Form 3 PMR Science Chapter 9 Stars and Galaxies

1. Science Form 3 Chapter 9
STARS AND GALAXIES

2. QUESTIONS
1. Name the 3 layer of the Sun’s atmosphere. Corona,
chromosphere and photosphere
2. Which layer of the Sun’s atmosphere can only be seen in a total
eclipse of the Sun? Why? Corona and
chromosphere, because they are not bright
enough
3. Why do sunspots appear to move across the Sun’s surface from
the west to the east? Because the Sun rotates from
west to east

3. QUESTIONS
4. What causes aurorae in the Earth’s north and south poles?
Charged particles deflected to the poles of the
Earth by the Earth’s magnetic field collide with air
molecules
5. How is heat generated in the Sun’s core? By
thermonuclear reaction in the core

10. STARS
 A star is a natural hot ball of gases in
space which gives out its own heat
and light
 A star is usually made up of almost
entirely of hydrogen and helium

12. STARS
 A star gets its heat and light from
thermonuclear reactions.
 A thermonuclear reactions happens
when the hydrogen atoms of a star
combine to form helium atoms. This
process releases plenty of heat and
light.

13. THERMONUCLEAR REACTION

14. STARS
 The Sun is a star. It is our nearest star.

17. SUN’S ENERGY: NUCLEAR FUSION
When the star’s core gets hot enough (millions of
degrees) Nuclear Fusion starts.
Hydrogen + Hydrogen  Helium + ENERGY (+ neutron)
Fusion continues to make heavier & heavier elements.
Stops at Iron

19. NUCLEAR FUSION
Nuclear Fusion is the process that releases energy on stars. It
takes place in the core of the star.
It starts when gravity pulls nebula together and temperatures
increase to millions of degrees.
In nuclear fusion, the nuclei (protons and neutrons) of atoms
fuse together to make heavier elements and release energy.
H + H  Helium + (neutron) + ENERGY

20. STARS
 Stars are huge balls of hot gases. But
they look small to us because they
are very far way.
 The distance of a star from the Earth
is measured in light years
 One light year is the distance light
travels in one year (light travels at
300 000 km/s)

21. STARS
 One year = (300,000 X 60 X 60 X 24
X 365) km =9,460,800,000,000 km
9,500,000,000,000km

22. STARS
 After the Sun, the nearest star is Alpha Proxima. It is about 4.2 light
years away and can only be seen through a telescope.
Proxima Centauri, the nearest known
star to our solar system, is shown here
in an image taken on April 6, 2007, at
15:12 UTC. This is a composite of 3
exposures each 30 seconds long
through I (infrared), R (red), and V
(visible) photometric filters. The image
was rendered in SAOImage ds9 with
red, green and blue corresponding to
those three filter bands. For the 650x480
pixels in this cropped image, the field of
view shown is 5.8x4.3 arc minutes with
north up and west to the right. Proxima
is the 11.05 V magnitude spectral type
M5.5V red star at the center.

25. ALPHA CENTAURI (半人马座阿尔法星)
 Alpha Centauri is 4.3 light years away from the Earth, but it can
be seen with the naked eye.

26. STARS
 Two very bright
stars which can be
easily be identified
are Sirius (天狼星)
and Rigel (猎户座
-参宿七)

27. STARS
 In ancient times,
travelers used
stars to guide
them in their
journeys across
the deserts and
the seas.

36. CLASSIFICATION OF STARS
 Stars can be classified according to
their characteristics such as:
 Temperature and colour
 Brightness
 Chemical composition
 Size
 density

37. 1. TEMPERATURE AND COLOUR

38. 1. TEMPERATURE AND COLOUR
 The temperature of a star affects it
colour.
 The hottest star is blue while the
coolest is red
 Blue stars are young stars
 Red stars are old stars which are
cooling

39. 2. BRIGHTNESS

40. 2. BRIGHTNESS
 Stars can be
classified according
to their brightness.
 The brightest star,
i.e. the Sun, has a
magnitude of -26.5
 The dullest star is
given a magnitude
of +26.0
 The brighter the
star, the lower its
magnitude is

42. 3. CHEMICAL COMPOSITION

43. 3. CHEMICAL COMPOSITION
 The chemical composition of a star can be
determined by analysing its light with a
spectroscope

44. SPECTROSCOPE

45. SPECTROSCOPE

46. 3. CHEMICAL COMPOSITION
 Most stars are composed of hydrogen and
helium
 Some stars contain other elements such
as iron and carbon

47.  Spectra - Jan 1, 2007
 This image (hi-res version) shows absorption lines in the sun's visible
spectrum. To find out more, see Atomic Absorption and Emission Spectra.

48.  Analysis of these lines determines the chemical composition of a star. For an
explanation of why the lines of calcium--rather than hydrogen--dominate the
solar spectrum, go to "Analysis of absorption Lines" in Spectra, from the
University of Illinois (the other sections are interesting, too).

49. 4. SIZE

50. 4. SIZE
 Stars vary a great deal in size
 White dwarfs are smaller than the
Earth
 Red giants are about 100 times
bigger than the Sun
 Supergiants are about 400 times
bigger than the Sun

52. 5. DENSITY
 The densities of stars vary a lot because of their sizes
 A big star has a low density (density = mass/volume)
The spiral arms are thought to be areas of
high density matter, or "density waves". As
stars move through an arm, the space
velocity of each stellar system is modified
by the gravitational force of the higher
density. (The velocity returns to normal
after the stars depart on the other side of
the arm.) This effect is akin to a "wave" of
slowdowns moving along a highway full of
moving cars. The arms are visible because
the high density facilitates star formation,
and therefore they harbor many bright and
young stars.

53. The spiral arms are thought to be areas of high density matter, or "density waves". As stars move through
an arm, the space velocity of each stellar system is modified by the gravitational force of the higher
density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is
akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because
the high density facilitates star formation, and therefore they harbor many bright and young stars.

55. FORMATION OF STARS

56. FORMATION OF STARS
 Stars are constantly being formed in outer space
 Scientist believe that a star is formed from a nebula (星云)

57. FORMATION OF STARS
 A nebula is a large cloud of dust and gas, mainly hydrogen, in
outer space.
 The dust and gas in the nebula condense and become
compressed due to the gravitational attraction of the particles

58. FORMATION OF STARS
The compressed dust and gas become very
hot and dense until a very high temperature is
reached.
The very high temperature causes
thermonuclear reactions to take place. In a
nuclear reaction, hydrogen atoms combine or
fuse together to form helium atoms

59. FORMATION OF STARS
This nuclear reaction produces a tremendous
amount of heat and light. So the ball of dense
compressed dust and gas gives out its own
heat and light. Thus a star is formed.
A star takes millions of years to form.

62. DEATH OF STARS
When the hydrogen in a star is used
up, the nuclear reactions in the star will
stop
The star will cool and eventually die
When a star dies it will leave behind:
A white dwarf,
A neutron star, or
A black hole

64. 1. WHITE DWARF
 When a star of similar size as the Sun has almost completely
used up its hydrogen, the outermost layer of hydrogen burns
fiercely and causes the star to expand.
 The star becomes red and is called a red giant.

65. 1. WHITE DWARF
 The outer layer of the red giant is unstable and diffuses into
space.
 The inner gases collapse and form a hot core known as a white
dwarf

67. 1. WHITE DWARF
 A white dwarf is about the size of the Earth
 Eventually the white dwarf cools and does not give out heat and
light. It becomes a black dwarf in space

70. 2. NEUTRON STAR
 If a dying star is about 10 times the size of the Sun, it expands
and becomes a red giant.
 The red giant formed is very big.

71. 2. NEUTRON STAR
 The inner gases collapse so quickly that the red giant explodes,
causing a supernova explosion

73. 2. NEUTRON STAR
 The remaining core shrinks with such force that the electrons and
protons combine to form neutrons.
 The core left is called a neutron star

74. 2. NEUTRON STAR
 A neutron star:
 has a mass of 1.5 and 2.5 times the
mass of the Sun
 Is very dense (because of its
neutrons)
 Rotates very fast
 Releases pulsing electromagnetic
waves (because of this, it is also
called a “pulsar” 脉冲星)

77. 3. BLACK HOLE
 A black hole is formed from a very big red giant. It is called a
supergiant.
 A supergiant has a size of about 500 times the size of the Sun.

78. 3. BLACK HOLE
 The explosion of a supergiant causes a gigantic supernova explosion.
 The core left is so dense and its gravitational force is so great that it
attracts even light.

79. 3. BLACK HOLE
 The attraction of light by the core makes the region around it dark.
Hence the core is called a black hole
 A black hole cannot be seen but its effect can be felt

84. CONSTELLATIONS
 A constellation is a group of stars which forms a particular pattern
that can be seen from the Earth.
 Astronomers have recognized 88 constellations.

85. CONSTELLATIONS
 Some constellations can be seen throughout the year, while some
can be seen only at certain times of the year. This is because the
Earth is revolving around the Sun.

86. CONSTELLATIONS
In ancient times, people used these constellations
to guide them in their journeys and to tell the
seasons.
12 of the constellations form the zodiac. This
name means the path of the animals, because
many of the constellations are named after
animals.

91. GALAXIES
 A galaxy is an assembly of millions of stars in outer space.
 The stars in a galaxy revolve around the centre of the galaxy.

92. GALAXIES
 Galaxies can be classified according to their shapes:
 Spiral
 Elliptical
 Irregular
 Barred spiral

93. 1. SPIRAL GALAXY

95. 1. SPIRAL GALAXY
Examples are the Milky Way (银河系) and the
Andromeda (仙女座)
A spiral galaxy is very large
It has a thick bright centre and thins out at the
edge
The centre has more stars which are older those
at the edge.

96. 2. ELLIPTICAL GALAXY
Elliptical Galaxy M87

97. 2. ELLIPTICAL GALAXY
It is the most common type of galaxy
It is smaller than a spiral galaxy
It consists of very old stars
It cannot form new stars because it
does not contain any more dust and
gas.
Elliptical Galaxy M87

98. 3. IRREGULAR GALAXY
Utah Skies
Challenge
Object, Irregular
Galaxy
NGC1275(mag11.6)
Irregular Galaxy
NGC1275 and
neighbors This
galaxy is located
near the center of
the Perseus Galaxy
Cluster -- a group of
some 530
members.
Depending on the
size of your
telescope and the
quality of your
skies, you'll see
many, many
galaxies clumped
together in this

99. 3. IRREGULAR GALAXY
 It does not have a fixed shape
 It can form new stars because it possesses dust and gases
(nebula)
 It consists of young stars

100. Big, beautiful, barred spiral
galaxy NGC 1300 lies some 70
4. BARRED SPIRAL GALAXY million light-years away on the
banks of the constellation
Eridanus. This Hubble Space
Telescope composite view of the
gorgeous island universe was
released at this week's meeting of
the American Astronomical Society
as one of the largest Hubble
images ever made of a complete
galaxy. NGC 1300 spans over
100,000 light-years and the
Hubble image reveals striking
details of the galaxy's dominant
central bar and majestic spiral
arms. In fact, on close
inspection the nucleus of this
classic barred spiral itself shows a
remarkable region of spiral
structure about 3,000 light-years
across. Unlike other spiral
galaxies, including our own Milky
Way, NGC 1300 is not presently
known to have a massive central
black hole.

101. 4. BARRED SPIRAL GALAXY
 This type of galaxy is uncommon
 It has a cross-like formation in the centre

102. WHEN GALAXY OVERLAPS

108. THE MILKY WAY
 Our Solar System belongs to a galaxy called the Milky Way
 The Milky Way is a spiral galaxy
 It contains about 10,000 million stars together with dust and
gases.
 It is very big. It takes 100,000 light years to cross its diameter

109. THE MILKY WAY
 The Solar System is located towards the edge of the galaxy
 The Sun rotates from west to east and at the same time, orbits
the centre of the Milky Way