Telescope parameters. Light-gathering power (ability to see faint objects) Resolving power (ability to see fine details) Magnification (least important). Other factors:. Optical quality Atmospheric conditions Light pollution. Seeing.
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Telescope parameters • Light-gathering power (ability to see faint objects) • Resolving power (ability to see fine details) • Magnification (least important)
Other factors: • Optical quality • Atmospheric conditions • Light pollution
Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing
The Best Location for a Telescope Far away from civilization – to avoid light pollution
The Best Location for a Telescope (2) Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence and other weather effects
Well-known astronomer at Cerro-Tololo, Chile Now Physics Professor at TAMU Nick Suntzeff Cerro-Tololo observatory Supernova in Centaurus A
Traditional Telescopes (1) Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions.
Advances in Modern Telescope Design Modern computer technology has made possible significant advances in telescope design: 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror 2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers
Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence
Examples of Modern Telescope Design (1) Design of the Large Binocular Telescope (LBT) The Keck I telescope mirror
Interferometry Recall: Resolving power of a telescope depends on diameter D: amin = 1.22 l/D. This holds true even if not the entire surface is filled out. • Combine the signals from several smaller telescopes to simulate one big mirror • Interferometry
Examples of Modern Telescope Design (2) The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes
CCD Imaging CCD = Charge-coupled device • More sensitive than photographic plates • Data can be read directly into computer memory, allowing easy electronic manipulations Negative image to enhance contrasts False-color image to visualize brightness contours
The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object
Exploring other wavelengths • Radio • Infrared • UV • X-ray • Gamma-ray
Radio Astronomy Recall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.
Science of Radio Astronomy Radio astronomy reveals several features, not visible at other wavelengths: • Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe. • Molecules (often located in dense clouds, where visible light is completely absorbed). • Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.
Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna) Amplified signals are stored in computers and converted into images, spectra, etc.
The Largest Radio Telescopes The 300-m telescope in Arecibo, Puerto Rico The 100-m Green Bank Telescope in Green Bank, WVa.
Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light Use interferometry to improve resolution!
Radio Interferometry (2) The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!
Radio observations with Very Long Baseline Interferometry (VLBI) are thousands of times more precise than optical observations (good enough to easily pinpoint a source the size of a pea in New York when sitting in Paris)
Discovery of the Infrared Frederick William Herschel 1738-1822 He directed sunlight through a glass prism to create a spectrum (the rainbow created when light is divided into its colors) and then measured the temperature of each color. Herschel used three thermometers with blackened bulbs (to better absorb heat) and, for each color of the spectrum, placed one bulb in a visible color while the other two were placed beyond the spectrum as control samples. As he measured the individual temperatures of the violet, blue, green, yellow, orange, and red light, he noticed that all of the colors had temperatures higher than the controls. Moreover, he found that the temperatures of the colors increased from the violet to the red part of the spectrum. After noticing this pattern Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region where no sunlight was visible. To his surprise, he found that this region had the highest temperature of all.
Cups with cold and hot water chameleon
Infrared Astronomy ( ~ 1-300 m) Most infrared radiation is absorbed in the lower atmosphere. However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed. NASA infrared telescope on Mauna Kea, Hawaii
NASA’s Space Infrared Telescope Facility (Now Spitzer Space Telescope)
The Hubble Space Telescope • Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s • Avoids turbulence in the Earth’s atmosphere • Extends imaging and spectroscopy to (invisible) infrared and ultraviolet
Hubble Deep Field 10 day exposure photo! Over 1500 galaxies in a spot 1/30 the diameter of the Moon 1011 galaxies in the observable universe Farthest and oldest objects are 12-13 billion ly away! Space observations as a time machine
Ultraviolet Astronomy • Ultraviolet radiation with l < 290 nm is completely absorbed in the ozone layer of the atmosphere. • Ultraviolet astronomy has to be done from satellites. • Several successful ultraviolet astronomy satellites: IRAS, IUE, EUVE, FUSE • Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.
X-Ray Astronomy • X-rays are completely absorbed in the atmosphere. • X-ray astronomy has to be done from satellites. X-rays trace hot (million degrees), highly ionized gas in the Universe. NASA’s Chandra X-ray Observatory
Gamma-Ray Astronomy Gamma-rays: most energetic electromagnetic radiation; traces the most violent processes in the Universe The Compton Gamma-Ray Observatory
Stars as black-body emitters It takes 10,000 years for a photon emitted in the core to reach the surface!
Black Body Radiation (1) The spectrum of a star’s light is approximately a thermal spectrum called a black body spectrum. A perfect black body emitter would not reflect any radiation. Thus the name “black body”. The spectrum of a black body emitter is described by a universal formula first suggested by Planck. It depends only on surface temperature.
Two Laws of Black Body Radiation 1. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases. Wien’s displacement law: lmax≈ 3x106 nm / T(K) (where T(K) is the temperature in Kelvin).
Color and Temperature Stars appear in different colors, from blue (like Rigel) via green / yellow (like our sun) to red (like Betelgeuse). These colors tell us about the star’s temperature! Orion Betelgeuse Rigel
L = A*s*T4 Two Laws of Black Body Radiation 2. The hotter an object is, the more luminous it is. The Stefan-Boltzmann law: Radiation Flux, or power emitted by unit area of a black-body emitter, is proportional to the fourth power of its surface temperature: s = Stefan-Boltzmann constant Luminosity, or total power: whereA = surface area
Note units!! Wien’s law: The Stefan-Boltzmann law
Example of black-body emitter: our sun Yellow light: ~ 520 nm Maximum of the black-body spectrum: Wien’s law Surface temperature T =3x106 nm/520 nm 5800 K The Stefan-Boltzmann law Radius = 7x105 km Total radiated power (luminosity) L = T4 4R2 = 4x1026 W
The Spectra of Stars Inner, dense layers of a star produce a continuous (blackbody) spectrum. Cooler surface layers absorb light at specific frequencies. => Spectra of stars are absorption spectra.
Bad news: stars are too far away to scoop their matter for testing Good news: they consist of the same atoms as the stuff on the Earth Fraunhofer in early 1800’s measures solar spectrum and identifies it with the spectrum of hydrogen in the lab English astronomer Lockyer, in the late-1800's, discovered an unknown element in the Sun, i.e. a set of spectral lines which did not correspond to elements in the lab. He named this element helium (Latin for Sun element).
Light and Matter Spectra of stars are more complicated than pure blackbody spectra. characteristic lines, called absorption lines. To understand those lines, we need to understand atomic structure and the interactions between light and atoms.