Astro 201: Sept. 23, 2010

Transcription

1 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, conznued = 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 1

2 Laminar vs. Turbulent Fluid Flow Air becomes turbulent when it encounters a barrier e.g. a mountaintop bad seeing Turbulent Flow Laminar flow 2

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4 Figure 5.16: Bubbles of warmer or cooler air distort light The Hubble Space Telescope is 600 kilometers above the Earth s surface. 4

5 Hubble Space Telescope has great angular resoluzon; 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: VisiZng this December 5

6 Figure 5.18: Atmospheric windows in the spectrum X-rays are completely absorbed in the atmosphere. X-ray astronomy has to be done from satellites. NASA s Chandra X-ray Observatory 6

7 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, aier Hubble Space Telescope ends: James Web Space Telescope (JWST) 7

8 Radio telescopes detect radio frequency radiazon which is invisible to your eyes. Parabolic dish of a radio telescope acts as a mirror, refleczng radio waves to the focus. Radio telescopes can be huge because they don t have to be as smooth as opzcal 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 8

9 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) 9

10 STARS What is a star? Why do they shine? How old are they? 10

11 Mass of Stars: Periodic Doppler Shii Summary of Stellar Proper2es: Spectral Type MASS (solar masses) Luminosity (solar luminosi2es) Surface Temperature (degrees K) Radius (solar radii) O ,000 40, B 15 13,000 28, A , F , G , K , M , Also there are Giants, Supergiants and white dwarfs: Same Temperature as stars in the table, but different luminosity and radii. 11

12 Thanks again to Barbara Ryden, OSU Kelvin = Celsius Water boils: 373 Kelvin (K) Water freezes: 273 K Absolute zero: 0 K Room temperature: ~300 K Surface of Sun: ~5800 K 12

13 B star is much larger, brighter and horer 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 scarer plot Hertzsprung and Russell first realized this, and the diagram they made is szll an important tool in astronomy for understanding stars 13

14 H- R Diagram Hertzsprung- Russell Diagram Plot Luminosity versus Surface Temperature (or equivalently, Luminosity versus spectral classificazon) 14

15 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 relazvely 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 L sun. 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 = to 0.01 L sun. 15

16 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: 16

17 Arcturus is an orange giant star: Betelgeuse A very large red giant in Orion 17

18 White Dwarfs: about the size of the Earth Stellar LifeZmes on the Main Sequence: More Massive Stars are more luminous, and are burning hydrogen more efficiently. They therefore have shorter lifezmes on the Main Sequence before they burn up the Hydrogen in their core Mass of the Star (M sun ) Main Sequence Life2me 1 10 billion years million years million years 18

19 Aier 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 gravitazonal collapse. No more hydrosta+c 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 All the stars in a cluster are (1) at the same distance, and (2) were formed together, so are the same age. STAR CLUSTERS Open Clusters: Young (Less than a billion years old) found in the disk of the Milky Way typically 100's 's of stars oien have gas and dust Globular Clusters: Contain oldest stars in the Milky Way billion years old stars in orbit around center of cluster, gravitazonally bound Typically 100,000 - million stars never have gas and dust 19

20 PLEIADES Open Cluster H & Chi Persei, a Double OPEN Cluster 20

21 The Jewel Box: 10 million years old M46 and M47: Open Clusters 21

22 Charles Messier ( ) French Astronomer Made a catalog of 103 nebulous i.e. fuzzy objects Nebula = cloud Purpose: help comet hunters M47 = 47 th object on Messier s list Some are clusters of stars, some are galaxies, some are gas clouds M55: Globular Cluster 22

23 Omega Cen: Globular Cluster M80: Globular Cluster 23

24 SchemaZc of Milky Way Galaxy Open clusters: In the Disk only Midterm #1 covers material up to here. 24