# وسط بين نجمي

]]).

الوسط بين نجمي Interstellar medium تطلق كلمة الوسط بين النجوم على الحيز الكامن بين النجوم في المجرة وتشمل دراسته في الفيزياء الفلكية على معرفة طبيعة الجزيئات المبعثرة بشكل عشوائي في الفضاء بين النجوم وبين المجرات.

Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025. Its twin, Voyager 2 entered the ISM in November 2018.

Voyager 1 is the first artificial object to reach the ISM.

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## Interstellar matter

Table 1 shows a breakdown of the properties of the components of the ISM of the Milky Way.

Table 1: Components of the interstellar medium[1]
Component Fractional
volume
Scale height
(pc)
Temperature
(K)
Density
(particles/cm3)
State of hydrogen Primary observational techniques
Molecular clouds < 1% 80 10–20 102–106 molecular Radio and infrared molecular emission and absorption lines
Cold Neutral Medium (CNM) 1–5% 100–300 50–100 20–50 neutral atomic H I 21 cm line absorption
Warm Neutral Medium (WNM) 10–20% 300–400 6000–10000 0.2–0.5 neutral atomic H I 21 cm line emission
Warm Ionized Medium (WIM) 20–50% 1000 8000 0.2–0.5 ionized emission and pulsar dispersion
H II regions < 1% 70 8000 102–104 ionized emission and pulsar dispersion
Coronal gas
Hot Ionized Medium (HIM)
30–70% 1000–3000 106–107 10−4–10−2 ionized
(metals also highly ionized)
X-ray emission; absorption lines of highly ionized metals, primarily in the ultraviolet

### The three-phase model

Field, Goldsmith & Habing (1969) put forward the static two phase equilibrium model to explain the observed properties of the ISM. Their modeled ISM consisted of a cold dense phase (T < 300 K), consisting of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T ~ 104 K), consisting of rarefied neutral and ionized gas. McKee & Ostriker (1977) added a dynamic third phase that represented the very hot (T ~ 106 K) gas which had been shock heated by supernovae and constituted most of the volume of the ISM. These phases are the temperatures where heating and cooling can reach a stable equilibrium. Their paper formed the basis for further study over the past three decades. However, the relative proportions of the phases and their subdivisions are still not well known.[1]

### The atomic hydrogen model

This model takes into account only atomic hydrogen : Temperature larger than 3000 K breaks molecules, lower than 50 000 K leaves atoms in their ground state. It is assumed that influence of other atoms (He ...) is negligible. Pressure is assumed very low, so that durations of free paths of atoms are larger than the ~ 1 nanosecond duration of light pulses which make ordinary, temporally incoherent light .

### Structures

Three-dimensional structure in Pillars of Creation.[2]

## Heating and cooling

### Heating mechanisms

Gas heating via grain-gas collisions is dominant deep in giant molecular clouds (especially at high densities). Far infrared radiation penetrates deeply due to the low optical depth. Dust grains are heated via this radiation and can transfer thermal energy during collisions with the gas. A measure of efficiency in the heating is given by the accommodation coefficient:

${\displaystyle \alpha ={\frac {T_{2}-T}{T_{d}-T}}}$

where T is the gas temperature, Td the dust temperature, and T2 the post-collision temperature of the gas atom or molecule. This coefficient was measured by ( [[#CITEREF|]]) as α = 0.35.

Other heating mechanisms
A variety of macroscopic heating mechanisms are present including:

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## History of knowledge of interstellar space

Herbig–Haro 110 object ejects gas through interstellar space.[3]

This light-year-long knot of interstellar gas and dust resembles a caterpillar.[4]

## Notes

1. ^ أ ب خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة Ferriere2001
2. ^ "The Pillars of Creation Revealed in 3D". European Southern Observatory. 30 April 2015. Retrieved 14 June 2015.
3. ^ "A geyser of hot gas flowing from a star". ESA/Hubble Press Release. Retrieved 3 July 2012.
4. ^ "Hubble sees a cosmic caterpillar". Image Archive. ESA/Hubble. Retrieved 9 September 2013.

## References

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