Astronomy

Simple definition of Bondi Radius

Simple definition of Bondi Radius


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Can anyone help me by defining what is exactly Bondi Radius? I have searched far and wide but the results are very complicated and so far I have only rough idea of it but I just cant explain it very well


From the mysterious site Vaporia, which is a fantastic index for astrophysics terms (and other things I'm seeing now):

The Bondi radius is analogous to the Hill radius, both giving an idea of how far from an object, a medium (gas, dust) is likely to be drawn in and accreted. The Bondi radius takes into account the (relative) speed of the object through the medium, and the medium's density and sound-speed. Accretion due to material falling within the Bondi radius is called Bondi accretion.

Try to picture a heavy object, perhaps a planet, travelling through a gaseous or dusty medium. Now, some of the gas is eventually going to be the victim of accretion to the planet. The planet is going to pull gas/dust from the medium and the Bondi radius tells you how far from the planet gas/dust is likely to be accreted to the planet.

Hopefully this is more clear!


Galaxy

A galaxy is a group of many stars, with gas, dust, and dark matter. [1] [2] [3] [4] The name 'galaxy' is taken from the Greek word galaxia meaning milky, a reference to our own galaxy, the Milky Way.

Gravity holds galaxies together against the general expansion of the universe. [3] In effect, the expansion of the universe takes place between groups of galaxies, not inside those groups. Gravity holds the galaxy together. The same applies to groups and clusters of galaxies, such as our Local Group where the Milky Way is, and the Virgo Cluster, a collection of more than 1,000 (might even be 2,000) galaxies. The gravitation is produced by the matter and energy in a galaxy or group of galaxies. Everything in a galaxy moves around a centre of mass, which is also an effect of gravity.

There are various types of galaxies: elliptical, spiral and lenticular galaxies, which can all be with or without bars. Then there are irregular galaxies. All galaxies exist inside the universe. The observable Universe contains more than 2 trillion (10 12 ) galaxies [5] and, overall, as many as an estimated 1 × 10 24 stars [6] [7] (more stars than all the grains of sand on planet Earth). [8]


Atomic Radius Periodic Table Trends

No matter what criteria you use to describe the atomic radius, the size of an atom is dependent on how far out its electrons extend. The atomic radius of an element tends to increase the further down you go in an element group. That's because the electrons become more tightly packed as you move across the periodic table, so while there are more electrons for elements of increasing atomic number, the atomic radius may decrease. The atomic radius moving down an element period or column tends to increase because an additional electron shell is added for each new row. In general, the largest atoms are at the bottom left side of the periodic table.


Notes

Atomic Radii: values are calculated from:

E Clementi, D L Raimondi, W P Reinhardt (1963) J Chem Phys. 38:2686.

Ionic Radii: these data are taken from an empirical system of unified atomic-ionic radii, which is suitable for describing anion-cation contacts in ionic structures. The data were derived by the comparison of bond lengths in over 1200 bond types in ionic, metallic, and covalent crystals and molecules by:

J C Slater (1964) J Chem Phys 41:3199

J C Slater (1965) Quantum Theory of Molecules and Solids. Symmetry and Bonds in Crystals. Vol 2. McGraw-Hill, New York.

Note that calculated data have been used for the following elements: He, Ne, Ar, Kr, Xe, At and Rn. These data were taken from:

E Clementi, D L Raimondi, W P Reinhardt (1963) J Chem Phys 38:2686

Covalent Radii: Data given here are taken from WebElements, copyright Mark Winter, University of Sheffield, UK.

Van-der-Waals Radii: Van der Waals radii are established from contact distances between non-bonding atoms in touching molecules or atoms. Most data here are from:

A Bondi (1964) J Phys Chem 68:441

"Crystal" Radii: These data are taken from Shannon & Prewitt's (S&mpP) seminal work on "physical" ionic radii, as determined from measurements of real structures.

Note that in most cases S&mpP quote different radii for the same element: the radii vary according to charge and coordination number. We have chosen the most-common charges (oxidation states) and coordination numbers. The details are given in the element text file after each data entry.