Transcript Chapter 24: Studying the Stars

How do Astronomers know what they know?
 Almost everything we know about Astronomy was learned by gathering and
studying light from distant sources
 Properties of light
1.
Behaves like a wave; has wavelength (distance from crest to crest)

Visible light can be separated into distinct wavelengths by a prism

Radio waves = a few km Gamma rays = less than a billionth of a cm
2.
Behaves like a particle; photons – small packet of light energy

Photons from the sun push the tail of a comet away.

Light with shorter wavelengths have more energetic photons!
Electromagnetic Spectrum
 Electromagnetic Radiation(EMR) - the flow of energy at the speed of light in the
form of electric & magnetic fields.
 Radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays
 Electromagnetic spectrum – arrangement of EMR according to their wavelengths
and frequencies
 Most wavelengths are too long or short for us to see
Continuous spectrum 
A spectrum of radiation distributed over
an uninterrupted range of wavelengths.
Absorption vs. Emission Spectrum
 Emission spectrum – a series of bright lines of particular wavelengths
produced by a hot gas under low pressure releasing energy
 Absorption spectrum – “dark line spectrum,” produced when white light
passes through a gas and certain wavelengths of light are absorbed
 Most often used by astronomers to identify stars.
 The specific wavelengths of absorption spectral lines of elements are like
“fingerprints” used to identify the specific elements present in a star
Spectrum of stars
 Stars are made of a mixture of elements.
 The line spectra will contain all the lines from each element
in the mixture.
 Line spectrum of the sun
The Doppler Effect
 Doppler effect – the perceived change in wavelength of a wave emitted from a
source moving away or toward an object (wavelength doesn’t actually change)
 Sound:
 Higher pitch when approaching
 Lower pitch when receding
 Light:
 Light source moving away—the wavelength is stretched & causes a Red Shift
 Spectral lines shift toward red end
 Light source moving toward—wavelength is compressed & causes a Blue Shift
 Spectral lines shift toward blue end
Tools For Studying Space
Telescopes are used to collect the light energy
(EMR) from distant galaxies and stars
Types of telescopes: Optical, Radio, & Space
Optical Telescopes: Properties
 Optical telescopes – contain mirrors or lenses, collect visible light
1. Light-gathering power – intercepts light from distant objects
 larger lens or mirror = “see” farther
2. Resolving power – allows for sharp images and fine detail
 larger lens or mirror = sharper image
3. Magnifying power – ability to make an image larger, depends on
focal lengths of the objective and eyepiece
Optical Telescopes: Refracting
• Refracting telescope – use lenses to refract or bend light
• First used by Galileo
• Mostly smaller telescopes
• The objective lens produces an image by bending light from
distant objects so that light converges on a focus (central point)
•
The image is inverted as light passes through both the objective lens
and the eyepiece.
Optical Telescopes: Refracting (continued)
• Optical defect of refracting telescopes:
• Chromatic aberration – light of different colors are focused at
different locations.
• Chroma = color
Aberration = to go astray
• when red light is in focus there is a blue halo
• when blue light is in focus there is a red halo
Optical Telescopes: Reflecting
 Reflecting telescope – uses a concave mirror to focus light
 Newton built reflecting telescopes
 Focus is in front of the mirror.
 Advantages:
 Glass doesn’t need to be optical quality
 Can be supported from behind the mirror
 Can be made much larger
 Disadvantage:
 secondary mirror blocks some light. Solution…make it bigger!
Detecting invisible
radiation
 Radiation we can’t see:
 gamma, X-ray, UV, IR, microwave, & radio
 Most radiation from the sun cannot pass through our
atmosphere.
 Objects can be sent above the atmosphere to detect it
(balloons, rockets, satellites)
 A narrow band of radio waves does pass through our
atmosphere and can be detected on Earth’s surface
Radio Telescopes
 Focuses incoming radio waves on an antenna, absorbs and
transmits waves to an amplifier
 Interferometer - several radio telescopes linked together
• Advantages = can run all the time,
cheaper than space telescopes, can
detect clouds of gas cooler than
visible light
• Disadvantages = take up a lot of
space, hindered by human-made
radio interference
Space Telescopes
Orbit above the atmosphere to produce clearer images
 Hubble Space Telescope was the 1st
 Used to study everything from birth of stars to extrasolar
planets (planets orbiting other stars)
 Chandra X-ray Observatory
 Studies black holes by collecting X-Rays
 Compton Gamma-Ray Observatory
 Studies gamma rays from exploding stars &black holes
 James Webb Space Telescope (planned space observatory)
 Will study IR radiation from the oldest stars