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PHY3: Astrophysics. Lesson 1 Introduction to Astrophysics, Observing stars, determining distances. Astrology, Astronomy, Cosmology or Astrophysics?. Astrology

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PHY3: Astrophysics

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PHY3: Astrophysics

Lesson 1

Introduction to Astrophysics, Observing stars, determining distances


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Astrology, Astronomy, Cosmology or Astrophysics?

  • Astrology

    • Astrology began about 4000 years ago in the religions of Babylonia that believed the future of the nation and ruling class depended on the planets, Sun, and Moon and their motions.

    • People came to believe that the position of the Sun, Moon, and planets at a person's birth was especially significant.


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Astrology, Astronomy, Cosmology or Astrophysics?

  • Astronomy

    • While most astrologers were developing ways to predict the future of human events by careful observations of the sky, early astronomers were developing ways to predict the motions of the planets, Sun, and Moon.

    • Motivated by the idea that if they could accurately predict the motions of the planets then they would be able to accurately predict the future of persons.

    • Astronomy broke away from astrology and became a science when astronomers became more interested in explaining what made the planets move the way they do and not in divining the future and interactions of individuals.


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Astrology, Astronomy, Cosmology or Astrophysics?

  • Cosmology is:

    • The study of the large-scale structure, the origins and the future of the Universe.

  • Astronomy is:

    • The art of observation and the measurement

    • radio, optical, IR, UV, x-ray, gamma-ray, neutrino, gravity-wave studies

    • measure positions, brightnesses, spectra, structure of gas clouds, planets, stars, galaxies, globular clusters, clusters of galaxies, superclusters, quasars, etc.


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Astrology, Astronomy, Cosmology or Astrophysics?

  • Astrophysics is:

    • The application of Physics to these observations to understand and interpret them.

    • The subject of this module!


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Observing Stars

  • Astronomers commonly make observations of stars by taking photographs.

  • Electronic charge coupled devices (CCDs) produce an electrical signal which depends on how much light they receive.


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Total Pixels:  11 millionArray:  4008 x 2672 pixelsPixel Size:  9 microns square

CCDs

  • CCDs are silicon chips divided into a grid of millions of identical pixels.

  • Semiconductor so normally very few free electrons. When light shines on a pixel, electrons are released with the number depending on the brightness.

  • Potential well traps the electrons underneath each pixel.

  • Once picture is taken electrons are shunted from one potential well to another so they all come out in sequence from one corner of the CCD.

  • Animation - http://faulkes-telescope.com/resources/animations/large/3d_ccd


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Sensitivity versus clarity

  • Large grains on film or CCD picture cells (pixels) are sensitive to light as they collect much light. But the image is unclear and “grainy”.

  • Small grains or pixels produce a sharper image, but less sensitive.

  • Astronomers balance sensitivity (needing large detecting areas) and clarity (needing small detecting areas).


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Efficiency and linearity

  • Astronomers need film and CCDs to be efficient, so the detect very small amount of light but need to detect difference between different levels of light.

  • CCDs must give a linear signal so output signal is proportional to light received.


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Twinkle, twinkle little…

  • Changes in density of the Earth’s atmosphere causes stars to twinkle. Air and dust absorb and scatter > 30% of visible light incident on Earth.


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Space observatories

  • Infrared Astronomical Satellite (IRAS)

    • launched ’83 and lasted 10 months.

    • Mapped 96% sky 4 times and discovered around 500,000 sources

  • Cosmic Background Explorer (COBE)

    • Launched ’89 and operational 4 years.

    • Detected fluctuations in cosmic microwave background remnant from Big Bang and density ripples are believed to have produced structure formation as observed in the universe today: clusters of galaxies and vast regions devoid of galaxies.


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Space observatories

  • Hubble Space Telescope (HST)

    • Launched ’90 and likely to re-enter orbit after 2010.

    • Optical, ultraviolet, near-infrared instruments.

    • Main mirror suffered from spherical aberration, severely compromising the telescope's capabilities.

    • Serviced in ‘93 and the telescope was restored to its planned quality and became a vital research tool as well as a public relations boon for astronomy.


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Space observatories

  • Chandra X-ray Observatory

    • Launched ’99 and still operational

    • Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes, requiring a space-based telescope to make these observations.

  • Spitzer Space Telescope

    • Launched ’03 and still operational

    • IR instruments targets forming stars, planets and other galaxies.

    • In ’05 SST was first to capture light from extra-solar planet.


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Space observatories

  • James Webb Space Telescope (JWST)

    • Projected to replace HST ~2013.

    • Will operate in IR 1-27 micrometres.

    • Mission has 4 main components:

      • to search for light from the first stars and galaxies which formed in the Universe after the Big Bang

      • to study the formation and evolution of galaxies

      • to understand the formation of stars and planetary systems

      • to study planetary systems and the origins of life


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Astronomical distances

  • A scale model of the Solar System shows how BIG stuff is and how REALLY BIG distances are!


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Astronomical Unit

  • Copernicus ~1500 measured the relative size of planetary orbits to within 1%.

  • 1 AU is the mean distance between the Earth and the Sun.

  • Halley, in 1716 (age 60) pointed out that the transit of Venus in 1761 and 1769 could be used to work out the size of 1 AU.

  • 1 AU = 1.496 x 1011 m


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Light year

  • All electromagnetic waves travel at the speed of light, c, in a vacuum.

  • A light year (ly) is the distance that an EM wave travels in one year.

    • c = 3.00 x 108 ms-1.

    • By c = d/t and t = 1 year = 31,536,000 seconds

    • So d = 1ly = 9.46 x 1015 m.

    • 1 ly is also equal to about 63,000 astronomical units (AU)

    • The nearest star is about 4.3 ly away which means that it takes light 4.3 years to travel from Proxima Centauri to Earth.


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Parallax

  • The distance to nearby stars can be measured using parallax.

  • tan p = 1 AU / d

  • So distance (in AU), d = 1 AU / tan p

  • [this also defines the parsec (pc).

    • 1 pc is the distance of a star that has a parallax of one arc second (1/3600 º) using a baseline of 1 AU.

    • 1 pc = 3.26 ly = 3.09 x 1016 m]


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Problems

  • Parallax angles down to 1/360000° can be measured. What distance is this in AU, metres and ly?

  • Complete the following table:


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