Astrophysical Techniques. opt nd Year Astrophysics Group Research Project

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1 Astrophysical Techniques opt nd Year Astrophysics Group Research Project Paul O Brien & Rhaana Starling For this lecture and lab handout see

2 Aim of this course: Work as a research group in order to become familiar with the techniques of CCD photometry as applied to various key problems in astrophysics. Study three examples of astronomical phenomena: 1. Cepheid variables measuring the size of the Universe 2. Planetary transits how to find an extra-solar Earth 3. The Hertzsprung-Russell diagram making sense of stars Topics 1 and 2 done together by taking CCD images of multiple powerpoint slides which simulate a set of variable stars. Topic 3 done by taking 3 colour images of a single (different) slide.

3 DO NOT USE DO NOT USE 1 3

4 DO NOT USE DO NOT USE

5 Course Outline Lecture 1 Facilitation Introduction to the astrophysics Feedback, presentation & discussion You will have 3 lab sessions of 2 afternoons each. Work in groups; 3 groups per lab session (so you alternate, working every other week in the lab). Pace and individual responsibilities (measurements, background reading, etc.) are up to you. Each student must produce a written report which is assessed individually. Marks are also given for group work in the lab and the facilitation sessions.

6 Measuring the Universe Distance ladder less accurate each step you take. Cepheid variables are a key step on the ladder.

7 Cepheid Variables What are they? Stars that vary in a regular way, with a brightness that approximately doubles on a timescales from days. Where are they? The original set, including the first δ-cephei were population I stars. Now know there are 2 populations. Why are they? Find out why a Cepheid pulsates.

8 The Computer AAVSO Henrietta Swan Leavitt ( ) Employed at Harvard College Observatory (by Director, Edward Pickering) to measure brightness of stars on photographic plates. Worked on the LMC/SMC plates and studied some curious variable stars (the Cepheids, named after δ-cephei, known since 1784). In 1908 she showed that brighter Cepheids had longer periods. She confirmed this in 1912, this is the Period-Luminosity relation.

9

10 The Period-Luminosity relation If you know the P-L relation you can get absolute magnitudes using the period. M = -a Log 10 P d b (period in days) where a & b are constants and M is the absolute magnitude the magnitude an object would have at a distance of 10 parsecs. The relation between absolute magnitude and apparent magnitude, m, is given by the relation: m M = 5 log 10 d 5 where d is the actual distance in parsecs. Once the relation is calibrated, P gives M and then by using m you can get d. This is why the P-L relation is absolutely vital!

11 The Period-Luminosity relation Ejnar Hertzsprung determined the distance to Delta-Cephei in 1913 and hence (incorrectly) calibrated the P-L relation. Today M v = -2.8 Log 10 P d 1.43 (period in days) The exact relation depends on metallicity (e.g. Type I & II stars)

12 The distance scale today In 1924, Edwin Hubble found Cepheids in several spiral nebulae, proving they were distant galaxies. The rest is history Hubble s constant, H 0 Locally, vel. = H 0 x distance NGC4603 in Centarus, HST/PC H 0 is not a constant in time. Must allow for cosmology. HST key project (2012): H 0 = 74.3 ± 3.6 km s -1 Mpc -1 WMAP 9-year (2012/2013): H 0 = ± 0.8 km s -1 Mpc -1 Planck initial results (2013): H 0 = 67.3 ± 1.2 km s -1 Mpc -1

13 Study questions on Cepheids (each group should do some background research and present the results in your reports and answer questions in facilitation sessions) Why do Cepheid variables pulsate? Why don t all stars pulsate? How far can we measure distances using Cepheid variables? What do we do then? How do we calibrate the period-luminosity relation? What else can alter the apparent brightness?

14 1995 First extra-solar planet discovered by Michel Mayor and Didier Queloz

15 Finding planets using transits 1999: first extra-solar planetary transit for HD The planet itself was already known from radial velocity work. Planet: 70 M J, 1.3 R J, AU from star example of what became known as a hot Jupiter.

16 Why use transits? Transists can detect planets which are less massive and/or further away from the star than detectable using radial velocity. These include planets in the so-called habitable zone. Combination of data (light curves, imaging, spectra ) can give much finer detail on the nature of the planet.

17 Study questions on transits What other methods are there for finding planets? What limits the mass you can measure using the transit or doppler methods? What masses are possible for a planet? What is the habitable zone? How far away from the Sun could an alien observer see the Solar System? How could you detect life? Do we need space missions to study extra-solar planets?

18 Spectral types Different stars have different spectral characteristics and colours depending on their temperature. Basis of the stellar classification scheme.

19 Another computer Annie Jump Cannon ( ) Started at Harvard in 1896 as one of Pickering s women. Built on the earlier spectral classification schemes to develop the one we use today (although now expanded and enhanced). Based primarily on the strength of the lines of Hydrogen. Scheme runs from the hottest stars (type O) to cool stars O B A F G K M Oh be a fine girl, kiss me Or, Only boys advocating feminism get kissed meaningfully

20 Hertzsprung Russell diagram Luminosity or absolute magnitude (Can also use apparent magnitude if the stars are at the same distance the basis of another distance indicator involving fitting clusters) Colour, spectral type or temperature

21 Hertzsprung Russell diagram If stars emit as blackbodies and are perfect spheres, their luminosity should vary as L = 4 π r 2 σ T 4. This would be a diagonal line in the HR diagram. Reality is somewhat different...

22 Cepheid variables in the HR diagram

23 Study questions on the HR diagram When was the HR diagram first used? How many different regions of the diagram contain stars? How can the main sequence be used to find distance or age? What happens to stars when they leave the main sequence? Where are proto-stars (pre main-sequence stars) located on the HR diagram?

24 Practicalities Decide who will do which task (taking data, photometry, internet search etc.) but be flexible if required. Preliminary results will be discussed at facilitation sessions. The demonstrator will explain how to calibrate magnitudes. Communicate with each other sharing information is absolutely crucial (it s a GReP!) and you all need to know the data processing tasks. You will have to explain what happened in your own individual report (and in facilitation sessions). You will be judged in part on how your group plans the activities (see the GReP notes for assessment details). At the facilitation sessions, groups will be asked to show preliminary results, discuss future plans and answer questions so come prepared. This carries marks.

25 Lab sessions are mondays and tuesdays 2pm, starting next week or the week after (second group will do thursday and friday for their last lab). Facilitations are every other Thursday at 11am, starting on 30 th. Reports (individual) must be handed in by Friday 14 th March. Any problems with the course or meeting the deadline please talk to us as soon as you can:

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