For simplicity we will consider only the geometric effects on

calculated radius, and also for an observed case where the expected Energy is re-emitted, not as a single photon, but rather as a series of photons of lesser energy. In supernova remnant 1987A, the Strömgren shell is strangulated into an hourglass whose limbs are like three pearl necklaces. is the total recombination rate and has an approximate value of, Using I , the ionization rate After a while, free electrons recombine with those hydrogen ions. The photons lose energy as they travel outward from the star's surface, and are not energetic enough to again contribute to ionization.

It shows a dotted circle which gives its name. The term is used for a star's (theoretical) sphere within a particular calculated radius, and also for an observed case where the expected phenomenon is seen to occur. S

The model. , and the numerical density of neutral hydrogen is S

It draws, however, on his earlier similar efforts published in 1937. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Outside the sphere, radiation of the atoms' frequencies cools the gas strongly, so that it appears as a thin region in which the radiation emitted by the star is strongly absorbed by the atoms which lose their energy by radiation in all directions. This is important, as the electric dipole mechanism always makes the ionization up from the ground level, so we exclude n=1 to add these ionizing field effects. {\displaystyle N_{I}}
e {\displaystyle N_{n}}



The resulting images more closely resemble many actual H II-regions than the original model. The model . They suggested the UV radiation of the O- and B-stars to be the required energy source.

We will consider the recombination rate of all energy levels, which is. The Strömgren sphere is a theoretical construct which describes these ionized regions. to the electromagnetic radiation and the expected random recombination

This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. n {\displaystyle n_{h}=(1-x)n} So after doing the sum, we arrive at: where is the total recombination rate and has an approximate value of: Using as the number of nucleons (in this case, protons), we can introduce the degree of ionization so , and the numerical density of neutral hydrogen is .

In 1939 Bengt Strömgren took up the problem of the ionization and excitation of the interstellar hydrogen.This is the paper identified with the concept of the Strömgren sphere. {\displaystyle J} The above-mentioned relationships are as follows: In Strömgren's model, the sphere now named Strömgren's sphere is made almost exclusively of free protons and electrons.

) ), so we have an inverse square law: We are now in position to calculate the Stromgren radius

The Necklace Nebula is a beautiful Strömgren's sphere.

e Thus a Strömgren system appears as a bright star surrounded by a less-emitting and difficult to observe globe.

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) T Calculating the radius is much simpler than Wikipedia makes it out to be, and it's worthwhile to figure this out on your own rather than look up the formula. From Infogalactic: the planetary knowledge core, The Rosette Nebula, an example of a Strömgren sphere, https://infogalactic.com/w/index.php?title=Strömgren_sphere&oldid=2507673, Creative Commons Attribution-ShareAlike License, About Infogalactic: the planetary knowledge core.

The reason we have excluded n=1 is that if an electron recombines directly to the ground level, the hydrogen atom will release another photon capable of ionizing up from the ground level. It draws, however, on his earlier similar efforts published in 1937. so Both Strömgren's original model and the one modified by McCullough do not take into account the effects of dust, clumpiness, multiple stars, detailed radiative transfer, or dynamical effects.

{\displaystyle R_{S}} Such regions are strong and characteristic emitters of radiation at radio wavelengths, and their dimensions are well calibrated in terms of the luminosity of the central star. N



Strömgren sphere (strohm -grĕn) An approximately spherical region of ionized gas, mainly ionized hydrogen (), that surrounds a hot O or B star.If a hot star were embedded in very low-density gas of uniform distribution, the ultraviolet radiation from the star would almost completely ionize the gas out to a particular radius – the Strömgren radius. The resulting images more closely resemble many actual H II-regions than the original model. n This is caused by the fact that the transition region between gas that is highly ionized and neutral hydrogen is very narrow, compared to the overall size of the Strömgren sphere.[1]. {\displaystyle 0\leq x\leq 1} [2], Let's suppose the region is exactly spherical, fully ionized (x=1), and composed only of hydrogen, so that the numerical density of protons equals the density of electrons (

To simplify calculations, the interstellar medium is taken to be homogeneous and consisting entirely of hydrogen. The boundary between the FrEGG and the sphere is seen in the Hubble image as a glowing purple region as the heat from the hot nearby star photoevaporates the outer layer of gas. FrEGGs are dense clumps of cooler gas clustered in the Strömgren sphere, and many of them are busily forming stars of their own. …II region, known as a Strömgren sphere. given a hydrogen density of a few thousand per cm3 and

[3], In 1939 Bengt Strömgren took up the problem of the ionization and excitation of the interstellar hydrogen.
Strömgren did not know Einstein's theory of optical coherence.

[4], In 2000 Peter R. McCullough published a modified model allowing for an evacuated, spherical cavity either centered on the star or with the star displaced with respect to the evacuated cavity.

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