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27.1: Characteristics of Stars STAR NOTES: Part 1 What is a Star? – A body of gases that gives off energy in the form of light and heat. 27.1: Characteristics of Stars Are all stars the same? No 1. Stars range in diameter from about 20 km to about 1.4 billion km. 2. The sun is a medium-sized star with a diameter of about 1,392,000 km. 1. Stars differ in mass. 2. Stars differ in color. 3. Stars differ in composition. 4. Stars differ in temperature. 5. Stars differ in distance from the earth. 6. Stars differ in brightness. 7. Stars differ in motion How Do Astronomers Determine Composition and Surface Temperature of a Star? The surface temperature of a star tells us what color it will be. Surface Temperature of a Star Red stars are the coolest stars with surface temperatures less than 3,500 °K Surface Temperature of a Star Yellow stars, such as our Sun, have surface temperatures ranging from 5,000 - 7,500°K Surface Temperature of a Star White stars have a surface temperatures ranging from 7,500 – 10,000°K White stars are NOT the same thing as white dwarfs! White dwarfs happen at the end of a star’s life, and they can be as hot as 100,000 °K! Surface Temperature of a Star Blue stars are the hottest stars with surface temperatures ranging from 10,000 - 60,000°K. Size of a Star The size of a star is based on its diameter and its mass. For this class the most important thing to remember about a star’s size is that the mass of the star determines how that star will evolve and eventually die. – – – The greater the mass of the star, the shorter its lifetime Low mass stars with an original mass of less than 8 times the sun (8 solar masses) will become Planetary Nebula. The remnant state of these stars is a white dwarf. High mass stars containing more than 8 solar masses will explode violently as a supernova. The remnant state of these stars is a neutron star or black hole. How Do Astronomers Determine Composition and Surface Temperature of a Star? Composition is determined by the bright-line spectra emitted by the star. A spectrometer is an instrument used to analyze the bright-line spectra. – – The colors and lines of the spectrum determine what the star is made of. There are 3 types of spectra. How Do Astronomers Determine Composition and Surface Temperature of a Star? Continuous Emission Absorption A. Continuous spectra are made by energy at all wavelengths _____________________. B. C. Emission spectra ___________________ are produced by heated elements ____________________. Each element has its own bright-line spectra which acts as its fingerprint _________________. Absorption spectra _______________ are produced by The absence of bright-line spectra __________________________________________ __________________________________________ How Do Astronomers Determine Composition and Surface Temperature of a Star? How do we determine the elements that are in a star? known bright-line spectra of We compare the ___________________ emission spectra elements to the ____________________ of stars. Hydrogen is the most common element in most stars, and helium is the second most common. Why Do Stars Appear to Move to an Observer on the Earth? Actual Motion – how stars really move! This can be measured only with highpowered telescopes and other instruments. Apparent Motion – how stars look like they are moving to us on Earth! (Visible to the unaided eye in a dark sky) Actual Motion 1. Rotate (spin) (just like Earth). Stars _____________ 2. Stars revolve around ___________________ (binary star other stars system). 3. Stars move toward or away from the Earth _______________________ (blue or red shifting). Apparent Motion ALL THE TIME 1. The Earth moves _______________. A. B. – rotates The Earth _____________________. revolves around the Sun The Earth _____________________. This makes the stars in the sky look like they are moving, BUT THEY ARE _____ NOT REALLY MOVING! The Motion of Stars – The rotation of the Earth makes the stars look westward like they are moving _________________. – The North Star, Polaris, does not appear to move in the night sky. Other stars seem to circle Polaris, moving in a westward direction. But, this movement is due to the Earth’s rotation, not actual motion… The Motion of Stars The revolution of the Earth around the Sun makes different stars visible during different seasons _______________. This is because the position of the _________________ Earth changes relative _________________ _________________ to the Sun during different times of the year. 27.1: Characteristics of Stars STAR NOTES: Part 2 Distance to Stars 1. Light Years _______________________ (____): LY Distance to stars A. Units used to measure _________________. B. __________________ are closer so we use Planets ______________________________. Astronomical Units 2. Astronomical Units __________________________ (____): AU measure distances that are A. Method used to ________________ closer than ________________. one light year 27.1: Characteristics of Stars STAR NOTES: Part 2 Distance to Stars Light Years 1. 2. 3. great distances. Units used because of _______________________ 300,000 9.5 trillion km/yr. Light travels ___________km/s OR ________ Sun, then Proxima Centauri (4.2 LY) Closet star: our ______________________________ Name and Describe the Way Astronomers Measure the Distance From the Earth to the Stars Parallax This method is used to measure the distance of a star from the Earth. How does Parallax work? Take a photograph of certain star Wait 6 months and re-photograph the same star. Measure the shift between the two locations. The closer a nearby star the greater the shift. This only works for stars that are closer than 1000 light-years. The Difference Between Absolute Magnitude and Apparent Magnitude Approximately 6000 stars are visible to the unaided eye from earth About 3 billion can be seen through ground-based telescopes Over 1 trillion can be observed from the Hubble Space Telescope The visibility of a star depends on its brightness and its distance from the Earth. Astronomers use two scales to describe the brightness of a star: apparent magnitude and absolute magnitude. Apparent Magnitude How bright a star appears from Earth. Positive #s = dimmest stars (dimmest seen with unaided eye is +6). Negative #s = bright star (Sun: –26.8). Absolute Magnitude **** This is figured out by moving all stars to a distance 32.6 light years from Earth. When they are all at the same distance then compare how bright they really are. Example: The Sun’s apparent magnitude is –26.5, but absolute magnitude is +5. Like apparent magnitude, smaller numbers are used for brighter stars **** The H-R Diagram uses Absolute Magnitude