science f3 chapter 9-stars and galaxies-part 2-ppt

Choong XH, Catholic High School (2010)

STARS AND GALAXIESScience Form 3 Chapter 9

Last update: 26/8/2010


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


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


Part A Stars

F3 C9 STARS AND GALAXIES9.2 Stars and Galaxies


Questions

1. What is a star?2. What is a light year?3. What is the use of a spectroscope?4. Which is older, the Sun or a red giant?5. What can be formed as a result of a

supernova explosion?6. What is a galaxy?7. In which galaxy is our Solar System?


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


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.


Thermonuclear Reaction


Stars

• The Sun is a star. It is our nearest star.


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)


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


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.


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.


Stars

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


Stars

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


Sirius (天狼星 )

Canis Major (大犬星座 )


Rigel (猎户座 -参宿七 )

Orion (猎户座 )


Betelgeuse (猎户座 -参宿四 )


Polaris (北斗七星 )


Part B Classification of Stars



Classification of Stars

• Stars can be classified according to their characteristics such as:– Temperature and colour– Brightness– Chemical composition– Size– density


1. Temperature and Colour


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


2. Brightness


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


3. Chemical composition



• The chemical composition of a star can be determined by analysing its light with a spectroscope


Spectroscope



• Most stars are composed of hydrogen and helium

• Some stars contain other elements such as iron and carbon


• 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.

http://www.compadre.org/informal/items/detail.cfm?ID=1011

http://www.compadre.org/informal/images/features/solarabsoprtlineslarge-12-0.jpg

http://www.colorado.edu/physics/2000/quantumzone/fraunhofer.html

http://csep10.phys.utk.edu/astr162/lect/light/absorption.html


• 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).

http://www.astro.uiuc.edu/~kaler/sow/spectra.html


4. Size


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


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.


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.


Part C Formation of Stars



Formation of Stars

• Stars are constantly being formed in outer space

• Scientist believe that a star is formed from a nebula (星云 )


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


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


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.


Part D Death of Stars



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


1. White dwarf (白矮星)


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.


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


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


2. Neutron star (中子星 )


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.


2. Neutron star

• The inner gases collapse so quickly that the red giant explodes, causing a supernova explosion


Supernova explosion (超新星爆发 )


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


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 )


The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration. Click image for enlargement. Credit: Casey Reed


3. Black holeAstronomers using the National Science Foundation's Very Large Array (VLA) radio telescope have greatly strengthened the case that super massive black holes at the cores of galaxies may have formed through mergers of smaller black holes. Their VLA studies showed that a globular star cluster in the galaxy M31 probably has a black hole with 20,000 times the mass of the Sun at its core.


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.


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.


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


Chandra observations of the galaxy NGC 1365 have captured a remarkable eclipse of the supermassive black hole at its center. A dense cloud of gas passed in front of the black hole, which blocked high-energy X-rays from material close to the black hole. This serendipitous alignment allowed astronomers to measure the size of the disk of material around the black hole, a relatively tiny structure on galactic scales. The Chandra image (shown in the inset) contains a bright X-ray source in the middle, which reveals the position of the supermassive black hole. An optical view of the galaxy from the European Space Observatory's Very Large Telescope shows the context of the Chandra data.


Part E Constellations (星座 )



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.


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.


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.


Part F Galaxies (銀河系 )



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.


Galaxies

• Galaxies can be classified according to their shapes:– Spiral– Elliptical– Irregular– Barred spiral


1. Spiral galaxy


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.


2. Elliptical galaxy

Elliptical Galaxy M87


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


3. Irregular galaxyUtah 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 region


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


4. Barred spiral galaxyBig, beautiful, barred spiral galaxy NGC 1300 lies some 70 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.

http://heritage.stsci.edu/2005/01/supplemental.html

http://www.hawastsoc.org/deepsky/eri/index.html

http://www.hawastsoc.org/deepsky/eri/index.html

http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/01/

http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/01/image/a+warn

http://heritage.stsci.edu/2005/01/fast_facts.html

http://apod.nasa.gov/apod/image/0501/ngc1300_hst_core.jpg

http://csep10.phys.utk.edu/astr162/lect/galaxies/hubble.html

http://apod.nasa.gov/apod/ap030925.html

http://hubblesite.org/newscenter/newsdesk/archive/releases/1997/01/text/


4. Barred spiral galaxy

• This type of galaxy is uncommon• It has a cross-like formation in the centre


The Milky Way


• http://www.atlasoftheuniverse.com/localgr.html

http://www.atlasoftheuniverse.com/localgr.html

http://www.atlasoftheuniverse.com/localgr.html


The Universe within 12.5 Light YearsThe Nearest Stars


The Universe within 50000 Light YearsThe Milky Way Galaxy


The Universe within 500000 Light YearsThe Satellite Galaxies


The Universe within 5 million Light YearsThe Local Group of Galaxies


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


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• 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


Questions

1. What is a star?2. What is a light year?3. What is the use of a spectroscope?4. Which is older, the Sun or a red giant?5. What can be formed as a result of a

supernova explosion?6. What is a galaxy?7. In which galaxy is our Solar System?

science f3 chapter 9-stars and galaxies-part 2-ppt

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