How do Astronomers know what they know? Almost everything we know about Astronomy was learned by gathering and studying light from distant sources Properties.

Chapter 24: Studying the Sun (and other stars)

How do Astronomers know what they know?Almost everything we know about Astronomy was learned by

gathering and studying light from distant sourcesProperties 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 SpectrumElectromagnetic 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 SpectrumEmission 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 absorbedMost 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 starsStars 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 EffectDoppler 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 approachingLower 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 starsTypes of telescopes: Optical, Radio, & Space

Optical Telescopes: Properties Optical telescopes – contain mirrors or lenses, collect

visible light1. 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 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: Refracting (continued)

Optical Telescopes: ReflectingReflecting 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 TelescopesFocuses 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 TelescopesOrbit 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 ObservatoryStudies black holes by collecting X-Rays

Compton Gamma-Ray ObservatoryStudies gamma rays from exploding stars &black holes

James Webb Space Telescope (planned space observatory)Will study IR radiation from the oldest stars