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Glossary

Absolute Magnitude M: The apparent magnitude the star would have if it were placed at a distance of 10 parsecs from Earth, where the difference in absolute and apparant magnitude m-M = 5 log(R[pc]/10[pc]) is called the distance modulus.

Active Galactic Nuclei (AGN): Galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core may be highly variable and very bright (much brighter than normal galaxies) compared to the rest of the galaxy. Models of active galaxies concentrate on the possibility of a supermassive black hole which lies at the center of the galaxy. The dense central galaxy provides material which accretes onto the black hole releasing a large amount of gravitational energy. There are several types of active galaxies: Seyferts (jet not at all pointed toward us), Quasars (jet pointed towards us), and Blazars (jet pointed directly towards us).

Alpha Process: ¹²C(α,γ)¹⁶O

Apparant Magnitude m: A number that tells how bright that star appears at its distance from Earth. The scale is "backwards" and logarithmic, i.e. larger magnitudes correspond to fainter stars.<> On this magnitude scale, a brightness<> (the flux of light<> comming towards us in W m-²) ratio of 100 is set to correspond exactly to a magnitude difference of 5. As magnitude is a logarithmic scale, one can always transform a brightness ratio B2/B1 into the equivalent magnitude difference m2-m1 by the formula m1/m2 = - 2.50 log(B2/B1).

Alpha Rich Freez-Out: xxx

Atomic Number Z = # protons

Atomic Mass Number (or Nucleon Number) A = # protons + # neutrons

Broadening, Line Broadening: A widening of the absoprtion and emission lines in a spectrum due to any of several factors. These include Doppler broadening, caused by movements within the emitting gas, pressure broadening (or Stark effect), due to collisions between atoms and molecules, and the Zeeman effect, due to a strong magnetic field.

Brown Dwarf: Object less massive than 0.08 Msol (the least massive stars), are supported by electron-degeneracy and become black dwarfs. Jupiter is a low mass (~ 0.001 Msol) object of this class. Although they never burn H, they may burn light elements such as D and Li, and during this period they are known as brown dwarfs.

Carbon: Carbon can be produced in stars of all masses (essentially by helium burning). Reliable carbon measurements in QALs are very rare.

-> Thuan: Carbon is produced by both intermediate (3 M_sun≤M≤8 M_sun) and high-mass (M≥9 M_sun) stars. Since C is a product of hydrostatic burning, the contributions of SNe Ia and SNe II are small. Therefore, the C/O abundance ratio is sensitive to the particular star formation history of the galaxy. It is expected that, in the earliest stages of galaxy evolution, when metallicity is still very low, carbon is mainly produced by massive stars, so that the C/O abundance ratio is independent of the oxygen abundance, as both C and O are primary elements. At later stages, at slightly higher metallicities, intermediate-mass stars add their carbon production, so that an increase in the C/O ratio is expected with increasing oxygen abundance. Earlier studies did not conform to these expectations. Garnett et al. (1995) found a continuous increase of log C/O with increasing log O/H in their sample of metal-deficient galaxies, a relationship which could be fitted by a power law with slope 0.43. Subsequent HST FOS observations of I Zw 18 (Garnett et al. 1997) have complicated the situation even more. It was found that I Zw 18 bucks the trend shown by the other low-metallicity objects. Although it has the lowest metallicity known, it shows a rather high log C/O, significantly higher than those predicted by massive stellar nucleosynthesis theory. This led Garnett et al. (1997) to conclude that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass stars and, hence, despite its very low metallicity, I Zw 18 is not a ``primeval'' galaxy. We have reanalyzed the data for I Zw 18. The use of new high signal-to-noise ratio MMT spectroscopic observations of I Zw 18 yields a much higher electron temperature (by ∼2000 K) within the FOS aperture and in a much lower C/O abundance ratio. Figure 11a shows log C/O against 12 + log O/H for the BCDs in the Izotov & Thuan (1998c) sample. It is clear that, in contrast to previous results, log C/O is constant in the extremely low-metallicity range, when 12 + log O/H varies between 7.1 and 7.6, as expected from the common origin of carbon and oxygen in massive stars. Furthermore, the dispersion of the points about the mean is very small: < log C/O > = -0.78±0.03. This mean value is in very good agreement with that of ∼-0.8 predicted by massive stellar nucleosynthesis theory (Woosley & Weaver 1995). Two models with Z = 0 and Z = 0.01 Z_sun are shown by horizontal lines in Fig. 11a. They are in good agreement with the observations. At higher metallicities (12 + log O/H > 7.6), there is an increase in log C/O with log O/H and also more scatter at a given O/H, which is attributed to the carbon contribution of intermediate-mass stars in addition to that of massive stars

Charbon measurements in QAL systems:

Erni et al. (2005, in prep.): The damped Ly alpha system toward Q0913+072: Possible Constraints on the First Stars in the Universe
D'Odorico & Molaro (2004): Hints of star fomation at z>6: the chemical abundances of the DLA system in the QSO BRI 1202-0725 (z_abs = 4.383)
Ledoux et al. (1998): On the kinematics of damped Lyman-alpha system (Q0913+072)
Levshakov et al. (2003): Extremely metal-poor Lyman limit system at mathsf zabs = 2.917 toward the quasar HE 0940-1050

Carbon Fusion Cycle: The main theme of the carbon cycle is the adding of protons, but after a carbon-12 nucleus fuses with a proton to form nitrogen-13, one of the protons decays with the emission of a positron and a neutrino to form carbon -13. Two more proton captures produce nitrogen-14 and then oxygen-15. Another neutron decay leaves nitrogen-15. Another proton capture produces oxygen-15 which emits an energetic alpha particle to return to carbon-12 to repeat the cycle. This last reaction is the main source of energy in the cycle for the fueling of the star.

Chemical Abundances: Some facts from observation

Observation show, that [C/Fe] ~ 0.0 (with large scatter at all metallicities) for halo and (!) disks stars in our Milky Way, i.e. C and Fe show identical behaviours, although they do not share the same nucleosynthesis origin. Observations show too, that [N/Fe] ~ 0.0 (with large scatter at low metallicites). (Goswami & Prantzos 2000)
Chemical Abundances from Stars: Generally, the convective atmosphere of a cool star is a good tracer of the chemical composition of the interstellar gas at the time and place of its formation. However, in giant stars material from deeper layerz may be dreged to the surface and thereby alter the initial composition (Gratton et al. 2000). So, one must carefully check if such mixing has occured.
On the Cosmic Origins of Carbon and Nitrogen (Henry et al. 2000)k

CNO-Cycle: A nuclear-fusion-reaction sequence in which hydrogen nuclei are combined to form helium nuclei, and in which other nuclei, such as isotopes of carbon, oxygen, and nitrogen, appear as catalysts or by-products. The CNO cycle is dominant in the cores of stars on the upper main sequence. A nuclear-fusion-reaction sequence in which hydrogen nuclei are combined to form helium nuclei, and in which other nuclei, such as isotopes of carbon, oxygen, and nitrogen, appear as catalysts or by-products. The CNO cycle is dominant in the cores of stars on the upper main sequence.

=> N will be produced at the expense of C (and O).

Collapsar: A rotating massive star whose core collapses to a BH and produces an accretion disk.

Colour Index: The colour index is defined to be the difference between the magnitudes at two different wavelengths. The convention is to subtract the longer wavelength magnitude from the short wavelength magnitude, i.e. colour index = (short-wavelength magnitude) - (long-wavelength magnitude). A red (blue, white) star will have a colour index that is > 0.00 (< 0.00, =0.00). For example, the bright star Rigel has a colour index of -0.03, meaning it is just slightly bluer than white. The sun has a colour index of +0.63, in keeping with its yellowish appearance. The bright red star Betelgeuse has a colour index of +1.85. There are many different systems of magnitudes and colours in use in astronomy, the most commonly used is the UBV-System. Column Density N(X): The number of atoms, ions or molecules of the species X found per cm² along the line of sight.

Cosmic Dawn: At z=1000 the Universe has colled down to 3000 K and the hydrogen becomes neutral (recombination). Then, at approx. z<20 the first stars (Pop III stars) form and these gradually photo-ionize the hydrogen in the IGM (reionization). These epoch, witnessing the return of light in the Universe after the Big Bang, is usually dubbed as Cosmic Dawn. At z<6, galaxies form most of their stars and grow by merging. Finally at z<1, the massive clusters are assembled. (see also feedback types).

Damped Lyman alpha (DLA) system: See Lyman Absorption

Dark Matter (DM):

Deflagration: A process of subsonic combustion that usually propagates through thermal conductivity (hot burning material heats next layer of cold material and ignites it). Deflagration is different from detonation.

Detonation: A process of supersonic combustion that involves a shock wave and a reaction zone behind it. The shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration that propagates with a subsonic speed and without a shock. Detonations generate high pressures and are usually much more destructive than deflagrations.

Doppler Broadening: The broadening of spectral lines caused by the thermal, turbulent, or mass motions of atoms along the line of sight.

Dust depletion: Si is easily locked onto dust grains. Molecular Hydrogen form on dust grains. …

Echelle Spectrum: In "traditional" spectrographs a dispersing element - typically a diffraction grating or prism - is used to produce the spectrum. This results in a single spectrum which can be imaged using a CCD or other type of camera. The data can then be extracted using a suitable program. The recordable part of the wavelength range covered by this type of spectrograph is limited by the size of available image sensors, i.e., CCDs. A quick inspection of a CCD image of such a spectrum will also reveal that much of the detector area away from the spectrum itself is unused. One method of optimising the use of the available detector area is to use an échelle spectrograph. An échelle is a diffraction grating in which the rulings are much further apart than usual. This leads to spectra of very high dispersion, but only over a short wavelength range in each order. As well as being `short', the high orders will overlap. To overcome this effect a cross-dispersing element is used to produce an order separation. The figure below shows a small part of such an échellogram recorded with a CCD camera. You can see a short part of three orders which run from the top to the bottom of the image at a slight angle. In the order to the right you can see a couple of absorption features. Several cosmic-ray events (bright spots) are also visible. Echelle spectrographs for astronomy are designed so that the wavelength coverage in one order will overlap the coverage of the adjacent orders. (That is at least for the middle orders in the full échellogram - there may be some gaps at the extremes of the image.) Using a suitable detector-usually a CCD - these spectral orders can be recorded.

Eddington Limit: One of the representative astrophysical application of the Thomson scattering. The Eddington Limit is the possibly maximum luminosity of a star, which can keep electrons from being blown out of the sphere. Ledd = 1.2×1e38 (M/Msol) erg s-¹.

Electromagnetic Spectrum:

Band	Wavelength [Å]	Energy [eV]	Blackboy Temp. [K]
Gamma-Ray	< 0.1	> 1e5	3e8
X-Ray	0.1 - 10	1e3 - 1e5	3e6 - 3e8
Ultraviolet	10 - 4000	1e3 - 3	7300 - 3e6
Visible	4000 - 7000	2-3	4100 - 7300
Infrared	7000 - 1e6	0.01 - 2	30 - 4100
Microwave	1e6 - 1e9	1e-5 - 0.01	0.03 - 30
Radio	> 1e9	< 1e-5	< 0.03

Electron Degeneracy: A stellar application of the Pauli Exclusion Principle (as is the neutron degeneracy). No two electrons (neutrons) can occupy identical states, even under the pressure of a collapsing star of several solar masses. For M < 1.44 Msol, the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star, so the collapse is halted by electron degeneracy to form a WD. This maximum mass for a WD is called the Chandrasekhar limit. As the star contracts, all the lowest electron (neutron) energy levels are filled and the electrons (neutrons) are forced into higher and higher energy levels, filling the lowest unoccupied energy levels. This creates an effective pressure which prevents further gravitational collapse. However, for masses greater than 2-3 Msol, even neutron degeneracy can't prevent further collapse and it continues toward the BH state.

Extremely Metal Poor Stars: See XMPs

Feedback Types: which are fundamentally shaping the Universe at cosmic dawn:

Stellar Feedback: Star formation (i.e. destruction of cold gas) can hinder, temporarily block or bring starformation (or even galaxy formation) to a halt.
Chemical Feedback: Massive stars pollute the ambient gas with metals what causes the star formation mode to be shifted toward low mass stars (self-killing process).
Radiative Feedback: Stars are sources of UV radiation and ionizing photons. Thus, stars conteract (or erase) the necassary condition for star formation which in return is necassary to maintain the reionization.

First Stars: Often believed to be very massive stars (VMSs). Recent work show, that the first stars might not have ben VMSs (Tumlinson 2004). Many uncertainties remain about the cosmological context and feedback effects (duration of the metal-free phase, IMF, etc.). Despite these unknowns, theoretical progress on the first stars has arrived at six point of consensus (Tumlinson 2004):

The first stars form from metal-fee (i.e. primordial) gas in the first collapsed DM halos around z ~ 20.
The first stars may be very massive (M > 100 Msol) because they are resticted during their formation to inefficent cooling by molecular hydrogen belo 1e4 K.
These very massive first stars seed their own halos and possibly enrich nearby ones by releasing metals from pair-instability SNe (PISNe).
At critical metallicity Z > ~ 1e-3.5 Zsol, protostellar clouds are able to cool and fragment more efficiently, leading to a "normal" IMF. Where this metallicity has not been reached, the IMF retains its unusual properties. Halo and IGM enrichment probably occur inhomogeneously in time and space during the transition to "normal" star formation.
The first stars begin and may complete the reionization of intergalacti HI and HeII. The total first-star contribution to the global ionizing photon budget is unknown, but the efficient ionization of VMSs is thought to be required. There may be partial recombination during the transition phase in metallicity, followed by a second reionization near zr = 6.
The first stars epoch ends when all star-forming regions have achieved the critical metallicity. This is thought to occur before the final stages of HI reionization at zr = 6.2.

Stellar evolution and SN models (incl. yield tables) are given by Heger & Woosely (2002): The Nucleosynthetic Signature of Population III.

Fraunhofer lines: Absorption lines in the spectrum of the Sun, or of another star, first studied and named by Joseph von Fraunhofer in 1814. The nine most prominent he labeled with capital letters A to K, starting at the red end. The A and B bands are now known to be caused by absorption in Earth's atmosphere, while the rest are due to absorption in the Sun's photosphere. C and F are now better known as H-alpha and H-beta; the D lines are of sodium, the H and K lines of calcium, and the G band by neutral iron and the CH molecule. All these features occur generally in stars of spectral types F, G, and K.

A Selection of Fraunhofer lines:

Lines	Due To	Wavelengths Å]
A band	O₂ (molecular oxygen in Earth's atmosphere)	7594 - 7621
B band	O₂ (molecular oxygen in Earth's atmosphere)	6867 - 6884
C (H-alpha)	H (hydrogen)	6563
a band	O₂ (molecular oxygen in Earth's atmosphere)	6276 - 6287
D1 & D2	Na (sodium)	5896 & 5890
E	Fe (iron)	5270
b1, b2, b3, b4	Mg (magnesium)	5184, 5173, 5169, 5167
c	Fe (iron)	4958
F (H-beta)	H (hydrogen)	4861
d	Fe (iron)	4668
e	Fe (iron)	4384
f	H (hydrogen)	4340
G	Fe	4308
g	Ca	4227
h	(H-delta) H (hydrogen)	4102
H & K	Ca (calcium)	3968 & 3934

Globular Cluster (GC): Compact, dense, spherical clusters of very old stards, typically containing 1e4-6 stars within a diameter of about 50 pc. In our Milky Way, around two-thirds of GCs belong to the stellar halo and on-third to the disc. While GCs are easy to recognise, they account for only about 1% of all stars in the stellar halo. The distribution of GCs in the Milky Way gives a good indication of the shape of the stellar halo and how fundamentally it differs from the disc.

Gravitational Collapse: The source of the energy for star formation is gravitational collapse - this collapse must provide enough energy to heat the gas of the protostar to the ignition point of hydrogen fusion, some 15 million Kelvins. Knowledge of the mass and distribution of the gas cloud permits some fairly detailed modeling, because half of the energy from gravitational collapse goes into kinetic energy according to the virial theorem.

Gunn-Peterson (GP) Effect: After the recombination epoch, the universe was full of a thick gas of hydrogen atoms. Hydrogen atoms absorb ultraviolet light well, so any light traveling through early the universe was quickly absorbed by a hydrogen atom, this epoch is called the Dark Ages. Over time, the gas clumped together to form the first stars, which began to emit light an thus - but this light too was quickly absorbed. Eventually, the stars became bright enough that their light had enough energy to break the hydrogen atoms into protons and electrons. After this happened, light could pass freely through the universe (reionization epoch). Gunn and Peterson predicted that one should see a "trough" in the ultraviolet part of an high-redshifted object's spectrum. Becker et al. discovered in 2001 a quasar which spectrum was truncated below the Lyman limit, i.e. there was no Lya forest to see but one large trough. See e.g. GPE1 or GPE2.

Heavy Elements: The most important heavy elements are C, N, O, Mg, Si and Fe.

Hyper Metal-Poor Stars (HMP): See also metal-poor stars. The most metal poor star know to date is the halo star HE1327-2326 () with [Fe/H] = -5.6 only, i.e. contains alomost 400'000-times less iron than our sun.

Hypernovae: Hyper-energetic (E > 1e52 ergs) Supernovae. Explosion caused bey core-collaps (approx. Mms < 25Msol). Hypernova explosions are expected to produce larger amounts of 56Ni (=>56Co=>56Fe) than ordinary SNe II (Umeda et al. 2002).
N.B.: Pair-intability SNe (Mms ~ 130-300Msol) explode in the regime of E~1e52 ergs too but are usually not classified as Hypernovae.

HI Regions: Clouds of neutral hydrogen in interstellar space.

HII Regions: Clouds of iniozed hydrogen gas in a galaxy, known to be regions of heavy starbirth.

Iron: ....
-> Thuan: The iron abundance in BCDs was first discussed by Thuan et al. (1995). Their small sample of 7 galaxies has been considerably increased (38 BCDs) by Izotov & Thuan (1998c) who found that oxygen in these galaxies is overproduced relative to iron, as compared to the Sun: [O/Fe] = log (Fe/O)_sun - log (Fe/O) = 0.40±0.14 (Fig. 11c). This value is in very good agreement with the [O/Fe] observed for Galactic halo stars, implying that the origin of iron in low-metallicity BCGs and in the Galaxy prior the formation of halo stars is similar, and supporting the scenario of an early chemical enrichment of the galactic halo by massive stars.

Iron-Peak Elements: Elements with atomicnumber near Fe (for example Sc, V, Cr, Mn, Co, Ni, Cu, Zn)

Jean's Mass: A collapsing gas cloud will cause ratiation pressure which opposes the collaps. A gas cloud will collaps under its own gravity if its mass is above the Jean's Mass.

James Webb Space Telescope (JWST): The James Webb Space Telescope is a large, infrared-optimized space telescope scheduled for launch no earlier than June 2013. JWST is designed to study the earliest galaxies and some of the first stars formed after the Big Bang. These early objects have a high redshift from our vantage-point, meaning that the best observations for these objects are available in the infrared. JWST's instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range.

Life and Death of different Stars: (still to be completed ...)

Main Sequence

Late Stage

Final Stage

Remnant

Mass [Msol]

Metallicity [Zsol]

Identifyer

Properties

Total Lifetime [Gyr]

Mass [Msol]

Metallicity [Zsol]

Identifyer

Properties

Mass [Msol]

Metallicity [Zsol]

Identifyer

Properties

0.08-0.26

pp cycle, fully convective

no He burning

0.26-1.5

pp cycle, radiative core, konvective hull

He flash, He core burning

hull pushed of and becomes a PN

mass < 1.44

X=0.70
Y=0.28
Z=0.02

Sun

pp cycle
radiative core, konvective hull

He flash, He core burning

hull pushed of and becomes a PN

1.5-8

CNO cycle, convective core, radiative hull

He core burning without He-flash

typical for 5 Msol

AGB

He shell burning, C-flash => SN

none (?)

8-30

CNO cycle, convective core, radiative hull

He core burning without He-flash

AGB

He shell burning, central C burning. After O and Si burning: collaps => SN

NS
(M<2)

or

BH
(M>2)

WD+accreted mass = MChsol) (=1.44M

binary system
=> SNeIa

explosive C and He burning, E~1e51 ergs

none (?)

WD with CO-core

8 - 70

SN 2,3)

core-collaps SN

NS or BH

40-130

SN 2,3)

core-collaps SN, complete disruption

none (?)

130-300

primordial

VMS

inefficient cooling by H2 only

PISN

none 1)

~ 600

primordial

extremely massive stars

might directly collaps into a BH

-----------------------> Franchesca's book pg 83/84
1) Eventually BH for very high-mass progenitors (~ 300Msol)
2) All types of Supernovae except Type Ia (i.e. Ib/IIb, Ic, II-L and II-P)
3) If the explosion energy is greater thatn ~ 1e51 ergs, the Supernova will be called Hypernova

Lookback-Time: xx

Lorentz Force: Both, the electric field E and the magnetic field B can be defined from the Lorentz Force Law: F = qE + qv × B

Lyman Alpha Absorption & Emission:

Lyman-Serie: n=1 -> n=2, 3, 4, ...
Balmer-Serie: n=2 -> n=3, 4, 5, ...
Paschen-Serie: n=3 -> n=4, 5, 6, ...
Bracket-Serie: n=4 -> n=5, 6, 7, ...

Energy Levels: En = -13.6 eV / n²
Absorption = CST × Cross-Section × Density
Cross-Section sigma = 6.3e-18 × (Ephot/13.6eV)-³ cm²
Note: lambdaabs = lambdalab (1 + zabs)

Transition ni -> nj		Eij = Ej - Ei [eV]	lambda = hc/Eij [Å]
n1 -> n2	Lyman α	10.2	1216
n1 -> n3	Lyman β	12.08	1026
n1 -> n4	Lyman γ	12.75	972
n1 -> ninfinity	Lyman Limit	13.6	912

Different Lya systems:

optically thin

Lya forest: N(HI) < 1e17.2 cm-²

optically thick

Lyman Limit System (LLS): 1e17.2 cm¯² < N(HI) < 1e19 cm¯²
sub-DLA: 1e19 cm¯² < N(HI) < 1e20 cm¯²
Damped Lya System (DLA): N(HI) > 1e20 cm¯²

Lyman Alpha Forest Absorbing System: Quasar Absorption Line (QAL) system with a column densitit N(HI) > 1e15 cm¯², no metal lines. Information about the Lyman alpha forest and many more related topics can be found here.

Lyman Break Galaxies (LBGs): The regime beyond z ~ 1 belonged longtime only to luminous active galaxies (AGNs). However, broad-band colours can be used to select young star-forming Galaxies at high redshifts, so called Lyman Break Galaxies. At z ~ 3, the Lyman Break (at 912Å) has moved to the center of the U band, and will produce galaxies that are red in B-U. Different to other Galaxies which appear also red in B-U, Lyman Break Galaxies will disappear when observed through an ultraviolat filter (measuring the part of the spectrum below the Lyman break). This is because most of the spctrum below the Lyman Break in Lyman Break Galaxies has been truncated by intergalactic hydrogen clouds (see also Syano

An active star-forming galaxy will have a flat spectrum down to the Lyman limit at (Lyman Break), beyon which most of the spectrum will be truncated by intervening absorptoin du to intergalactic hydrogen clouds. High-z Galaxies showing star-forming activity, so called Lyman Break Galaxiesk, will therefore appear read in U-B but

Steidel & Hamilton (1992): technique which revolutionized the field, finding first galaxies at z>3 (spectroscopically confirmed by Steidel et al. 1995). Based on imaging below Ly break (912A), above Lya (1216) and with intermediate filter.

Lyman Limit System (LLS): See Lyman Absorption

Lyman Serie: See Lyman Absorption

Main-Sequence (MS) Star: Once a protostar starts burning hydrogen in its core, it quickly passes through the T-Tauri stage (in a few million years) and becomes a main sequence star where its total mass determines all its structural properties. The three divisions in a stellar interior are the nuclear burning core, convective zone and radiative zone. Energy, in the form of gamma-rays, is generated solely in the nuclear burning core. Energy is transfered towards the surface either in a radiative manner or convection depending on which is more efficient at the temperatures, densities and opacities.

Properties of Main-Sequence Stars:

Mass [Msol]	Spectral Type	MV	log Lbol [Lsol]	log Teff	tMS
60	O5	-5.7	5.90	4.65	3.4×10e6
40	O6	-5.5	5.62	4.61	4.3×10e6
20	O9	-4.5	4.99	4.52	8.1×10e6
18	B0	-4.0	4.72	4.49	1.2×10e7
10	B2	-2.4	3.76	4.34	2.6×10e7
8	B3	-1.6	3.28	4.27	3.3×10e7
6	B5	-1.2	2.92	4.19	6.1×10e7
4	B8	-0.2	2.26	4.08	1.6×10e8
2	A5	1.9	1.15	3.91	1.1×10e9
1.5	F2	3.6	0.46	3.84	2.7×10e9
1	G2	4.7	0.14	3.77	1.0×10e10
0.8	K0	6.5	-0.55	3.66	2.5×10e10
0.6	K7	8.6	-1.10	3.59	...
0.4	M2	10.5	-1.78	3.54	...
0.2	M5	12.2	-2.05	3.52	...
0.1	M7	14.6	-2.60	3.46	...

Source: "The Formation of Stars", Stahler & Palla

Mass-Cut: Used in model calculations of explosive nucleosynthesis (i.e. Supernovae), indicating the zone between the ejecta and the remnant. The mass-cut ist typically located somewhere close to the border of complete and incomplete Si-burning regions. Therefore, the deeper mass-cut leads to lager Co/Mn. The upper bounds of the complete Si-burning region is defined by the Massfraction X(56Ni)=1e-2 and of the incomplete Si-burning region by X(28Si)=1e-4.

Massive Stars: Massive Stars are responsible for the creation of most of the heavy elements (exceptions: iron-peak elements and s-process) and in particular oxygen, the dominant element in the global metallicity Z. Although different model calculations are in surprisingly good agreement, one has to be aware of many uncertainties like the treatement of convection, fall-back, mass loss, the rxn rate of ¹²C(α,γ )¹⁶O and the explosion mechanism (prompt or delayed).

Metal-Line Absorbing System: Quasar Absorption Line (QAL) system with a column densitit N(HI) > 1e15 cm¯², associated with metal lines.

Metal-Poor Stars: Observation support the popular assumption for model calculation that [Zn/Fe] ~ 0 in the range of [Fe/H] ~ -3 to 0. But, positive [Zn/Fe] values might arise below [Fe/H] ~ -3. This could be due to Hypernovae which are thought to produce positive mounts of [Zn/Fe] (Umeda et al. 2002). We distinguish:

Description	Abbreviation	metallicity [Z/H] = [Fe/H]
Extremely Metal-Poor Stars	XMP	-4.0 ... -2.7
Ultra Metal-Poor Stars	UMP	-5.0 ... -4.0
Hyper Metal-Poor Stars	HMP	below -5.0

Metallicity: Various notations exsit to quantify metallicity. In Astronomy, all elements heavier than helium are called "metals".

Z - The fractional abundance (by mass) of metals in a gas. In other words: Z := (mass of elements heavier than He in the object)/(total mass of all elements in the object). The metallicity of the Sun is Z=0.02, i.e. 2% of the Sun's mass comes from elements heavier than He.
X,Y,Z - Denote the fraction by mass of hydrogen (X), helium (Y) and all elements heavier than helium (Z) where Xsol = 0.70, Ysol = 0.28 and Zsol = 0.02, respecively.

Most commonly, abundances relative to solar:
[Fe/H]: Less restictively, the metallicty is often given by [Fe/H], where [A/B]:=log(N(A)/N(B))-log(N(Asol)/N(Bsol)).
[Mg/H]: Indication the metallicity of a star by [Fe/H] might not be appropriate when considering SN nucleosynthesis because the yields of Fe from SNe models to date do not converge due to uncertainties in the explosion mechanism, fallback dynamics and mass-cut. [Mg/H] might be used instead of [Fe/H] to specify the metallicity because (1) Mg is less affected by the mass-cu in SN models, (2) Mg is not synthesized or broken by the SN shock, (3) the mass of ejected Mg increases with increasing progenitor mass, and (4) the abundance of Mg is observationally known for many stars with [Fe/H] < -2.5.

Neutron Degeneracy: See Electron Degeneracy

Nitrogen: In contrast to O and C, however, the initial formation of N is still not well understood.

-> Thuan: The basic nucleosynthesis process is well understood - nitrogen results from CNO processing of oxygen and carbon during hydrogen burning - however the nature of the stars mainly responsible for the production of nitrogen remains uncertain. If oxygen and carbon are produced not in previous generation stars, but in the same stars prior to the CNO cycle, then the amount of nitrogen produced is independent of the initial heavy element abundance of the star, and its synthesis is said to be primary. On the other hand, if the ``seed'' oxygen and carbon are produced in previous generation stars and incorporated into a star at its formation and a constant mass fraction is processed, then the amount of nitrogen produced is proportional to the initial heavy element abundance, and the nitrogen synthesis is said to be secondary. In this case, the N/O ratio should increase linearly with the O abundance. This behavior is seen in high-metallicity H II regions in spiral galaxies with 12 + log O/H ≥ 8.4. As all of the BCDs discussed here are less metal-rich, only primary N concerns us here.

Figure 11b shows the behavior of the N/O abundance ratio as a function of the O/H ratio. Two remarkable facts can be seen. First, at low abundances (12 + log O/H ≤ 7.6), log N/O is constant (∼ -1.60) and with an extremely small scatter (±0.02 dex) at a given O abundance, implying that nitrogen is produced as a primary element by massive but not by intermediate-mass stars as commonly thought (Thuan et al. 1995; Izotov & Thuan 1998c). N production by intermediate-mass stars would introduce a time-delay as large as 5·10⁸ yr with respect to the primary production of oxygen by massive stars, which would introduce a larger than observed scatter in N/O. Second, the value of log N/O increases above ∼ -1.60 along with the scatter at a given O abundance in BCDs with 7.6 < 12 + log O/H < 8.2. This increase in log N/O and its larger scatter is interpreted as due to the additional contribution of primary nitrogen produced by intermediate-mass stars, on top of the primary nitrogen produced by massive stars.

N in QLAs:

Molaro (2003): Nitrogen Abundances in High-z DLAs

N Production, Primary: Forms from seed nuclei which are in situ (i.e. within the star itself) produced from He. Primary production does not require pre-existing seed nuclei and is therefore independent of the stars metallicity. The major producers of primary N are thought to be intermediate mass stars (during the third dredge-up phase). At present it cannot be decided whether primary N is produced primarily in massive stars (mixing between the helium-burning layer can induce primary N formation, but the yield may be very variable), which will expose as SNe II, or in intermediate-mass stars (~ 4-8Msol) during the ABG phase, or both (Spite et al. 2005). The [N/O] ratio should remain constant for increasing metallicity.

N Production, Secondary: Forms from pre-existing seed nuclei (carbon) and is therefore obviously dependent on the stars metallicity. The mayor producers of secondary N are all stars (with not too small masses) which reach the core temperature to initiate the CNO cycle. The yield is expected to be proportional to the stars initial metallicity. The [N/O] ratio should increase for increasing metallicity.

Nucleosynthesis:

Sites of production:

1H: Created during the BB and only destroyed afterwards
D, 3He, 4He, 7Li: Produced during the BB (although He and 7Li are also partly produced in stars)
D: Only destroyed after the BB
3He: Mainly destroyed but also produced in stars in the mass rang of 1-2 M_sun.
4He: Produced in stars with masses between 1-100 M_sun. Its production strongly depends on the mass loss.
7Li: Probably produced in AGB stars (5-8 M_sun) and perhaps in novae and SNe II. A fraction of 7Li is produced by spallation but the exact amount is still uncertain.
6Li, 9Be, 10B, 11B: Produced by spallation of cosmic rays on atoms of the ISM.
12C: Produced during the quiescent He-burning in high and intermediate mass stars.
13C: Produced during the quienscent and explosive H-burning, both in single and binary intermediate mass stars (novae). 13C can be a partially primary element as a result of the AGB phase of intermediate mass stars.
14N: Produced during the quiescent H-burning (cold CNO-cycle) in low and intermediate mass stars. 14N can also be a primary element like 13C.
15N: Produced in explosive H-burning occuring in SNe and novae.
16O: Produced during the He-burining in massive stars.
17O: Produces from destruction of 14N occuring in stellar regions suffering He-burning. Restored into the ISM from SNe II. Produced also in quiescent and explosive H-burning.
20Ne, 24Mg: Produce during the C-burning in massive stars and the C-deflagration in CO-WDs (only a very small amount). Mg is produced also during the quiescent and explosive Ne-burning.
28Si, 32S: Produced during the O-burning in massive stars (quiescent and explosive) and C-deflagration in CO-WD.
56Fe: Produced during the Si-burning (quiescent and explosive) in massive stars and C-deflagration in CO-WDs (substantial contribution).
s-process elements: Produced during the He-burning in massive stars (A < 90) and He-shell flashes in low mass stars (A > 90).
r-process elements: Produced during the explosive He, Co, O or Si burning in SNe II or in the very rich neutron matter originating from NSs.

Elements	# Protons	Production Site
H	1	Big Bang
He	2	Big Bang & stars
C	6	low- & high-mass stars
N	7	low- & intermediate-mass stars
O	8	low- & high-mass stars
Ne - Fe	10 - 26	hig-mass stars
Co - Bi	27 - 83	s- and r-process, AGB & SNe
Po - U	84 - 92	r-process in SNe

Oxygen: There is general consensus that oxygen is almost entirely contributed by masive SNe II. The oxygen-production from low- and indermediate-mass stars (primarlily during the central hydrogen burning with some contribution from neon burnig) is pratically irrelevant.

p-process: Was believed to be a proton capture process which occurs during supernovae explosions. Then it was realized that the proton densities are too small and the observed isotopic abundances could not be explained by this kind of reaction. Today its believed that photodisintegration reactions are responsible for the production of the proton-rich p-isotopes with masses A>100. The black body radiation produces a photon bath that can disintegrate the seed nuclei from the s-process and r-process. This is the reason why the p-process is sometimes called gamma-process.

The p-process contribution to isotopic abundances of elements that can also be produced in the s-process or r-process is usually very small. However there are p-only isotopes that cannot be produced in the s- or r-process (e.g. 190Pt or 168Yb). These isotopes have very small abundances compared to neighbour nuclei.

Sometimes the term p-process includes also the rp-process (rapid proton capture process). The astrophysical scenario for this process is still not firmly established but it is believed that a neutron star in a binary system which is accrediting mass from a main sequence star could be one possible scenario. During X-ray bursts the temperature and the proton density is high enough to start proton capture reactions and proton rich elements up to mass A=100 can be produced.

Pair (-Production) Instability SNe (PISNe): Between ~ 140 - 260 Msol (Umeda: 130 - 300 Msol) lies the domain of PISNe. After central He burning, stars have high enough central entropy that they enter a temperature and and density regime in which electron/positron pairs are created in abundance, converting internal gas energy into rest mass of the pairs without contributing much to the pressure. When this instability is encountered, the star contracts rapidly (quasi-dynamically). The central temperature increases to 3-6 1e9 K, which is so high that central oxygen- or silicon-burning (depending on the mass of thes star) takes place explosively, bein much faster than neutrino energy losses. The generated nuclear energy is large enough for internal energy to exceed the gravitational binding energy (i.e. to revert the collaps). The star then disrups (becomes a PISN), ejects metals and gets completely disrupted, leaving no compact remnant (Heger & Woosely 2002). Its possible that the total energy in very massive progenitor-stars (M~300 Msol) does not become positive after central oxygen and silicon burning and hence collapses into a black hole. Unlike their lighter cousins (core-collaps SNe), the explosion mechanism in PISNe is well understood, and there are no issues of mass-cut or fall back (but rotation is still an issue).

Population Parameters: The three key parameters for stellar population are age, metal content and location. The German-American astronomer Walter Baade observed colours and location: Baade noticed a difference in colour between the stars in the disc and spheroid (i.e. the stellar halo and bulge) of M31. He named these two stellar types Population I (resemble the brightest stars in the disk, which are predominantly blue) and Population II (resemble the brightest stars in GCs, which are predominantly red), respectively. We note that third population, so called Pop III stars, unknown to Baade, which describes a theoretical population of the very first stars (not yet observed).

Pop I stars: Stars like our sun with typically 70% H, 28% He and 1-4% metals (Zsun = 2). Pop I stars are associated with a disk, including many very young stars, only a few million years old (star-birth regions), but also including some stars as old as 10 Gyr. The stars of Pop I move in essentially circular orbits (around the Galactic center), which do not take them far above or below the Galaxy, and hence they are confined to the flat, circular structure that constitutes the Galactic disc. The sun takes roughly 240 million years to move once around the Galactic center. This value is typical for other Pop I stars near the sun.

Pop II stars: Stars with 0.1% (or less) metals, the rest is pure H and He. Pop II stars occupy the spheroid (i.e. the stellar halo and bulge) and turn out to be the oldest stars known, with ages in the range of 12-15 Gyrs. Conspicuous examples are GC stars. Little or no interstellar gas is still associated with Pop II stars, which is consistent with star formation in the spheroid ceasing long agon. Because this population is so old, only low-mass stards still remain as main sequence stars buring hydrogen in their cores. For a long time it was thought that all Pop II stars had much lower metallicities than Pop I stars, but it is now known that this only applies to the stellar halo (approx. 2x1e-6 < Z < 0.002). Some Pop II bulge stars, on the other hand, have up to solar metallicities. Unlike disc stars, Pop II stard do not follow circular orbits, nor are they confined to the Galactic plane. They move in eccentric orbits, although still attracted to the Galactic center, and may travel many kpcs from the Galactic plane. Pop II stards belonging to the spheroid, briefly passing through the disc as they move from one side of the Galactic plane to the other, are known as high-velocity stars. In contrast to the disc, there is almost no net rotation of the halo, so almost half of all halo stars travel in retrograde (i.e. in the opposite sense) orbits to the more orderly disc stards, which all orbit in the clockwise direction as viewed from the Galactic north pole.

Pop III stars: The very first stars which formed in the Universe and therefore stars that formed out of the unprocessed gast from the primordial nucleosynthesis (primordial abundances usually assumed for simplicity to be 75% H and 25% He). Even lithium, the next most abudant element after H and He in the primordial gas, would only consitute one particle in one billion, i.e. Z~1e-9, much lower as the lowest metallicity found in Pop II stars (Z~2x1e-6). No Pop III star has been observed but they surely existed as the first stellar generation in the Universe.

pp-Chain: p(p,²H)e++nu and p(²H,³He)gamma

Primordial Abundances: Usually assumed for simplicity to be 75% H and 25% He. After the primordial nucleosynthesis theory predicts He/H = 20-26% (observation: He/H = 22-25%) and D/H = 0.0001-0.1% (observation: 0.001-0.02%) plus minimal traces of ³He, Li, B and Be. The values of X = 0.765, Y = 0.235 and Z = 0.00, respectively, are assumed in many stellar-evolution model calculations.

Primordial Stars: See First Stars

Quasar (QSO): Acronym for Quasi Stellar Radio-Object. An extremely distant and luminous astronomical objects that are much smaller than a galaxy and much more luminous. Quasars may be the central regions of certain very energetic galaxies at an early stage of their evolution. It is believed that the power of a Quasar derives from a massive black hole at its center.

Quasar Absorption Line (QAL) system:

r-process (r for rapid): A neutron capture process for radioactive elements which occurs in high neutron density, high temperature conditions (in contrast with p- and s-process). In the r-process nuclei are bombarded with a large neutron flux to form highly unstable neutron rich nuclei which very rapidly decay to form stable neutron rich nuclei. r-process elements: A=?...?.

Radiation Pressure: Stars can maintain fairly stable sizes because of the radiation pressure exerted by the radiation coming from the hot core. This radiation pressure comes into play in a major way at point during the : Stars can maintain fairly stable sizes because of the radiation pressurestellar evolution where the collapsing gas cloud becomes opaque to electromagnetic radiation. Striking this opaque ionized region, the radiation is said to "scatter" off the ions, exerting a net outward pressure which halts the gravitational collapse. There is a strong connection between the transparency point and radiation pressure. Trefil makes the analogy to the air in a tire - the pressure exists because the molecules bounce back from the tire "the tire remains inflated because the rubber walls are very efficient at scattering air molecules." Before the transparency point of the "big bang", the ions and electrons of the plasma were efficient scatterers of light, but after they condense into atoms, they are very inefficient scatterers of light - you can easily see 100 miles through air on a clear day.

Red Giant (RG): For a sun-like star: After H in the core is depleted, the stars starts to contract. This leads to an increasing fusion rate in the shells, causing expansion. Expansion takes energy, so the surface cools an reddens. Due to this enormous expansion (considrable mass loss du to the weaker gravity at the edge), the stars luminosity increses even though the temperatures decreses. The core continues to heat to 1e8K where the triple-alpha process starts (He-flash), this lasts for a few years only. The outer shell may be unstable, show contractions and variability. After He is depleted, the star has not enough mass to ignit C fusion and collapses, cools and becomes a WD.

Relaxation Time: Any isolated many particle system will eventually reach equilibrium, irrespective of its initial state. Note that the principle of equal a priori probabilities from statistical mechanics is only valid for equilibrium states. The typical time-scale to reach equilibrium is called the relaxation time, and depends in detail on the nature of the inter-particle interactions. The relaxation time for the air around you is very much less than one second, where the relaxation time of the Milkyway is about 10^13 years. Compared to the estimated age of the Milkyway of about 10^10 years, it is clear that our Milyway has not benn around long enough to reach an equilibrium state, hence, the observed velocity distribution of the stars is not governed by the principle of equal a priori probabilities.

s-process (s for slow): is a neutron capture process in the decay of radioactive elements that occurs in lower neutron density, lower temperature conditions. This process tends to produce stable isotopes by moving along the valley of beta stability. The s process is believed to occur red giant stars. In contrast to the r-process which is believed to occur over time scales of seconds, the s-process is believed to occur over time scales of thousands of years. The extent to which the s-process moves up the elements on the periodic table is essentially determined by the degree to which the star in question produces iron, which is the "starting material" for this neutron capture - gamma-ray emission - beta decay method of synthesizing new elements. This is why the largest stars with the longest lifetimes are the most likely candidates for significant elemental production via the s-process. Because of the relatively low neutron fluxes expected to occur during the s-process, this process does not have the ability to produce any of the heavy radioactive isotopes such as Thorium or Uranium.

Solar abundances for some of the most abundant elements:

Element	log(N(X)/N(H))sol+12
C	8.69
N	7.95
O	8.52
Mg	7.58
Al	6.49
Si	7.56
P	5.56
S	7.20
Fe	7.50

Taken from Morton (2003) except for O,
this was taken from Allende Prieto (2001)

Spallation: Process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy proton, thus greatly reducing its atomic weight.

Spectroscopy: One can design a spectrograph to sample the sky in various ways. A typical spectrograph is a long-slit spectrograph. That is, the sample one has of the sky is a thin slice, with a typical width of one to a few arcseconds, and a length than will range from tens of arcseconds to a few arcminutes, depending on the spectrograph design. Standard terms and definitions:

Dispersion: #Å / per pixel
Resolution: The number of Angstroms per resolution element. Put another way, the minimum wavelength seperation between features such that the features are still distinguishable. Δλ = #Å / resolution element.
Resolving Power: R = λ/Δλ
Velocity Resolution: Δv=cΔλ/λ
Signal to Nois Ratio: The ratio of the strength of a signal to the strength of any background noise.

Stellar Evolution: Standard tool TSV03: Cosmological Effects of the First Stars: Evolving Spectra of Population III (Tumlinson 2003)

Strömgren sphere: The sphere of ionized hydrogen (H-II) around a young star of the spectral classes O or B. The most prominent example is the Rosette Nebula.

Sub-Damped Lyman alpha (sub-DLA) system: See Lyman Absorption

Sub-Dwarfs: These extremely metal-poor MS stars owe their name to the fact that they lie in the CM-diagram below the ZAMS of the Hyades cluster, which is the standard for stars of approximately solar metallicity.

Supernovae (SNe): A Supernova is the explosion (typically E ~ 1e51 ergs) of a massive star at the end of its life. Except for type Ia Supernovae, which are explained by exploding white dwarfs in binary systems, all other Supernovae types (Ib/IIb, Ic, II-L and II-P) seem to be linked to massive stars with main-sequence masses Mms > 8Msol. The material which is ejected enriches the interstellar medium and will be used in the formation of new stars. The remnant, an extremely dense object, will form a neutron star or a black hole. Sta rs draw their energy from nuclear fusion, converting hydrogen to helium. When its nuclear fuel in the core is exhausted, the star begins to move towards the red giant star stage. Its core contracts while its envelope expands. Through this contraction, the core temperature rises causing further nuclear reactions, the fusion of helium to form carbon, followed by the fusion of carbon to form oxygen and successively, until iron is synthesized. Iron has a very stable nucleus, it is, therefore, not possible to gain more energy by fusion of iron. The lack of energy in the core which used to stabilize the star against gravitational forces causes the collapse of the centre of the star. The outer layers of the star also collapse afterwards until complex physical mechanisms reverse this collapse in an explosion. The explosion of a star as a Supernova produces a flash of light as bright as a whole galaxy. Supernovae models suppose the formation of radioactive elements during the explosion which later emit gamma-rays. Unfortunately Supernovae are rare phenomena and it is very difficult to validate the models through observations. See also the nice summary by Mike Guidry.

Core-Collaps SNe, Core-Collaps Hypernovae & Pair-Instability SNe Nucleosynthesis:

Chieffi et al. (1998): The Evolution of a 25 Msun Star from the Main Sequence Up to the Onset of the Iron Core Collapse (with ejecta tables for "Set L")
Chieffi & Limongi (2002): The Explosive Yields Produced by the First Generation of Core Collapse Supernovae and the Chemical Composition of Extremely Metal Poor Stars (with ejecta tables for "Set L")
Thielemann et al. (1996): Core-Collapse Supernovae and Their Ejecta (with ejecta tables)
Thielemann et al. (2000): Type II Supernova Nucleosynthesis and Early Galactic Evolution
Nakamura et al. (2001): Explosive Nucleosynthesis in Hypernovae (with ejecta tables)
Umeda et al. (2000): Evolution and Nucleosynthesis of Metal Free Massive Stars (with ejecta tables)
Umeda et al. (2000): Pop III Hypernova Nucleosynthesis and Abundances in Very Metal-Poor Halo Stars
Umeda et al. 2002: Nucleosynthesis of Zinc and Iron Peak Elements in Population III Type II Supernovae: Comparison with Abundances of Very Metal Poor Halo Stars

Star Formation induced by SNe:

Shigeyama & Tsujimoto (1998): Fossil Imprints of the First-Generation Supernova Ejecta in Extremely Metal-deficient Stars

Supranovae: First an otherwise normal SN occurs, leaving behind a NS whos existence depends on a high rotation rate. The high rotation rate (~ 1ms) is braked by pular-like ratiation until a critical angular momentum is reached. the star then collapses on a dynamical timescale into a BH, leaving behind a disk of ~ 0.1 Msol (this is still controversial). Accretion of this disk produces a delayed GRB (time scale in the order of a year), much as in the merging NS model.

Supernovae Ia: It is believed that binary snstems can give rise to supernova explosions producing a relevant amount of heavy elements, in particular to type I (Ia, Ib and Ic) and to nova outbursts which can produce non negligible amounts of 7Li and perhaps 15N, 13C and even some Ne, Na, Mg Al and Si.

SNe Ia are thought to originate from white dwarfs in binary systems exploding by C-deflagration when they achief the Chandrasekhar mass as a result of accreting or merger with the companion star. In fact, C-deflagration in CO-WDs whose mass exceeds the Chandrasekhar limit can reproduce many of the observed features in typical SNe Ia, since it produces the right amount of 56Ni->56Co->56Fe able to power their light curve and allows for the formation of intermediate mass elements (from C to Si) which are observed in their spectra.

Up to now, the two main models of binary systems leading to a SNe Ia explosion have been proposed:

Degenerated CO-WD & RG or MS companion: A C-deflagration occurs when the WD has accreted enough mass from the companion (the secondary star, namely the primordially less massive) to reach the Chandrasekhar mass limit. The clock for the explosion in this scenario is given by the lifetime of the secondary star which is ≥ 0.8 Msun and therefore varies from the age of the universe to several 1e7 years.
Two degenerated CO-WDs: Merging after gravitational wave emission, the so called "double generated model". The explosion mechanism is again C-deflagration. The clock for the explosion can vary from 1e8 yrs to several time the age of the universe.

Another possible scenario would be:

Sub-Chandrasekhar WD in a binary system: A sub-Chandrasekhar explosion might be the explanation for sub-luminous SNe Ia. the proposed scenario is a CO-WD accumulating a He-layer at low rates from a binary companion and ignites He off-center (at the bottom of the accreted layer) which results in a detonation before reaching the Chandrasekhar mass.

Sunyaev-Zeldovic Effect (SZE): The Sunyaev-Zeldovich effect (SZ effect or Sunyaev-Zeldovich theory) is due to high energy electrons distorting the cosmic microwave background radiation (CMB) through the inverse Compton effect, in which some of the high energy of the electrons is transferred to the low energy photons. It determines disk accretion and photon-electron scattering. This inverse Compton effect between the photons and electrons alters the CMB spectrum observed. This has been observed through many telescopes, for example XMM-Newton. The Sunyaev-Zeldovich theory can be divided into: thermal effects, kinetic effects and polarization). The Sunyaev-Zeldovich effect is of major astrophysical and cosmological interest. It can help determine the value of the Hubble constant.

Telluric Lines: Lines or bands in the spectrum of a celestial object that are due to absorption by gases such as oxygen, water vapor, or carbon dioxide in Earth’s atmosphere. The most prominent are the A and B bands (see Fraunhofer lines).

Thomson Depth: The distance over which the electron scattering optical depth is unity.

Thomson Scattering: When an electromagnetic wave is incident on a charged particle, the electric and magnetic components of the wave exert a Lorentz force on the particle, setting it into motion. Since the wave is periodic in time, so is the motion of the particle. Thus, the particle is accelerated and, consequently, emits radiation. More exactly, energy is absorbed from the incident wave by the particle and re-emitted as electromagnetic radiation. Such a process is clearly equivalent to the scattering of the electromagnetic wave by the particle.

Transparency Temperature/Point: At temperatures above about 3000 K where the average kinetic energy of particles is about 0.26 eV, the formation of stable atoms is hindered. Above that temperature, matter exists in a plasma state of ionized atoms, which strongly absorbs electromagnetic radiation of all wavelengths, i.e., the plasma is opaque. When the plasma cools below that temperature, it is cool enough for hydrogen and helium nuclei to collect electrons and become stable atoms. Stable atoms absorb only those frequencies characteristic of those atoms or those high enough to ionize them. This means that a cooling gas cloud has a point at which it becomes transparent to almost all wavelengths, at least for photons with quantum energy less than the ionization energy of the atoms. This transparency point is a crucial concept in the modeling of the expanding universe and in the modeling of star formation. Key information about it is provided by the 3 K background radiation.

Triple Alpha Process: Helium can fuse to form beryllium and then carbon.

UBV - System: The most widely used standard system (colour index) was developed by Harold Johnson and W. W. Morgan in 1953. They selected three filters to use in front of their photomultiplier detectors.

UBV - Johnson & Morgan 1953 (photoelectric USA version)
Filter	Central Wavelength	Wavelength Width
U (ultraviolet)	3499 Å	639 Å
B (blue)	4380 Å	928 Å
V (visual)	5470 Å	843 Å

The central wavelengths of the three filters were chosen to record certain information present in the light of stars. Because the bands are broad, they enable the measurement to be made to very faint magnitudes. However, the very width of the bands creates certain problems (The effective central wavelength changes with the colour of the star, the short-wavelength side of the U filter extends below the limit of atmospheric transmission or the bands are so wide that they do not cleanly record the information in the light that they were designed to measure).

Astronomical Bands:
blue (0.4mm) B-band
green (0.55mm) V-band
red (0.7mm) R-band
infra-red (0.85mm) I-band
infra-red (1.3mm) J-band
infra-red (1.6mm) H-band
infra-red (2.1mm) K-band
U,B,V -> optical
H,K,J -> near infra-read

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Last updated: 10.03.2005