Download
1 / 47
470 likes | 586 Vues
P7.4 The Sun, the stars and their surroundings. Images. The Pleiades is a group of bright stars about 500 light-years away. With a spectrometer, the light from each star is broken up into a spectrum, revealing slight differences in their colour.
E N D
The Pleiades is a group of bright stars about 500 light-years away With a spectrometer, the light from each star is broken up into a spectrum, revealing slight differences in their colour.
All objects (including stars) emit some electromagnetic radiation. As the temperature rises, the amount of radiation at high frequencies increases
A graph of the spectrum tells us about intensity as well as showing which frequencies are present. What TWO things do you think will happen to this graph as the object gets hotter ?
Robert Bunsen used his burner to heat different elements and investigate the light given off. Not all colours of the spectrum are seen. In fact different elements gave off (emitted) different colours
What causes an element to produce its own unique emission spectrum?
An electron gives out a single photon of light as it drops from one energy level to another.
Only certain energy levels are allowed so only certain colours are emitted
The spectrum of the Sun – its like the opposite of an emission spectrum – see the dark lines?
Light photons from the surface of the Sun must pass through its atmosphere before it reaches us. Gases in the Sun’s (or star’s) atmosphere absorb some of these light photons
An electron jumps from one energy level to another when the atom absorbs a single photon of light. All elements have different energy levels
Only those photons with particular energy values will be absorbed 6 5 3 4
The missing frequencies in the absorption spectrum of hydrogen correspond exactly to the frequencies seen in its emission spectrum.
Lines in the emission spectrum of hydrogen (bottom) correspond to some of the dark absorption bands in the Sun’s spectrum (top). Other lines in the absorption spectrum are due to other elements in the solar atmosphere, including helium.
You should be able to... recall that the removal of electrons from atoms is called ionisation and explain how electron energy levels within atoms give rise to line spectra recall that specific spectral lines in the spectrum of a star provide evidence of the chemical elements present in it use data on the spectrum of a star, together with data on the line spectra of elements, to identify elements present in it
The Gas Laws and Absolute Zero Mass of gas Pressure of gas Volume of gas Temperature of gas
Particles of a gas collide with the walls of its container. This causes pressure.
Keep this constant The Gas Laws and Absolute Zero Keep this constant Mass of gas Pressure of gas Volume of gas Temperature of gas
Understand that the volume of a gas is inversely proportional to its pressure at a constant temperature and explain this using a molecular model Flash animation
Here we keep the mass of gas constant again and the VOLUME of gas Flash
Here we keep the mass of gas constant again and the PRESSURE of gas Flash
understand that both the pressure and the volume of a gas are proportional to the absolute temperature explain why the pressure and volume of a gas vary with temperature using a molecular model
You should be able to recall that –273°C is the absolute zero of temperature, and convert temperatures in K to temperatures in °C (and vice versa) 0°C = 273K 350K = 77°C 27°C = 300K Can you interpret absolute zero using a molecular model and kinetic theory ?
As long as the temperature does not change...., pressure × volume = constant P1V1 = P2V2 = P3V3 = ....
As long as the volume does not change.... pressure / temp (K) = constant P1 = P2 = P3 T1 T2 T3
As long as the pressure does not change.... Volume / temp (K) = constant V1 = V2 = V3 T1 T2 T3
A star is born - Protostars A protostar forms from a cloud of gas (mostly hydrogen and helium) which is pulled together by gravity. Gravity ‘compresses’ the gases (increased pressure) so increasing the temperature. Every particle in the dust cloud attracts every other particle. As they ‘fall inwards’ they move faster and particles collide with each other. The fastest particles are in the centre of the cloud and faster moving particles means a higher temperature. The young protostar attracts even more dust and gas. Eventually its core becomes so dense and hot that nuclear fusion starts and a star is born.
The core of a star is where the temperature and density are highest and where most nuclear fusion takes place ....and radiated into space as electromagnetic radiation from the star’s surface (photosphere) Energy is transported by photons of radiation through the radiative zone.... ...and by convection currents through the convective zone....
The surface of the Sun, showing the granularity caused by the presence of convective cells.
About 100 years ago newer ideas still made the Sun only 25 million years old. Would have burned out in only 50000 years!! Nuclear processes discovered in the early 20th Century provided a possible explanation of the Sun’s energy source 18th Century Sun powered by chemical reactions, burning like a fire. If brought close enough together, hydrogen nuclei can fuse into helium nuclei releasing energy. This is called nuclear fusion The fusion process is a little more complicated than this
During the fusion process, one of the protons becomes a neutron. This releases two tinier particles — a positron and a neutrino The positron immediately meets an electron and the two self-annihilate in a POOF! of energy, while the neutrino passes through the sun and into space. The positron immediately meets an electron and the two self-annihilate in a POOF! of energy, while the neutrino passes through the sun and into space. Gamma rays
Can you complete & interpret nuclear equations for fusion in stars to include the emission of positrons to conserve charge 2 protons (hydrogen nuclei) fusing to make deuterium Mass numbers balance. Emission of positron (+vely charged electron) keeps atomic number the same before and after. Surprisingly mass on the right is slightly less than on the left. At each stage in the fusion, mass changes into energy Use Einstein’s equation E = mc2 is to calculate the energy released during nuclear fusion and fission (where E is the energy produced, m is the mass lost and c is the speed of light in a vacuum) NB energy is liberated when light nuclei fuse to make heavier nuclei with masses up to that of the iron nucleus
Watch the two animations HR_an Star Life Cycle _an
recall that ..... ...the Hertzsprung-Russell diagram is a plot of temperature and luminosity and identify regions on the graph where supergiants, giants, main sequence and white dwarf stars are located ...in a main sequence star, hydrogen fusion to helium takes place in the core ...a star leaves the main sequence when its core hydrogen runs out; it swells to become a red giant or supergiant and its photosphere cools ...the more massive the star, the hotter its core and the heavier the nuclei it can create by fusion
...and also.... ... in a red giant or supergiant star, helium nuclei fuse to make carbon, followed by further reactions that produce heavier nuclei such as nitrogen and oxygen ...a low-mass star (similar to the Sun) becomes a red giant, which lacks the mass to compress the core further at the end of helium fusion; it then shrinks to form a white dwarf ...in a white dwarf star there is no nuclear fusion; the star gradually cools and fades ...in a high-mass star (several times the mass of the Sun) nuclear fusion can produce heavier nuclei up to and including iron; when the core is mostly iron, it explodes as a supernova creating nuclei with masses greater than iron and leaving a dense neutron star or a black hole.
Astronomers have found convincing evidence of planets around hundreds of nearby stars but even if only a small proportion of stars have planets, many scientists think that it is likely that life exists elsewhere in the Universe ...but no evidence of extraterrestrial life (at present or in the past) has so far been detected.
