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Definitions of Astronomical Objects

What is a "Messier"?

Charles Messier was a French astronomer who, back in the 1800's was using his telescope to observe comets.  Other observers sent him a message every time they saw a fuzzy object and he quickly realised that lots of these objects were not comets because they were not moving, so he compiled a list of them, assigning them numbers. Since then we have learned that these objects are actually nebulae, open clusters, globular clusters, planetary nebulae and galaxies.  Messier did not actually discover many of them but accepted observations from other observers, and a number of other objects were added to the list after Mr Messier was finished. All are designated by an "M" number - hence M1 is the Crab Nebula in Taurus, the great Andromeda Galaxy is M31, the "Ring" nebula in Lyra is M57 and 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.

What is a Planet?

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 now known that many stars throughout the galaxy have formed solar systems.  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.  The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which is in orbit around the Sun, has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and has "cleared the neighborhood" around its orbit. For this reason Pluto was downgraded to a "dwarf planet" in 2006.

What is a Star?

Stars are formed from large accumulations of hydrogen gas, usually within galaxies.  Galaxies usually contain many such concentrations of gas and dust. This material 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, the temperature inside the cloud rises and rotation begins.  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 groups of stars which appear close together in the same part of the sky.  The ones which we can easily see are quite close to us in our neighbourhood in the Milky Way galaxy, and the stars they contain can often be 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 a very small part of our galaxy.  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 interesting objects because all the stars formed at the same time, from the same cloud of gas, and this can tell us a lot about the way different sized stars live, age and die. Our Milky Way has in excess of 140 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, and will become a red giant 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" (Messier 57) 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 and Reflection 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 these dusty regions.  Emission nebulae are formed of ionized gases which emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. The absorbed energy excites nearby atoms to higher energy states and when those atoms revert to a lower energy state they emit light of different wavelengths (and hence colours).  Reflection nebulae are clouds of interstellar dust which are reflecting the light of a nearby star or possibly several stars. The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. 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. 

Dark Nebulae

Dark nebulae are areas of gas and dust, which are 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 at the end of its life 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 by Chinese astronomers in 1054 AD.  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.

What is a Galaxy?

Galaxies are huge collections of stars, gas and dust, all gravitationally bound together.  Galaxies 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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