A model for Coronal Phase Hydrogen Plasma of Interstellar Medium part 1

7/28/2019 A model for Coronal Phase Hydrogen Plasma of Interstellar Medium part 1

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Chapter 1

1.1 Introduction

Astronomy is a natural science which deals with the objects outlying beyond the

atmosphere of the Earth. In ancient times, observers who lived in the Egyptian and

Babylonian eras were able to intuit that there were some objects in nature situated

beyond the sky, and worshiped these as Gods. In medieval times people who lived in

Europe discovered that these objects are merely constituents of the universe and in 16 th

century Sir Isaac Newton revealed the basic relationship underlying their motion.

In the dawn of the modern age due to the development of science people have realized

the complicated truth about the universe. But still there are many questions to be

answered, and the most crucial one among them was the question of formation of the

universe. According to the some beliefs of religions, the universe was created by their

God. Despite the fact, in accordance with the theories of physics universe is an

accidental result of an event which is known as Big Bang. Due to Big Bang an

enormous amount of energy was released into the newborn universe which created a

super-hot dense soup of energy. As the universe grew older the hot dense soup of

energy started to cool down and then it triggered the creation of matter according to the

Einsteins equation [1]

(1.1)

These new born matter particles initially possessed great velocities and they are not

subjected to any kind of interactions. But as the universe got cooler these particles have

bonded between each other in order to produce molecules and dust particles throughout

the universe. And after billion years later these particles started creating stars, planets,

Nebulas and Galaxies, and still the process is continuing. The stars which glow within

the galaxies are born inside the nebulas. The nebulas are large gas clouds and in some


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occasions the dimensions of such a cloud may encompass several parsecs [1]. And these

Nebulas and dust particles are commonly known as the interstellar medium, uniformly

distributed throughout the Galaxies. Therefore when a star dies it is often replaced with

another one which is in a different position. But for a distant observer outside the

relevant Galaxy will not identify any difference in total light received from it.

But after finite time in the future this process will terminate. Then all the Galaxies will

fade away and if the density of the universe is large enough to contract the universe in

to a small region, again another Big Bang will create another universe which will be

similar to the existing one, and if the density of universe is less than the critical density,

the gravity will not counteract to overcome the ever-growing expansion of the universe,

therefore the universe will remain as a dark cold place for infinite period of time.

Nevertheless this whole process reveals that the universe controls almost everything in

the nature. It implies that gaining profound knowledge of the universe is the key to

revealing the destiny of everything.

1.1.1 The Interstellar Medium

The word Interstellar Medium (ISM) means the medium among the stellar objects. It is

collectively used to define the clouds of interstellar gas and dust which fill the galactic

disk. The ISM has smog like behavior. As a result of this nature it obscures the view of

the galactic Centre and its features. In accordance with the above fact, it is conclusive

that visible light does not peer through these dusty clouds, because of the scattering of

light due to the large dust particles which are almost comparable in size to the wave

lengths of visible light. As a result of this nature, interstellar medium becomes totally

opaque to the visible light emerging from the distant stars.


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In contrast though the ISM can be identified as the dust between the Stars,

quantitatively the mass confined to the stars is much greater than the mass which

distributed through the ISM. Evidently the ISM holds only a fraction of Galaxies total

luminous mass, this fraction is ' 125% for a spiral, and 1550% for an irregular galaxy

[3].

1.1.2. Organization of Interstellar Medium

The distribution of the interstellar medium throughout the Galaxy is not uniform, due to

the numerous interactions between matter and radiation the mater distribution changes

from one region to another drastically. For an example in the plane of Milky-way

Galaxy, where the Galactic gas is at its densest, the particle number density is ranging

from ' 103 to 109 atomic nuclei per cubic meter. Despite the fact that the interstellar

Figure 1.1: The main features of Milky way galaxy [2]


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medium represents all the dust and matter placed between stars it is convenient to

categorize them into several phases. All these phases are mainly defined by their

inherent temperature and density [4].

Coronal gas: Gas that has been shock-heated to temperatures byblastwaves racing outward from supernova explosions. The gas is collisionally

ionized, with ions such as O VI ( present. Most of the coronal gas has

low density, being an appreciable fraction approximately half of the volume of

the galactic disk. The coronal gas regions may have characteristic dimensions of

around 20 parsecs, and may be connected to other coronal gas volumes. The

coronal gas cools over Million year time scales. Much of the volume above and

below the disk is thought to be pervaded by coronal gas. It is often referred to as

the hot ionized medium, or HIM.

H II gas: Gas where the hydrogen has been photo-ionized by ultraviolet photonsfrom hot stars. Most of this photo-ionized gas is maintained by radiation from

recently formed hot massive O-type stars the photo-ionized gas may be dense

material from a nearby cloud (in which case the ionized gas is called an H II

region) or lower density intercloud medium (referred to as diffuse H II). The

Roman numeral II represents that one electron has repelled from neutral

Hydrogen atom. When expelled electrons recombined with ions they emit

radiation with a wavelength of 656.3 nm which gives the H II nebulas its

characteristic red colour.

Warm HI: Predominantly atomic gas heated to temperatures around in the local interstellar medium, it fills a significant fraction of the volume of the

disk perhaps 40%. Often referred to as the warm neutral medium, or WNM


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Cool H I: primarily atomic gas at temperatures about 100 this region fillsnearly1% of the volume of the local interstellar medium. Often referred to as

the cold neutral medium, or CNM.

Diffuse molecular gas: Similar to the cool H I clouds, but with sufficiently largedensities and column densities so that self-shielding allows molecules to

be abundant in the cloud interior.

Dense molecular gas: These clouds are often dark. In these dark clouds, thedust grains are often coated with mantles composed of and other

molecular ices. It is within these regions that star formation takes place.

It should be noted that the gas pressures in these dense clouds would qualify

as ultra-high vacuum in a terrestrial laboratory.

Stellar outflows: Evolved cool stars can have mass loss rates as high assolar masses per year and low outflow velocities less than ,

leading to relatively high density outflows. Hot stars can have winds that are

much faster but far less dense.

Interstellar Dust: Interstellar dust consists of particles of silicates or carboncompounds, which are relatively small, but have a comprehensive range in size.

The largest are 0.5m in size (i.e. 5 107 m) with nearly 104 atoms, but some

appear to have. 100 atoms and thus are not significantly different from large

molecules. Dust has a deep observational effect, it absorbs and scatters light.

Dust reduces the light of background sources, a process known as interstellar

extinction. Examples of this are dark nebulae, and the zone of avoidance for

galaxies at low galactic latitudes.

Hot Gas Planetary Nebulae:A planetary nebula is like a compact H II region,except that it surrounds the exposed core of a hot, highly evolved star rather than

a hot young star. The gas is ejected from the star through mass loss over time.


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Ultraviolet photons from the star ionise the gas in way similar to HII regions,

and the gas emits photons like H II region. Emission processes are similar to

H II regions, but the density, temperature and ionisation state of the gas around a

planetary nebula can be somewhat different to the H II region.

Hot Gas Supernova Remnants: Supernovae eject material at very high velocitiesinto the interstellar medium. This gas shocks, heats and disrupts the ISM. Low

density components of the ISM can be significantly affected, but dense

molecular clouds are less strongly affected. Hot gas from supernovae can even

be ejected out of the galactic disc into the halo of the galaxy.

Component Temperature

(K)

Density

(atoms/cm)

State

ofhydrogen

Primary observational techniques

Molecularclouds

1020 102106 molecularRadio and infrared molecular emission

and absorption lines

Cold NeutralMedium(CNM) 50100 2050 neutral atomic H I 21 cm line absorption

Warm Neutral

Medium

(WNM)

600010000 0.20.5 neutral atomic H I 21 cm line emission

Warm IonizedMedium (WIM)

8000 0.20.5 ionized Hemission and pulsar dispersion

H II regions 8000 10-210-4 ionized Hemission and pulsar dispersion

Coronal gas

Hot Ionized

Medium (HIM)

106107 102104

ionized

(metals also

highly ionized)

X-ray emission; absorption lines of

highly ionized metals, primarily in the

ultraviolet

Table (1.1) ISM phases in Milky Way Galaxy [5]
http://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Molecular_cloudhttp://en.wikipedia.org/wiki/Molecular_cloudhttp://en.wikipedia.org/wiki/Radio_astronomyhttp://en.wikipedia.org/wiki/Infrared_astronomyhttp://en.wikipedia.org/wiki/Hydrogen_linehttp://en.wikipedia.org/wiki/Hydrogen_linehttp://en.wikipedia.org/wiki/H%CE%B1http://en.wikipedia.org/wiki/H%CE%B1http://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion_in_pulsar_timinghttp://en.wikipedia.org/wiki/H_II_regionhttp://en.wikipedia.org/wiki/H%CE%B1http://en.wikipedia.org/wiki/H%CE%B1http://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion_in_pulsar_timinghttp://en.wikipedia.org/wiki/Galactic_coronahttp://en.wikipedia.org/wiki/X-ray_astronomyhttp://en.wikipedia.org/wiki/Ultraviolet_astronomyhttp://en.wikipedia.org/wiki/Ultraviolet_astronomyhttp://en.wikipedia.org/wiki/X-ray_astronomyhttp://en.wikipedia.org/wiki/Galactic_coronahttp://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion_in_pulsar_timinghttp://en.wikipedia.org/wiki/H%CE%B1http://en.wikipedia.org/wiki/H_II_regionhttp://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion_in_pulsar_timinghttp://en.wikipedia.org/wiki/H%CE%B1http://en.wikipedia.org/wiki/Hydrogen_linehttp://en.wikipedia.org/wiki/Hydrogen_linehttp://en.wikipedia.org/wiki/Infrared_astronomyhttp://en.wikipedia.org/wiki/Radio_astronomyhttp://en.wikipedia.org/wiki/Molecular_cloudhttp://en.wikipedia.org/wiki/Molecular_cloudhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Kelvin


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1.2 Origin of ISM

Prior to answer the above question it is necessary to examine the interior and the

various stages of life cycle of stars. Since the Sun is the nearest star to the Earth, the

analysis of the interior of a star can be done solely based upon the Sun. Hence the

spectroscopy data reveals that the Sun mainly compound with Hydrogen and Helium,

where 70% of mass consist with Hydrogen and almost 28% of mass is consist with

Helium and all other elements contains only 2% of mass of the Sun. As other stars in

the universe the Sun produces its energy by using the fusion reaction which converts

Hydrogen into Helium and as the result of the mass deflect of the relevant reaction the

energy is released according to the equation (1.1). The total power output of the Sun is

about as the Sun gets older these exact numbers will change and which

will be discussed in detail at the end of this chapter.

The interior of the Sun can be categorized into 6 regions as follows

Solar coronaSolar corona is the uppermost layer of the solar atmosphere and it lies around

few million kilometers above the solar surface. The temperature of this region is

about one million Kelvin therefore it can emit hard X-rays hence this region is

responsible for the emission of X-rays from the Sun, nevertheless the density of

this region is very much low when comparing to the Earths atmosphere.

ChromosphereChromosphere is a region which lie closer to the solar surface, which has

temperature about 10000 K, and this region produces the Ultra Violet (UV)

radiation.


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PhotosphereThe visible surface of the Sun is called the photosphere, where the average

temperature is about 6000 K. Although the photosphere looks like well defined

surface from the Earth, it contains gas far less dense than the Earths

atmosphere.

Convection zoneThis region contains hot gas encircling inside the core of the Sun, where spouts

of hot gas rising upward and cooler gas cascade downward into the core of the

Sun. These rising gas transfer the energy produced in the core to the upper

layers of the solar atmosphere. Due to tremendous temperature the gas

encircling in this region consists of fully ionized particles forming a plasma

region. And the nature of this plasma is governed by the intense magnetic fields

inside the solar core.

Radiation zoneIn comparison to the convection region radiation zone comprise with calmer

plasma region, and the energy is carried out by the photons throughout the

region. But the temperature of the region is much intense than the upper layers

of the Sun. It is about 10 million Kelvin ( and the region is covered with

X-rays trillions of times more intense than the visible light at solar surface.

Solar coreThe solar core is the source of the Suns energy, it produces energy by using the

Hydrogen fusion reaction, in this region the temperature is about 15 million

Kelvin (1.5 and the density is more than 100 times higher than the

water,


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And the pressure is 200 billion times than on the surface of the Earth. However

the energy produced inside the core takes approximately 1 million years to reach

the surface.

The life time and the some remarkable features of stars are indistinguishably dependent

on the rate of its Hydrogen burning, in Astronomy sole meaning of burning is referred

to the Hydrogen conversion into Helium under nuclear fusion. Primarily the Hydrogen

burning rate is depend on the mass of the star, thus for the sake of simplicity the life

cycle of stars can be categorized into two major groups

Life cycle of low mass stars Life cycle of high mass stars

Figure 1.2: The solar interior [6]


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1.2.1 The life cycle of low mass stars

In common usage stars having masses in comparable to the mass of the Sun or less are

known as the low mass stars. These stars use proton-proton chain to produce their

energy. Hence the fusion of these stars is in steady slow rate. Therefore, they are

eligible to light up for several billion years. In proton-proton chain, stars tend to convert

their inherent Hydrogen into Helium in its core. When the star eventually burns up the

Hydrogen accumulated in its core, tends to expand its outer layer. As a result of this star

becomes extremely luminous, these stages of the cycle subdivided as Sub Giant [7] and

Red Giant [8] stages. Lighting up for several hundred million years in this red giant

stage, the stars eventually repels their outer layer into the interstellar medium. While

this process occurs the dust particles confined in the exterior of the star continuously

emits to the space with solar winds. And simultaneously heavy elements (specially

carbon) are compiled in the core. Having repelled the outer layer of the star, the

remaining tends to burn its very last drop of fuel. In this stage the core mainly consists

of carbon [1]. The carbon burning core continuously ejects its interior matter with

stellar winds. Thus due to the lower temperatures ranging from 2000 K - 3000 K of

these carbon burning stars the carbon atoms tends to create clusters by combined with

one another and growing into dust particles. Having blended with the stellar winds the

dust particles are flown through the interstellar space and deposited in the interstellar

medium.

Before low mass star dies it treats one last spectacle, through winds and other processes

with ejecting its outer layers into space, makes huge shell of gas expanding away from

the inert, and the degenerate carbon core. The exposed core is still very hot and emits

intense ultra violet radiation that ionizes the gas in the expanding shell glowing brightly

and specified as a Planetary Nebula [9].


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Figure 1.3: Planetary nebula, Ring Nebula (M57) in Lyra/Hubble telescope image [10]

1.2.3. The life cycle of high mass star

Star with mass larger than 10 solar masses is often called high mass star. Unlike low

mass stars, high mass stars burn its fuel more rapidly by proton-proton cycle to

overcome the greater gravitational contraction. But this will cause a massive destruction

for the star in very short period of time. Since the burning of Hydrogen will pile Helium

into its core much rapidly than low mass star and increasing temperature starts Helium

fusion eventually. Unlike low mass stars, high mass star tends to fusion heavy elements

into much heavier elements such as Gold, Silver and Platinum. And star tends to pile up


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these heavy elements in its core which will cause enormous thermal pressure inside the

core.

Having completed the burning of Hydrogen star tends to fuse Helium and other heavy

elements, nevertheless when in formation of iron due to fusion, will be the catastrophic

disaster for the star. Since neither fission nor fusion be subjected to iron, piled up iron in

the core will cause rapid increment of mass. Consequently this will initiate the

gravitational collapsing into its verge. Then the super hot core starts to shrink up and be

exploded, causing massive destruction. This is known as Super Nova [11]. All the

heavy elements compiled in the core will burst into space with enormous inherent

velocities in magnitude of several thousand kilometres per second. If it was not the

interstellar medium these heavy elements would left the galaxy due to their high

velocities. These particles emitted by supernova explosion will fly through the space

with coherent group of velocities creating hot expanding cloud of debris from

supernova explosion is known as Supernova Remnants [12].

When these high velocity particles travel through the ISM will suffer with large number

of collisions with the contents of the medium, causing huge decrement of velocities of

the intruder particles. Eventually after large number of collisions they tend to rest and

merged with the medium. With each collision described above, particles will transfer

fraction of its inherent kinetic energy to the medium. This transferring energy is stored

as thermal energy. Thus medium tends to increase its internal energy and it will radiate

as high frequency X- rays and gamma- rays known as the After Glow [13].


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Figure 1.4: Supernova remnants found in crab nebula [14]

Substantially the afterglow is very important event in Astronomy. Usually observations

of supernova or its remnants can reveal lot of information about the history of galaxies

and the distance between them. The deposition of intruder substances in the ISM is

known as the Chemical Enrichment [15]. Despite this process the new born stars and

planetary systems will not have heavy materials and also the carbon which necessary to

originate the life forms.


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1.3. The Importance of Interstellar Medium

Since the ISM obscures our view of universe it is not worthy to condemn the existence

of it or abandon the studies about this concept. The ISM plays a major role in the

process of the formation of galaxies, stars, planets and the life forms in inhabitable

planets. It is responsible for the origination of iron which is represent in human blood,

and the various machineries used in day to day life and all other rare elements found in

the Earth. Nevertheless it implies that ISM acts like storage room for all the matter

originally created by the Big bang. Hence the objective is to understand the mechanism

of the ISM how it is organized and distributed throughout the Milky Way and other

galaxies, what are the conditions (temperature, density, ionization ...) in different parts

of it, and how it dynamically evolves.

1.3.1.Formation of stars from Interstellar medium

It is known that stars create their energy by fusion of the light elements which are

originally present in the interstellar medium, and the stars will release the core materials

to interstellar medium through their death events. It is observed that the dust clouds in

the galactic plane are found more often in low temperature levels typically about 10 K-

30 K, and low concentration of ions and molecules when compared with the atmosphere

of the Earth. These interstellar clouds are basically composed of Hydrogen and helium,

approximately 99% of total volume of interstellar gas. Star forming clouds are usually

called molecular clouds because of their low temperatures, which allow hydrogen atoms

to pair with each other to form Hydrogen molecules. And other heavier atoms having

low abundance can form molecules such as , , etc [1]


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Due to cold temperatures which are inherent by dust clouds, gravitation starts to pull the

gas molecules to concentric regions, in order to create high dense regions relatively to

the rest of the gas clouds. If the thermal pressure in a molecular cloud is feeble to

counter interact with the compressing force generated by gravity, the cloud be subjected

to the gravitational contraction. Due to these gravitational interactions gas cloud starts

to form dense lumps in its Interior, these are known as molecular cloud cores [17]. A

cloud thus fragments into numerous pieces, each of which will form one or more new

stars

Figure 1.6: Star forming cloud of molecular Hydrogen gasses, Eagle nebula [18]

Fi ure 1.5: Dust clouds in the alactic lane lying in cocoon nebula [16]


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1.3.2. Formation of Protostar

In the early stages of the gravitation contraction the gas quickly radiates away its

thermal energy. In these stages the star has its own inherent temperature which is below

100 K and it glows in long wave length infrared light.

The gravitational contraction cannot continue rapidly, due to the increasing density of

gas core preventing the escape of thermal radiation for some extent. But eventually it

will grow completely opaque to infrared radiation. Hence it will trap the thermal energy

produced by gravitational contraction. Due to the prevention of radiation burst both

thermal pressure and gas temperature at the centre of the contracting region increase

dramatically. Consequently this increasing pressure starts to fight back against the

gravitational contraction. And the dense cloud fragments become a Protostars [19].

Figure 1.7: Spitzer telescopes infrared view of W5, Infrared photograph of protostar [20]


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1.3.3. The Disk and Jets

The matter surround by the protostar tends to spin around the core. Thus this spinning

disk of matter is named as Protostellar Disk [21]. This protostellar disk will eventually

become planets and other planetary features such as satellites and asteroids and

planetary remnants.

Thus above facts denote how and when the stars and solar systems originate. In

conclusion it denotes that the interstellar medium is playing dominant role in this

scenario, acting as the factories which produce stars and planetary systems throughout

the universe.

1.3.4. Eagle Nebula

The Eagle nebula has been a benchmark of ISM studies. Its resemblance with all the

facts which has been discussing in previous content makes it ironical example for ISM.

The Eagle Nebula (catalogued as Messier16 or M16, and as NGC 6611) is a

young open clusterofstars in the constellation Serpens, its name has derived due to its

remarkable shape which resembles an Eagle and it was discovered by Jean-Philippe de

Cheseauxin 1745-46.

The Eagle nebula is distributed over 9.5 light years in height, it can be identified as a

gas cloud which consist of cold dust particles, the cluster associated with this nebula

contains about 460 stars with 1 million times luminous than the Sun.

Images made using the Hubble Space Telescope in 1995, by Jeff Hester and Paul

Scowen, greatly improved scientific understanding of processes inside the nebula. One

of these, a famous photograph known as the "Pillars of Creation" which is shown in

Figure 1.6, depicts a large region of star formation. Its small, dark areas are believed to
http://en.wikipedia.org/wiki/Messier_objecthttp://en.wikipedia.org/wiki/Messier_objecthttp://en.wikipedia.org/wiki/Open_clusterhttp://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Constellationhttp://en.wikipedia.org/wiki/Serpenshttp://en.wikipedia.org/wiki/Jean-Philippe_de_Cheseauxhttp://en.wikipedia.org/wiki/Jean-Philippe_de_Cheseauxhttp://en.wikipedia.org/wiki/Jean-Philippe_de_Cheseauxhttp://en.wikipedia.org/wiki/Hubble_Space_Telescopehttp://en.wikipedia.org/wiki/Pillars_of_Creationhttp://en.wikipedia.org/wiki/Pillars_of_Creationhttp://en.wikipedia.org/wiki/Hubble_Space_Telescopehttp://en.wikipedia.org/wiki/Jean-Philippe_de_Cheseauxhttp://en.wikipedia.org/wiki/Jean-Philippe_de_Cheseauxhttp://en.wikipedia.org/wiki/Serpenshttp://en.wikipedia.org/wiki/Constellationhttp://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Open_clusterhttp://en.wikipedia.org/wiki/Messier_object


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be protostars. Observations verify that there are some stars inherited in Eagle nebula

forming in a environment similar to the solar system thus this leaves rather high

probability to ignite the life forms, hence one can assume Eagle Nebula might be a

dwelling for a distant Earth in near future.

Figure 1.8: Hubble Telescope image of Eagle nebula [22]
http://en.wikipedia.org/wiki/Protostarhttp://en.wikipedia.org/wiki/Protostar

A model for Coronal Phase Hydrogen Plasma of Interstellar Medium part 1

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