QUESTIONS1. Name the 3 layer of the Sun’s atmosphere. Corona, chromosphere and photosphere2. 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 enough3. 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
QUESTIONS4. 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 molecules5. How is heat generated in the Sun’s core? By thermonuclear reaction in the core
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.
STARS The Sun is a star. It is our nearest star.
SUN’S ENERGY: NUCLEAR FUSIONWhen the star’s core gets hot enough (millions ofdegrees) Nuclear Fusion starts. Hydrogen + Hydrogen Helium + ENERGY (+ neutron) Fusion continues to make heavier & heavier elements. Stops at Iron
NUCLEAR FUSIONNuclear Fusion is the process that releases energy on stars. Ittakes place in the core of the star.It starts when gravity pulls nebula together and temperaturesincrease to millions of degrees.In nuclear fusion, the nuclei (protons and neutrons) of atomsfuse together to make heavier elements and release energy. H + H Helium + (neutron) + ENERGY
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 knownstar to our solar system, is shown herein an image taken on April 6, 2007, at15:12 UTC. This is a composite of 3exposures each 30 seconds longthrough I (infrared), R (red), and V(visible) photometric filters. The imagewas rendered in SAOImage ds9 withred, green and blue corresponding tothose three filter bands. For the 650x480pixels in this cropped image, the field ofview shown is 5.8x4.3 arc minutes withnorth up and west to the right. Proximais the 11.05 V magnitude spectral typeM5.5V 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.
CLASSIFICATION OF STARS Stars can be classified according to their characteristics such as: Temperature and colour Brightness Chemical composition Size density
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 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 suns visible spectrum. To find out more, see Atomic Absorption and Emission Spectra.
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).
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 ofhigh density matter, or "density waves". Asstars move through an arm, the spacevelocity of each stellar system is modifiedby the gravitational force of the higherdensity. (The velocity returns to normalafter the stars depart on the other side ofthe arm.) This effect is akin to a "wave" ofslowdowns moving along a highway full ofmoving cars. The arms are visible becausethe high density facilitates star formation,and therefore they harbor many bright andyoung stars.
The spiral arms are thought to be areas of high density matter, or "density waves". As stars move throughan arm, the space velocity of each stellar system is modified by the gravitational force of the higherdensity. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect isakin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible becausethe high density facilitates star formation, and therefore they harbor many bright and young 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 STARSThe 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 STARSThis 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.
DEATH OF STARSWhen the hydrogen in a star is used up, the nuclear reactions in the star will stopThe star will cool and eventually dieWhen a star dies it will leave behind: A white dwarf, A neutron star, or A black hole
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 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
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)
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
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.
CONSTELLATIONSIn 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.
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 GALAXYExamples are the Milky Way (银河系) and the Andromeda (仙女座)A spiral galaxy is very largeIt has a thick bright centre and thins out at the edgeThe centre has more stars which are older those at the edge.
2. ELLIPTICAL GALAXYIt is the most common type of galaxyIt is smaller than a spiral galaxyIt consists of very old starsIt cannot form new stars because it does not contain any more dust and gas. Elliptical Galaxy M87
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, youll see many, many galaxies clumped together in this
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
Big, beautiful, barred spiral galaxy NGC 1300 lies some 704. 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 weeks 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 galaxys 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.
4. BARRED SPIRAL GALAXY This type of galaxy is uncommon It has a cross-like formation in the centre
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 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