Archive | December 2014

Disk Detective Glossary

Lily (voyager1682002) suggested that we put together a glossary of some of the jargon we use in Disk Detective.  Shigeru, TED91 and Pini2013 all contributed words and John Debes and John Wisniewski wrote up the definitions.  Nice work, everybody! If you think of any more words you want to see defined here, put them in the comments or send ’em in to

Marc Kuchner

ALADIN. ALADIN is an interactive sky atlas that allows users to visualize digitized astronomical images, and overlay data from astronomical catalogs (retrieved via SIMBAD or VIZIER).  ALADIN can be accessed both via a web-portal, as well as via a stand-alone java app.

ALLWISE. Processing the data from the WISE mission was a huge task, and the data was released to the public in several stages.  The most recent release is the ALLWISE data release. That’s the source of the WISE images you see here at Disk Detective.

Active Galactic Nuclei (AGN).  At the center of each galaxy is a supermassive black hole.  When one of these monster black holes starts accreting a lot of material, this process heats the material around the black hole, causing it to emit light across a broad range of wavelengths from the X-rays to radio, a phenomenon we call an AGN.  Because AGN can contain a lot of dust, they sometimes can mimic the dusty stars we’re searching for.  But we can often weed out these AGN using catalogs like SIMBAD or follow-up spectroscopy.

Blended object. This refers to any object observed in an image that is actually comprised of two or more distinct astronomical sources.  This can occur for images that have low spatial resolution, or in areas of the sky where many stars or galaxies reside together.  Here at Disk Detective we are trying to weed out blended objects.

Contamination/Contaminated object. A “contaminated object” in Disk Detective is simply an object that represents a mix of signals from more than one astrophysical source.  Since we do not have an easy way to isolate the contributions of each of the object contributing to a “contaminated object”, we do not consider such sources as viable candidate objects of interest.

Debris Disk.  After a star finishes collapsing onto the main sequence, a disk of rock and dust can sometimes linger around the star for hundreds of millions of years.  This kind of disk is called a debris disk.  One example of a debris disk is the asteroid belt around the Sun.  Most of the extrasolar planets that have been imaged to date orbit within debris disks. Disk Detective is primarily a search for Debris Disks and YSO disks.

Disk Detective Object of Interest (DDOI). When you and other users classify an object as none of the above/good candidate, the science team reviews it and if we agree that it’s OK, we add it to the list of Disk Detective Objects of Interest. These objects then get sorted and the most interesting ones get sent out for follow-up observing.

IR Excess. A star surrounded by a circumstellar disk will exhibit an excess of brightness in the infrared above that expected for a bare star, caused by the warm dust emitting thermal heat at these wavelengths.  A common way to identify disk systems in the sky is to search for evidence of this Infrared (IR) excess. Every object you see in Disk Detective has been preselected to have at least a small amount of infrared excess, specifically in the WISE 4 band.

IR Source.  An IR-source is simply an astrophysical object that exhibits a detectable signal at infrared wavelengths. When SIMBAD doesn’t reconize an object in as being a star or a galaxy or something else familiar, it sometimes calls it an IR-Source.

J-magnitude.  Magnitudes are a logarithmic scale that comes from when astronomers measured the brightness of a star with their own eyes.  The idea is that a star that looks 100 times fainter than another is considered to be 5 magnitudes fainter.  The “J” moniker refers to one of many different filters astronomers use to observe the Universe at different wavelengths.  In particular, the J-filter is centered on light redder than what the human eye can perceive, with a sensitivity that peaks roughly around 1.25 micron (or 12500 Angstrom) wavelengths of light.  The star Vega is commonly used as the reference point; by definition, Vega has a magnitude of zero in J band, so a star 5 magnitudes fainter than Vega would have a magnitude of J=5.  Currently, all the objects in Disk Detective have J magnitude < 14.5.  (I.e. they are no fainter than 1/631,000 as bright as Vega.)

noAO. This acronym stands for “no astronomical object found in SIMBAD”, and simply means that there is no cataloged astronomical source in the SIMBAD database at a specific set of coordinates.  This doesn’t mean that there is nothing at that position; rather, is a by-product of the fact that the SIMBAD database only catalogs stars down to a certain brightness and misses faint sources.  You’ll find that about half of the objects in Disk Detective are NoAOs.  That’s part of why we’re doing this search!  But when you come across an object that is a NoAO, you can often find more information about it by typing its coordinates into VizieR.

Parallax.  Since the Earth orbits around the Sun, we see nearby stars from a different point of view at different times of year.  The apparent shift in the star’s position that we measure is called the star’s parallax, an angle usually ranging from about 1 milliarcsecond to about 300 milliarcseconds.  Presently, the biggest and best catalogs of parallaxes come from from ESA’s Hipparcos mission. That’s where the parallaxes quoted in SIMBAD come from.  If you divide 1000 by the parallax (measured in milliarcseconds) you get the distance to the start measured in parsecs.  Just remember that Hipparcos was not about to make measurements more accurate than about 1 milliarcsecond,  So when the measured parallax is less than 1 milliarcsec, it’s better to simply consider the distance to be > 1000 parsecs because when you divide you’ll get nonsense.  ESA has launched a new mission called GAIA to measure more precise parallaxes for a larger sample of stars.

Pre-main sequence Star. A pre-main sequence star is an object which is in the process of becoming a star.  Stars are born from the collapse of a portion of a cold cloud of molecular gas.  Objects in the process of collapsing from a molecular cloud are called pre-main sequence stars.  Once the collapsing object has heated to a sufficient internal temperature, hydrogen fusion will begin in its core, and the object is thereafter formally classified as a “star” that resides on the “main sequence”.

Proper Motion. Proper motion is the apparent movement of a star on the sky once you have subtracted the effects of the Earth’s rotation and Earth’s orbit around the Sun (actually, orbit around the solar system’s center of mass, a point that is inside the Sun).  The motion arises because the sun and other stars all have different orbits around the Galaxy.  Of course, when a star is quite far away (say 1000 parsecs or more) it’s very hard to see the star move at all.  So the amount of proper motion a star has depends on how far away it is from Earth.  Sometimes we use proper motion as a kind of proxy for distance; when a source has high proper motion, say > 30 milliarcseconds per year, we expect it to be relatively close to the sun, roughly within 300 parsecs or so.  It’s better when you can measure the parallax, though.

Quasi Stellar Object (aka QSO or Quasar). This name refers to an object that resides external to the Milky Way but appears point-like in most telescopes, like a star.  These are usually galaxies where the central supermassive black hole is accreting a lot of gas and dust so it shines many times brighter than the underlying galaxy.  See also AGN.

Saturated. Just like your digital cameras and iPhones at home, astronomical digital cameras and detectors record signal across a grid of many pixels (light bucket receptacles).  If one tries to take a picture of too bright of a source, these receptacles will overflow and we refer to these pixels as being saturated.  One can see saturation in everyday life when you try to take a picture of an object in the day-time sky too close to the Sun (this will produce a vertical white stripe on your digital camera).  Saturated images are undesirable as one typically can not extract accurate measurements of a star’s brightness from a saturated image. This very bright (sixth magnitude) star is saturated in the DSS2 images and somewhat in the WISE 1 band as well.

Spectral Energy Distribution (SED).  This term refers to the amount of light an object emits as a function of frequency or wavelength.  If one were to measure how much light was emitted from an object across the full electromagnetic spectrum, on in principle could deduce the physical mechanisms involved in how that object shines.  For stars, the nuclear fusion in their cores is emitted as thermal radiation, which has a very characteristic shape across the various wavelengths of light (also known as black body radiation).  Dust also emits thermally, usually by absorbing a small fraction of light coming from the host star.  You can view an SED for each object in Disk Detective on its talk page.

Shifting. Images of objects in Disk Detective can sometimes appear to move around slightly from image to image.  This can occur from slight misregistrations between images or the fact that an object might have a large amount of yearly motion on the sky (DSS images were taken in the 1950s to the 1980s, 2MASS and SDSS images were taken in the late 90’s to early 2000’s, and WISE images were taken in 2010-2011).  Sometimes you’ll see a light shift at WISE 1 caused by saturation. Large shifts usually are caused by contamination. See Contamination.

SIMBAD. SIMBAD is a web-based astronomical database that enables one to search for stars by their names or coordinates, and retrieve a wealth of information such as the brightness (magnitude) in various filters, updated coordinates, the speed at which the star is moving (proper motion and radial velocity), the distance (parallax), and scientific papers that mention the star. SIMBAD stands for Set of Measurements, Identifications and Bibliography for Astronomical Data.

V-magnitude.  The letter V refers to one of many different filters astronomers use to observe the Universe at different wavelengths.  In particular, the V-filter is centered on light close to the peak of our own Sun’s spectrum near 0.55 micron (or 5500 Angstrom) wavelengths of light.  See J-magnitude for our attempt to define the term “magnitude” or see Wikipedia.

Variable Star.  A star is classified as a variable star if its brightness at any wavelength or band-pass changes as a function of time. If you could measure it precisely enough, you would find that every star is variable at some level.  What do you expect for a rotating ball of plasma undergoing nuclear fusion?  But realistically most surveys can not detect variations in brightness less than a few percent. So most of the time you would say a variable star is one that changes in brightness by more than a few percent from one observation to the next.  Often you would quote the variability in magnitudes.  One magnitude of variability represents a change of about a factor of 2.5.  That’s pretty big.  Imagine what your day would be like if the Sun changed in brightness by that much.  But this level of variability and more is common among young stars and among giant stars, for example.  If a star’s name stars with two capital letters, like RR Tau, then it’s known to be variable star.  (Though there are plenty of variable stars that have escaped this designation.)

VIZIER. VIZIER is a web-based astronomical database that enables one to query over 13,000 astronomical data catalogs from refereed literature by astronomical position or target name.  The type of data which can be accessed via these catalogs include the brightness of the object at a wide range of wavelengths (x-ray to infrared), amongst other properties.

WISE. The Wide-field Infrared Survey Explorer (WISE) is a NASA infrared space-based telescope that was launched in 2009 to perform an all-sky survey at 4 infrared band-passes, 3.4 microns, 4.6 microns, 12 microns, and 22 microns.  The Disk Detective project is mining the WISE catalog to detect new protoplanetary and debris disks.  The WISE mission has a website at NASA and a website at Berkeley.

Young Stellar Object (YSO).  This is shorthand for any star that appears to be in the process of forming.  You can think of these as baby stars that usually aren’t too far from a molecular cloud, which is where new stars form.  They can have jets, large disks, and can have variable luminosity as they accrete gas and dust on their surfaces.  YSOs can be  classified according to their Spectral Energy Distributions into Class 0, Class I, Class II and Class III objects. Also included in this group are transitional disks and “pre-transitional” disks and other kinds of interesting critters. See also Pre-Main Sequence Object.