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Chapter 28 Stars and Their Characteristics Spectral Analysis • We can’t always get a sample of a piece of the Universe. • So we depend on light ! Spectral Analysis • Light is a form of Electromagnetic Radiation. – Electromagnetic radiation = energy that travels in waves (radiowaves, x-rays, etc) – Length of the waves determine the characteristics of the electromagnetic radiation. – The types of electromagnetic radiation can be arranged in a continuum called the Electromagnetic Spectrum (longest wavelengths at one end and shortest wavelengths at the other end) Electromagnetic Spectrum • Visible white light is actually made up of light of various colors, each with a different wavelength. (colors seen in rainbow or when light passes through a triangular prism.) – Red light has the longest wavelength, violet has the shortest wavelength. • Electromagnetic waves emitted by an object provide information about elements within it or its motion. (use this to learn about distant stars) Spectroscope • Spectroscope – tool astronomers use to separate starlight into its colors (uses a prism to split light, gathered by a telescope, into a spectrum) • Break light into 3 different types of spectra: – Continuous Spectrum – Emission Spectrum – Absorption Spectrum Types of Visible Spectra • Continuous Spectrum – unbroken band of colors, which shows that its source is emitting light of all visible wavelengths. – Emitted by: • Glowing solids, such as the hot filament of an electric light • Glowing liquids, such as molten iron • The hot, compressed gases inside stars Types of Visible Spectra • Emission Spectrum – series of unevenly spaced lines of different colors and brightnesses. The bright lines show the source is emitting light of only certain wavelengths. – Emitted by: • Glowing thin gases (every element has its own color signature) Types of Visible Spectra • Absorption Spectrum – a continuous spectrum crossed by dark lines. – Dark lines form when light from a glowing object passes through a cooler gas, which absorbs some of the wavelengths. – Elements absorb the same wavelengths that they would emit if they were in the form of glowing gases. – A stars absorption spectrum indicates the composition of the star’s outer layer. Spectral Analysis • Each element has a unique spectral signature: • Determined by arrangement of electrons. • Lines of emission or absorption arise from re-arrangement of electrons into different energy levels. Hydrogen (Developed by Shirley Burris, Nova Scotia) Spread a rainbow of color across a piano keyboard Then, “play” an element Hydrogen More Musical Elements Now play another element Helium And Another Carbon Getting a Handle on Water Oxygen Hydrogen All together now ... Water Doppler Effect • If lines on the spectrum are shifted toward the red side then object is moving away = Red Shift • If lines on the spectrum are shifted toward the blue side then the object is moving toward you = Blue Shift Now that we can tell if a star is moving toward or away from us lets learn more about stars … measuring their brightness, their distances, their life cycles… If we imagined that the distance from the Earth to the Sun was 1 Centimeter….. Sun Earth 1 Centimeter How far away do you think the next nearest star would be??? ? How far away do you think the next nearest star would be??? 2.5 Kilometers 1.5 miles In real distance, the next closest star would be 300,000 times the distance from the Earth to the Sun, or…… Earth Sun Proxima Centauri 39 Trillion miles (that’s 4.24 Light Years!) What does 39 trillion miles look like???? Objects in Space are so far apart that units of measurement used on Earth are not useful. The distance to the next nearest big galaxy, the Andromeda Galaxy, is 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance. Andromeda Spiral Galaxy Earth 21,000,000,000, 000,000,000 kms The basic unit of measurement of distance inside of our solar system is the… An Astronomic Unit is equal to the distance from the Sun to the Earth, which is about 93 million miles. Sun 93 million miles Earth Planets inside Earth’s orbit have distances from the Sun of less than 1 AU. (Mercury is .4 AU’s from the Sun.) .4 AU’s Sun Mercury Planets outside the orbit of the Earth have distances from the Sun of greater than 1 AU. (Mars is 1.5 AU’s and Pluto is 39 AU’s from the Sun.) But, Astronomic Units are too small for measuring distances outside of our own Solar System. The closest star to the Sun, Proxima Centauri, would be more than 300,000 AU’s from our star, and that’s the closest! Astronomers use to measure distances outside our Solar System. A Light-Year is a unit of Distance. A Light Year is equal to the distance that light can travel in one Earth year. A Light Year is equal to 5.3 trillion miles. Use of Light Years makes the units used in measuring distances in Space smaller, but “small” is pushing it! The Speed of Light is 186,000 miles per second. Peeoooummmmmmm!!! That is almost 8 times around the Earth in 1 second! The Crab supernova remnant is about 4,000 light-years away. The Milky Way Galaxy is about 150,000 light-years across. The Andromeda Galaxy is 2.3 million light-years away. The background radiation from the big-bang is 14 billion lightyears away. Types of Stars and Their Organization in Space How are Stars and Planets different? • Stars emit light, due to nuclear fusion in their center, while planets only reflect light. A Star is a “self-luminous” (it is giving off light as opposed to reflecting it) sphere of gas that is undergoing Nuclear Fusion in its center. Not all stars are the same. In fact, they vary in many ways. Stars Vary in “Brightness”. Magnitude-How bright an object in Space is, or appears to be. Luminosity- Luminosity is the true brightness of an Individual unit of a star. The Luminosity of a Star depends on a star’s temperature. 5 5 5 5 A. 3 3 3 3 3 3 B. 3 3 Which star is hotter? Which is brighter? Absolute Magnitude- Absolute Magnitude is a measurement of the true “brightness”of stars as if all stars were viewed from the same distance. The Absolute Magnitude of a star depends on its Volume and Luminosity. 1 1 1 1 1 1 1 1 A. 1 1 1 1 3 3 3 3 1 3 3 3 B. 3 Which star is larger? Which is brighter? Apparent Magnitude- Apparent Magnitude is a how bright a star “appears” to be from Earth. The Apparent Magnitude of a star is affected by Absolute- Magnitude (Volume x Luminosity) and Distance from Observer. Betelgeuse, one of the brightest stars in the Universe, does not appear to be as bright as our Sun, because of its distance from us compared to the Sun’s distance. Stars also vary in their mass, density and volume, interior and surface temperature, rate of fuel-consumption, color, Main Sequence life-span, what they do when they “die” and what they become after they “die”. Stellar Mass When comparing the masses of different stars, we will use the mass of our star, the Sun, as the standard. A star that is identical to ours would be a star of 1 “Solar Mass”. Stars vary in mass from a fraction of 1 solar mass, up to 50 times the mass of our Sun, or “50 Solar Masses”. Red Dwarf star 50 solar mass star The Sun Stars vary even more in their volume/density Number represents xSun volume White Dwarf Earth Star Density Stars vary in their Main Sequence and Giant life-span Stars vary in what they become when they are no longer fusing Hydrogen. Blue Supergiant Red Supergiant Betelgeuse Orion’s Belt Orion Nebula Rigel Supernova explosion Stars vary in how they “die” Supernova Supernova Planetary Nebula White Dwarf in Binary System Stars vary in what they become when they “die” (Run out of material that can be fused to create outward pressure). White Dwarfs Neutron Stars Neutron Star Pulsars Pulsar Cone Black Holes Volume Life-Span Temperature Density A star’s mass determines every other characteristic of the star that we mentioned earlier. Rate of “Fuel” consumption How it “dies” Luminosity HR Diagram Main Sequence Main Sequence Line; Core Fusion of H at constant rate; Volume directly related to mass .. 25-50 M sun Masses/Luminosity of Main Sequence Stars Giants Core fusion of He Supergiants .. . Supergiants White Dwarfs White Dwarfs “Dead” Star; High temps. Due to compression Black Holes, Pulsars and NeutronStars are not identified on the HR Diagram because they are either very dim or do not give off energy in the visible wavelengths. Star Life Cycles As we have discussed, stars are not all the same. All of the characteristics of a star are determined by their mass. Stars with different masses have different life cycles. Based upon their masses, stars can follow three main “pathways” and fit into three “candidate” groups during the course of their “lives”. These groups include: White Dwarf Candidates (less than one “solar mass” to 15 solar masses) Neutron Star and Pulsar Candidates (16 to 30 solar masses) Black Hole Candidates (Greater than 30 Solar Masses) White Dwarf Candidates Pulsar and Neutron Star Candidates Supergiant Pulsar Black Hole Candidates Supergiant Black Holes