CHAPTER 28 STARS AND GALAXIES
28.1 A 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 a stars allow for astronomers to learn about the star s elements and motion. Spectra studied using a spectroscope
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 Shift towards the blue end of the visible light spectrum, object moving towards Earth - Blueshift Doppler effect determined that the universe is expanding
TELESCOPES Optical Telescopes gather far more light than an unaided eye and magnify imagers Reflecting Uses one lens at back to gather and focus light Image reflected on to a small mirror and then the eye piece Refracting 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)
28.2 Stars and Their Characteristics 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)
Constellations (continued) Constellations will change 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
The Constellations that dominate the night sky change from month to month. This is the result of the Earth s change in position as it orbits the Sun.
Plain of the Ecliptic (path of Sun, Moon and Planets across the sky) Curve in Plain of the Ecliptic is due to change in tilt of the Earth over the course of a year.
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) Parallax change in an object s direction due to a change in the observer s position Parsec short for parallax second equal to 3.258 lightyears.
Parallax The further the object from the viewer, the less the parallax shift.
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 Gravitational effect 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 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 ton on Earth
Star size comparison
Temperature of stars vary Stars have Different colors which indicate different temperatures Cool stars are redder in color Ex. Betelguese with a surface temperature of 3000 o C Mid-temperatures are yellower in color Ex. The Sun with a surface temperature of 5500 o C Hot stars are blue in color Ex. Sirus Astronomers group stars by temperature and color into spectral classes. System is called the Harvard Spectral Classification Scheme
Actual brightness of a star is Luminosity Dependent on size and temperature of the star Distance from Earth 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 observer on Earth Does not factor in distance Absolute magnitude is the measure of how bright the star would be if all stars were the same distance from Earth (10 parsec)
Variable stars These stars show a variation in brightness over cycles Can last days to years Some change brightness as they expand and contract One important class is called Cepheid variables Yellow supergiant stars with a cycle of brightness ranging from 1-50 days. Most have cycle of 5 day. Nonpulsating star may change in brightness due to the fact that is more than one star.
28.3 LIFE CYCLE OF STARS Stage 1 Stage 2 Stage 3
Main Sequence Star (stars like our Sun) less than 8 solar masses Nebula spins & flattens To form a protostar Protostar shrinks due to gravitional attraction. As temperature increases and pressure increases, light and heat are emitted Energy is produced from Nuclear fusion of H 2 into He begins moves to next stage Main sequence stage, last the longest Star stays until all H 2 fuel is used up (converted to He) He core shrinks and Contracts to produce additional heat. H 2 fusion starts in the outer Layers causing the star to swells moves to next stage. Core temperature increases to a point that He can fuse into heavier elements (O 2 and C) H 2 into He continues in the outer layers. When Hydrogen is exhausted the outer layers blow away leaving only the carbon-oxygen core White Dwarf. Expelled layer absorbs the white dwarfs ultraviolet emission causing a halo affect called a Planetary Nebula
Massive Star 8 solar masses or more Same process however much more dust and gas present Same process however due to the more massive size it swells to an even larger size than the main sequence star does Fusion in core continues until iron forms Iron does not release energy so it absorbs it, thus leading to a quick collapse resulting in an explosion Supernova. Massive star that is 8-15 solar masses, creates a neutron star following the supernova Massive star that is more than 15 Solar masses, creates a black hole following the supernova.
HR Diagram HR diagram gives you a picture of a star s life. There are thousands of variations of the HR diagram All plot luminosity against temperature Some include spectral class and absolute magnitude. Spectral class is based on the temperature of the star Luminosity is the actual brightness of a star, where as absolute magnitude is the brightness if all stars were the same distance from Earth.
HR Diagram Groups Stars fall into one of several distinct groups on the HR diagram. 90% of stars fall in a band called the main sequence, which runs from the upper left corner to the bottom right corner. Stars in this band are called main sequence stars. Giant stars grouping is found in the upper right corner just above the main sequence. Larger stars are more luminous and have diameters from 10 to 100 times greater than the sun
HR Diagram Groups Supergiants grouping is found in the upper right corner just above the giant stars. Diameters that are more than 100 times that of the sun Relatively cool stars but their size gives them their luminosity. White Dwarfs grouping is found in the lower left corner, these stars are near the end of their lives. Were once red giants The red giants outer lost their outer atmosphere, leaving only a glowing stellar core. Black holes and Neutron stars do not appear on the HR diagram because their luminosity is not great enough.
28. 4 Galaxies and the Universe The universe is everything that exists. The observable universe is 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 and the Universe Galaxies are hard to separate from stars without telescopes. 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.
Galaxies and the Universe There are three classifications Spiral (ex. Milky Way) Elliptical are near spherical to lens-shape Irregular (ex. The two Magellanic Clouds) Spiral Elliptical Irregular
Origin of the Universe Big Bang Model 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
Origin of the Universe Evidence for the Big Bang Model: The universe s apparent expansion, 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.
Origin of the Universe It is impossible to know for certain how the universe began as long as the Big Bang Theory (model) survives 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