STARS CHAPTER 10.1 the solar neighborhood The distances to the nearest stars can be measured using Parallax => the shift of an object relative to some distant background as the observer s point of view changes. knowing the to the stars is essential to understanding many of their properties. Position 1 Position 2 6 after position 1 Distance between positions 1 and 2 is the Bigger = bigger parallax (Ɵ) parallax 1 o = 60 = 3600 Parallax of proxima centauri Stellar motion The velocity of a star is how fast it is moving directly towards or away from us. The velocity of a star is its velocity perpendicular to us. Motion is the apparent angular motion of a star across the sky with respect to more distant stars. Why can t astronomers use simultaneous observations from different parts of Earth s surface to determine stellar distances? d = distance in p = parallax in arc seconds
10.2 Luminosity Inverse Square Law of Brightness a star s apparent is proportional to the square of the distance from the star How "Bright" is a Star? Luminosity: Measures the Total Output by the star in Watts Distance
Apparent magnitude: Measures how bright the star to be as seen from a distance. on the distance to the star Absolute Magnitude what magnitude a star would have if it were at a distance of 32.6 light years from Earth Modern Magnitude System The modern system is based on observations of stellar brightness. The modern system of magnitudes defines them as follows: 5 steps of magnitude = of 100 in brightness magnitude = star. The standard of is the star Vega (0th magnitude) How much brighter? Example 1 Example 2 Two stars are observed to have the same apparent magnitude. Based on this information, what, if anything, can be said about their luminosities? Spectroscopy Each element in the periodic table can appear in form and will produce a series of bright lines to that element. H will not look like He which will not look like which will not look like iron... and so on. Thus, astronomers can identify what kinds of are in stars from the lines they find in the star's spectrum. This type of study is called 10.3 Stellar Temperatures 3 ways to determine stellar temperature: 1. Stellar Spectra 2. 3.
Stars of different temperatures show varying of lines in their spectra Spectral Classification early researchers classified stars primarily according to their -line intensities. They an A, B, C, D,... Scheme A stars had the H lines, B stars the second brightest H lines, etc Modern astronomers classified stars according to surface. But they kept the old system in order of temperature, the letters now run O, B, A, F, G, K, M. These stellar designations are called spectral (or spectral types). Astronomers each lettered spectral class into 10 subdivisions, zero to nine. the the number, the the star. Sun is classified as a star (a little cooler than G1 and a little hotter than G3) Why does a star s spectrum depend on its temperature? Stellar sizes Direct Measurement Only works for close, stars Indirect measurement Stefan-Boltzmann Law L = Can we measure the radius of a star without knowing its distance? Stellar Life Cycles Stars start from Nuclear At 15 million Celsius in the center of the star, fusion ignites 4 ( 1 H) --> 4 He + 2 e + + 2 neutrinos + energy Where does the come from? Mass of four 1 H > Mass of one 4 He A Balancing Act Energy released from nuclear fusion counter-acts force of gravity. The Beginning of the End: Sun type stars After is exhausted in core... Core, Kinetic energy of collapse into heat. This heat expands the layers. Meanwhile, as core, Increasing Temperature and Pressure... More At 100 million degrees Celsius, fuses: 3 ( 4 He) --> 12 C + energy Energy sustains the expanded outer layers of the Giant
The end for solar type stars White dwarfs At center of Nebula lies a White Dwarf. Size of the Earth with Mass of the A ton per teaspoon Inward force of gravity by repulsive force of electrons. Type I supernova Fate of high mass stars After Helium exhausted, core collapses again until it becomes hot enough to fuse into Magnesium or Oxygen. Through a combination of processes, successively elements are formed and burned. The End of the Line for Massive Stars Massive stars burn a succession of elements. Iron is the most element and cannot be fused further. Instead of releasing energy, it energy. What s Left After the Supernova Neutron Star (If mass of core < 5 x Solar) Under collapse, protons and electrons combine to form. 10 Km across Black Hole (If mass of core > 5 x Solar) Not even compacted neutrons can support of very massive stars. HR Diagram The Main Sequence most stars are found in a band stretching diagonally from left to right. All are turning into helium White Dwarfs Remnants of -like stars Balls of hot that will cool and become invisible overtime Giants and supergiants Giants are stars that are nearing They are no longer turning hydrogen into helium, but are fusing elements Giants will nova and become white
10.6 Stellar Masses we measure a star s mass by observing its influence on some nearby object Super giants will supernova and become stars or holes. If we know the distance between the two bodies, then we can use laws to calculate their masses. Binary Stars Lucky for us most stars are members of multiple-star The majority are found in binary-star systems, which consist of stars in orbit about their common center of mass, held together by their mutual attraction. So we can use Newton s law to their masses. 3 types of binary stars 1. 2. 3. binaries are systems where the stars are far enough apart we can see them with our eyes (or telescopes) binaries are seen due to the Doppler shift of their light. shifted = moving shifted = moving In binaries, the orbital plane of the pair of stars is almost edge-on to our line of sight. we observe a periodic changes in brightness as one member of the binary passes in of the other Luminosity classes How do we know the masses of stars that aren t members of binaries?