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Astrophysics

Astrophysics. Contents. Lenses & Optical Telescopes Radio Astronomy Classification of Stars Cosmology. Lenses & Optical Telescopes. Convex Lens : light rays converge Power (Dioptres, D) = 1 / focal length (m). Lenses & Optical Telescopes. Lens Formula :

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Astrophysics

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  1. Astrophysics

  2. Contents • Lenses & Optical Telescopes • Radio Astronomy • Classification of Stars • Cosmology

  3. Lenses & Optical Telescopes • Convex Lens: light rays converge Power (Dioptres, D) = 1 / focal length (m)

  4. Lenses & Optical Telescopes • Lens Formula: 1/f = 1/v + 1/u v = image distance, u = object distance M = v/u M = magnification (no units)

  5. Lenses & Optical Telescopes • Refracting Telescope: bends light with lenses - objective lens: produces small real image - eyepiece: magnifies image Angular Magnification: M = α/β • α: angle (rads) subtended by the object to the unaided eye • β: angle (rads) subtended by the image to the eye

  6. Reflecting Telescopes • Disadvantages: - refract light of different colours by different  distorted image - do not transmit 100 % of the light; some is lost - large lenses are complex to make - for good magnification, the objective lens must have a very long focal length  telescope becomes very long

  7. Reflecting Telescopes • Resolving Power: how good a telescope is at distinguishing between two objects that are close to one another • Resolving power of eye: angle = 3x10-4radians • Observations from Earth: are poor because of atmosphere turbulence (light is scattered) and light pollution • Diffraction: As light enters the telescope, a small gap diffracts the light wave  Fraunhofer Diffraction Pattern • Overlapping of fringes can occur: θ = λ/D θ = angular separation (rads), D = aperture width (m)

  8. Reflecting Telescopes • Recording the Image: Charged Coupled Device (CCD) - films have fine grain for high resolution - CCD (size of a stamp) connects to computer - CCD has millions of pixels on it The eye has a quantum efficiency of 1%; CCD is approx. 70%:

  9. Radio Telescopes • Radio Astronomy: - revealed the existence of radio sources, such as quasars and pulsars - analyses chemical elements in stellar objects - tracks the movement of planets using the Doppler effect - looked at microwave radiation which gives evidence for the Big Bang • Radio waves penetrate dust (nebulae etc.) so radio telescopes allow us to view inwards, into the centre of the galaxy • Radio telescopes have poor resolution and are absorbed by light pollution on Earth

  10. Classification of Stars • Astronomical Unit (AU): - Distance between Earth and Sun: 1AU = 1.5 x 1011m • Light Year (ly): - Distance travelled by light in 1 year: 1ly = 9.46 x 1015 m • Parsec (pc): - An object at 1pc subtends an angle of 1 arc second for a distance of 1AU: 1pc = 3.086 x 1016m - 1 pc = 3.26 ly

  11. Classification of Stars • Apparent Magnitude: apparent brightness of stars on a scale • Very bright stars have negative magnitudes • Magnitude 1 star = 100 times brighter than magnitude 6 star • Difference in magnitudes = n - ratio of brightnesses = 2.512n • Absolute Magnitude: stars at an arbitrary distance of 10pc m: apparent magnitude M: absolute magnitude d: distance (pc)

  12. Classification of Stars • Classification by Temperature: spectroscopy - Balmer lines arise from electron transitions in hydrogen atoms - As the electron drops from high levels to the second energy level, photons are emitted - Emissions of photons appear black  absorption spectrum Emission Spectrum Absorption Spectrum

  13. Classification of Stars • Low Temperatures: - energy levels are rare as electrons remain in ground state • High Temperatures: - Electron transitions occur at higher levels so there are comparatively few Balmer transitions • Intermediate Temperatures: - Many electrons perform Balmer transitions, so there are strong absorption lines

  14. Classification of Stars • Black Body: an object that absorbs all radiation • Perfect absorber = perfect emitter (of all radiation… including visible) • Stars are approximated to black bodies: - hot object emits radiation across range of λ - peak in intensity at a given λ - hotter object = higher peak - hotter object = shorter peak λ(λmax) Wien’s Displacement Law: λmaxT = constant = 0.00289 mK

  15. Classification of Stars • Luminosity of Stars: power; energy given out per second • Stefan’s Law: P = σAT4 σ = 5.67x10-8Wm-2K-4 star = sphere…  A = 4πr2 • For a star… P = 4πr2σT2 • Observing Stars is difficult due to: - light pollution which is problematic for viewing dim stars - poor weather such as clouds - dust (from pollutants) - turbulent atmosphere (which is why stars “twinkle”  put telescope in orbit, or failing that, on a very high mountain

  16. Life & Death of a Star • Hertzsprung-Russell Diagram:

  17. Life & Death of a Star • Hertzsprung-Russell Diagram: - most stars lie along the main sequence - very bright = blue; very dim = red - Luminosities: Sun = 1, Betelgeuse = 20 000, Sirius = 0.002 - Surface Temp: Sun = 5800K, Sirius = 20 000K

  18. Life & Death of a Star • Stage 1: Stars are born in a region of high density Nebula, which condenses into a huge globule of gas and dust and contracts under its own gravity. Huge amounts of thermal energy and IR radiation are emitted

  19. Life & Death of a Star • Stage 2: A region of condensing matter will begin to heat up and start to glow forming Protostars. If a protostar contains enough matter, the internal temperature can reach 15 million degrees Celsius

  20. Life & Death of a Star • Stage 3: Ignition temperature is reached… fusion begins. Hydrogen fuses into Helium • Stage 4: Energy is released which prevents the star from collapsing further. Star stabilises and shines  main sequence lasts approx. 10 billion years

  21. Life & Death of a Star • Stage 5: Helium core contracts and reactions occur in a shell around the core • Stage 6: Core is hot enough for the Helium to fuse to form Carbon. Outer layers begin to expand, cool and shine less brightly. Expanding star is now called a Red Giant

  22. Life & Death of a Star • Stage 7: Helium core runs out, and the outer layers drift away from the core as a gaseous shell. This gas that surrounds the core is called a Planetary Nebula

  23. Life & Death of a Star • Stage 8: Approx. 20% of original star mass has been lost. Remaining mass (core) becomes a White Dwarf as the star cools and dims. Once it ceases shining  Black Dwarf. It is believed that up to ½ the mass of the galaxy is due to Black Dwarfs White Dwarf Black Dwarf

  24. Life & Death of a Star • Novae & Supernovae: With massive stars, electrons & protons will combine to form neutrons. The inner core will spontaneously collapse (approx. 1 sec duration) as neutrinos are ejected at which point  Neutron Star. The outer layers then collapse onto the core, which cannot be compressed further. The pressure of this outer layer collapse causes the wave of movement to reverberate and a violent shock wave outwards occurs. This explosion is called a Supernova

  25. Life & Death of a Star • Black Hole: With very massive stars, the inner core collapses but continues to do so until it becomes nothing more than a point mass. Point mass  singularity, and this breaks the laws of Physics. The strength of gravity inside a black hole is so massive that nothing can escape, not even light (which is why they are not visible). The perimeter at which light can/cannot escape is called the Event Horizon, but far away from this point, everything else is sucked in. Black holes are invisible, but they can be found, as nearby stars will be sucked in

  26. Cosmology • Doppler Effect: shift in frequency and wavelength of waves that causes its properties to change • Objects that travel with v<<c obey this law: and - Moving towards observer = positive speed; away = negative - Moving away  frequency decreases  wavelength increases - Moving towards  frequency increases  wavelength decreases • Moving away = red shift • Moving towards = blue shift

  27. Cosmology • Big Bang: age of universe = 1/H0 • If galaxies are moving away from us, they must have been closer together millions of years ago. If Hubble’s graph is used, the origin of this movement = approx. 10 000 million years old • Earth = approx. ½ age of universe (5 000 millions years old) • The universe will either: • Continue to expand forever • Return in on itself back to its origin (Big Crunch)

  28. Cosmology • Quasars: Extremely bright objects that are most distant from us that we know of. Smaller than a galaxy, they are more luminous and contain elements we do not know exist. They have a huge red shift • They could be: • massive black holes? • responsible for consuming 10 solar masses per year • responsible for ejecting jets of matter at high velocity (Several hundred light years is probably a safe distance!)

  29. Summary • Lenses & refracting telescopes • Reflecting telescopes • Radio telescopes • Classification of stars • Life & death of a star • Cosmology

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