Author Topic: Glossary  (Read 51400 times)

waveney

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Glossary O
« Reply #15 on: October 07, 2008, 02:48:02 pm »
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OIII Objects
OIII objects have a significant OIII emission line on their spectrum chart. The reason for this has not yet been satisfactorily explained. A selection of OIII objects are being collected and analysed by Galaxy Zoo astronomers.
Links: OIII Objects thread

OOTD
The Objects of the Day are chosen by the zookeepers, moderators, waveney and invited zooites in rotation. There's no selection criteria; each chooses their favourites from the Zoo to share with you and maybe sneak in a bit of science on occasion.
See What is the Object of the Day
and How to nominate an Object of the Day

Open Cluster
An open cluster is a physically related collection of stars held together by their mutual gravitational attraction. Each cluster is thought to have formed from the same giant molecular cloud and numbers up to a few thousand.
Links: Wikipedia - Open Cluster
          Forum Thread - Star Clusters Index


« Last Edit: October 24, 2008, 08:05:32 pm by waveney »
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Glossary P
« Reply #16 on: October 07, 2008, 02:48:24 pm »
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Parsec
Parsec is a measurement of length that is about 3.3 light years or just equal to over 30 trillion kilometres.
Links:
Wikipedia link on Parsec

Peas
Peas are a Galaxy Zoo name for bright (almost featureless) galaxies with very prominent O[III] emission lines that dominate all others, most are bright green but depending on the redshift they can be other colours.

Links: There is an active project looking at them The Peas Project
      For wider discussion about Peas see Give Peas a Chance

Photometric
The measurement of an objects brightness viewed through various filters. On the SDSS Navi page the objects with a photometric circle will generally have an explore page with information about that photometric point.
Links:
Wikipedia link on Photometric Redshift

Planetary Nebula
A planetary nebula is a glowing shell of gas and plasma formed by certain stars when they reach the end of their lives. They were called this because they looked a bit like giant planets when seen through early telescopes (they have no relation to planets).
See “Nebula”.
Links: Wikipedia - Planetary Nebulae
          Forum Thread - Planetary Nebulae pictures

Plasma
An ionized gas containing free charged particles - ions and electrons - and therefore capable of conducting electric currents.
Wikipedia Link

Polar-ring
A polar-ring galaxy is a type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring seen in the polar-ring galaxy. The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar ring structure.
Links: Forum Thread - Possible Polar Rings

Pseudo Ringed Galaxies
Galaxies with symmetrical rings that loop round and rejoin the core on each side.
Links: OOTD: Ringed and Pseudo Ringed Galaxies
         Ringed And Pseudo-Ringed Galaxies

Pulsar
The word pulsar is a contraction of “pulsating star”. A pulsar is a neutron star that is rotating and emitting beams of radiation. If the poles are aligned towards the earth we will observe the pulses which are extremely regular.
Links: Wikipedia Pulsar
          NASA - Pulsars


« Last Edit: April 09, 2009, 10:41:53 am by waveney »
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Glossary Q
« Reply #17 on: October 07, 2008, 02:48:43 pm »
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Quasar
The term quasar is created from “Quasi-stellar radio source” and refers to a very powerful and distant active galactic nucleus (see also AGN).
A quasar is a compact region around the supermassive black hole of a galaxy and is believed to be powered by the accretion of matter into the black hole.
Links: Wikipedia - Quasar
          Forum Thread - Quasars Introduction
          Forum Thread - Quasars


« Last Edit: October 07, 2008, 03:36:35 pm by waveney »
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Glossary R
« Reply #18 on: October 07, 2008, 02:49:04 pm »
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RA and DEC - Right Ascension and Declination
RA (right ascension) and DEC (declination) are to the sky what longitude and latitude are to the surface of the Earth. RA corresponds to east/west direction (longitude) while DEC measures north/south directions (latitude).
If you need to convert the RA & DEC values used by SDSS into the usual Hours/Minutes/Seconds values used elsewhere, see the Glossary entry “Conversion of RA & DEC”.
Links: What are RA and DEC?

Red Shift
Red shift is the systematic displacement of individual lines in the spectrum of an object towards the red end of the visible spectrum when the object is moving away from the observer (Doppler red shift). All galaxies, apart from some in the local group, show red shifts in their spectrum. This galactic red shift is proportional to their distance from the earth as a result of the general expansion of space-time (cosmological red shift).
Links: Wikipedia - Redshift
          Forum Thread - Redshift = Distance

Red Shift Calculator
Redshift z is essentially a velocity. It's the sum of the recession velocity due to the expansion of the universe and whatever local "peculiar velocity" a galaxy has; motion relative to its neighbours. Thus, if two galaxies are physically apart but moving relative to each other, their redshifts might give the impression that they're at the same distance [zookeeper Kevin].
If z=
   0.000, it's in this galaxy, and maybe out to .002 or so.
   0.050, medium distant, outside the local cluster.
   0.150, getting pretty far out there, far enough for that QSO to really be a quasar.
   0.400, way the heck out there.
   0.5 or 0.6, approaching the limits of the telescope. [suprtrkr].
Links: Wikipedia - Distance Graph to 0.5z
          Ned Wright’s Cosmology Calculator
          Distance Scale of the Universe
          Forum Thread - All about z’s

Ring Galaxy
A ring galaxy has a ring-like appearance. The ring contains luminous blue stars, but relatively little luminous matter is present in the central regions. It is believed that such a system was an ordinary galaxy that recently suffered a head-on collision with another galaxy.
Links: OOTD: Ringed and Pseudo Ringed Galaxies

« Last Edit: January 07, 2009, 03:02:33 pm by waveney »
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Glossary S
« Reply #19 on: October 07, 2008, 02:49:31 pm »
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SDSS - Sloan Digital Sky Survey
The Sloan Digital Sky Survey is the most comprehensive survey of the sky yet undertaken.
Link: SDSS at Chicago

Saber's Beads
Detached 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 Galaxies
Seyfert galaxies are lower-luminosity active galactic nuclei. See links under “AGN” for more info.
Link: Caltech Glossary

SIMBAD - Astronomical Database
The 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 Beginners
If 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.
Forum Links: History of spectra - absorption spectra
                     History of spectra - emission spectra

Spectra (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 Profiles


Spectral Lines
Dark 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.
Links: Wikipedia Spectral Line

Speed of Light
The 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.
Links: Wikipedia - Speed of Light

Starburst Galaxy
This 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 Galaxies

Star Cluster
See “Globular Cluster” and “Open Cluster”.

Stellar Population
Three 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 Population
Cross reference link to Metallicity
OOTD on Stellar populations

String Theory
String 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.
Links: Official String Theory Site
          Wikipedia - String Theory
          Stringing us along

Strong 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.
Links:
Berkeley - Gravitational Lensing
Wikipedia link on Strong Gravitational Lensing

Supernova
A 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.
Links: Wikipedia - Supernova
          NASA - Supernovae
          Forum Thread - Supernovae


« Last Edit: January 13, 2011, 02:48:13 pm by waveney »
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waveney

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« Reply #20 on: October 07, 2008, 02:49:51 pm »
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« Last Edit: October 07, 2008, 03:21:15 pm by waveney »
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« Reply #21 on: October 07, 2008, 02:50:18 pm »
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« Last Edit: October 07, 2008, 03:21:23 pm by waveney »
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« Reply #22 on: October 07, 2008, 02:50:40 pm »
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Variable star
A variable star changes in brightness over time as seen from Earth. They are classified as either:
an intrinsic variable where its luminosity actually changes due to the star growing larger then smaller;
or an extrinsic variable where the change in luminosity is caused by a companion that blocks its light every so often.
Links: Wikipedia - Variable Star
Forum Thread - Cataclysmic Variables


« Last Edit: October 07, 2008, 03:38:09 pm by waveney »
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« Reply #23 on: October 07, 2008, 02:50:57 pm »
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Weak Lensing
This is when a lens isn't strong enough to form multiple images or arcs, but the source can still appear distorted and stretched.
Links:
Berkeley - Gravitational Lensing
Wikipedia link on Gravitational Lensing 

White Dwarf
A white dwarf is a small or medium-sized star that has come to the end of its life and has shrunk to a much smaller size and become very dense. When our sun ends its life it will shrink to about the same size as the earth. A white dwarf is composed of degenerate matter, one of the densest forms of matter known, surpassed only by neutron stars.
Links: Wikipedia - White Dwarf
           NASA - White Dwarf

Wolf-Rayet star/galaxy
Wolf-Rayet stars are massive, exceptionally hot stars, usually over 20 solar masses. They lose mass rapidly via a strong stellar wind.
Wolf-Rayet galaxies are galaxies that contain a higher percentage of Wolf-Rayet stars than a normal galaxy.
See the Forum thread for further information.
Links: Forum Thread - Wolf-Rayet Galaxies


« Last Edit: October 07, 2008, 03:38:42 pm by waveney »
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« Reply #24 on: October 07, 2008, 02:51:19 pm »
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« Last Edit: October 07, 2008, 03:21:45 pm by waveney »
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« Reply #25 on: October 07, 2008, 02:51:39 pm »
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« Last Edit: October 07, 2008, 03:21:53 pm by waveney »
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« Reply #26 on: October 07, 2008, 02:52:00 pm »
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Zodiacal Light
Zodiacal light is a faint glow seen in the night sky when there is no moon or light pollution. It is produced by sunlight reflecting off dust particles which are in the interplanetary dust cloud.
The Queen guitarist, Brian May, published his doctoral thesis on zodiacal light.
Links: Wikipedia - Zodiacal Light


« Last Edit: October 30, 2008, 10:12:20 am by waveney »
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