Astro 201 Sept 23 2010 Turn in IR Camera write up in front of class Pick up graded HW along side of classroom will talk about grading in class First MIDTERM Tuesday Sept 28 covers through the end of today s lecture see web page for info Office Hours Monday 2 5pm Reading Hester Chapter 13 Taking the Measure of Stars 14 Our Sun Today Finish talking about telescopes Stars continued Seeing twinkling Weather conditions and turbulence in the atmosphere set limits to the quality of astronomical images from ground based observatories Mountain top observatories are put on peaks where the Atmospheric turbulence is minimal Bad seeing Good seeing Laminar vs Turbulent Fluid Flow Air becomes turbulent when it encounters a barrier e g a mountaintop bad seeing Turbulent Flow Laminar flow Figure 5 16 Bubbles of warmer or cooler air distort light The Hubble Space Telescope is 600 kilometers above the Earth s surface Hubble Space Telescope has great angular resolution it s above the turbulent atmosphere Light gathering ability Not as great it s only D 2 4 meters in diameter Problem It costs a lot of money to put a telescope in space Problem 2 It s really hard to repair telescopes in space only Hubble was designed to be repairable John Grunsfeld Visiting this December Figure 5 18 Atmospheric windows in the spectrum X Ray Astronomy X rays are completely absorbed in the atmosphere X ray astronomy has to be done from satellites NASA s Chandra X ray Observatory Gamma Ray Astronomy Gamma rays most energetic electromagnetic radiation traces the most violent processes in the Universe The Compton Gamma Ray Observatory Infrared Astronomy Although short wavelength IR gets through the atmosphere longer wavelength IR does not In space can cool the telescopes so it s not a source of high background Spitzer Space Telescope Next Huge NASA mission after Hubble Space Telescope ends James Web Space Telescope JWST Radio telescopes detect radio frequency radiation which is invisible to your eyes Parabolic dish of a radio telescope acts as a mirror reflecting radio waves to the focus Radio telescopes can be huge because they don t have to be as smooth as optical telescopes the wavelength of radio light is several cm s and mirrors only have to be smooth to about 1 20 of a wavelength to focus the light well Surface of mirror Arecibo Radio Observatory in Puerto Rico Radio Interferometry The Very Large Array VLA 27 dishes are combined to simulate a large dish of 36 km in diameter Even larger arrays consist of dishes spread out over the entire U S VLBA Very Long Baseline Array or even the whole Earth VLBI Very Long Baseline Interferometry STARS What is a star Why do they shine How old are they Mass of Stars Periodic Doppler Shift Summary of Stellar Properties Spectral Type MASS solar masses Luminosity solar luminosities Surface Temperature degrees K Radius solar radii O 40 400 000 40 000 13 B 15 13 000 28 000 4 9 A 3 5 80 10 000 3 0 F 1 7 6 4 7 500 1 5 G 1 1 1 4 6 000 1 1 K 0 08 0 46 5 000 0 9 M 0 05 0 08 3 500 0 8 Also there are Giants Supergiants and white dwarfs Same Temperature as stars in the table but different luminosity and radii Thanks again to Barbara Ryden OSU Kelvin Celsius 273 Water boils 373 Kelvin K Water freezes 273 K Absolute zero 0 K Room temperature 300 K Surface of Sun 5800 K B star is much larger brighter and hotter than the Sun An example is HD93129A shown below The Hertzsprung Russell Diagram When you plot LUMINOSITY versus Temperature for stars in the sky the result is not a scatter plot Hertzsprung and Russell first realized this and the diagram they made is still an important tool in astronomy for understanding stars H R Diagram HertzsprungRussell Diagram Plot Luminosity versus Surface Temperature or equivalently Luminosity versus spectral classification Main Sequence Stars fusing hydrogen to helium Example The Sun Giants are more luminous than a main sequence star of the same temperature Giants tend to be relatively cool T 6000 Kelvin but luminous L 100 to 1000 L sun Supergiants are even more luminous than giants Supergiants can have any temperature but they are always VERY luminous with L 100 000 to 1 000 000 Lsun White Dwarfs are less luminous than a main sequence star of the same temperature They are called WHITE dwarfs because they are fairly hot white hot in fact with temperatures of T 5000 Kelvin The are low in luminosity with L 0 0001 to 0 01 Lsun In a sample of 1 000 000 stars from the Milky Way on average you d find 900 000 main sequence stars 96 000 white dwarfs 4000 giants 1 supergiant So now we have a range of stellar colors and sizes For example Aldebaran is a red supergiant star Arcturus is an orange giant star Betelgeuse A very large red giant in Orion White Dwarfs about the size of the Earth Stellar Lifetimes on the Main Sequence More Massive Stars are more luminous and are burning hydrogen more efficiently They therefore have shorter lifetimes on the Main Sequence before they burn up the Hydrogen in their core Mass of the Star Msun Main Sequence Lifetime 1 5 10 billion years 100 million years 10 10 million years After the hydrogen fuel in the core of the main sequence star is used up There is no longer enough thermal pressure in the core to balance gravitational collapse No more hydrostatic equilibrium What happens next Star rearranges itself outer layers expand and cool Star becomes a red giant or supergiant Eventually more processes happen and the red giant becomes a supernova or planetary nebula and then a white Dwarf neutron star or black hole more on this later STAR CLUSTERS All the stars in a cluster are 1 at the same distance and 2 were formed together so are the same age Open Clusters Young Less than a billion years old found in the disk of the Milky Way typically 100 s 1000 s of stars often have gas and dust Globular Clusters Contain oldest stars in the Milky Way 12 13 billion years old stars in orbit around center of cluster gravitationally bound Typically 100 000 million stars never have gas and dust PLEIADES Open Cluster H Chi Persei a Double OPEN Cluster The Jewel Box 10 million years old M46 and M47 Open Clusters Charles Messier 1730 1817 French Astronomer Made a catalog of 103 nebulous i e fuzzy objects Nebula cloud Purpose help comet hunters M47 47th object on Messier s list Some are clusters of stars some are galaxies some are gas clouds M55 Globular Cluster Omega Cen Globular Cluster M80 Globular Cluster Schematic of Milky Way Galaxy Open clusters In the Disk