ASTRONOMICAL OBJECTS

Messier - what exactly is a "Messier"? And what does it mean? A word of explanation is first needed about something called "Nomenclature" - or what do astronomers mean by "Messier" this, and "NGC" that?  Well, Charles Messier was a French astronomer who, back in the 1800's used his telescope to search for comets.  He ended up cataloging every faint fuzzy object he could see, and so the list of objects bearing his name contains many different types of object, from nebulae and open clusters to globular clusters, planetary nebulae and galaxies (for explanations about what all these objects are, look elsewhere on this site).  The objects which Messier saw (and a number of others which were added to the list afterwards) are designated with an "M" number - hence M1 is the Crab Nebula in Taurus, the great Andromeda Galaxy is M31, the "Ring" nebula in Lyra is M57and so on.  NGC stands for New General Catalogue, and it is a much larger list of objects which were catalogued after Messier's initial effort.
Planets
Planets are bodies which circle around (orbit) a star.  The planet on which we live is of course called the Earth, and we are third planet out from our star, which we call the Sun.  There are eight planets in our "solar system" and it is thought that many stars throughout the galaxy have formed solar systems of their own.  Planets can be made of rocky material (like Mercury, Venus, Earth and Mars) or mostly gas (like Jupiter, Saturn, Uranus and Neptune).  During formation of a solar system, gas and dust left over from formation of the star  itself condenses to form planets.  Rocky material condenses first to form the inner planets, and the lighter gases are driven to greater distances by solar radiation before condensing into the outer gas giants. This means that rocky planets would be close to the star and the gaseous ones further out.    There is no clear limit of size below which an object orbiting a star is no longer called a planet.  Some of the asteroids are called "minor planets" and Pluto has recently been downgraded to a "dwarf planet".
Stars
Stars are formed from dust.  Everywhere you look in the universe you will see clouds of gas and dust.  This dust is mostly hydrogen, and if there is enough of it in one place at one time, then over a long period of time, gravity will cause the cloud to contract.  As it contracts it becomes more dense, and the temperature inside the cloud rises.  This process continues over time until the temperature and gas density at the center of the cloud rise to very high levels, sufficient for a process called nuclear fusion to begin.  Basically, this is when the hydrogen gas "burns" in a nuclear reaction to produce another gas - helium -  at the same time releasing large quantities of energy.  This energy is emitted as heat and light - and "presto" a new star is born.  We can see this process of star formation taking place in gas clouds within our Milky Way galaxy, and some famous Hubble space telescope photographs have shown this very well.  It is not commonly known that a majority of stars in the galaxy exist in multiple systems of two or more stars which form together and are gravitationally bound together.  For a detailed description of how stars evolve over time, visit this page
Open Clusters
Open clusters are the groups of stars which appear close together in the same part of the sky.  These clusters normally lie quite close to us in the disk of the Milky Way galaxy, and the stars are usually fairly young, often younger than our sun.  The clusters can have tens or hundreds of member stars, and they are all a part of our own Milky Way galaxy.  Remember that all the stars we can see in the sky with our naked eye are part of the Milky Way.  A fine example of a nice open cluster is Messier object 45 - also known as the Pleiades, or "Seven Sisters".  This cluster is in the constellation of Taurus and although you can see six or seven stars with the naked eye, a pair of binoculars reveals many fainter stars, and in a dark sky it can look like diamonds sprinkled on black velvet.
Globular Clusters
Globular clusters are very different collections of stars from open clusters.  They are giant aggregations of tens or hundreds of thousands of stars and they take the form or shape of a sphere or globe - hence "globular".  They are comprised of very old stars - perhaps up to 15 billion years old - which is as old as the Milky Way galaxy itself.  They are normally found associated with spiral galaxies like our Milky Way and our galaxy has in excess of 100 of them, forming a kind of "halo" above and below the central bulge of the galaxy.  Some of the best examples of globular clusters are only visible from the southern hemisphere, but some nice examples we can see in northern skies are Messiers 3, 4, 13, 15 and 22.  In a small telescope or binoculars these clusters are visible as small, fuzzy patches but in a large telescope they are resolved into thousands of closely packed stars.  Life in a globular cluster would be very interesting, with thousands of very bright stars in the sky at night, plus a grandstand view of the spiral arms and center of our Milky Way galaxy.
Planetary Nebulae
Planetary Nebulae are rings of dust surrounding aging stars.  As a star like our sun ages and burns it's hydrogen fuel, it reaches a point where this fuel begins to run out.  The star then undergoes a series of changes which cause  it to expand significantly and become what is called a red giant.  Our sun will evolve in this way, in approximately four billion years.  Eventually, further changes take place which cause the giant star to contract again, leaving behind as a disk a significant amount of it's outer material.  The new evolutionary phase which the star has entered drives this material away to form strange shapes, which the discover of the planet Uranus, William Herschel, thought looked like planets.  He named them planetary nebulae because they looked round and "planet-like" in his telescope, but now we know them for what they are.  There are planetary nebulae all over the sky, and perhaps the most famous is the "Ring Nebula" in the constellation of Lyra.  See the section on stellar evolution on this web site for a more detailed explanation of the formation of planetary nebulae.
Emission Nebulae
"Nebula" is the Latin word for "cloud" and as we have said there are indeed clouds of gas and dust scattered throughout our Milky Way galaxy.  In fact, all galaxies have dust in them, and if you look at some of the photographs of galaxies on this website you will be able to see some of the gas, which appears as dark lines in the images.  Emission nebulae are the most colorful nebulae of all.  They shine by absorbing energy from stars which are embedded within them, and then re-emitting this energy at different wavelengths - and hence colors.  A telescope will sometimes reveal these colors, and if the telescope is large enough it will gather enough light to enable the colors of the nebula to be seen.  Perhaps the most famous nebula in the northern sky is Messier object 42, the great nebula in the sword of the Orion the hunter.  To find M42, find the belt stars and let your eyes move downwards to the stars of his "sword".  In addition to the faint stars of the sword, you can also see a faint fuzzy patch in reasonably dark skies.  That's M42. 
Reflection Nebulae
Reflection Nebulae, as their name suggests, shine by the reflected light of nearby stars.  They are perhaps less spectacular than the emission nebulae, but they can be very beautiful.  The main star in the Pleiades cluster, Merope is surrounded by a beautiful but faint, blue reflection nebula which can only be seen in a telescope in dark skies.  Another reflection nebula in Orion is Messier 78, and the photograph of M78 to the left was taken by us in west Texas.  It is one of the many reflection nebulae in the constellation of Orion, the Hunter.
Dark Nebulae
Dark nebulae occur because our galaxy contains a great deal of gas and dust, and a lot of it is not illuminated by stars.  This is why, when you look up at the Milky Way at night, stretching like a cloud over your head and reaching from horizon to horizon, you can see dark areas where there are no stars.  Be assured the stars are there, but their light is obscured by dust, and we call the large dark areas in the Milky Way "The Great Rift".  In fact, our view of the center of the Milky Way galaxy is completely blocked by gas and dust, and only recently have special techniques been used to "view" our galaxy's center.  There are several smaller dark nebulae, and they can look very interesting in a telescope.
Supernova Remnants
Supernova remnants are the debris left behind when a star undergoes the titanic explosion known as a supernova.  In the description of Planetary Nebulae - above - it was stated that a star "like the sun" creates a planetary nebula as it ages.  However, stars which are considerably more massive than the sun - perhaps ten times the mass - age in a very different way.  They become red supergiants as they exhaust their fuel, but then they explode suddenly, and in a massively destructive way.  They shine for a brief period as brightly as the galaxy itself, before fading away into relative insignificance.  The material which is ejected in this explosion spreads through the universe, becoming gradually more and more tenuous, in a process which lasts for thousands of years.  We can see some of these supernova remnants if we look carefully.  One of the most famous is Messier object 1, in the constellation of Taurus.  It has been linked to a supernova which was seen and recorded in 1054AD.  The star shone brightly enough to be visible in the daytime and the explosion remnant has been expanding for nearly 1,000 years.  Another famous remnant is the Veil Nebula in Cygnus, but the explosion which caused it was so long ago that it has become very dispersed and tenuous.  Perhaps even more interesting is what is left behind, because the parts of the star which are left become highly compressed and rotate at a very fast speed.  These objects are called "Pulsars" and imagine if you can a star with the mass of our sun, rotating at 1,000 times a second and emitting intense beams of light and radio waves.  For more information about supernovae and pulsars read this page
Galaxies

Picture by R. Gendler

Galaxies are huge collections of stars, all gravitationally bound together and mixed together with masses of gas and dust - as we have seen.  They come in all shapes and sizes, and the most common types are "Spirals", "Barred Spirals" and "Ellipticals".  The names describe them well, the spirals having bright centers with spiral arms coiling outwards, barred spirals having a bar through the middle and arms coming off the ends of the bar, and ellipticals having no real structure, appearing as just a fuzzy sphere or ellipse.  There are other galaxy types, including "Peculiars" which have no obvious shape and have probably been disrupted at some stage by "collision" with another galaxy, and "Irregulars" which are usually much smaller than other types.  There are also galaxies known as "Seyferts", which have very bright and active centers.  Messier object 77 is a good example of a Seyfert galaxy, and it is not known why they are so violently active.

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