WHY STUDY ASTROPHYSICS ?

1. Caroline Chisholm College Physics ASTROPHYSICS • To gain an understanding of our universe and our role in it Learn about how the universe operates --> modern science • Study of our solar system allows us to study data from other planets and assess the nature of our planet, its origins and our resources WHY STUDY ASTROPHYSICS? • Space technology gives us communication satellites, accurate weather forecasts, GPS, minerals exploration, long term monitoring of earth • Observations lead to Laws such as Newtonian mechanics, which had applications for machines, construction and Industrial Revolution Technology (e.g. medicine, materials, techniques) developed for space have valuable uses on earth

2. Contextual Outline Caroline Chisholm College Physics The wonders of the Universe are revealed through technological advances based on tested principles of physics. Our understanding of the cosmos draws upon models, theories and laws in our endeavour to seek explanations for the myriad of observations made by various instruments at many different wavelengths. Techniques, such as imaging, photometry, astrometry and spectroscopy, allow us to determine many of the properties and characteristics of celestial objects. Continual technical advancement has resulted in a range of devices extending from optical and radio-telescopes on Earth to orbiting telescopes, such as Hipparcos, Chandra and HST. Explanations for events in our spectacular Universe, based on our understandings of the electromagnetic spectrum, allow for insights into the relationships between supernovae, star formation and evolution, and extreme events, such as high gravity environments of a neutron star or black hole. This module increases students’ understanding of the nature and practice of physics and the implications of physics for society and the environment.

3. Caroline Chisholm College Physics 1. Our understanding of celestial objects depends upon observations made from Earth or from space near the Earth Discuss Galileo’s utilisation of the telescope to identify features of the Moon Based on what he had heard of a Dutch idea, Galileo ground and assembled lenses to construct his own telescopes. Galileo used the telescope to observe the Heavens. After careful observation of dark patches (shadows) on the Moon, he noted that they changed with the angle of the sun. He deduced and wrote that the Moon was not smooth, uniform and perfectly spherical (as had been believed), but uneven, rough and full of cavities and prominences. He was able to work out a way of calculating mountain heights on the Moon using their shadows. Galileo was the first to scientifically record and publish observations of the heavens and he clearly demonstrated the advantages of using the telescope for detailed observations. He used scientific reasoning to interpret his results and was able to put forward important ideas about the nature of celestial bodies which contradicted the accepted views and had implications that the Earth, with its imperfections, may also be a heavenly body.

4. Caroline Chisholm College Physics Discuss why some wavebands can be more easily detected from space Electromagnetic radiation of different wavelengths is absorbed by different amounts in the atmosphere. Wavebands which are absorbed are therefore not easily detected from ground-based systems, so space telescopes (such as the Hubble Space Telescope) are preferable. Very short wavelength radiation such as X-rays and Gamma rays interact with and are absorbed by oxygen and nitrogen in the atmosphere Most Ultraviolet (UV) radiation interacts with ozone and so cannot reach the ground. Infrared radiation interacts with water vapour and carbon dioxide and so is (incompletely) absorbed by the atmosphere Long waves interact with electrons in the ionosphere So only near-UV, visible light, near-Infrared, radio and microwaves make it to the ground with out much absorption by the atmosphere. All others are more easily detectable from space

5. Define the terms resolution and sensitivity of telescopes Caroline Chisholm College Physics sensitivity resolution the light-gathering power of the telescope (directly proportional to the square of the diameter of the objective) the minimum angular separation between two equal point sources such that they can be just barely distinguished as separate sources Identify data sources, plan, choose equipment or resources for, and perform an investigation to demonstrate why it is desirable for telescopes to have a large diameter objective lens or mirror in terms of both sensitivity and resolution

6. Discuss the problems associated with ground-based astronomy in terms of resolution and absorption of radiation and atmospheric distortion Caroline Chisholm College Physics Limitations of ground-based astronomy Atmospheric distortion Local changes in the temperature and density of the atmosphere cause problems called ‘atmospheric seeing’ Defocusing atmospheric ‘cells’ act as concave or convex lenses, changing focal length Image motion or blur ‘wedges’ of air moving along change refractive index and position of image. The random refraction of light causes ‘scintillation’ or ‘twinkling’. Stars are so far away that they are almost ‘point sources’ of light. Planets tend to be a larger angular diameter than the effects of atmospheric turbulence, hence they don’t twinkle! Astronomers refer to a ‘seeing disk’ which represents the angular diameter of the star’s image, which has been increased by the turbulence in the atmosphere. The smaller the diameter, the better the seeing conditions. Most earth-based optical telescopes can do no better than one arcsecond, though there are some (e.g. one on a mountain top in Hawaii) that can get the seeing disk as small as 0.5 arcseconds.

7. Discuss the problems associated with ground-based astronomy in terms of resolution and absorption of radiation and atmospheric distortion Caroline Chisholm College Physics Limitations of ground-based astronomy Absorption of radiation As discussed previously, some wavebands are absorbed by the atmosphere and can be more easily detected from space. This obviously means they are not suitable for ground-based astronomy. Space telescopes such as Hubble are more suitable for detecting far-infrared, far-UV, X-ray and gamma-ray wavebands, although some are detectable from telescopes on high mountains and Antarctica, where atmospheric effect are less. Molecules of air (Nitrogen and Oxygen) and dust particles also scatter light, which reduces its intensity and thus further limits the effectiveness of ground-based optical telescopes. Other resolution problems: THE OBJECTIVE LENS IN GROUND-BASED REFRACTING TELESCOPES CAN ONLY BE SO BIG BEFORE IT STARTS TO SAG UNDER ITS OWN WEIGHT AND DISTORT THE IMAGES. RADIO TELESCOPE IMAGES DECREASE IN RESOLUTION AS THE WAVELENGTH INCREASES. LONGER WAVELENGTHS PRODUCE BLURRIER IMAGES. OTHER PROBLEMS WITH ABERRATION, DISTORTION OF FIELD, MATERIAL IMPERFECTIONS ETC. CAN AFFECT BOTH GROUND-BASED AND SPACE TELESCOPES.

8. Outline methods by which the resolution and /or sensitivity of ground-based systems can be improved, including:– adaptive optics – interferometry – active optics Caroline Chisholm College Physics • Interferometry • combine signals from 2 or more radio telescopes to create a resolution equivalent to a telescope of diameter equal to antennae separation • different distances travelled cause destructive or constructive interference • antennae separations of 20 km give resolutions of 0.1 arc seconds • VLBI can give 0.00001 arc secs (1,000,000 x optical telescope) Adaptive optics position of star measured and corrected via computer and moving mirrors size compared to a point source and changes corrected to maintain focus • Active optics • uses a slow feedback system that samples the light using a wavefront sensor to send signals to correct sagging or other deformities in the mirror • back of the mirror is fitted with actuators that push or pull the mirror into the correct shape • Lightweight Mirrors • can give bigger diameters without problems of bending and flexing under own weight • multimirrors are used with active optics to also counteract this problem

9. Gather, process and present information on new generation optical telescopes Caroline Chisholm College Physics Read about (gather and process- yes you!) these and write down (present - yes you!) information from pages 280 and 281 of the text. (actually I think that means someone else has done the gathering - lucky you!) ...now process and present!