Module 4 Hertzsprung Russel Diagram .pptx

Basic Astrophysics

1. 6B15PHY ASTRONOMY & ASTROPHYSICS MODULE 4

2. Module 4 - Syllabus Harvard system of spectral classification: The H-D Catalogue—H-R diagram (Minimum marks: 6) Ref: An Introduction to Astrophysics - Baidyanath Basu - Chapter 4

3. THE HYDROGEN SPECTRA

4. THE HYDROGEN SPECTRA

5. THE HYDROGEN SPECTRA

6. Spectral Classification of Stars Continuous , Absorption, Emission Spectra

7. Spectral Classification of Stars Absorption Spectra

8. Spectral Classification of Stars Beginning of spectral studies of stars – Discovery of dark absorption lines of different elements in the solar spectrum by Joseph Fraunhofer (1814) – The Fraunhofer lines. Common features of all stars : Dark absorption lines superposed on the brighter background of a continuous spectrum. Lines of some elements are much stronger than those of other – different for different stars. Strength of the lines of same element is different in spectra of different stars. Balmer series of Hydrogen lines dominate over the entire spectrum. He I and HE II lines are present in some, weak/absent in others.

9. Spectral Classification of Stars In some of the stars, He lines are absent, H lines are weak but lines of neutral or ionized metals are predominant. In many others, H lines are absent, ionized metal lines are weak while neutral metal lines and molecular bands dominate. Before the atomic theory, this sequence was interpreted as a result of the difference in stellar composition. i.e , if He lines dominate – star composed mainly of He! Another conclusion – Variety due to various stages of stellar evolution.

10. Spectral Classification of Stars Relation between Spectral sequence and temperature dependence – pointed out by Sir Norman Lockyer based on Lab experiments. 1920 – M.N. Saha – Ionization theory – theoretically proved that state of ionization in the sun and other stars is dependent on the temperature and pressure in stellar atmosphere!! Supported by Rutherford’s model(1911) , Bohrs model(1913) and Planck’s quantum hypothesis (1901). Saha’s work – Theoretical basis for stellar spectral classification. Low gas density/pressure reduces chance of recombination and hence favours ionisation at a given temperature.

11. Spectral Classification of Stars ABSORPTION SPECTRA

12. Harvard Classification System: The HD Catalogue Stars are classified into several spectral classes. Earlier , Harvard group of astrophysicists led by E.C. Pickering, Miss. A J Cannon and H. Shapley classified the spectra of around 400,000 stars. Data was compiled as Henry Draper catalogue (H. D catalogue) – criterion-strength of the hydrogen lines . Included all stars upto magnitude 8.25 and some stars upto magnitude 10 observable from the Harvard observatory. Alphabetical system. Class A – Strongest H-lines, Class B - next strongest and so on…

13. Harvard Classification System: The HD Catalogue Later: Inspired by Saha’s & Russel’s idea the sequence was re classified according to the surface temperatures- lost the alphabetical order. In the order of decreasing temperature - O, B, A, F, G, K, M. Each class is subdivided into 10 subclasses- O0,O1, O2, ……O9, B0, B1, ………, B9 etc. O- Hottest, M-coolest.

14. Harvard Classification System: The HD Catalogue Modified again to include more classes as side branches: WC R N (W) O B A F G K M WN S W class- Wolf Rayet stars later discovery - two sub classes - WC (Carbon lines) & WN (Nitrogen)

15. Harvard Classification System: The HD Catalogue Side branches R, N, S : Rare stars with T similar to that of G to M but different spectra. Classes W to A are called early types and F to M including the side branches are called late .

16. The Harvard Classification System : Conclusions Stars are nearly uniform in composition. They are composed of mainly hydrogen and helium. M.N Saha’s suggestion- Spectra & Surface Temperature Low temperature stellar spectra contain molecular bands - insufficient T to split molecule to atoms. Metals- low IP-Dominant in F & G Ionized He lines only in hottest O stars. Note: IP of H = 13.60 eV , He = 24.58 eV , Na = 5.14 eV , Mg = 7.64 eV , Al = 5.98 eV , Cs = 3.89 eV (lowest) Greater the I. P of an atom, the more energy will be needed to remove an electron from its outermost orbit.

17. Harvard Spectral Sequence - Main Characteristics Spectral Class Temperature Range (K) Main characteristics Colour Typical Example O5 -O9 40000 - 25000 Ionized He lines dominant (emission and absorption lines) Blue Zeta Puppis B0 -B9 25000-11000 Neutral He lines dominant, no ionized He. Blue or Bluish White Rigel , Spica A0 -A9 11000-7500 Balmer lines of Hydrogen lines dominant He lines absent, Lines of Fe, Ti, Ca weak. White “Hydrogen stars” Sirius, Vega F0-F9 7500-6000 Ionized Ca lines, Many metal lines ( Mn , Fe, Ti, Sr etc.) White to Yellowish white Canopus, Polaris G0-G9 6000-5000 Very large no. of metal lines, Strong ionized Ca lines, ionized and neutral Fe. Yellowish white to yellow Sun (G2) , Alpha Centauri K0 -K9 3500-4500 Large no, of neutral metal lines Deep yellow to Orange Red Arcturus , Aldebaran M0 -M5 2000-3500 Band spectra of molecules ( mainly Titanium Oxide), H lines Scarcely present. Red Betelgeuse, Antares

18. SPECTRUM OF EACH CLASS

19. ENTROPY ENTROPY 25/01/2017, 2.00 P.M. 25/01/2017, 5.00 P.M.

20. STELLAR EVOLUTION

21. A graphical representation of the absolute magnitude(or luminosity) of the stars plotted against the spectral class ( or temperature). OR A plot of some measure of temperature (spectral class or colour) on horizontal axis and some measure of luminosity (absolute magnitude) on vertical axis. HERTZSPRUNG- RUSSEL DIAGRAM

22. HERTZSPRUNG- RUSSEL DIAGRAM

23. HERTZSPRUNG- RUSSEL DIAGRAM

24. HERTZSPRUNG- RUSSEL DIAGRAM

25. By E. Hertzsprung and H.N.Russel independently (1913) Shows the relationship between the intrinsic luminosity or absolute magnitude and Spectral class or Surface temperature of stars Stars whose distances were known are plotted according to some measure of their temperature (Spectral type/ colour ) v/s luminosity (absolute magnitude) Helps to interpret stellar evolution - Birth, age, death Stars appear only in certain parts of the diagram: A regular pattern was observed. Bright stars are near the top and dimmer stars are near the bottom Most of the stars lying in a narrow band from top left to bottom right -Main-sequence stars or dwarf sequence - About 90% belongs to this group . HERTZSPRUNG- RUSSEL DIAGRAM

26. Their M ranges from -8 to +15. Famous stars like the Sun, Sirius, 61 Cygni , Procyon , Barnards star etc belong to Main Sequence Their temperature ranges from more than 30000 K to about 3000 K. The sun is only a mediocre member of this sequence belonging to class G2 with M = +4.8 and surface temperature of about 6000 K. Brightness of main-sequence stars are related to mass Hotter, more massive blue stars --> cooler, less massive red stars HERTZSPRUNG- RUSSEL DIAGRAM

27. Giants Above and to right of main-sequence stars in classes from F to M More luminous than dwarfs of the same classes with M ranging between -1 to +1. Size --> compare them with stars of known size that have same temperature Supergiants = bigger (M: -3 to -8) - Ex: Betelgeuse Subgiants : Between giants and dwarf sequence, a small group with M: +1 to +5 in class F to K HERTZSPRUNG- RUSSEL DIAGRAM

28. White dwarfs Lower-left corner, well below the main sequence, a group of faint stars of classes from B to G with M: +10 to +15 Fainter than main-sequence stars of same temperature Masses of the order of solar masses and sizes comparable to that of earth - High density! Sub Dwarfs Below the main sequence and above the white dwarfs Hertzsprung gap - A region almost free from stars between giant branch and the main sequence. Note: Giants are more luminous than the dwarfs of the same temperature. Ie . Giants have larger surface area ( L -R relation) HERTZSPRUNG- RUSSEL DIAGRAM

29. Stellar Evolution The life cycle-How stars are born, mature, age and die. Main Stages ( for a Sun like star): Star formation-Interstellar Cloud Proto-star stage Main Sequence stage Red giant stage White dwarf stage A star’s mass determines what life path it will take.

30. Stellar Evolution The life of any star can be described as a battle between two forces: Gravity vs. Pressure Gravity always wants to collapse the star. Pressure holds up the star. the type of star is defined by what provides the pressure So, in different stars the pressure can be provided by: Gas (as in the Sun) Radiation (in hotter stars than the Sun) ‘ degeneracy pressure ’ (in very dense stars) Principle is the same though, this balances gravity, -else the star will collapse!!

31. Birth: Gravitational Collapse of Interstellar Cloud "Hayashi Contraction" of Protostar Life: Stability on Main-Sequence Long life - energy from nuclear reactions in the core (E = mc2) Death: Lack of fuel, instability, variability expansion (red giant, then white dwarf) Life Cycle of the Sun