Light and Telescopes Chapter 5

Radio Interferometry The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Just as for optical telescopes, the resolving power of a radio telescope is  min = 1.22 /D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light. → Use interferometry to improve resolution!

The Largest Radio Telescopes The 100-m Green Bank Telescope in Green Bank, WVa. The 300-m telescope in Arecibo, Puerto Rico.

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.

Observatories in Space (I) Avoids turbulence in the Earth’s atmosphere Extends imaging and spectroscopy to (invisible) infrared and ultraviolet Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s. The Hubble Space Telescope

Observatories in Space The Spitzer Space Telescope Launched in 2003 Infrared light traces warm dust in the Universe. The detectors need to be cooled to -273 o C (-459 o F).

The Future of Space-Based Optical/Infrared Astronomy: The James Webb Space Telescope

High-EnergyAstronomy: X-rays and gamma-rays from space can also be observed from astronomy satellites: Combined visual + X- ray image (taken by Chandra) of a supernova remnant The Chandra X-ray Observatory

Atoms and Starlight Chapter 6

The Amazing Power of Starlight Just by analyzing the light received from a star, astronomers can retrieve information about a star’s… 1.Total energy output 2.Surface temperature 3.Radius 4.Chemical composition 5.Velocity relative to Earth 6.Rotation period

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.

Atomic Structure An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it. Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud.

If you could fill a teaspoon just with material as dense as the matter in an atomic nucleus, it would weigh ~ 2 billion tons!!

Different Kinds of Atoms The kind of atom depends on the number of protons in the nucleus. Helium 4 Different numbers of neutrons ↔ different isotopes Most abundant: Hydrogen (H), with one proton (+ 1 electron). Next: Helium (He), with 2 protons (and 2 neutrons + 2 el.).

Electron Orbits Electron orbits in the electron cloud are restricted to very specific radii and energies. r 1, E 1 r 2, E 2 r 3, E 3 These characteristic electron energies are different for each individual element.

Atomic Transitions An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy. All other photons pass by the atom unabsorbed. E ph = E 4 – E 1 E ph = E 3 – E 1 (Remember that E ph = h*f) Wrong energy The photon is absorbed, and the electron is in an excited state.

Color and Temperature Orion Betelgeuze Rigel Stars appear in different colors, from blue (like Rigel) via green / yellow (like our sun) to red (like Betelgeuze). These colors tell us about the star’s temperature.

Blackbody Radiation The light from a star is usually concentrated in a rather narrow range of wavelengths. The spectrum of a star’s light is approximately a thermal spectrum called Blackbody Spectrum. A perfect blackbody emitter would not reflect any radiation. Thus the name “Blackbody”.