lect07 04

Chapter 5: 

Chapter 5 Light, Telescopes, and Spacecraft Light and its properties Telescopes and Observatories Spacecraft Typical test questions

Light and Matter: 

Light and Matter The amount of light is called intensity Studying spectra of celestial bodies one can learn a wealth of information about them

Absorption and Emission in Gases: 

Absorption and Emission in Gases Since electrons in atoms can have only specific energies, the atoms can absorb or release energy only in these amounts Electron gets energy, jumps to an excited state, release the energy, and falls back down The energy is emitted as a photon of light The photon has exactly the same energy that the electron has lost

Types of Spectra: 

Types of Spectra Emission line spectrum consists of photons emitted as each electron falls back to lower levels Absorption line spectrum appears when photons are absorbed, causing electrons to jump up in energy Each element or molecule produces its own distinct set of spectral lines Examples of spectra

Emission by Hydrogen: 

Emission by Hydrogen

Hydrogen lines in the visible: 

Hydrogen lines in the visible

Examples of Spectra: 

Examples of Spectra

Thermal Radiation: 

Thermal Radiation ``Complex’’ objects - planets, stars, people - produce thermal radiation Its spectrum depends only on the object’s temperature Hotter objects emit more total radiation per unit surface area The radiated energy is proportional to the fourth power of the temperature Hotter objects emit photons with a higher average energy

Temperature and Color: 

Temperature and Color

Temperature and Intensity: 

Temperature and Intensity

Reflected light: 

Reflected light When the light (for example, sunlight) strikes an object (ground, clouds, people), we see only the wavelengths of light that are reflected Different objects (fruits, rocks, atmospheric gases) reflect and absorb light at different wavelengths

The Doppler Shift: 

The Doppler Shift Radial motion of a distant object can be determined due to the Doppler effect The Doppler effect causes shifts in the wavelengths of light If an object is moving toward us, its entire spectrum is shifted to shorter wavelengths Because shorter wavelengths of the visible light are bluer, the Doppler shift of this object is called a blueshift The Doppler shift of a moving away object - redshift

Doppler Effect: 

Doppler Effect Demo Doppler effect

Collecting Light with Telescopes: 

Collecting Light with Telescopes Telescopes are giant eyes, collecting more light than we could with our naked eyes Telescopes are characterized by 2 key properties Light-collecting area (depends on the telescope size) Angular resolution (how much detail we can see in the telescope’s images)

Telescope Design: 

Telescope Design Two basic designs: Refracting and Reflecting telescopes Refracting telescope uses transparent glass lenses to focus the light (from Galileo’s small telescopes to a 1-m refractor)

Refractors: 

Refractors

Refractors: 

Refractors

Telescope Design: 

Telescope Design Reflecting telescopes use precisely curved mirrors Most contemporary telescopes are reflectors Primary mirror gather and focuses the light Secondary mirror reflects the light to a convenient location

Reflectors: 

Reflectors

Reflectors: 

Reflectors

Uses of Telescopes: 

Uses of Telescopes Imaging - pictures of celestial objects Spectroscopy - dispersing light into a spectrum Timing - tracking time variations of the light Atmosphere affects observations - light pollution, turbulence Turbulence can be corrected by adaptive optics

Types of Telescopes: 

Types of Telescopes Optical and Infrared telescopes Radio telescopes (use metal “mirrors”) Interferometeres (link several separate telescopes together to improve angular resolution)

Observatories: 

Observatories

Radiotelescopes: 

Radiotelescopes

Satellites: 

Satellites First satellite  1957 Soviet Sputnik First astronomical satellites  late 1960’s The Hubble Space Telescope (HST)  1990 The X-ray Chandra Observatory  1999 The Spitzer Space (IR) Observatory  2003

Satellites: 

Satellites

Slide27: 

Summary Spectral information gives us more knowledge about the objects (composition, surface temperature, moving properties) The visible light is only a small portion of the electromagnetic spectrum Telescopes work as giant eyes, enabling us to see the Universe in great detail. The ultimate place to observe the Universe is space

Typical questions for quizzes: 

Typical questions for quizzes Questions requiring to memorize something (names, numbers) Example: Who was the longest influential figure in ancient astronomy? Hint: We studied the Ptolemaic model Questions requiring to figure something using studied laws of nature Example: A comet orbits the Sun in 4 years (P), what is its mean distance (A) from the Sun? Hint: 3rd Kepler’s law: P2 ~ A3

Particular questions: 

Particular questions The Moon always shows nearly the same face to Earth because: Everything is rotating (Earth, Moon) Study Fig. 1 (page 96) in the course book The period of its rotation on its axis is exactly the same as its period of revolution around Earth

Particular questions: 

Particular questions Galaxy 1 is closer that Galaxy 2, so what: You do not know anything about their physical properties, but we see them simultaneously You only know that the light is traveling with the same speed Thus, we see Galaxy 1 at a later time than Galaxy 2 (light has left Galaxy 1 later)

Particular questions: 

Particular questions Where is our solar system located with the Milky Way galaxy? Think: How do we see Milky Way? As a wide band of faint light over the sky Also, there are other stars opposite the Milky Way So, we are neither in the center, nor at the very edge.

Particular questions: 

Particular questions Lunar eclipses can occur only at: Think: What is a lunar eclipse? It is passing through a shadow. What is get shaded: the Moon Whose shadow is it: the Earth’s What does create the shadow: the Sun Where is the shadow: behind the Earth

Hints for a successful quiz taking: 

Hints for a successful quiz taking 2. Do not look at others. You will spend more time figuring the system of answers. 3. Go through the questions and answer those you remember or can figure out quickly first. Then start answering the others. 1. Refresh your memory before the quiz by reviewing the studied chapters and retaking quizzes from the website.