Transcript 13Oct_2014
Reading Unit 28, Unit 29, Unit 30 Will not be covered by the first exam As a blackbody object becomes hotter, it also becomes ____________ and _____________ • • • • a. more luminous, redder b. more luminous, bluer c. less luminous, redder d. less luminous, bluer • • • • Compare two blackbody objects, one at 200K and one at 400K. How much larger is the flux from the 400K object, compared to the flux from the 200K object? a. Twice as much b. Four times as much c. Eight times as much d. Sixteen times as much Star A and star B appear equally bright in the sky. Star A is twice as far away from Earth as star B. How do the luminosities of stars A and B compare? • • • • a. Star A is 4 times as luminous as star B b. Star A is 2 times as luminous as star B c. Star B is 2 times as luminous as star A d. Star B is 4 times as luminous as star A Which of the following factors does *not* directly influence the temperature of a planet? • • • • a. The luminosity of the Sun b. The distance from the planet from the Sun c. The color of the planet d. The size of the planet The strangest telescope [tele-scope (look from a distance)] • IceCube Observatory to indirectly detect neutrinos photomultiplier tubes Cherenkov Effect blue Cherenkov light in a nuclear power reactor charged particle traveling in transparent medium faster then light (in the medium) Sonic boom condensates moisture Cherenkov Effect charged particle traveling in transparent medium faster then light (in the medium) neutrinos produce energetic charged particles that emit Cherenkov light Sonic boom condensates moisture Telescopes • Telescopes have been used for hundreds of years to collect light from the sky and focus it into an eyepiece. An astronomer would then look through this eyepiece at planets, nebulae, etc. • The human eye is not very sensitive to dim light, and was replaced in astronomy by the film camera. • Film is sensitive to only around 10% of the impinging light, and is usually replaced by a… The Charge-Coupled Device (CCD) • • The CCD, similar to those found in commercial digital cameras and phones, utilizes the photoelectric effect to collect around 75% of the visible light that is focused on it! It has revolutionized astronomy – images can be recorded and downloaded to a computer anywhere in the world for analysis • The science of developing new methods for sensing, focusing and imaging light in astronomy is called instrumentation Outside the visible spectrum • • • • Many objects of astronomical interest are visible only in wavelengths other than the visible! Much can be learned from studying a star, planet or nebula in multiple wavelengths. Radio telescopes can be used from the ground to image pulsars and other bodies • Observations in other wavelengths require instrumentation to be lifted above the Earth’s atmosphere. X-ray, Gamma ray and infrared wavelength telescopes are currently in orbit! Modern Telescopes • • Modern telescopes are designed to collect as much light as possible, and must be built to exacting standards. Collected light is of nanometer wavelength, so the telescopes must be extremely precise to keep the waves coherent for maximum efficiency Radio Telescopes • Radio telescopes, like the one in Arecibo, Puerto Rico, collect radio waves from astronomical objects and events Radio Telescopes • Radio telescope arrays to achieve large collecting areas National Radio Astronomy Observatory (U.S.A.) Size Matters! • • • Aperture size is very important when collecting light! A large collecting area allows astronomers to image dim and distant objects. For a telescope with an aperture a distance D in diameter, Collecting Area = p 4 ´ D2 Refraction • • • Light moves at a fixed speed, c. The value of c changes depending on what substance, or medium, it moves through. The speed of light in vacuum is around 300,000 km/s. Its speed through glass or water, however, is slightly slower • • If a beam of light (or light ray) enters a new medium at an angle, light on one side of the ray enters first, and slows. This slowing of one part of the ray causes the ray to change direction, similar to driving a car from asphalt onto sand can make a car swerve. This bending of a light ray’s path is called refraction Refraction in Water Dispersion • • • • The amount a light is diffracted depends on its wavelength A prism spreads the light out, using this effect This dispersion of light is a problem in refracting telescopes, as the focal plane will be at a slightly different location for each wavelength of light This leads to chromatic aberration, a blurring effect. Lenses • A lens is a specially shaped piece of glass that bends light rays passing through it so that they focus a particular distance away (the focal length) at a particular location (the focal plane). • A sensor such as a human eye, a camera or CCD, if placed in the focal plane can image the light Refracting Telescopes • Telescopes that use lenses to focus light are called refracting telescopes, or refractors. • Large refractors are difficult to build! – Glass is heavy, and glass lenses must be supported only by their rims, a difficult engineering problem – Glass sags under its own weight, defocusing the light! – Refractors suffer from chromatic aberration, a blurring effect due to changes in the focal plane of the lens for different wavelengths of light Reflecting Telescopes • Reflecting telescopes, or reflectors, use a curved mirror to focus light • Mirrors can be supported from behind, and so can be much larger than refractors • Larger sizes mean that more light can be collected and focused, allowing astronomers to image dimmer or more distant objects • Most modern telescopes are reflectors. Different styles of reflectors X-Ray reflectors • • X-rays only reflect at glancing angles, otherwise they are absorbed or pass through the mirror! X-Ray mirrors are designed to gently reflect the incoming photons, focusing them at the end of a long tube-shaped array of mirrors Very Large Mirrors • Reflectors can be made very large if multiple mirrors are used as the primary mirror. • The Keck Telescope uses 36 large mirrors to create a single huge primary. • The positions of the mirrors are precisely measured by lasers, and can be individually adjusted to keep them perfectly aligned.