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The gold in your ring formed in a stellar explosion, and the carbon in your diamond was forged in the heart of a star. Do not have a diamond ring? The iron in your blood was formed in the same way.As a resultof this cosmic alchemy, aside from the metals that are produced, galaxies can contain large amounts of ‘dust’. Dust is a general term used to describe the carbonaceous and silicate grain-like material that is also created during stellar evolution. When a star dies, either by shedding its layers in a nova or explosively dispersing in a supernova, this dust is spread out into the interstellar medium. Dust tend to be accumulated in thick patches where the most active sites of star formation occurred, or are still occurring, and becomes obvious when we make large maps of the galaxy in visible bands of light. The dust is generally opaque to the visible light photons, which get absorbed and scattered by the dust grains, just as it is hard to see clearly through a smoky room. The dust is better at absorbing and scattering photons of shorter, or bluer,wavelengths, so that preferentially the ‘redder’ photons make it through-this effect is called ‘reddening’. Reddening means that in regions of high dust concentration optical observations alone will give us an incomplete view, because a lot of the light is blocked out. The effect can easily be seen in the disc of the Milky Way. Any longexposure image of the apparently starry plane will reveal darker patches and whirls within it. This is dust concentrated in the plane of the galactic disc, obscuring some of the light from stars behind it. To see through this dust, we have to turn to slightly longer wavelengths of light, as these can penetrate the dust more easily. What is the ‘dust’, exactly?. Solving the problem of how to measure the star-formation rate of a galaxy that is heavily blanketed by dust was a cue for astronomers to develop telescopes to detect and map this infrared emission. The most successful infrared telescope of recent years is the Spitzer Space Telescope. Spitzer carries instruments that could detect radiation at around 4 to 160 microns .When we look at images of spiral galaxies taken by Spitzer and compare the infrared emission with the optical light, it is totally clear how the dust- which appears as dark occluding patches in the optical- becomes visible in the infrared, tracing the obscured star-forming regions.What it means is that about half of the star-formation activity in the universe is actually traced by infraredemitting dust, rather than direct ultraviolet and optical emission from stars and gas. That is averaged over all galaxies .When flown on a spacecraft, the dust telescope enables us to touch interstellar. Thus, Dust Astronomy opens up a new, supplemental window to investigate astrophysical questions. The dust can also emit its own characteristic glow, because, as it absorbs those ultraviolet and blue photons from stars, it heats up. This ‘thermal’ energy is re-emitted as infrared photons. Rather than the near-infrared, these 358 photons tend to have much longer wavelengths, tens to hundreds of microns, in a region called the mid-to farinfrared. If you view the galaxy in these wavelengths, then the dust suddenly becomes bright, because it is the strongest emitter of those infrared photons. The stars themselves do not emit much radiation at these long wavelengths. Some of the most active galaxies in the universe- those that are forming the most stars- tend also to have the most dust, which blocks out the majority of the visible visible light coming from the stars, but the reprocessed infrared emission can dwarf all the other light coming from the galaxy and so these dust-obscured galaxies can blaze brightly at infrared wavelengths. The dusty detritus formed in previous generations of stars. This galactic cirrus poses a problem for extragalactic studies at ultraviolet and visible frequencies, because before contending with the gauntlet of absorption presented by the Earth’s atmosphere (where water and other molecules can easily snuff out photons that are passing through), the ultraviolet and visible-light photons from distant extragalactic sources must pass through the rest of our own galaxy. If a photon coming from some distant galaxy encounters some of the dust within our own galaxy, then it can also get absorbed. This is called galactic extinction, and we have to correct for its dimming and reddening effect on the observed intensity of extragalactic light. To make the extinction correction, we need detailed maps of where the galactic dust is and how thick it is (we can get this from the all-sky maps made at infrared wavelengths, for example). Combined with a formula for expressing how strong the absorption is for different frequencies of light (called a reddening law), we can add back the emission that the galactic dust took away. In some directions, like the galactic plane, the extinction is so extreme that no extragalactic light can make it through. So, when we perform very deep observations of the distant universe, ideally we want to peer out of the disc of the galaxy where the amount of intervening cirrus is low, so the galactic extinction of extragalactic light is minimized. This is another disadvantage of being an extragalactic astronomer living within the disc of a galaxy. It is curious to think, however, that astronomers in other societies in the galaxy will be able to explore the extragalactic universe to varying extents, depending on whether they reside in a very dense part ( such as closer to the bulge), or in the galactic suburbs- perhaps even in one of the Magellanic Clouds. The dust was once solely an annoyance to astronomers, as it obscures objects they wish to observe, When infrared astronomy began, those dust particles were observed to be significant and vital components of astrophysical processes. Their analysis can reveal information about phenomena like the formation of the Solar System. For example, cosmic dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and from planets. In the Solar System, dust plays a major role in the zodiacal light, Saturn’s B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune and comets. The evolution of dust traces out paths in which the Universe recycles material, in processes analogous to the daily recycling steps with which many people are familiar: production, storage, processing, collection, consumption, and discarding. Observations and measurements of cosmic dust in different regions provide an important insight into the Universe’s recycling processes; in the clouds of the diffuse interstellar medium, in molecular clouds, in the circumstellar dust of young stellar objects, and in planetary systems such as the Solar System, where astronomers consider dust as in its most recycled state. The astronomers accumulate observational snapshots of dust at different stages of its life and, over time, form a more complete movie of the Universe’s complicated recycling steps. References: 1. Дагаев М.М., Чаругин В.М.: Астрофизика. Учебное пособие. – М., Просвещение. 1990. 2. Шкловский И.С. Звезды: их рождение 6 жизнь и смерть. – М., Наука,1980. 3. Douglac.Giancoli. (2008). "Physics for scientists & engineers with Modern Physics". Upper Saddle River. 558. 4. Fishbane, Gasiorowicz, Thornton. (2005) "Physics for scientists & engineers". Upper Saddle River. 546. 5. Kirshner, 1991, Physical Cosmology, 2, 595.

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