A B C D E F G H I J K L M N O P Q R S T U V W X Y ZSDSS - Sloan Digital Sky SurveyThe Sloan Digital Sky Survey is the most comprehensive survey of the sky yet undertaken.
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SDSS at ChicagoSaber's BeadsDetached points of light seen along the limb of very young and old lunar crescents. The necklace of staggered brightness peaks near New Moon is reminiscent of the moments before and after a total solar eclipse.
Seyfert GalaxiesSeyfert galaxies are lower-luminosity active galactic nuclei. See links under “AGN” for more info.
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Caltech GlossarySIMBAD - Astronomical DatabaseThe SIMBAD database provides basic data, cross-identifications, bibliography and measurements for astronomical objects outside the solar system.
There is a link to the SIMBAD database on the SDSS Object Explorer page.
Spectra for BeginnersIf you're completely new to spectra, you should read through the two threads below, which are Objects of the Day created by Alice and are about the history of spectroscopy.
Once you've done that you could tackle the "Spectra (within SDSS)" topic.
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History of spectra - absorption spectra History of spectra - emission spectraSpectra (within SDSS)The SDSS spectra can be considered to contain three general properties.
1. A roughly horizontal line that constitutes the background radiation. This would be best characterized as the blackbody radiation given off by all of the background stars. This is typically referred to as the "baseline" although it does not need to appear at the bottom of the plot.
2. Emission is the physical process corresponding to light being given off by some body. For example, a light bulb emits light. Easy enough. In the SDSS spectra, emission features show up as narrow lines that point up from the base line towards the top of the plot.
3. Absorption is the physical process corresponding to some body literally taking up light from a background source. For example, if you hold a piece of colored, transparent plastic between a light bulb and your eye, everything looks colored because of absorption. This means that less background light reaches your eye than it would if the sample were not in your line of sight. In the SDSS spectra, absorption features are narrow lines that point down from the base line towards the bottom of the plot.
The SDSS spectra also show a green line near the bottom. That is a correction for atmospheric effects.
Of course the real objective is to interpret the spectrum. By that I mean to assign the various lines to their respective atomic species and, in conjunction with more complex analyses of the relative amplitudes of the lines and sometimes the shapes of those lines, determine the chemical and physical conditions of a region of space. Interpretation is always much more difficult than simply reading the spectrum.
[EigenState]Links:
Introduction to Electronic Spectroscopy Introduction to Spectral Line ProfilesSpectral LinesDark lines visible in an absorption spectrum, or bright lines that make up an emission spectrum. They are caused by the transference of an electron in an atom from one energy level to another; strong lines are produced at levels at which such transference occurs easily, weak where it occurs with difficulty.
Each kind of molecule can be recognised by its own particular patterns of spectral lines – they form a ‘signature’. By looking for spectral lines, astronomers have found many different kinds of molecules (such as water and many carbon-based molecules) in space.
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Wikipedia Spectral LineSpeed of LightThe speed of light, usually denoted by the letter “c”, is 299,792,458 metres per second in a vacuum. In imperial units it is 670,616,629 miles per hour.
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Wikipedia - Speed of LightStarburst GalaxyThis is an imprecise term used for galaxies whose rate of star formation is exceptionally high and unsustainable. This is a temporary state, triggered by a collision or close encounter with another galaxy. M82 is archetypal, following a close encounter with M81.
Wikipedia link on Starburst GalaxiesStar ClusterSee “Globular Cluster” and “Open Cluster”.
Stellar PopulationThree stellar populations are recognised according to their metallicity. Metallicity has increased as the universe has aged due to nucleosynthesis, hence:
Population III - the earliest stars were metal free;
Population II stars were low metal; and
Population I stars - such as our own sun - is metal rich.
Wikipedia link for Stellar PopulationCross reference link to MetallicityOOTD on Stellar populationsString TheoryString theory posits that elementary particles are made of vibrating one-dimensional strings. There are many flavours of string theory, some with strings which are connected and cannot be broken and others with strings that connect and unconnect. Many of the theories call for extra dimensions within space-time, up to eleven or more.
It attempts to provide a complete, unified, and consistent description of the fundamental structure of the universe.
There are many critics of string theory who say that it is not testable and therefore is not scientific.
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Official String Theory Site Wikipedia - String Theory Stringing us alongStrong Lensing This is the most extreme bending of light when the Lens is massive and is close to the source of the lens. This can cause light to take different paths to the observer and will manifest as more than one image of the source.
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Berkeley - Gravitational Lensing Wikipedia link on Strong Gravitational Lensing SupernovaA supernova is an enormous stellar explosion which for a brief time can out-shine the light from a galaxy. There are two types of supernova;
type I where a white dwarf star accumulates sufficient matter from a companion star that it results in an uncontrolled fusion of carbon and oxygen and the star explodes,
type II where a massive star runs out of fuel to keep fusion going in the core and implodes.
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Wikipedia - Supernova NASA - Supernovae Forum Thread - Supernovae
