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Chapter 28 Stars and Galaxies
28.1 Closer look at Light
- Light is a form of electromagnetic radiation, which is energy that travels in waves.
- Waves of energy travel at 300,000 km/sec (speed of light
- Ex: radio waves and x-rays
- Electromagnetic waves do not need a medium to travel, they travel through space
- Electromagnetic waves emitted by an object indicated what elements are present and its motion
- Electromagnetic radiation waves are arranged into a continuum call the electromagnetic spectrum.
- Wide range of wavelengths
- long wavelengths with low frequencies at one end, short wavelengths with high frequencies at the other end
- wavelength measured crest to crest/trough to trough
- frequency the number of that crests of the same wavelength that pass a point in one second.
- Scientists study the visible light portion of the electromagnetic spectrum.
- spectra of stars allow for astronomers to learn about the star’s elements and motion.
- Spectra studied using a spectrascope
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- Three types of visible spectra
- Continuous spectrum: unbroken band of colors, emitting all colors of visible light
- glowing solids, such as a light bulb filament
- glowing liquids, molten iron
- hot, compressed gases inside a star
- Emission spectrum: unevenly spaced lines of different colors, emitting light of only some wavelengths
- glowing thin gases
- every element has its own unique emission spectrum, element’s finger print
- Absorption spectrum: dark lines that cross a continuous spectrum.
- forms when light from a glowing object pass through a cooler gas which absorbs some wavelengths
- can determine what is present in the cooler gas by comparing emission and absorption spectrum
- Doppler Effect is the apparent change in the wavelength of radiation or sound due to relative motion between the object and the receiver.
- Doppler effect applies to light as well as sound.
- Shift of the emission spectra can indicate if the object is moving towards or away from Earth
- Shift towards the red end of the visible light spectrum, object moving away Earth – redshift
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- Shift towards the blue end of the visible light spectrum, object moving towards Earth - blueshift
- Doppler effect determined that the universe is expanding
- Optical Telescopes – gather far more light than an unaided eye and magnify imagers
- Reflecting Telescopes
- Uses one lens at back to gather and focus light
- Image reflected on to a small mirror and then the eye piece
- Refracting Telescopes
- Uses two lenses
- Lens at the front gathers light
- Eyepiece magnifies image
- Radio Telescopes
- Technical term for really big satellite dishes
- Use to detect energy waves at frequencies lower than visible light
- Other
- Detects energy waves at frequencies higher than visible light
- Usually satellites in space
- Gamma ray
- Background radiation
- X-ray
- Hubble (infrared)
- Observation of stars has been going on for over 5000 years
- The grouping of stars are called Constellations
- Constellations
- human grouping of stars
- only appear together as viewed from Earth from a different angle they do not look like the constellation
- Grouped together due to looking like they all have the same brightness (apparent magnitude)
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- Constellations will change shape over shape over thousands of years due to the universe expanding
- Some constellations have been around for thousands of years other were recently conceived
- Move across the sky from east to west (though Earth rotates west to east)
- Big Dipper and Little Dipper two of the best know constellations point to the North Star
- North Star
- Current north star is called Polaris
- Sits directly over the north pole
- Does not move to the naked eye
- Very powerful tool for navigation
- Due to Precession, Polaris will not always be the “North Star”
- Circumpolar Constellations – constellations around the north star
- Create star trails evidence for Earth’s rotation
- North star appears fixed in the sky as Earth rotates
- star trails from circumpolar constellations rotate counterclockwise around the north star
- Circumpolar constellations seem to never set from some northern hemisphere latitude
- Distances to stars and other objects in space
- Unit of measurements on Earth are not sufficient for space measurements
- Astronomical Unit (AU) is used for the distance from Earth to the Sun (150 million kilometers)
- Light Year is the distance light travels in one year (9.5 trillion kilometers)
- It is a distance measurement
- Example - 4.2 light-years means that the light we see has been traveling for 4.2 years before we can see it (4.2 X 9.5 trillion km)
- Parallaxchange in an object’s direction due to a change in the observer’s position
- Parsec short for “parallax second” equal to 3.258 light-years.
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- Stars
- Elements
- Hydrogen ~69%
- Helium ~29%
- Heavier elements ~2%
- No two stars have the same proportions of elements
- light that radiates is dependent on composition and temperature, this differs in every star
- Star spectrum is its fingerprint
- Mass, Size and Temperature
- Stars vary greatly in masses, size, and temperature
- Cannot observe directly so we are estimating what the mass might be
- Gravitationaleffect on bodies around the star help with estimating its mass
- Star mass is expressed as multiples of the mass of our Sun (which has a stellar mass of 1)
- Size varies more than mass
- Smallest stars are smaller than Earth
- Largest have diameters more than 2000 times that of our Sun
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- Stars differ even more in density
- Betelguese is about one ten-millionth of our Sun
- One star is so dense that one teaspoon would weight more than a tonon Earth
- Temperature of stars vary
- Stars have different colors which indicate different temperatures
- Cool stars are red in color
- Mid-temp are yellower in color
- Hot stars are blue in color
- Astronomers group stars by temperature and color into spectral classes.
- Star Brightness
- Actual brightness of a star is Luminosity
- Dependent on size and temperature
- Distance from Earth is not a factor
- If two stars are the same size the hotter star would be more luminous
- Apparent magnitude is how bright a star appears to an observe on Earth
- Does not factor in distance from earth
- Absolute magnitude is the measure of how bright a star would be is all stars were the same distance from Earth (10 parsec)
See foldable for Star life cycle and HR Diagram
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28.4 Galaxies and the Universe
Galaxies:
- The universe is everything that exists.
- The observable universeis everything we can observe.
- Astronomers are not sure how old the universe is since the light from the beginning has not yet reach us. The estimate is between 10-20 billion years
- Galaxies are hard to separate from stars without a telescope.
- Hazy patches of light which when viewed through a telescope reveals thousands to billions of stars.
- Astronomers estimate there are 50- 100 billion galaxies in the observable universe.
- No two galaxies are the same, however, they are classified based on shape.
- There are three classifications
- spiral (ex. Milky Way)
- ellipticalare near spherical to lens-shape
- irregular (ex. The two Magellanic Clouds )
- Big Bang Models (Theory) – explains history of the universe from a tiny fraction of a second after it came into being up to the present
- Best explanation for how the universe came to being
- Developed due to observations of stars, galaxies and other objects with telescopes and experimenting with matter on Earth
- Approximately 10-20 billion years ago all matter in the universe existed in an incredibly hot and dense state, from which it expanded and cooled slowly condensing into stars and galaxies.
- It is expanding at a very slow rate and is still expanding today
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- Evidence for the Big Bang Model:
- The universes apparent expanstion, distance between galaxies and groups of galaxies seems to be increasing with time.
- Edwin Hubble’s discovery of the redshift in the spectra of galaxies supports the expansion of the universe
- Discovery of radiation called cosmic background radiation apparently left over from the universe’s beginning.
- Continues to be tested and examined to seek further evidence to support it
- Some astronomers are considering alternative ways the universe has reached its present state.
- It is impossible to know for certain how the universe began
- as long as the Big Bang Model survives (passes) crucial tests, it remains the best explanation for the origin of the universe
- If it were to ever fail a test, then astronomers will have to look for a new theory/model
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