Star overview Properties of Stars • Do Inquiry activity pg. 699 in Earth Science book – Answer the questions in your notebook Class discussion: What does these mean about astronomy and how we see stars Characteristics of Stars • Color, Temperature and Mass are three properties of stars • Color is a clue to a star’s temperature • Hot stars appear blue because they emit short-wave lengths • Red stars are cooler because they emit longer wavelengths • Yellow stars like the sun emit wavelengths between 5000-6000 K Temperatures of the Sun and Stars • We determine the temperatures of stars from an analysis of their light (spectra). Both the continuous and absorption line spectra of stars can be used to infer the surface temperatures of stars. • Consider the spectrum of the Sun made using a prism: Sunlight through a prism Binary star motion demonstration • What does the first demo represent? • What will happen if I replace one ball with a smaller ball? • What does the second model represent? Binary stars and Stellar mass • • • • Pairs of stars are called binary Pulled toward each other by gravity >50% of stars are binary or multiple Binary stars are used to determine star’s mass • The mass of a body can be calculated if it is attached by gravity to another body Binary Star Motion – Binary stars orbit around a common point called “center of mass” – The center of mass lies in the middle when the mass of each body is equal – If one star is larger the center of mass will lie closer to the bigger star – The star masses are then determined by the size of the orbits Video Field Trip Answer the following after viewing the film 1. What happens when a star runs out of fuel? 2. Describe what will happen to the sun when it dies? Parallax • Basic way to measure star distances • Shifting of the position of a nearby star because of earth’s orbital motion • A picture of a star is taken, then six months later another picture is taken and compared. The nearby star appears to shift against the backdrop of more distant stars • Nearer stars have larger parallax angles Parallax • What causes the star to appear to shift? • How is the parallax angle related to the distance between the star and the earth? Parallax • Parallax angles are very small • Nearest star, Proxima Centauri is less than 1 second of an arc (1/3600 of a degree) • Your finger when your arm is stretched is about 1 degree!!! Light Year • A unit of interstellar distance, defined as the distance light travels in a period of one year. The speed of light is constant, at about 300,000 km per second or 9.5 trillion km/ year: From an Earthbound perspective, this is a vast distance our entire Solar System, out to the orbit of Pluto, is only one eight-hundredth of a light year across. Meet the neighbors: these are the star systems that lie within ten light years of the Sun and Solar System. Of these, only Alpha Centauri and Sirius are visible to the naked eye: the others are faint red dwarf stars. Our entire Galaxy is 100,000 light years across or more. Important points about light years • A light year is not a time period but a distance • The sun is a star we don’t measure in light years because it is only 93 million miles away and its light only takes 8 minutes to get here!! • Imagine if the distance between the sun and earth were an inch than a light year would be one mile and the star Sirius would be 8 ½ miles away. Stellar Brightness • Magnitude is the measure of a star’s brightness • Over 3 billion stars can be seen through ground based telescopes • 3,000 can be seen without a telescope • Hubble telescope can see over a trillion stars • Visibility is on the stars brightness and its distance to earth Apparent magnitude is the star’s brightness as it appears from earth. – How big the star is, how hot it the star is and how far away from Earth the star is controls the apparent magnitude. – Astronomers rank stars by apparent magnitude • the larger the number the dimmer the star – A star of the first magnitude is about 2.5 x the magnitude of a star of second magnitude The faintest star that can be seen without a telescope has an apparent magnitude of 6 and is called a sixth magnitude star Some stars, the sun, the moon and some planets have apparent magnitudes that are in the negative numbers because they are brighter than first magnitude stars. The sun has an apparent magnitude of -26.8. The most powerful telescopes can detect stars with an apparent magnitude of 29 Absolute magnitude • Absolute magnitude is how bright a star actually is • Two star with the same absolute magnitude usually do not have the same apparent magnitude because they would not be the same distance away • To compare absolute brightness, astronomers determine what magnitude the stars would be if they were 32.6 light-years away. • The sun’s absolute magnitude is +5, which is middle range for a star • Most stars have an absolute magnitude of -5 to +15 • Stars that are more than 32.6 light years away have apparent magnitudes that are higher than their absolute magnitudes Vsauce • http://www.youtube.com/watch?v=FJB7gbj iJKw&feature=c4overview&list=UU6nSFpj9HTCZ5tN3Rm3-HA The apparent and absolute magnitude demo 1. Which flashlight has the greater apparent magnitude? Which flashlight has the lower apparent magnitude? 2. How do the flashlights’ apparent magnitudes compare? How do the flashlights absolute magnitudes compare? Demo 3. Now which flashlight has the greater apparent magnitude? Which flashlight has the greater absolute magnitude? What does this mean? Hertzsrpug-Russell Diagram • Shows the relationship between the absolute magnitude and temperature of stars • Main sequence stars are linear with the hottest stars being the brightest and the dimmest being the coolest. The hottest are also the most massive and the coolest are the least massive • Red giants are massive, bright but cooler stars • White dwarfs are small and faint, although not all are white Hertzsprung-Russell Diagram Look at Figure five on pg. 704 in your book • To what group of stars does the sun belong? • How are absolute magnitude and temperature related in the main sequence? • What types of stars have high absolute magnitude but low temperatures? • What type of stars have low absolute magnitudes and medium temperatures? Variable stars • Read pg 705 and answer the following questions: • What is a Cephaid variable? • What is a light period? What is a nova? • How long are most novas? • Why do scientists think Novas occur in certain binary systems? • Describe the process of a nova flare-up Building a model • Work in a group of three to four • Design a model to represent the different types of stars-on paper would be fine – It must be proportional – The diameters of the stars are: • • • • • Super Giants are about 696 million km Giants are about 160 million km. Medium-sized star are about 1.4 million km. White dwarf are about are about 13,000 km Neutron stars are about 20 km. Interstellar matter: Go to Nebula Power Point • 1. 2. 3. 4. 5. Read pg. 706 and answer the following: What is a nebulae? What are the two types of nebulae? Describe an emission nebulae Describe a reflection nebulae What are dark nebulae Star Spectrum and type Stellar Classification • stellar classification is a classification of stars based on their spectral characteristics, what color of light is given from their photosphere • We determine this by seeing what atomic excitations are most prominent in the light, giving an objective measure of the photosphere's temperature. Letters used to represent star type • O, B, A, F, G, K, and M • where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. • O stars are called "blue", B "blue-white", A stars "white", F stars "yellow-white", G stars "yellow", K stars "orange", and M stars "red", Star Classification • In the current star classification system, the Morgan-Keenan system, the spectrum letter also has a number from 0 to 9 indicating tenths of the range between two star classes, so that A5 is five tenths between A0 and F0, and A2 is two tenths of the full range from A0 to F0. Lower numbered stars in the same class are hotter. • Another dimension that is included in the MorganKeenan system is the luminosity class expressed by the Roman Numbers I, II, III, IV and V, expressing the width of certain absorption lines in the star's spectrum