1993 issue of the NightTimes