# Guide to this presentation Introduction Stars in the Night Sky What is a Star

## Guide to this presentation Introduction Stars in the Night Sky What is a Star

Dailey, Don, News Director has reference to this Academic Journal, PHwiki organized this Journal Star in a BoxExploring the lifecycle of starsWhite slides are section headings, in addition to are hidden from the presentation. Show or hide the slides in each section as appropriate to the level required.Rough guide to the levels:Beginner: KS3Intermediate: KS4 (GCSE)Guide to this presentationIntroductionBasics of what a star is in addition to how we observe them.Level: Beginner +

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Stars in the Night SkyWhat is a StarA cloud of gas, mainly hydrogen in addition to helium.The core is so hot in addition to dense that nuclear fusion can occur.This is where the energy comes from that makes the stars shine.The fusion converts light elements into heavier ones.This is where all the atoms in your body have come from.All the Stars in the Night Sky are DifferentLuminosity:Tells us how bright the star is, i.e. How much energy is being produced in the core.Colour:Tells us the surface temperature of the star.Rigel

Units of LuminosityWe measure the luminosity of every day objects in Watts.How bright is a light bulb 10-20WBy comparison, the Sun outputs: 380,000,000,000,000,000,000,000,000 Watts (380 million million million million Watts!) or 3.8 x 1026 Watts This is the amount of energy it emits per secondWe measure the luminosity of other stars relative to the Sun.Units of TemperatureTemperature is measured in Kelvin.The Kelvin temperature scale is the same as the Celsius scale, but starts from -273o. 0 K (or -273oC) is known as absolute zero-273 oC-173 oC0 oC100 oC0 K100 K273 K373 K1000 oC1273 KKelvin = Celsius + 273Measuring the TemperatureThe colour of a star indicates its temperature.Red stars are cold, in addition to blue stars are hot.The Sun is a yellow star, its temperature is 5800 K.

Stefan-Boltzmann LawBlack Body RadiationMore detail about the colour in addition to temperature of a star, using black body radiation.Level: Advanced +Black Body radiationA black body is A body that absorbs all wavelengths of EM radiation in addition to can emit all wavelengths of EM radiation.A star is a good approximation of a black body.The intensity of each wavelength of radiation a star emits depends on its temperature.

Black Body RadiationHow hot is the SunThis is a graph of the Suns energy output  its Blackbody CurveIts peak wavelength is around 0.5m, this is in the visible region of the Electromagnetic SpectrumWiens LawThe peak intensity of the radiation is related to the surface temperature of the star.Temperature (K) = Wiens constant (m.K) / peak wavelength (m)b = 2.898 x 10-3 m.KLooking at the graph on the previous slide, can we determine how hot the surface of the Sun is

Wiens LawHertzsprung-Russell DiagramAn introduction to the H-R diagram, on which various stars will be plotted  try to get the students to suggest where they might appear be as long as e they are plotted.Level: Beginner +The Hertzsprung Russell DiagramWe can compare stars by showing a graph of their temperature in addition to luminosity.Where do the stars in the night sky fit on this graph

Luminosity (relative to Sun)110010,0000.010.0001Temperature (Kelvin)25,00010,0007,0005,0003,000We start by drawing the axes:Luminosity up the vertical axis (measured relative to the Sun)Temperature along the horizontal axis (measured in Kelvin)Where would you mark the Sun on the plotIt has Luminosity of 1 relative to itselfIts temperature is 5800 KThe stars Vega in addition to Sirius are brighter than the Sun, in addition to also hotter. Where would you put themSome stars are much cooler in addition to less luminous, such as the closest star to the Sun, Proxima Centauri. Where would you plot these These stars are called red dwarfs.SunSiriusVegaProxima CentauriIn fact, most stars can be found somewhere along a line in this graph. This is called the Main Sequence.Main SequenceLuminosity (relative to Sun)110010,0000.010.0001Temperature (Kelvin)25,00010,0007,0005,0003,000SunSiriusVegaProxima CentauriMain SequenceRigelBetelgeuseDenebArcturusAldebaranSirius BBut not all stars lie on the main sequence. Some, such as Arcturus in addition to Aldebaran, are much brighter than the Sun, but cooler. Where would these lie on the diagramThese are red giant stars.The bright star Betelgeuse is even more luminous than Aldebaran, but has a cooler surface.This makes it a red supergiant.Even brighter than Betelgeuse are stars like Deneb in addition to Rigel, which are much hotter.These are blue supergiants.Some of the hottest stars are actually much fainter than the Sun. Which corner would they be inThese are white dwarfs, such as Sirius B which orbits Sirius.Luminosity (relative to Sun)110010,0000.010.0001Temperature (Kelvin)25,00010,0007,0005,0003,000Main SequenceGiantsSupergiantsWhite DwarfsAlmost all stars we see are in one of these groups, but they change groups during their lives.As stars evolve they change in luminosity in addition to temperature.This makes them change position on the Hertzprung-Russell diagram.SunSiriusVegaProxima CentauriBetelgeuseArcturusRigelDenebSirius B

Luminosity (relative to Sun)110010,0000.010.0001Temperature (Kelvin)25,00010,0007,0005,0003,000SunThe Sun has been on the Main Sequence as long as billions of years, in addition to will remain there as long as billions more.But eventually it will swell into a giant star, becoming more luminous but cooler.Luminosity (relative to Sun)110010,0000.010.0001Temperature (Kelvin)25,00010,0007,0005,0003,000SunAt this point it is a red giant star.It will get then hotter in addition to slightly brighter.Luminosity (relative to Sun)110010,0000.010.0001Temperature (Kelvin)25,00010,0007,0005,0003,000SunFinally nuclear fusion in the core will cease.The Sun will become a white dwarf, far less luminous, but with a hotter surface temperature.

Star in a BoxAt this point, run star in a box to explore the Hertzsprung-Russell diagram as long as different mass stars.Level: Beginner + Nuclear fusionThe processes taking place in the centre of a star.Level: Intermediate +Nuclear FusionThe luminosity of a star is powered by nuclear fusion taking place in the centre of the star converting hydrogen into helium.The temperature in addition to density must be high enough to allow nuclear fusion to occur.Stars are primarily composed of hydrogen, with small amounts of helium.

The proton-proton chainAt temperatures above 4 million Kelvin hydrogen nuclei fuse into heliumStable StarsWhile the star is on the Main Sequence, it is in a stable state.The inward as long as ce of gravity trying to collapse it, in addition to the radiation pressure outwards from Hydrogen fusion are balanced.Running out of HydrogenAs the hydrogen runs out, the energy released from fusion decreases which reduces the outward as long as ce.The as long as ces are now unbalanced, in addition to the larger as long as ce of gravity causes the star core to collapse.If the star is massive enough, the core temperature increases until helium fusion starts.

Energy lost in a Helium fusion reaction From last slide, Mass lost = m = 7.50 x 10-30 kg E = m c2 Energy lost = E = 7.50 x 10-30 kg x (3 x 108 ms-1)2 = 6.75 x 10-13 J = 4.22 MeV

## Dailey, Don News Director

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