- 2819 - INTERSTELLAR DUST - where stars are born. Interstellar dust is not a very exciting astronomical topic, But, it happens to be the birth place of all the stars and the ingredients for the creation of all life. So, we better learn something about this “dust“. Interstellar refers to the space between the existing stars, but it is where new stars are born.
--------------- 2819 - INTERSTELLAR DUST - where stars are born.
- Interstellar dust is like a soot, almost chimney soot. It is microscopic grains of carbon. It is also crystals of water ice and silicates, or sand. Some of the dust grains are actual metals, like iron.
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- Interstellar dust that gathers in clumps and clouds is called a “molecular cloud” because the hydrogen gas in the dust clouds is cool enough to form molecular hydrogen, H2 versus H. “H” is simply an atom with one proton with one electron. H2 is two hydrogen atoms linked together.
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- Energetic starlight among and around these clouds can make the hydrogen gas fluoresce to a pink or red color. If the stars are energetic enough their stellar winds of radiation and particles can clear out voids in the molecular clouds This can occur rather quickly, say 500,000 years. The edge of the cloud can become a thin excitation zone, or a “rim nebula“.
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- Interstellar space is not a perfect vacuum. Great patches of dust absorb light form distant stars. In fact, dust completely obscures from view the visible light emanating from the center of our Milky Way Galaxy. You can see these dark dust lanes in the Milky Way on a good seeing night looking toward Sagittarius, the Tea Pot, where the center of the Milky Way lies and stretches out going north across the sky.
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- Dust clouds in the temperature range of 100 Kelvin contain atomic hydrogen (H). When the gas and dust cools to 30 Kelvin molecular hydrogen forms,(H2). The density of the cloud increases from 100 atoms per cm^3 to 300 atoms per cm^3.
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- At the center of these clouds the cores increase in temperature to 60 Kelvin and a density of 10,000 atoms per cm^3. At this point if the cloud is large enough gravity will begin forming a star. By comparison the average value for density or interstellar dust in our Milky Way is 100 particles per cm^3. The temperature of empty space is 3Kelin.
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- Atoms heavier than helium make up about 2% of the mass of the interstellar medium. Tiny, solid, dust grains are about 1% of a molecular cloud’s mass. Individual dust grains are less that 1 micrometer in size, smaller that a single cell of bacteria which is 10 micrometers in size.
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- These small dust grains tend to scatter the shorter wavelengths ( the blue light) causing the longer-wavelength photons to make stars appear redder and dimmer. Consequently, clouds that appear dark in visible light photos often glow red when observed in the infrared light spectrum.
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- The interstellar medium is turbulent and full of structure and spatial scales The medium begins where the interplanetary medium of the solar systems ends. The solar wind extends out 100 astronomical units ( 100 times the Earth-Sun distance which is 93,000,000 miles.) 100 astronomical units is about 80 minutes at the speed of light.
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- Voyager I and II are space craft launched in September, 1977. In May, 2008 Voyager I passed this termination shock region between the solar wind and the interstellar medium, at 106 AU. The termination shock boundary fluctuates in the distance from the Sun as a result of fluctuation in solar flare activity.
- The shockwave arises because the solar wind is traveling 895,000 miles per hour and the speed of sound in the interstellar medium is about 223,700 miles per hour, the exact speed depends on the medium density. Even though the interstellar medium has a very low density it still has a constant pressure associated with it.
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- Whenever the solar wind is slowed down due to this pressure to below the speed of sound a shockwave is created. The termination shock is where the solar wind particles are stopped by the interstellar medium. Past this bow shock particles in the interstellar medium are no longer being excited by solar energy.
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- These same dust phenomenon’s occur around all the stars. That is why the oldest stars appear as a blue dot. Younger stars shine at the edges of the cavities or at tips of the dust pillars.
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- The dust clouds around the stars appear green at the densest part of the cloud and red at the thinly spread out dust in the cloud. The radiation and out flowing stellar winds carve out the cavities of less density in the clouds. Red light is usually coming straight to us. Blue light is usually reflected light.
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- Astronomers have determined that actual size of astronomical objects are the same as their apparent size. This implies that the shape of the Universe is flat. If spacetime had a positive curvature like a sphere then these objects would look bigger than they actually were. If spacetime had a negative curvature these objects would look smaller than they actually were.
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- This also means that if you add up the total density of the Universe ratio’ed to the Critical Density where it would neither collapse or expand you get 0.27 for the total mass and 0.73 for the total energy, the sum being 1.00.
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- Therefore the Universe must be nearly flat in shape. It is homogeneous meaning it looks the same no matter where you are and it is isotropic meaning it looks the same no matter which direction you look.
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- See Review 2799 “ The Homogeneous Universe”. - COSMOLOGICAL PRINCIPLE - a homogeneous universe? Astronomers “ assume” the Universe is homogeneous. That means on the grandest scales the universe is the same everywhere. From the dictionary something homogeneous is composed of parts or elements that are all the same, having a common property throughout.
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- September 8, 2020 1013 2819
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--------------------- --- September 9, 2020 --------------------------------------
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