Get ready for quiz # 5! Get out a ½ sheet and Calculator

Transcription

1 Get ready for quiz # 5! Get out a ½ sheet and Calculator

2 The above image shows the solar eclipse earlier this month as covered and uncovered by several different solar observatories. The innermost image shows the Sun in ultraviolet light. This image is surrounded by a groundbased eclipse image, reproduced in blue. Further out is a circularly blocked region used to artificially dim the central sun by the LASCO instrument aboard the Sun-orbiting SOHO spacecraft. The outermost image -- showing the outflowing solar corona -- was taken by LASCO ten minutes after the eclipse and shows an outflowing solar corona. Over the past few weeks, our Sun has been showing an unusually high amount of sunspots, CMEs, and flares -- activity that was generally expected as the Sun is currently going through Solar Maximum -- the busiest part of its 11 year solar cycle.

3 Weekly Schedule Today Hw # 6 Due Quiz # 5 Measuring Stars Lecture Tutorial Next Tuesday Star Characteristics Lab

4 We can make the following measurements on stars Distance Luminosity (energy) Brightness Temperature Size

5 Measuring distance to stars 10.1 The Solar Neighborhood Parallax: Look at apparent motion of object against distant background from two vantage points; knowing baseline allows calculation of distance: distance (in parsecs) = 1/parallax (in arc seconds)

6 Example calculation Proxima centurai is measured at a parallax angle of How far away in parsecs is it?

7 Example calculation Pollux, a star in the constellation Gemini, has a parallax angle of How far away is it? Betelguese, a star in the constellation Orion, has a parallax angle of How far away is it?

8 10.1 The The Solar Solar Neighborhood Neighborhood Nearest star to the Sun: Proxima Centauri, which is a member of a 3-star system: Alpha Centauri complex Model of distances: Sun is a marble, Earth is a grain of sand orbiting 1 m away. Nearest star is another marble 270 km (167 MILES) away. Solar system extends about 50 m from the Sun; rest of distance to nearest star is basically empty.

9 10.1 The Solar Neighborhood The 30 closest stars to the Sun

10 Luminosity is another measurement we make on stars Luminosity = energy per time (energy/time) AKA: Absolute brightness What affects luminosity? LT. p.55

11 Calculating luminosity What is the luminosity (as compared to the Sun) of a star with a radius 3 times that of the Sun and has a temperature of 10,000 K? (Sun temperature = 5800 K) Luminosity Sun = R 2 Sun x T 4 Sun

12 Question 1. A 2. B 3. C 4. D 5. E 10 20% 20% 20% 20% 20% A B C D E

13 Rigel is much more luminous than Sirius B. Rigel and Sirius B have the same temperature. Which star has the greater surface area? 1. Rigel 2. Sirius B 3. the same 4. not enough information 10 25% 25% 25% 25% Rigel Sirius B the same not enough information

14 You observe two stars with the same luminosity and determine that one is larger than the other. Which star has the greater temperature? 1. the smaller star 2. the larger star 3. The temperatures are the same. 33% 33% 33% the smaller star the larger star The temperatures are th... :10

15 What we see is the apparent brightness. a closer, dimmer star and a farther, brighter one. This is an example of an inverse square law. Apparent brightness is how bright a star appears when viewed from Earth; it depends on the absolute brightness but also on the distance of the star: apparent brightness luminosity/distance 2

16 The magnitude scale ranks stars by their brightness (luminosity/distance 2 ) Apparent magnitude ~ measure of brightness Dimmer Absolute magnitude ~ measure of luminosity Brighter

17 Differences in brightness As a group, select any two stars in Appendix 3, Table 4 listing of the sky s brightest stars and compare apparent magnitude to determine how many times brighter one is as compared to another. See page 274 in your textbook (2 nd paragraph under Magnitude scale for help). It is a logarithmic scale; a change of 5 in magnitude corresponds to a change of a factor of 100 in apparent brightness. It is also inverted larger magnitudes are dimmer.

18 Vega has an apparent magnitude of 0.03 and an absolute magnitude of If it were moved twice as far from Earth as it is now, which of the following would occur? 1. apparent magnitude number would increase 2. apparent magnitude number would decrease 3. apparent magnitude number would stay the same 4. absolute magnitude number would increase 5. absolute magnitude number would decrease 20% 20% 20% 20% 20% apparent brightness luminosity/distance 2 :10 apparent magnitude n... apparent magnitude n... apparent magnitude n... absolute magnitude nu... absolute magnitude nu...

19 10.3 Stellar Temperatures Measuring temperature The color of a star is indicative of its temperature. Red stars are relatively cool, whereas blue ones are hotter.

20 Measuring temperature

21 10.3 Stellar Temperatures Stellar spectra are much more informative than the blackbody curves. There are seven general categories of stellar spectra, corresponding to different temperatures. From highest to lowest, those categories are: O B A F G K M

22 Spectral classification

23 Measuring radius 1) Size is calculated using luminosity and temperature 2) Size is reported in units of solar radii luminosity radius 2 temperature 4

24 To calculate a star s size To calculate a star s radius use: Star s temp. in solar temps. (Sun = 5800 K) Luminosity in solar luminosity Result is in solar radii R = L/T 2 Classification: Sun size and smaller = dwarfs times Sun = Giants Over 100 times Sun = Supergiants

25 10.5 The Hertzsprung Russell Diagram The H R diagram plots stellar luminosity against surface temperature. This is an H R diagram of a few prominent stars. dia_astro/if/if.html?star_properties_hr_diag ram

26 10.5 The Hertzsprung Russell Diagram Once many stars are plotted on an H R diagram, a pattern begins to form: These are the 80 closest stars to us; note the dashed lines of constant radius. The darkened curve is called the main sequence, as this is where most stars are. Also indicated is the white dwarf region; these stars are hot but not very luminous, as they are quite small.

27 An 10.5 H R The diagram Hertzsprung Russell of the 100 brightest Diagram stars looks quite different. These stars are all more luminous than the Sun. Two new categories appear here the red giants and the blue giants. Clearly, the brightest stars in the sky appear bright because of their enormous luminosities, not their proximity.

28 Why do we need to learn H-R diagrams?

29 Measuring distance to stars >200 pc Measure apparent magnitude (BRIGHTNESS) Classify spectral type (TEMPERATURE) Assume star is main sequence. Use H-R diagram to estimate Absolute magnitude. Use Absolute/Apparent magnitude to estimate distance.

30 10.7 Measuring Stellar Masses Stellar Masses Many stars are in binary pairs; measurement of their orbital motion allows determination of the masses of the stars. Orbits of visual binaries can be observed directly; Doppler shifts in spectroscopic binaries allow measurement of motion; and the period of eclipsing binaries can be measured using intensity variations. _binary_system nary_system

31 Measuring 10.7 Stellar Stellar Masses Masses Mass is the main determinant of where a star will be on the main sequence.

32 10.7 Measuring Stellar Masses Stellar Masses Stellar mass distributions there are many more small stars than large ones!