Interdisciplinary "Space" Studying the Universe

for IJSO training course 1

Content

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Solar system – an overview Order of the planets Key features of each planet Asteroids, comets and meteoroids Stars and their colors Constellations Galaxy Space exploration Scale model of planets 2

1. Solar system

•

Sun:

its mass is about 300,000 times more massive than the Earth. Its radius is 700,000 km, about 110 times that of the Earth.

• Energy source: thermonuclear reactions ( 熱核反應 )at the core.

• Its atmosphere is divided into 3 layers: • Photosphere (~ 500 km thick) ( 光球層 ) • Chromosphere ( 色球層 ) • Corona ( 日冕 ) 3

1. Core 2. Radiative zone 3. Convective zone 4. Photosphere 5. Chromosphere 6. Corona 7. Sunspot 8. Granules 9. Prominence (Wikimedia Commons) 4

•

Sunspots

( 太陽黑子 ): cool, dark areas of the solar surface, each consists of a darker, cooler (~ 4,000 K) region called

umbra

( 本影 ), surrounded by a less cool region called

penumbra

( 半影 ).

A large group of sunspots in year 2004. The grey area around the spots can be seen very clearly, as well as the granulation of the sun's surface. (Wikimedia Commons) 5

•

Planets (

行星

):

8 planets and their satellites • lie close to a common plane.

• Planets move in

nearly

circular orbits around the sun in counter-clockwise sense as seen from “above”.

• The average distance between the sun and the earth is about 1.5  10 11 m, which is also called 1 AU (Astronomical unit).

6

• Self-rotation is also in the counter-clockwise sense as seen from “above”, except for Venus and Uranus.

• Orbits of planets are

not

evenly spaced distances between successive planets increase with their distances from the sun.

7

(Wikimedia Commons) 8

Scale model of planets

• • • • • • • •

Earth:

a grain of table salt (0.3 mm in diameter)

Moon:

A speck of pepper 1 cm away

Sun:

A plum 4 m away

Mercury, Venus, Mars:

grains of salt

Jupiter:

Apple seed 20 m from the sun

Saturn:

Smaller apple seed 36 m from the sun

Uranus:

lighter than salt grain

Neptune:

lighter than salt grain, 115 m from the sun 9

•

Dwarf planets (

矮行星

):

“Minor” planets.

The first three members are • Ceres ( 穀神星 ) --- in the Asteroid Belt • Pluto ( 冥王星 ) • Eris [formerly known as 2003 UB 313 ( 齊娜 )] or Xena 10

The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/grey). The arcs below the ecliptic are plotted in darker colours, and the red dot is the Sun. The diagram on the left is a polar view while the diagrams on the right are different views from the ecliptic.

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•

Small solar-system bodies:

include •

Asteroids (

小行星

):

Most can be found in the

Asteroid belt (

小行星帶

)

that lies between the orbits of Mars and Jupiter.

•

Comets (

彗星

):

“Dirty snow balls” moving in highly elliptical orbits around the sun In the solar system, there are the sun, the planets, the dwarf planets and small solar-system bodies.

12

Universal gravitation (

萬有引力

)

• In hammer throw ( 投鏈球 ), the tension in the chain keeps the ball moving around a centre. Without the tension, the ball will fly straight away.

(Wikimedia Commons) 13

• The gravitational force from the sun is just like the tension in an invisible chain that keeps a planet in its orbit around the sun.

14

• Inverse square law: The attractive force (

F

) between any two bodies is directly proportional to the product of their masses (

M

1 and

M

2 ) and is inversely proportional to the square of their separation (

r

).

F

 

GM

1

M

2

r

2

G

= 6.674  10 -11 Nm 2 kg -2 15

Circular motion (

圓周運動

)

• An object is moving in a circle of radius

r

. Its speed is

v

.

• What is the acceleration of the object?

r v

16

Time for the particle to travel from A to B is given by 

t

 2 

r

   2   1

v



r

 

v

To find the acceleration, we have to know the change in velocity.

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v c

 2

v

sin     2     Magnitude of acceleration

a

is then

a



v c



t



v

2

r

 2   sin 2  18

To find the instantaneous acceleration at A, we let   tend to 0. We also note that sin(

x

) =

x

when

x

is very small, hence

a



v

2

r

 2   sin 2  

v

2

r

 2      2 

v

2

r

Acceleration is

a



v

2

r

Direction: Perpendicular to the velocity and towards the centre, hence centripetal.

19

Example 1

• It is given that the gravitational acceleration on the Earth is 9.81 ms -2 and the radius of the Earth is 6373 km.

• Find the mass of the Earth.

Solution: Let

m

be an object on the Earth,

M

the mass of the Earth,

R

the radius of the Earth, the weight of the object is

W



GmM R

2 20

Hence

g M



W m



GM R

2 

gR

2 

G

9 .

81  ( 6373000 ) 2 6 .

674  10  11 kg  5.97

 10 24 kg From table: 5.97  10 24 kg 21

Example 2

• It is given that the mean radius

R

of the Earth’s orbit is 149600000 km. Mass of the sun

M

1.9891

 10 30 is kg. Find the period of revolution of the Earth around the sun.

Solution: The centripetal acceleration of the Earth is caused by the gravitational attraction from the sun.

v

2

R



F m



GM R

2 v: speed of the Earth m: mass of the Earth 22

v



GM R

The period

T

is therefore

T

 2 

R v

 2 

R

3

GM

 2  ( 1496000000 00 ) 3 6 .

674  10  11  1 .

9891  10 30 s  3 .

16  10 7 s  365.26

days 23

2. Order of the planets

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Radius Mass/Earth Rotation Period 2439 km 6052 km 6378 km 3397 km 71492 km 60268 km 25559 km 24764 km 0.055

0.815

1 0.107

317.8

95.2

14.5

17.2

59 days 244 days 1 day 24.7 hours 9.9 hours 10.7 hours 17.2 hours 16.1 hours Orbital Radius Revolution Period No. of Satellites 57.9  10 6 km 108.2  10 6 km 149.6  10 6 km 227.9  10 6 km 778.6  10 6 km 1433.5  10 6 km 2872.5  10 6 km 4495.1  10 6 km 88 days 224.7 days 365.2 days 1.88 yrs 11.8 yrs 29.4 yrs 83.8 yrs 164 yrs 0 0 1 2 63 56 27 13 24

Two kinds of planets: Terrestrial ( 類地行星 ) • • Mercury, Venus, Earth, Mars, all lie in the

inner

solar system • Relatively

dense

(~3-5 g cm -3 ), with cores of iron and nickel surrounded by a mantle of dense rocks.

Small

in size and mass 

weak

gravity  have a

few

satellites (e.g., one for Earth, two for Mars) and

thin

atmospheres,

no

ring systems • Their surfaces are scarred with craters.

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Two kinds of planets: Jovian ( 類木行星 )  Jupiter, Saturn, Uranus, Neptune, all lie in the outer solar system  Gaseous-like, mainly made up of hydrogen and helium, low-density (  1 g cm -3 )  They do not have solid surfaces, but have thick liquid layers inside, possibly with small rocky core of Earth ’s size.

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

Large

in size and mass 

strong

gravity  all have

ring systems (

光環系統

)

,

many

satellites and

thick

atmospheres of hydrogen, high atmospheric pressure and a lot of weather activities.

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3. Key features of each planet

A. Mercury ( 水星 )

• Too hot and gravity too weak to hold a thick atmosphere.

• Results: – retains a lot of craters – No thick atmosphere to retain heat, large temperature difference between day and night (-173 o C – 430 o C) (NASA) 28

• Mercury in fact has a thin layer of atmosphere, which is mainly made up of sodium and a little helium. The atmospheric pressure is almost zero. The presence of gaseous sodium means the temperature is high enough to allow sodium in rock be released. This is expected as Mercury is so near the Sun.

(NASA) 29

B. Venus ( 金星 ) Rotation of Venus

• Firstly, its self-rotation is in the

clockwise

sense.

(NASA) 30

• Secondly, the axis of rotation is almost perpendicular to the orbital plane. (For the Earth, the rotational axis tilts 23.5

o .) As a result, there is no seasonal change on Venus.

(NASA) 31

The atmosphere of Venus

• Venus has a thick atmosphere. The pressure is 90 times that of the Earth. The atmosphere consists of 90% of CO 2 , 3% of N 2 , and some SO 2 . The whole planet is completely covered by clouds made up of sulphuric acid (H 2 SO 4 ). As a result, the rain on Venus is acidic.

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• Much carbon dioxide 

Greenhouse effect (

溫室效應

) :

CO 2 traps the heat of solar radiation  very hot surface (470  C); the atmosphere is full of vapour of chemical compounds.

A schematic representation of the exchanges of energy between outer space, the Earth's atmosphere, and the Earth surface. The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect. (Wikimedia Commons) 33

Volcano Eista (NASA) Crator Cunitz: diameter ~ 48.5 km (NASA) 34

C. Mars ( 火星 )

• Like Earth, the axis of rotation tilts 24 o . Hence, there are seasonal changes on Mars. • Mars looks red because its soil contains minerals of iron (like rust).

(NASA) 35

The atmosphere of Mars

• Mars has a mass less than 11% of Earth’s, its gravity is weak • the atmosphere was much denser billions of years ago, but volatile gases escaped, leaving a thin atmosphere (1% of Earth’s). The chemical composition is mainly carbon dioxide (95%) and nitrogen (3%).

• Long ago water was dissociated by the solar radiation (unlike the earth, Mars has no ozone layer to shield the solar ultraviolet radiation) • no liquid water on surface, a little water combined with minerals in soil; polar caps ( 極冠 ) contain layers of frozen CO 2 (dry ice) with frozen water beneath.

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(NASA) 37

• Although the atmosphere consists mainly of carbon dioxide, it is too thin to trap heat. So, the surface temperature varies enormously, from -100 o C to -10 o C. Moreover, owing to the long distance from the Sun, the temperature is quite low on average.

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Features on the surface

• Mars is a cratered world having gigantic volcanoes (e.g. Olympus Mons 奧林匹斯山 ), deep canyons (e.g. Valles Marineris 水手谷 ), dry channels, and vast dust storm.

25 km above the surface and is 600 km in diameter (NASA) 5000 km long, 200 km wide and 7 km deep (NASA) 39

Large bodies of liquid water may have existed on Mars (NASA) 40

 Evidence of old channels and signs of erosion, seemingly carved by

running

liquid  billions of years ago Mars was much warmer (with a thicker atmosphere)  Large bodies of liquid water may have existed (NASA) 41

Its two satellites

Deimos Phobos (NASA) 42

D. Jupiter ( 木星 )

• The largest and most massive planet in our solar system. The mass of Jupiter is about 300 times that of the Earth, however its density is low. In fact, these are general features of Jovian planets.

• Almost completely made up of gases.

(NASA) 43

• The rotational period of Jupiter is about 10 hours, and such a high velocity flattens Jupiter at the two poles.

• Mainly made up of hydrogen, helium, and a small amount of methane and ammonia.

• The atmospheric pressure is extremely high, over 1000 times than that of the Earth. Because of the great pressure, the core of Jupiter is made up of metallic hydrogen. The rapid rotation of such metallic core explains the strong magnetic field of Jupiter. 44

Feature I: Dark belts and light zones

(NASA) 45

Feature II: Great Red Spot

(NASA) • A great cyclone lasting for at least 300 years.

• 3 times the size of the Earth.

• Red: presence of sulphuric compounds.

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(NASA) 47

Feature III: Ring system

(NASA) • The dark and thin ring of Jupiter. It is composed of small particles.

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Satellites

• Jupiter has 63 satellites, the four largest ones were discovered by Galileo.

(NASA) 49

Io ( 木衛一 )

(NASA) • Famous for its active volcanic activity that emits sulphuric compounds, and has a geologically young surface.

50

(NASA) 51

Erupting volcano (NASA) 52

A volcano spewing out gas.

(NASA) 53

Europa ( 木衛二 )

• A rocky world with an icy crust.

• There may be a lake under the icy surface.

(NASA) 54

Ganymede ( 木衛三 )

• The largest satellite in the solar system, its surface is old and is heavily cratered, crossed with grooved ( 有溝槽 ) terrain.

(NASA) 55

Callisto ( 木衛四 )

• A heavily cratered, dark surface.

(NASA) 56

E. Saturn ( 土星 )

• The second largest planet. It has 47 satellites.

• Atmospheric condition is similar to Jupiter, but the belts and zones seem less distinct.

• Average density is lower than water (0.7 g cm -3 ).

(NASA) 57

Ring system

• Three concentric rings (A, B and C) can be easily observed on Earth.

• Thickness ~ I km • Made of dust and ice.

• The most obvious gap is Cassini’s division.

(Wikimedia Commons) 58

Shepherd satellites for inner F ring Pandora (pan-DOR-uh) Prometheus (pra MEE-thee-us)

Mimas Rhea (NASA) Tethys Dione 60

Titan ( 土衛六 )

• The most famous satellite.

• It is cold enough to hold an atmosphere of nitrogen ( 氮 ) and methane ( 甲 烷 ).

(NASA) 61

F. Uranus ( 天王星 )

  Discovered in 1781 by William Herschel.

Uranus appears blue because of the methane in its atmosphere. It has much less distinct atmospheric circulation than Jupiter.

(NASA) 62

(NASA) 63

Shepherd satellites (NASA) 64

G. Neptune ( 海王星 )

 Astronomers in 19 th century found that Uranus ’ orbit deviated from a perfect ellipse, it was under the gravitational pull of an unknown outer planet.

 Newtonian mechanics predicted the mass and orbit of this planet.

 discovery of Neptune in 1846.

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(NASA) 66

 It is similar to Uranus in size, mass, and atmospheric condition.

 Cyclone patterns have been discovered (e.g.

Great Dark Spot

大黑斑 ).

(NASA) 67

(NASA) 68

Changing (NASA) 69

(NASA) 70

4. Asteroids, comets and meteoroids

Asteroids ( 小 行 星 )   Small rocky debris that revolve around the sun.

Most orbits lie in the

asteroid belt (

小 行 星 帶

)

between those of Mars and Jupiter.

 Only two dozens or so are larger than 200 km, most as small as  0.1 km, irregular in shape.

 Asteroids are either fragments of a planet broken up long ago, or primal rocks never managed to accumulate into a planet. Researchers favour the latter view.

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Gaspra ： 19  12  11 km Spins once every 7 hours (NASA) 72

Ida and its satellite Dactyl (NASA) 73

Comets ( 彗星 )

Comet Hale-Bopp (Wikimedia Commons) Comet Hyakutake (Wikimedia Commons) 74

• • • • They are dirty “snow balls”.

Nucleus (

彗核

):

is very small (a few km), it is the main solid body of a comet. Only this frozen part exists when a comet is far from the sun.

Coma (

彗髮

):

Dust and evaporated gas surrounding the nucleus. Its maximum size could be as large as Jupiter.

Tail (

彗尾

):

Vapourized materials directed away from the sun by solar wind (particles from the sun) and pressure of the sunlight. • Coma and tail are most pronounced when the comet is closest to the sun.

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Comets have highly elliptical orbits. Note the two distinct tails:Cyan for gas tail (controlled by the solar magnetic field), grey for dust tail ( bends due to the comet’s motion). (Wikimedia Commons) 76

Meteoroids ( 隕星 )

• Meteoroids are interplanetary debris hitting Earth, heated up by friction in Earth ’s atmosphere.

• appear as bright streaks of “shooting stars” called

meteors (

流星

).

• Most meteoroids are destroyed in the atmosphere; any parts that reach the ground are called

meteorites (

隕石

).

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 Some are fragments dislodged from comets, spreading along the comets ’ orbits.

Marília Meteorite, a chondrite H4, which fell in Marília, São Paulo state, Brazil, on October 5, 1971, at 5:00p.m. (Wikimedia Commons) 78

5. Stars and their colors

• When we heat something up, it will radiate electromagnetic waves. When the object is not very hot, it will be red. If it is hotter, it will be yellow, then white and finally blue. The color of a star depends only on its surface temperature, and nothing else.

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6. Constellations (

星座

)

•

Visual groupings

of stars.

• There are totally 88 constellations today, some added in modern days (e.g., Telescopium 望 遠 鏡 座 ).

• Usually

no real correlation

among the stars in the same constellations; they could be very far away from one another.

80

7. Galaxy (

星系

)

• Almost all the stars visible by naked eyes are in our galaxy, the Milky Way Galaxy.

• A galaxy is a collection of hundred billions of stars.

• Galaxies are categorized into three basic classes according to their shapes: Elliptical galaxies, spiral galaxies and irregular galaxies.

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Milky Way Galaxy

• About 200 billion stars whirling in a great wheel-like system; the sun is  8.5 kpc (1 pc  3.3 light years) from the galactic centre.

Artist's conception of the spiral structure of the Milky Way with two major, stellar arms and a bar. (Wikimedia Commons) 82

•

Disk component

contains almost all of the gas and dust in the galactic plane.

–

Spiral arms (

旋臂

):

Long spiral patterns of bright stars, star clusters, gas and dust.

Observed and extrapolated structure of the spiral arms (Wikimedia Commons) 83

•

Spherical component

–

Halo (

銀暈

):

Thin scattering of old, lower mass stars, globular star clusters; almost

no

gas and dust. –

Nuclear bulge (

核心

):

The most crowded part of spherical component around the galactic core; about 20000 light years in diameter; the center is obscured at visual wavelengths and requires radio or infrared observations.

84

• The universe contains  100 billion galaxies.

• Along the plane of Milky Way, dust clouds block our view of distant galaxies.

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Elliptical galaxies ( 橢圓星系 )

• Spherical or elliptical in shape, lacking in gas and dust, they contain relatively old, low-mass stars.

– Disk component is not obvious or missing The giant elliptical galaxy ESO 325-G004. (Wikimedia Commons) 86

Spiral galaxies ( 旋渦星系 )

An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457) (Wikimedia Commons) • contain gas, dust, and hot bright stars outlining spiral arms, having a mixture of star types.

• Obvious disk component.

• They are very luminous and therefore easy to find.

–  2/3 of all

known

galaxies are spiral, but they may make up only a small fraction of all galaxies 87

Irregular galaxies ( 不規則星系 )

• Irregular in shape, clouds of gas and dust mixed with both young and old stars.

– e.g., the Large Magellanic Cloud and Small Magellanic Cloud are neighbors of the Milky Way Galaxy. NGC 1427A, an example of an irregular galaxy about 52 Mly distant. (NASA) 88

Hubble classification of galaxies

Types of galaxies according to the Hubble classification scheme. An

E

indicates a type of elliptical galaxy; an

S

is a spiral; and

SB

is a barred-spiral galaxy. (Wikimedia Commons) 89

8. Space exploration

1957 1969 1972 1977 1989 1990 1996 1997 1998 2003 Sputnik Apollo 11 Pioneer 10 Voyager 1 and 2 Galileo Hubble Space Telescope Mars Pathfinder Cassini-Huygens International Space Station Shenzhou 5 First Earth orbiter First Manned Lunar Landing (Total six manned lunar landings, Apollo 17 shown below) First Jupiter Flyby Multiple Planet Flybys (still active) First Asteroid Flyby (on trip to Jupiter) First Mars Rover First Saturn Orbiter (date of first section) First Chinese Manned Earth orbiter 90