The word “nebula” comes from Latin which means “cloud.” It is apt, because nebulae (the plural of nebula) look like clouds of gases. Nebulae are made up mostly of hydrogen and helium gases, plus plasma and dust. These clouds are believed to be the birthplace of stars.

There are many kinds of nebulae, often determined by the gases that make them up and how they appear to observers on earth. One type is the diffuse nebula, which has no definite shape and no definite outlines. Another type is the emission nebula, made up of plasma or ionized gas, and emitting multi-colored lights. An example of an emission nebula is the Eagle nebula. Yet another type is a planetary nebula, such as the Cat's Eye Nebula. This is thought to be composed of the shells of “dying” stars called white dwarves.

A very fascinating type of nebula is the dark nebula, an example of which is the Horsehead Nebula. This kind of nebula is extremely dense, with very thick clouds that block out the light behind them and stars in the background. Aside from those mentioned, some famous nebulae are the Crab Nebula or M-1, M-57 which is a Ring Nebula, the Cygnus Loop, and the Orion Nebula. These are fascinating objects in the sky that can be watched through a telescope.

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Stars with 10 solar masses become red supergiants once they have used up their fuel source which is usually made up of hydrogen. Once this happens, the stars turn to other sources of fuel such as helium, which does not produce as much energy. As a result red supergiants have very cool surface temperatures that range from 3500-4500 K. As the star progresses on its evolution, it fuses heavier elements together until such a time when iron builds up in its core – this marks the life of a star that is at its very end. A red supergiant will take about a few hundred thousand years before it reaches this stage.

The largest known red supergiant is VY Canis Majoris, while the most popular is Betelgeuse. Betelgeuse belongs prominently in the constellation Orion and is part of the famous winter triangle seen in the night sky. This star is found to be about 370 times larger than the sun with a luminosity that is 10,000 times brighter.

Red Supergiant Star

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Currently there are seven main types of stars based on their temperature, O, B, A, F, G, K, M. Depending on their mass and stage of development however; stars may be classified as either dwarf stars or giants. Based on this grouping, scientists have found out that the most common stars in outer space are what they call red dwarf stars.

Red dwarf stars get their name because of their relatively smaller size and mass than the sun. These stars are known to have only a little less of the half of the weight of the sun. Their small size enables them to live for a very long time because they burn their fuel very slowly. However, because they burn only a little bit of fuel, red dwarf stars are also cooler compared to other stars and thus isn’t able to shine as brightly. Typically these stars only have a temperature below 4,000 K resulting in a faint light.

Though quite numerous in outer space, it will be very hard to see red dwarf stars in the night sky because of their small size and the faint glow that they give off. Probably the most popular red dwarf is the Proxima Centauri.

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All galaxies are capable of emitting electromagnetic radiation. In fact, even the Earth’s galaxy, the Milky Way, produces radio waves. A radio galaxy, however, compared to ordinary galaxies gives off as much as a thousand to a million times more energy per unit time. Basically, elliptical galaxies produce more radio waves than spiral ones and, as such, are perceived as the brightest in galaxy clusters.

Radio waves are produced when electrons move at light’s speed through the magnetic field, a process called synchrotron radiation. Because of the field being influenced by magnetism, it bends the electrons’ path leading to their release of energy.

With the gadgets scientists now have, they can detect radio wave emissions from the most distant parts of the universe. Interestingly, such emissions were produced in the past when the galaxies were still one-third of their current age. This means that such galaxies are so distant that the waves have only now reached the Earth. The ability to measure electromagnetic radiation makes possible the detection of how far such galaxies are from the Earth as well as their speed.

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Recently, short-period comets have been subdivided into the Jupiter-type comets, like that of Temple 2 and Encke, which typically have periods of just twenty years; and the Halley-type comets, with intermediate periods of between twenty to about two hundred years.

Experts believed that the Jupiter-type comets are to have originated from the Kuiper Belt that surround the sun at various distances ranging from thirty to fifty astronomical units, or 7.5 billion kilometers to 4.7 billion kilometers. The outer planets Neptune and Uranus and their gravitational influences is thought to affect some objects within the Kuiper belt occasionally, causing them to be flung and be caught up in orbits which characterizes those belonging to the Jupiter family. Halley-type comets on the other hand, together with those considered to be long-period comets, appear to originate from the Oort cloud.

The life spans of short-period comets are influenced by the loss of gas, rocks and dust every time they swing in their orbits around the sun. Many are visibly split into many pieces, while some disintegrate or fall away. Comet’s lives are calculated to be around ten thousand years. But for the average short-period comet, it only takes two hundred trips for its entire lifetime. Based on every 76-year return of Halley’s Comet, experts calculated that it will only live for 15,200 years before it dies out.

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The second closest star is the Proxima Centauri discovered by Robert Innes. It is 30 trillion km away, making the sun 250,000 times nearer. Proxima Centauri (also called Alpha Centauri C) is one of the three stars (the other two being Alpha Centauri A and Alpha Centauri B) of the triple star system, the Alpha Centauri. The Alpha Centauri is part of the constellation Centaurus and is considered the third brightest star. The three stars of the Alpha Centauri are considered as one since they are also seen as one. Alpha Centauri A and B are too close to each other without any clear distinction between them and C is not visible at all.

Alpha Centauri A is a type of yellow star while B is a yellow orange one. The Proxima Centauri, on the other hand, is a type of red dwarf star, which means that it is small in size and of low temperature. Because of this, you cannot see it with the naked eye despite its being second closest star to the Earth. You will need to use the aid of a powerful telescope to view it.

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Not all stars get to bow off in a fantastic display of planetary nebulae. Only those that are of low to medium mass—like our very own Sun—can create this type of spectacle when they die. Planetary nebulae don’t really last long compared to all other astronomical objects, but long enough to span a thousand generations of human life.

So, how does a planetary nebula form? It begins when the start reaches its Mira stage or that stage in its life when it turns into a pulsating red giant. The gradually increasing instability of this dying star eventually leads to its outer layer being expelled by the combined forces of strong stellar winds and the star’s own pulsations. The hot, but dead stellar core that is left behind emits ultraviolet radiations that cause the expelled outer layers of the star to radiate, thus forming a planetary nebula.

Planetary nebulae hold a very important role in the evolution of the universe. As they merge back into the interstellar space, they hand out generous amounts of the heavy elements they contain (carbon, nitrogen and oxygen) to the formation of new stellar lives.

Planetary Nebula

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Because it only reflects or scatters the stars from a nearby star, the absorption spectrum of the nebula is similar to the spectrum of the illuminating stars. The same spectrum is emitted by the presence of different compounds and elements such as carbon, nickel, and iron while the light reflected by the presence of the dust particles. A reflection nebula often has a bright blue color because blue light is easier to reflect than the other types such as red.

Reflection nebulae have gained popularity among astronomers because the Herbig-Haro or streams of gas ejected from very young stars have been captured beside it. Other than that, these reflections of light are also a sight to behold. Currently there 500 known reflection nebulae with the nicest being the reflection nebulae illuminated by the stars from Pleiades. Another magnificent blue reflection nebula can be observed in the same area of the sky as the Pleiades and is called the Trifid Nebula. Although blue lighted nebulae are more popular and abundant, red reflection nebulae can also be found in the sky as represented by the nebula surrounding the red giant Antares.

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Comets

Periodic Comets are often written with a prefix, which is determined after the second perihelion passage and some of the comets in this category are still unnumbered. The letter P written after the number indicates that the comet is periodic. Some comets have the same name and the prefixes are the only clues to determine that their orbital periods are different. Most comets are named after the people who discovered them and others take the names of those who calculated the orbital periods that some of them engaged in a lifelong career of computing these values. Comets that are considered periodic have already appeared to some observers on Earth and are expected to appear again after completing their orbital periods. The perihelion passage of comets in this category can now be computed with high accuracy.

Some of the most popular periodic comets that have appeared before the eyes of many enthusiasts of the night sky include Halley, Encke, Tempel-Tuttle and Swift-Tuttle. Seeing a comet is a once in a lifetime experience and many night sly observers look forward to seeing one in their own time.

Halleys Comet – A Periodic Comet

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