Investigating Astronomy Timothy F. Slater, Roger A. Freeman Chapter 2 Decoding the Hidden Messages in Starlight

Light Takes Time to Travel When close to Jupiter, the moons appeared to eclipse “too early.” When far from Jupiter, the moons appeared to eclipse “too late.” Light takes time to travel the extra distance! c = 300,000 km/s During Romer’s time, navigators intended to use the eclipses of Jupiter’s moons as a clock of sorts, with very carefully calculated tables of eclipses for each of the moons, for every day. What Romer discovered was that the tables were “slow” when Earth was close to Jupiter, and “fast” when Earth was farther away. For instance, if the table said that Io was to eclipse at 10 P.M. on a certain day, when Earth was close to Jupiter it would eclipse at 9:52 P.M., but when it was farther away, it would eclipse at 10:08 P.M. The error would oscillate according to the relative distances between the planets.

ConceptCheck Why has the speed of light been historically so difficult to measure?

Glowing objects, like stars, emit an entire spectrum of light. The Sun emits energy that: Your eyes can see Your skin can feel Damages your DNA

Sunlight Is a Mixture of All Colors Prisms don’t “add” colors to the sunlight. Each color of light “bends” as it passes through the material.

Light Travels in Waves Water waves show diffraction, addition, and canceling. So does light! A wave!

Our Eyes See Only Some of the Spectrum of Light

As Frequency Increases, Wavelength Decreases f is the symbol for frequency Hertz = 1 wave per second λ is the symbol for wavelength f = c / λ

Light Has Properties of Both Waves and Particles Shorter wavelength Higher frequency Higher energy More “particle-like” Longer wavelength Lower frequency Lower energy More “wave-like” Light Has Properties of Both Waves and Particles

Electromagnetic Radiation A “field disturbance” Both electric and magnetic properties A spectrum of waves, varying in wavelength and frequency

ConceptCheck If you cover a white light with a specially designed green plastic gel so only the green light passes through, which color plastic cover gel do you need to add to the pure green light to make it change to red? Do light waves move up and down or back and forth as they move through space? Which form of electromagnetic radiation has a wavelength similar to the diameter of your finger?

ConceptCheck How do the frequencies of the longest wavelengths of light compare to the frequencies of the shortest wavelengths of light? What is the wavelength of radio waves from your favorite FM radio station? If a photon’s wavelength is measured to be longer than the wavelength of a green photon, will it have a greater or lower energy than a green photon?

Infrared light can pass through interstellar clouds that visible light cannot. If our eyes can only see some parts of the spectrum, there must be things we can’t see. Infrared light can pass through clouds of dust and gas.

Objects emit specific amounts of light, revealing their temperatures. Wien’s Law: The higher the temperature, the more intense the light and the shorter the wavelength….

How much energy a star emits is determined by both temperature and surface area. As temperature increases, the energy released by the object increases.

ConceptCheck Which form of light is being emitted most intensely by a frozen ice cube at 0° Celsius? What single piece of information do astronomers need to determine if a star is hotter than our Sun? Which wavelength of light would our Sun emit most if its temperature were twice its current temperature of 5800 K?

ConceptCheck If astronomers observe a red star and a blue star in the sky, how do they distinguish which star is at a higher temperature? How many times more energy flux comes from a star that is three times hotter than the Sun?

Electrons Occupy Specific Orbits within Atoms Each orbit is a specific energy state. Electrons “leap” between orbits.

Electrons “leap” when they absorb the perfect amount of energy. Electrons “fall” and emit that same specific amount of energy.

Kirchhoff’s Laws Law 1: A hot, opaque body or a hot, dense gas produces a continuous spectrum—a complete rainbow of colors without any spectral lines.

Kirchhoff’s Laws Law 2: A hot, transparent gas produces an emission line spectrum—a series of bright spectral lines against a dark background.

Kirchhoff’s Laws Law 3: A cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum —a series of dark spectral lines among the colors of the continuous spectrum.

Kirchhoff’s Laws The wavelengths absorbed by the gas exactly match the wavelengths emitted by the gas.

Identifying Chemical Substances Using Spectral Lines The light from a burning chemical makes a special, unique pattern when it passes through a prism.

ConceptCheck What type of spectra would result from a glowing field of hot, dense lava as viewed by an orbiting satellite through Earth’s atmosphere?

Spectra Also Reveal Motion An object’s motion through space is revealed by the precise wavelength positions of its spectrum of light. The Doppler Effect

Exploiting the Doppler Effect The wavelength we observe The velocity of the object, toward or away from us = The wavelength we “should” observe The speed of light

ConceptCheck How is the spectrum changed when looking at an emission spectrum from an approaching cloud of interstellar gas compared to a stationary cloud? How fast and in what direction is a star moving if it has a line that shifts from 486.2 nm to 486.3 nm?

Telescopes Gather Light Telescopes aren’t primarily used to magnify stars. Light-gathering power is directly related to the size of the telescope’s objective lens― the gathering area.

Refracting Telescopes Use a lens to concentrate incoming light at a focal point Magnify near objects

Reflecting Telescopes Use a curved mirror to concentrate incoming light at a focal point. Are more durable and can be made bigger and less expensive.

Adaptive Optics Computers compensate for turbulence in the atmosphere.

Telescopes in Orbit Detect light that does not penetrate the atmosphere.

Looking toward the center of the Milky Way using the best of Earth-based and space telescopes

Charge-coupled devices record very fine image details.

ConceptCheck If someone says they are using an 8-inch telescope, to which dimension of the telescope’s size are they most likely referring? If a thick lens is able to bend light more than a thin lens, which lens has a greater focal length? How do the eyepieces with the largest focal length affect a telescope’s overall magnification? In large sizes, which type of telescope can be made lightest and most inexpensively?

ConceptCheck If astronomers are using an adaptive optics system on a night where the atmosphere is unusually turbulent, will the adaptive optics actuators be deforming the telescope’s mirror more rapidly or less rapidly than on a typical night? Look at Figure 2-28, which shows the transparency of Earth’s atmosphere. Would astronomers most prefer to have a new ground-based telescope constructed that is most sensitive in the X-ray region, the ultraviolet wavelength region, or in the microwave region? What is the primary advantage of an orbiting space telescope, compared to a ground-based telescope? Why can CCDs more efficiently observe faint stars than photographic film or photographic plates?

Chapter 3 Analyzing Scales and Motions of the Universe Next Chapter: Chapter 3 Analyzing Scales and Motions of the Universe