22PH503 - Astronomy and Astrophysics - Unit 3 - The Sun.pptx

Astronomy and Astrophysics
Ms Dhivya R
Assistant Professor
Department of Physics
Sri Ramakrishna College of Arts and Science
Coimbatore - 641 006
Tamil Nadu, India
1

Unit 3 – The Sun
 The Sun facts
 The Sun – A Typical Star
 The Photosphere
 The Limb Darkening
 Solar Granulation
 Faculae
 Solar Corona
 Prominence
 The 11 year solar cycle and sun spots
 The solar magnetic fields
 Solar Flares
 Solar Wind
2

3

 Equator circumference: 4,379,000km
 Radius: 695,700km
 Temperature: 5,973°C to 15,000,000°C
 Average orbital speed around the Milky Way:
720,000km/h (200km/s)
 Star type: Yellow dwarf
 Average time taken to rotate on axis: 27 Earth days
 Number of planets: 8
The Sun facts
4

 The Sun is the biggest object in our solar system, with a
distance of 695,508 kilometres from centre to surface. It
contains 99.86% of the mass of the entire solar system
and could contain roughly 1.3 million Earths.
 The Sun is an average-sized star. Some stars are just a
tenth of its size, while others are more than 700 times
bigger. Due to its huge mass and strong gravity, the Sun is
a near perfect sphere.
How big is the Sun
5

 The core is the hottest part of the Sun, at 15 million degrees Celsius.
This is hot enough to sustain thermonuclear fusion, when hydrogen
atoms are fused together to form larger helium atoms. This releases
an extraordinary amount of energy which in turn is released as heat
and light.
 The energy produced at the core takes up to a million years to reach
the outer layer known as the convective zone. At this point the
temperature drops to around two million degrees Celsius. By the
time it gets to the surface the temperature is down to a much cooler
5,973°C - but it's still hot enough to boil diamonds.
 In the Sun's atmosphere, known as the corona, the temperature
begins to rise again to roughly two million degrees Celsius. As
distance from the core of the Sun grows wider, the temperature
would be expected to drop. This dramatic increase in temperature in
the atmosphere is one of the star's biggest mysteries.
How hot is the Sun
6

The Sun at different ultraviolet wavelengths, (l-r) the bright spots are 60
000–80 000ºC, 1 million, 1.5 million and 2 million degrees
7

 The Sun is a ball of gas and plasma - around 91% of it is
hydrogen gas. Under intense heat and gravitational force
this is fused into helium during nuclear fusion.
 When the plasma is heated to the temperatures seen on
the Sun, it contains so much energy that the charged
particles can escape the star's gravity and blow out into
space. This is called solar wind - under certain conditions,
when it hits Earth's atmosphere it can cause auroras,
such as the Northern Lights.
 In addition to hydrogen and helium, scientists have
detected at least 65 other elements in the Sun. The most
abundant of these include oxygen, carbon, nitrogen,
silicon, magnesium, neon, iron, and sulphur.
What is the Sun made of?
8

 Yes.
 Even though the Sun is not solid like Earth, it still has a
rotation as the plasma swirls around its surface.
 On average, it takes 27 Earth days for the Sun to rotate
once on its axis, but different parts move at different
speeds.
 The equatorial regions take just 24 days to rotate and the
polar regions more than 3
Does the Sun rotate?
9

 Sunspots are cooler parts of the Sun's surface, and occur
in the photosphere.
 The temporary splodges across the surface appear darker
to us than the warmer plasma surrounding it.
 These cooler spots can be up to 50,000 kilometres across.
 They are thought to be caused by interactions with
particularly strong regions of the underlying magnetic
field, which slightly reduce the radiation coming up from
the core, cooling the surface.
What are sunspots?
10

Sunspots
11

 Solar flares are the largest explosive events in the entire
solar system.
 They occur when the magnetic fields associated with the
sunspots convert energy into heat and accelerating
particles, ejecting it into space.
 They are characterized by a burst of photons - or light - at
almost every wavelength.
 Scientists usually measure the ultraviolet rays, X-rays and
gamma-rays coming from the solar flares.
 These typically show up as bright flashes on the surface
of the Sun, and can last from just a couple of minutes to a
few hours.
What are solar flares?
12

Solar Flare
13

 The Sun is currently a yellow dwarf.
 Like all stars, it began as a contracting nebula - effectively a
cloud of dust and gas.
 As each particle has its own tiny gravitational pull, the dust and
gas began to coalesce into a protostar.
 As this mass is pulled together and collapses in on itself it
generates heat.
 If it becomes big enough it will eventually get sufficiently hot
to fuse hydrogen into helium. This was how the Sun formed.
 The celestial object then enters its main sequence stage,
during which the outward pressure of nuclear fusion is
balanced by the inward pressure of the stars own gravity.
 The Sun is currently in this stable phase
What type of star is the Sun?
14

 It won’t.
 When all of the hydrogen in its core is burned up, it exits the
main sequence stage.
 As a relatively small star, the outer layers of the Sun will
collapse in on themselves, creating temperatures hot enough
to fuse helium into carbon.
 At this point the pressure will cause the star to rapidly expand
outward to form a red giant.
 For the Sun, this is expected to occur in about five billion
years.
 It is likely to engulf Mercury and Venus - and potentially even
the Earth.
 It will eventually shrink to become a planetary nebula with a
white dwarf at its core.
When will the Sun explode?
15

 There are an estimated 100 billion galaxies in the known
universe.
 The Sun and its eight planets, including Earth, reside in one
known as the Milky Way.
 Our galaxy is around one quintillion kilometres across, or
100,000 light years.
 The Sun is in a spiral arm of the Milky Way called the Orion
Spur.
 This branches off from the galaxy's Sagittarius arm.
 The Sun and our solar system are orbiting around the centre of
the Milky Way at a speed of 720,000 kilometres per hour.
 It takes 230 million years to make one complete orbit.
Where is the Sun in the Milky
Way?
16

 Sometimes the Moon comes between the Sun and Earth. If all
three are aligned, the Moon can block the Sun entirely, casting
a shadow on Earth and causing a solar eclipse.
 This is because even though the Sun is 400 times larger than
the Moon, the Moon is 400 times closer to Earth.
 Solar eclipses occur roughly every six months, but total solar
eclipses, in which the Sun is completely obscured by the
Moon, are much rarer.
 They tend to occur around every two years, although often in
remote areas of the Earth.
 These total eclipses can last for as little as a few seconds, but
never longer than seven minutes.
What is a solar eclipse?
17

Solar Eclipse
18

 Due to its extreme temperatures studying the Sun with
spacecrafts is incredibly difficult.
 We have to rely on observations using telescopes and
cameras on Earth satellites.
 However, in 2020 the European Space Agency and NASA
launched the Solar Orbiter, which will enter an elliptical
orbit of the Sun allowing unprecedented observations.
 After taking three and a half years to reach the Sun, it will
begin a seven year mission to study the stars solar wind,
magnetic fields, and plasma.
 It will orbit as close as 42 million kilometres from the
Sun's surface, closer even than Mercury.
How do we study the Sun?
19

 Aditya L1 shall be the first space based Indian mission to
study the Sun.
 The spacecraft shall be placed in a halo orbit around the
Lagrange point 1 (L1) of the Sun-Earth system, which is
about 1.5 million km from the Earth.
 A satellite placed in the halo orbit around the L1 point
has the major advantage of continuously viewing the Sun
without any occultation/eclipses.
 This will provide a greater advantage of observing the
solar activities and its effect on space weather in real
time.
Aditya-L1
20

 The spacecraft carries seven payloads to observe the
photosphere, chromosphere and the outermost layers of the
Sun (the corona) using electromagnetic and particle and
magnetic field detectors.
 Using the special vantage point L1, four payloads directly view
the Sun and the remaining three payloads carry out in-situ
studies of particles and fields at the Lagrange point L1, thus
providing important scientific studies of the propagatory effect
of solar dynamics in the interplanetary medium
 The suits of Aditya L1 payloads are expected to provide most
crucial information to understand the problem of coronal
heating, coronal mass ejection, pre-flare and flare activities
and their characteristics, dynamics of space weather,
propagation of particle and fields etc.
Aditya-L1
21

Three features of the Sun that are typical of stars are fusion
reactions, sunspots, and solar wind.
 Fusion reactions: The Sun, like other stars, produces energy
through nuclear fusion, where hydrogen atoms combine to
form helium. This process releases a tremendous amount of
energy in the form of light and heat.
 Sunspots: Sunspots are dark, cooler regions on the Sun's
surface caused by increased magnetic activity. They are
common features on stars and are associated with the Sun's
magnetic field.
 Solar wind: Solar wind is a flow of hot, charged particles that
leave the Sun. It is a characteristic feature of stars and is
responsible for creating the Sun's extended atmosphere.
The Sun – A Typical Star
22

 Our Galaxy consisting of some 10 stars. The Sun acts just
as a mediocre member in every physical aspect.
 The Sun is the only star which shows a disc the different
parts of which can be studied in isolation, unlike any
other star which looks just like a point source.
 The use of modern sophisticated instruments and
efficient techniques of observation coupled with the
physical laws, have enabled us to gather considerable
insight into the structure and true physical characteristics
of the Sun.
23

 We, now know fairly accurately the values of pressure, density
and temperature all the way from the surface to the centre of
the Sun. Its radiant energy is generated by thermonuclear
transmutation of hydrogen into helium at a central
temperature of about 16 million degrees Kelvin.
 Although the Sun offers scope for detailed and most accurate
observations by virtue of its nearness, these observations are
restricted only to atmosphere and extreme superficial layers.
 The main body of the solar interior cannot be directly
observed. One has to extend knowledge to the interior by
applying the laws of physics governing the equilibrium of a
radiating gaseous sphere, in combination with the observed
results or the upper layers
24

 Sunspots and faculae in the photosphere, spicules and
plages in the chromosphere, great prominences, flares
and streamers in the corona, the solar wind and sporadic
radio bursts from the outer atmosphere of the Sun and
the most mysterious 11-year cycle of solar activity all
these call for their explanation a host of physical theories
25

 Starting from the visible solar surface upwards, these
layers are known as
 the photosphere
 the chromosphere
 the corona (merging into the interplanetary space)
 The temperature minimum is attained at the transition
layer between the photosphere and the chromosphere.
 The temperature increases rapidly both ways from this
layer which is often described as the base of the
chromosphere.
 This minimum temperature is about 4200 K while the
temperature at the base of the photosphere where our
view is obstructed is around 5800 K
The Photosphere
26

 The thickness of the entire photospheric layer which runs from
completely transparent to perfectly opaque layers of gas is
quite small, only of the order of 200-300 km.
 The continues absorption spectrum of the Sun produced by H
ions originates entirely in the photosphere It is now believed
that most of the Fraunhofer lines also originate in this layer.
 The current observations however indicate that most of the
solar absorption lines originate in the photosphere-the weaker
lines in the lower photosphere and the stronger lines in the
upper photosphere.
 The depth of the photosphere through which our view can
penetrate depends on the part of the disc we are observing
and the opacity of the photospheric gas. Deeper and hotter
photospheric layers are observed when we look to the Sun's
disc near the centre.
The Photosphere
27

 When we see the disc near the limb our view penetrates
through cooler superficial layers of the photosphere This
causes the well-known limb darkening effect in solar
observation.
 Light from near the edge of the disc comes from upper,
cooler and more tenuous layers of the photosphere than
that from near the centre .
 So we see the limb of the solar disk redder and dimmer
as compared to the bright and hot central part.
 The effect is most pronounced at the blue end or the
spectrum.
The Photosphere
28

 The decrease of brightness of the solar disk as we go across
from centre to the edge, points to the fact that there exists a
temperature gradient across the photospheric layers.
 The temperature decreases as we move from lower to upper
photospheric layers.
 The temperature again rises rapidly in the chromosphere.
 But the gas density in these upper layers is so low that it is
capable of absorbing or emitting very small amount of
radiation.
 The opacity thus being very low, these layers are almost
transparent compared to the photosphere.
 This is why the disc appears to terminate abruptly at the end
of the photosphere.
The Photosphere
29

 Limb darkening, in astrophysics, gradual decrease in
brightness of the disk of the Sun or of another star as
observed from its centre to its edge, or limb.
 This phenomenon is readily apparent in photographs of
the Sun.
 The darkening is greatest for blue light, amounting to a
drop of as much as 90 percent from the
Sun’s photosphere to its outer atmospheric regions.
 Such limb darkening occurs because the solar
atmosphere increases in temperature with depth.
The Limb Darkening
30

 At the limb, only the upper, cooler layers that produce
less light can be seen
 Observations of solar limb darkening are used to
determine the temperature structure of the Sun’s
atmosphere.
 Information derived from such observations is applied in
studying other stars.
The Limb Darkening
31

 The typical grain-like fine structures of the visible disc of
the Sun are known as photospheric granulations.
 In the envelope of stars, like Sun the radiative energy has
to be supplemented by convective transport of energy.
 Convective cells of gaseous mass carrying higher energy
from the hotter deeper layers of the envelope rise
through the upper cooler layers, transfer there the excess
energy and subsequently sink down again into the
deeper layers. The process is repeated incessantly.
 These rising and falling convective cells appear in the
photographs of the solar disk as bright granules of
various sizes bordered by darker regions.
Solar Granulation
32

 The diameter of granules vary from 300 km to 18,000 km
 Granules are found everywhere on the solar surface
including regions of Sunspots.
 The total number of granules on the visible disk of the
Sun is about 3.5 x 106 .
 They can be observed as close to the solar limb as 10’’,
but are not observed still closer as there one has to look
tangentially through somewhat higher layers.
 Granules a found to vary in both size and number with
the 11-year solar cycle.
Solar Granulation
33

Solar Granulation
34

 The works of Schwarzschild and others have established
that the lifetimes of granule occurring in different regions
of the solar disc are different.
 The photospheric granules (quiet region) have average
life-time of about 8 minutes.
 Granules are also observed in umbral and penumbral
regions of sunspots.
 The former have life-times in the range 15-30 minutes
while the later are frequently found to live more than an
hour. Granules occurring in facular regions have the
highest average lifetime of the order of 2 hours.
Solar Granulation
35

 The velocity pattern of the granules can be studied by
measuring the Doppler shifts of lines formed by gases
just above the granules.
 The detailed study of the velocity pattern in the
photospheric gases has revealed cellular pattern of much
larger scale.
 This large-scale pattern has been called supergranulation.
 The coarse supergranulation pattern is similar to the
pattern of fine photospheric granulation but the two are
believed to be physically independent.
Solar Granulation
36

 The average linear size of the former is about 20 times
that of the latter, with many of the super granules being
larger than 30,000 km.
 Within this structure the gas flows in the center-to-edge
directions.
 The photospheric gases thus possess two well-defined
flow patterns-the vertical current of the fine granules and
the horizontal current of the super granules.
 Superimposed on these two patterns of motion there is
also a third pattern of slow oscillatory motion up and
down which performs a complete cycle in about 5
minutes.
Solar Granulation
37

 Facula, in astronomy, bright granular structure on
the Sun’s surface that is slightly hotter or cooler than the
surrounding photosphere.
 A sunspot always has an associated facula, though faculae may
exist apart from such spots.
 Faculae are visible in ordinary white light near the Sun’s limb
(apparent edge), where the photospheric background is
dimmer than near the centre of the disk.
 The extensions of faculae up into the chromosphere become
visible over the entire disk in spectro heliograms taken at the
wavelengths of hydrogen or ionized calcium vapour.
 When seen in the chromosphere, they are called plages.
Facula
38

 The lifetime of spots hardly exceeds a month. that of
faculae generally lies in the range 200-300 days.
 Photospheric faculae are thus closely correlated with the
solar activity.
 Largest individual faculae generally occur at the time of
maximum activity.
 Two to three years after maximum, the individual faculae
become smaller in size and also fainter, although they
may be more numerous.
Facula
39

 The corona is the outermost part of the Sun's atmosphere.
 The corona is usually hidden by the bright light of the Sun's
surface.
 That makes it difficult to see without using special
instruments.
 However, the corona can be seen during a total solar eclipse.
 The spectrum of the corona was St studied in 1869 by the
American astronomers, W. Harkness and C.A. Young. and its
First success photograph was obtained in 1930 by the French
physicist B. Lyot, with his newly discovered instrument, the
Coronagraph
 The corona extends up to several solar radii above the
photosphere. The transition between the chromosphere and
Ro corona takes place at about a radial distance of R = 1.03
times the radius of the Sun.
Solar Corona
40

Solar Corona
41

 The inner corona which is also sometimes called as the
real corona extends between 1.03 R.< R < 2.5 Ro.
 This part of the corona is known as the K-corona.
 It imitates the continuous spectrum of the photosphere
(the name K-corona is deriyed from the German word
"Kontinuum. after W. Grotrian) but the Fraunhofer lines
are absent.
The Inner Corona or K-Corona
42

 The F-corona which lies at R > 2.5 R displays the solar
spectrum with Fraunhofer lines Superimposed on the
continuum.
 This is sometimes called the "false" part of the corona
and the prefix F stands for Fraunhofer.
 The spectrum of the F-corona is produced by the Sun’s
light scattered by tiny dust particles of the interplanetary
space.
The Outer Corona or F-Corona
43

 In the optical range of the coronal spectrum, about two
dozen emission lines are found to be superimposed on
the continuous background.
 The total light of these emission lines formed by highly
ionized atoms in the extremely hot inner part of the
corona constitute what may be called the E-Corona or the
emission corona
 The total radiation in this lines are however small, that is
less than even 1% of the total coronal radiation, but the
study of conditions under which this radiation is emitted
is very interesting.
 Until 1942. they remained quite unidentified and were
called coronium lines.
The Emission Corona or E-Corona
44

 The Sun’s fiery hot sphere produces a variety of special
features. Maybe the most intriguing is a so-called solar
prominence.
Prominence
45

 These prominences can last for days, sometimes even months,
and are often associated with a very powerful surge of gas that
flows outward in a giant solar “belch” — a coronal mass
ejection that can light up Earth’s skies with an aurora and even
interfere with our technology.
 For solar observers, prominences are a joy.
 A solar filter that permits seeing prominences is a so-called
Hydrogen-alpha (H-alpha) filter, which transmits the
wavelength of light prominences give off.
 If you have a telescope fitted with an H-alpha filter, you can
see these tongues of reddish gas blasting above the Sun’s limb,
and even watch them as they slowly change over minutes or
hours.
 It makes an incredible way to appreciate our home star, the
source of all the energy that makes life possible on Earth.
Prominence
46

 These prominences can last for days, sometimes even months,
and are often associated with a very powerful surge of gas that
flows outward in a giant solar “belch” — a coronal mass
ejection that can light up Earth’s skies with an aurora and even
interfere with our technology.
 For solar observers, prominences are a joy.
 A solar filter that permits seeing prominences is a so-called
Hydrogen-alpha (H-alpha) filter, which transmits the
wavelength of light prominences give off.
 If you have a telescope fitted with an H-alpha filter, you can
see these tongues of reddish gas blasting above the Sun’s limb,
and even watch them as they slowly change over minutes or
hours.
 It makes an incredible way to appreciate our home star, the
source of all the energy that makes life possible on Earth.
Prominence
47

 Scientists are still researching how and why prominences
are formed.
 The red-glowing looped material is plasma, a hot gas
comprised of electrically charged hydrogen and helium.
 The prominence plasma flows along a tangled and
twisted structure of magnetic fields generated by the
sun’s internal dynamo.
 An erupting prominence occurs when such a structure
becomes unstable and bursts outward, releasing the
plasma.
Prominence
48

 Prominences vary considerably in size, shape, and motion and
are of two main types, active and quiescent.
 Active prominences erupt quickly and have lifetimes lasting
from several minutes to a few hours.
 They are associated with sunspot groups and, like these, are
correlated in numbers and activity with the solar cycle.
 Quiescent prominences tend to emerge smoothly and subside
much more slowly, so they may be visible for several months.
 Prominences appear either as flame-coloured projections
when the disk of the Sun is totally eclipsed or as dark ribbons
(called filaments) when viewed through a spectroscope.
Prominence
49

 Less active – slower mass motion – greater longevity
 Appear dark
 Survival – few hours to several days or even months
 After a quite period of time these prominences may
evolve into active or eruptive prominence and merge into
space with a violent velocity.
 Magnetic field is a main reason for the prominence to
maintain the structure at such a high temperature of the
sun.
Quiescent prominences
50

 Occurs in sunspot zone.
 Occasionally found to develop from Quiescent
prominences.
 In the later phase they often become eruptive.
 They are characterized by huge mass movement which
are joined by curved filaments
 Large narrow filament are seen to be ejected
continuously from the main massive part of these
prominences.
Active Prominences
51

 Most violent of all prominences.
 Sometimes develop from active prominences.
 In these types, materials are thrown violently from the
solar body at a velocity of 1200km/sec.
Eruptive Prominence
52

 These appear above the sunspot activity as curved arches
or loops.
 Condensation or knot type (coronal prominences) is
another type of sunspot type prominences.
 Material condenses at a height of about 50,000 to
1,00,000 km above the suns surface.
 They are believed to be associated with the solar flares.
 Tornado type prominences are a rare type of
prominences.
Sunspot type Prominences
53

 Among the photospheric phenomena sunspots are the
most striking.
 These are regions of strong magnetic field and low
temperature (3800 K). Its about 2000 K cooler than its
surrounding.
 Spectrum of sun spot is similar to K0 star.
 First observed by Galilio in 1610.
 A sun spot first appears as a pore of diameter of the
order of 1000km.
 The size ad shape grows gradually.
The 11 year solar cycle and sun
spots
54

 A developed sunspot has two main regions – Umbra(dark
central region) and penumbra(relatively lighter region)
 Granulation have much longer lifetime in these.
 Leader spot followed by group of follower spots.
 Gradually the followers disappear followed by the
disappearance of the leader.
 Astronomers concluded that the number of visible spots
on suns disk varied with time.
 This time period was found to be 11.2 years.
 The cycle of activity of the sun is repeated nearly over
this period which is therefore known as solar cycle.
spots
55

 Sunspot minimum and sunspot maximum.
 Zurich sunspot number 𝑅 = 𝐾 10𝑔 + 𝑓
 The magnetic orientation of leader and follower spots
and the unipolar grp remain the same in each
hemisphere over a solar cycle.
 The bipolar grp in the two hemispheres have opposite
magnetic orientation.
 The magnetic orientation of bipolar grp reverses for each
cycle.
 Evershed effect – direction of gas flow from umbra to
penumbra is reversed in each hemispheres.
spots
56

The solar magnetic fields
57

 Solar flare, sudden intense brightening in the solar corona,
usually in the vicinity of a magnetic inversion near a sunspot
group.
 The flare develops in a few minutes, or even seconds, and may
last several hours. High-energy particles, electron streams,
hard X-rays, and radio bursts are often emitted, and a shock
wave occurs when the flare interacts with the interplanetary
medium.
 The flare occurs above the surface in the corona, and energy
deposited in the surface brings up a superhot cloud, about 100
million Kelvins (100 million °C, or 180 million °F), which is a
strong, long-lasting source of X-rays.
 Smaller flares do not show all these attributes, and flares
rarely occur in the three or four years of sunspot minimum.
Solar Flares
58

 The biggest flares occur in association with large sunspots
that have sharp magnetic gradients and large currents,
which are the source of the flare energy.
 There is a class of spotless flares associated with filament
eruptions; they are large and sometimes produce coronal
mass ejections but produce few high-energy particles.
 Flares are brighter than the whole Sun in X-rays and in
ultraviolet light. X-ray photons and high-energy particles
arrive immediately, but the main particle flux arrives a
few days later.
Solar Flares
59

 Solar wind, flux of particles, chiefly protons and electrons
together with nuclei of heavier elements in smaller numbers,
that are accelerated by the high temperatures of the solar
corona, or outer region of the Sun, to velocities large enough
to allow them to escape from the Sun’s gravitational field.
 The solar wind is responsible for creating the tail of Earth’s
magnetosphere and the tails of comets, both of which face
away from the Sun.
 At a distance of one astronomical unit, during a relatively quiet
period, the wind contains approximately 1 to 10 protons per
cubic centimetre moving outward from the Sun at velocities of
350 to 700 km (about 220 to 440 miles) per second; this
creates a positive ion flux of 108 to 109 ions per square
centimetre per second, each ion having an energy equal to at
least 15 electron volts.
Solar Wind
60

 During solar flares, the proton velocity, flux, plasma
temperature, and associated turbulence increase
substantially.
 There are two solar winds: a fast, uniform, and steady
wind, blowing at 800 km (500 miles) per second, and a
slow, gusty, and sporadic wind, with about half the speed
of the fast one.
 The two winds originate at different places on the Sun
and accelerate to terminal velocity at different distances
from it.
 The distribution of the two solar wind sources depends
on the 11-year solar activity cycle.
Solar Wind
61

 When the solar wind encounters Earth’s magnetic field, a
shock wave results, the nature of which is not fully
understood.
 As the solar wind spreads out into an increasing volume,
its density and pressure become less.
 Eventually the pressure of the solar wind becomes
comparable to that of the interstellar medium.
 The termination shock, where the solar wind slows
because it encounters the interstellar medium, has been
measured at about 94 and 84 AU by the Voyager 1 and 2
spacecraft, respectively.
Solar Wind
62

https://www.britannica.com/science/solar-prominence
Reference
63

22PH503 - Astronomy and Astrophysics - Unit 3 - The Sun.pptx

22PH503 - Astronomy and Astrophysics - Unit 3 - The Sun.pptx

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22PH503 - Astronomy and Astrophysics - Unit 3 - The Sun.pptx