- 1694 - Nebulae are like Snowflakes, no two are alike. The physics part of the story is even more interesting. Scroll the internet for some beautiful pictures and then try to imagine what is really going on with atoms and elements being created.
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--------------------------- 1694 - Nebulae are like Snowflakes.
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- Like snowflakes no two nebulae are alike Each color represents a different element. Hydrogen being the predominate red glow. Different colors are created by radiant emissions and also by reflections, absorption and reemission at different wavelengths.
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- Nebulae are a natural evolution in the lifetime of stars. But, the nebulae phase lasts only a short period of time, thousands of years to even only decades. Our Sun will be a nebula in another 5 billion years. Our Sun is a medium size star and has an expected lifetime of 10 billion years. Massive stars have a much shorter lifetimes, only a few million years in some cases and their nebula phase can be much different.
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- Check out the internet to witness some great examples of nebulae:
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------------------- Red Rectangle Nebula HD44179. composed of 4 spokes and ladder rings nested in the shape of a rectangle.
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------------------ Dumbbell Nebula M27 discovered in 1764.
------------------ Cat’s Eye Nebula NGC 6543
----------------- Eskimo Nebula NGC 2392
----------------- Spiro graph Nebula IC 418
----------------- Blinking Planetary Nebula NGC 6826
----------------- Ant Nebula Menzel 3
----------------- Ring Nebula M57
----------------- NGC 5189
----------------- Boomerang Nebula
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- Wherever we see hot, massive stars we are seeing a region of star formation. Because massive stars have short life spans they are never found very far from their birthplace.
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- Near hot stars are “ nebulae” colorful, wispy, blobs of glowing gas. The gas glows because electrons get accelerated out of their atoms due to the high temperatures and vibrating atoms ( the definition of temperature.) The gas becomes ionized, the charges become separated. Protons and electrons are no longer allowed to stay together and remain neutral.
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- The gas atoms get excited, energized because they are absorbing ultra-violet photons from the hot stars. The absorbed energy puts electrons in higher energy orbits. When these electrons fall back not lower energy orbits they emit visible light. Energy always seeks its lowest level.
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- When an electron in a hydrogen atom falls from an energy level of 3 to and energy level of 2 it emit’s a red photon with a wavelength of 656 nanometers.
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- The nebulae are multicolored because light reflected from dust grains produces blue light that gets reflected due to its shorter wavelength. Red light has wider wavelengths and passes through the dust without being reflected. This is the same reason the sky is blue. It is the same reason big waves pass by the boat and small waves splash against the hull.
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- Dark regions in the nebula is where dusty gas completely blocks the light. Dust grains can also absorb high-energy light and remit light at lower frequency, infrared light. Fast moving electrons nearing the speed of light can also emit radio wavelengths. The is the same as how a radio transmitter antenna works.
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- Among the dozens of nebulae discovered so far 25% have bipolar lobes and 20% have multiple lobes. Others are oval shape , or cylinders. Some even have spiral arms. If you count all of them discovered to date in the Milky Way you get 3,500. Counting those undiscovered the estimated total grows to 28,000 in our Galaxy.
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- The astronomical term “ planetary nebulae” was created because when some nebulae were first discovered they were thought to be planets. Only when telescopes got better did astronomers realize they were gas bubbles. The term is given to a very short phase in the evolution of a low-mass star. Our Sun will experience such a phase in about 5 billion years from now.
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- When a star has burned most of its fuel, hydrogen and helium, and the core is mostly fused oxygen and carbon surrounded by a shell of helium and an outer shell of hydrogen, the fusion moves from the core to the outer shell. The helium then sinks to the lower shell. The increased density of helium creates a “helium flash”.. The explosion plows through the hydrogen and the density decreases until the hydrogen sinks back down and the fusion starts producing helium again. The whole cycle repeats itself many times. You can see these shockwaves in the image history of each nebula.
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- The explosive flashes send out a wind of high density carbon and heavier elements in the dust. This wind can take 50% to 90% of the star’s mass out into a dense envelope surrounding the star. This phase of the dying star is the planetary nebulae .
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- The last phase of the star’s death ends as a White Dwarf of carbon and oxygen at the center. There is no more hydrogen or helium to restart fusion again. The smaller stars are not massive enough to produce the gravity pressure needed to fuse the heavier elements above carbon. Big stars do this in supernovae.
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- The star’s surface temperature gets to 30,000 Kelvin crating intense radiation ionizing the surrounding gas clouds causing them to glow. This final phase in the process only lasts a few hundred years, sometimes as short as a few decades. The planetary nebula phase in its entirety only lasts a few tens of thousands of years. Mere moments in astronomical terms.
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- As the star’s winds die off the nebula steadily dissipates leaving behind the small dense White Dwarf that continues to cool for billions of years.
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- It remains mysterious how the nebulae acquire the different and unusual shapes during the explosions. Perhaps the density regions in the outer envelope vary to the degree high speed winds flow more rapidly. If the wind flows out the poles it could produce the bipolar shapes we see.
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- The theories so far do not go far enough to explain the complex point-symmetric shapes where each point on one side matches a corresponding point on the other side. Maybe an interaction of a binary pair of White Dwarf stars could produce this shape?
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- Maybe the star’ s magnetic field interacts with the exploding gases to sculpture the shape in interesting ways. But, this would require an explanation for where the magnetic energy strong enough would come from?
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- Maybe it is not an energy gain but an energy loss. Maybe exoplanets interfere with the wind and change the angular momentum of the star when the planet is swallowed up?
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- Maybe all these things happen and each planetary nebulae is formed in its own unique way.
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- Astronomers explain the Red Rectangle Nebula as seeing two cone-shaped lobes where the edges of the cones stand out in the form of spikes. The ladder rings could be consecutive, repeated eruption and shockwaves. A Red Rectangle mystery is that its formation appears to be older than 14,000 years.
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- Better telescopes discover more unique nebulae and more details to mystify their formations. Stay tuned - there is more to learn. Study these images and try to imagine the myriad of elements created and thrown into the interstellar medium. Everything in our lives is made from this star dust.
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