1 A= one Angstrom = 1 10 cm

Transcription

1 Our Star : The Sun )Chapter 10) The sun is hot fireball of gas. We observe its outer surface called the photosphere: We determine the temperature of the photosphere by measuring its spectrum: The peak intensity of light as a function of wavelength of light received at earth : Using Wien's law max T =0.29 cm K 1 A= one Angstrom = 1 10 cm 8

2 Its surface temperature is 5700 deg K. Composition of the photosphere: This is determined by examining the dark lines in the spectra, caused by absorption by different elements and bright lines due to emission by elements. First observed by Fraunhofer.

3 Analysis of these spectral lines leads used to find composition. Element Helium was first discovered by this type of observation, before we isolated it on earth! The percentage of elements heavier than helium is called metallicity of the star and for the sun it is 1.7 %

4 Structure of the sun: Parts of solar atmosphere and interior: The core is where sun's energy is generated by nuclear fusion, the p p chain. Its temperature is 15 million degrees K. The corona is at high temperature supplied by magnetic energy and contains, prominences, filaments, streamers and coronal holes. Sunspots are magnetic with N and south pole strucures. Amount of energy radiated by the sun is Joules / sec or watts

5 Sunspots and Solar activity: First observed with telescope by Galileo in earth They are magnetic disturbances with cooler gas hence dark. They come in pairs north and south magnetic polar.

6 Flares, coronal mass ejections, filaments, prominences. Flares are tremendous eruptions ejecting energetic particles and radiation. Temperaturesreach 5 million degrees!! Coronal mass ejections (CME) cause magnetic storms on the earth could wipe out communications in northern latitudes. Produce what is called solar wind which travels out of the sun at supersonic speeds and produces an excited region called the Heliosphere which extends up to 100 AU. Voyager space craft has finally reached the end of the heliosphere called the heliopause. Some examples are:

7 Solar Corona during total eclipse: Superposed is an image of the sun taken at the same time by SOHO spacecraft an UV image.. corona The UV image is used to correlate coronal streamers back to their roots on the Sun's surface.

8 Solar prominence and coronal loop sunspots cooler regions

9 End Coronal Mass Ejection(CME) During the last solar maximum of activity. Time increases from bottom left, counter clockwise to the upper left image. Total time is 3 and half hours. start

10 Solar Activity Cycle: The number of sunspots varies with an 11 year cycle. The Sun like the earth is a magnet with north and south poles and this magnet reverses polarity every 11 years. So after 22 years the north pole is north pole again. When it reverses polarity we have a minimum number of sunspots. This is shown in: In the Maunder minimum, there were probably no sunspots. It was also the time when weather in Europe was extremely cold. Sunspots weather connection?

11 The outer layer of the sun: Chromosphere and the Corona can be best observed during solar eclipse when the main photosphere of the sun is covered exactly by the moon. (Moon and the sun subtend the same angle at the earth) and if the moon is in front of the sun and centered then we get full occultation of the sun or total solar eclipse. This outer layer can be observed in a range of wavelengths from radio to X rays either from the earth or by satellite instruments. The chromosphere, seen through hydrogen filter shows a lot of activity its temperature varies from 7000 to 15,000 deg K. See Figure 10 8.

12 Mass of the Sun: Newton's version of Kepler's III law can be used to find the mass of the sun. This version is given on page 105 of the text (Figure it out 5.2). If P is the period of revolution of the earth around the sun = 1 year, R the radius of the orbit which is 1 AU, P and R are related by: P 2= 4 2 G M sun m earth R 3 as m earth is << M sun we can solve for M sun neglecting m earth and get M sun = 4 G 2 R 3 2 P 30 putting in numbers we get M sun =2 10 kg 7 For the earth: P=1 year 3 10 sec 11 Earth's orbit : R=1 AU = m Newton's constant : G= N m2 kg 2

13 Important Numbers for the Sun: 8 Solar Radius: R sun = meters Solar Mass: M sun =2 10 kg 30 Surface Temperature: T surface =5700 deg K Solar Luminosity: 26 L sun = watts (Joules per sec ) Temperature of Core: 7 T core= deg K Mostly Hydrogen and Helium. Average density: av =1400 kg / m 3

14 More about the sun: Core is the region where a large fraction of solar mass resides. Central pressure in the sun is 250 billion times atmospheric pressure. Central density or the sun is about 100 times the density of water! It is still much less than the density of neutrons and protons inside a nucleus. Nuclear density is about grams per cc Density of water is 1 gram per cc!!

15 Our Sun and the solar system provides two tests of Einstein's General Theory of Relativity (GTR). 1. Advance of the perihelion of planet Mecury's orbit around the sun. The distance of closest approach of Mercury's orbit to the sun, varies with time. This is with respect to a fixed coordinate system of stars. It is called the advance of of the perihelion. It is a small angular shift (+ 0.45) seconds of arc per century! Newtonian mechanics (flat space and absolute time) predicts 532 arc sec, while General Relativity of Einstein predicts arc sec. 2. Deflection of light due to curvature of space time near the sun's surface. In GTR. freely moving objects in gravitational fields (produced by objects like Sun ) are described by motion in a curved space, making it look like they are being pulled by gravity. Time is also warped and the world is a curved space time manifold! Imagine a rubber membrane representing space, then the presence of a large mass like the sun at some point curves the space as shown in the figure.

16 A stable orbit of Mercury is shown in red. Path of light ray from a distant star which goes close to the sun on its way to the earth is shown, being deflected by the curvature of space near the sun.

17 Einstein calculated the expected deflection of light from a distant when it goes close to the sun's surface. The apparent position of the star in the sky gets shifted with respect to other fixed stars whose light does not go near the Sun's surface. It is less than a second of arc!! To observe this one must compare the stellar positions during total eclipse and at other times and measure the small predicted shift. Verified during total eclipse of 1919, by a team of astronomers observing in Egypt. Made Einstein immediately famous!