1. Rosetta’s comet:
The perfume of this comet is quite strong, with the odor of rotten eggs (hydrogen sulphide), of horse stable (ammonia) and the pungent, suffocating odor of formaldehyde. This is mixed with the faint, bitter, almond-like aroma of hydrogen cyanide. Add some whiff of alcohol (methanol) to this mixture paired with the vinegar-like aroma of sulphur dioxide and a hint of the sweet aromatic scent of carbon disulphide and you arrive at the perfume of our comet.
While this doesn’t probably make a very attractive perfume, remember that the density of these molecules is still very low and that the main part of the coma is made up of sparkling water (water and carbon dioxide molecules) mixed with carbon monoxide. “This all makes a scientifically enormously interesting mixture in order to study the origin of our solar system material, the formation of our Earth and the origin of life,” says Kathrin Altwegg…
… if Churyumov-Gerasimenko being a Kuiper belt comet differs from the better known Oort cloud comets and this will then shed light on the solar nebula from which our solar system emerged.

2. Lecture 20 Comet Siding Spring & Mars Read before coming to class:
ALL NOTES COPYRIGHT JAYANNE ENGLISH
Read before coming to class:
Stellar Mass: 17.7 & Box17-3.
Variable Stars P
Star Birth Chapt 18 and 19
Evolution of stars Chapt 20
HST composite image

3. Stars: Why Temperature is useful.
measure T & r  calculate L.
measure T & L  calculate r.
Temperature can be determined using
Spectral Classification
Photometry (i.e. “colour”)

4. Stars: Plot Luminosity versus Spectral Class
Plot L versus T (Spectral Type)
As if there were no relationship.
As if there is a 1-to-1 correlation.
Read about the Hertzsprung-Russell (H-R) diagram in your text.
Draw it out! But let’s have a quick look at it first.
Shows a correlation … almost 1-to-1.
This is a scatter plot.
What would you expect?

5. Stars: Hertzsprung-Russell Diagram
Because of the equation for Luminosity with radius and temperature, we can plot dashed lines for radii.
plotting 20,000 stars.

6. The Hertzsprung-Russell Diagram
80 closest stars
dashed lines of constant radius.
darkened curve is main sequence (MS); 90% of all stars
white dwarf (WD) region: hot but small. 1% of all stars.
Figure H–R Diagram of Nearby Stars Most stars have properties within the long, thin, shaded region of the H–R diagram known as the main sequence. The points plotted here are for stars lying within about 5 pc of the Sun. Each dashed diagonal line corresponds to a constant stellar radius, so that stellar size can be indicated on the same diagram as stellar luminosity and temperature.

7. The Hertzsprung-Russell Diagram
100 stars more luminous than the Sun.
red giants and the blue giants.
Figure H–R Diagram of Brightest Stars An H–R diagram for the 100 brightest stars in the sky is biased in favor of the most luminous stars—which appear toward the upper left—because we can see them more easily than we can the faintest stars. (Compare with Figure 17.14, which shows only the closest stars.)

8. Stars: Hertzsprung-Russell Diagram
Notice the luminosity and the temperatures.
General regions:
Main Sequence (MS)
White Dwarfs (WD)
Giants
Supergiants

9. mark on important regions place our sun on the H-R diagram
Draw the H-R diagram!
label axes
mark on important regions
place our sun on the H-R diagram

10. Stars: Hertzsprung-Russell Diagram
The highest density of stars is in the Main Sequence (MS).
HIPPARCOS measured
precise positions, parallaxes and motions
2.5 million stars in 3.5 years
Out to about 200 pc
2 colour photometry for 400,000 stars
Gaia will measure a billion stars out to 10kpc over 5 years!
Colour scale gives the number of stars in that point.
Notice in this version they plot “observables” – the things that astronomer measure. From these temperature and luminosity can be calculated. For temperature they plot the magnitude difference in colour between filters. Recall that the ratio between intensities in filters (i.e. this colour difference) gives the spectral class and hence temperature. For luminosity they plot the absolute magnitude.
Roughly stars (ESA Hipparcos).
2 colour photometry -> T
Gaia launched!

11. Stars: Hertzsprung-Russell Diagram
Don’t yell the answer out at first.
If we know only the temperature, can we use the H-R diagram to know a unique luminosity of a star?
Yes.
No.
We need to know their radii.
Now yell out the answer.

12. Stars: Hertzsprung-Russell Diagram
Colour scale gives the number of stars in that point.
Notice in this version they plot the magnitude difference in colour between filters. Recall that the ratio between intensities in filters (i.e. this colour difference) gives the spectral class.
Notice: no 1-to-1 correlation between T & L.

13. Stars: Radii L from Inverse square brightness law.
Distance from parallax (e.g. Hipparcos data).
Measure apparent brightness (F).
Collisional broadening of a line (due to atoms colliding while an electron is changing orbits) occurs more frequently in a dense gas. So a giant star, with a more diffuse atmosphere, will have narrower lines.
3. As well as spectral classes, there are luminosity classes which can be plotted on the HR diagram.

14. Stars: Radii L from Inverse square brightness law.
Distance from parallax (e.g. Hipparcos data).
Measure apparent brightness (F).
Periodically varying stars, e.g. Cepheid variables, have known luminosity. P

15. Stars: Radii L from Inverse square brightness law.
Distance from parallax (e.g. Hipparcos data).
Measure apparent brightness (F).
Periodically varying stars, e.g. Cepheid variables, have known luminosity.
Measure width of spectral lines in star’s atmosphere.
Line width depends on density.
Density correlates with luminosity,  luminosity classes.
 Distinguish supergiants, giants, MS, & WD.
Collisional broadening of a line (due to atoms colliding while an electron is changing orbits) occurs more frequently in a dense gas. So a giant star, with a more diffuse atmosphere, will have narrower lines.
3. As well as spectral classes, there are luminosity classes which can be plotted on the HR diagram.

16. Stars: Hertzsprung-Russell Diagram
If know Luminosity Class then get L from Spectral Class on HR diagram.
stellar radius
distance

19. Can we have a low surface temperature star with a high luminosity?
Stellar Radii:
Can we have a low surface temperature star with a high luminosity?
Yes, if the radius is large.
WRITTEN NOTES!

20. Cannot measure a star’s radius from an image!
Our nearest star (~4 ly) – Proxima Centauri
Cannot measure a star’s radius from an image!
Bright stars are big due to telescope optics.

21. E.g. Antares by David Malin.
Stars: Red Giants
E.g. Antares by David Malin.
Dust grains floating away  very tenuous surface.
Low T, high L  R very large (e.g. 100s x radius of sun).
Masses ~< 100 solar masses.

22. Red Giant “Toby Jug”
Very Large Telescope

23. E.g. Sirius B. Dot in left corner.
Stars: White Dwarf
HST
Binary system of Sirius A and Sirius B
Diffraction spikes and concentric rings are due to the optics. The large star is Sirius A, the brightest star in the night time sky.
Sirius A is a Spectral class A and the size of a main sequence star.
E.g. Sirius B. Dot in left corner.
High T, low L  R very small (e.g. size of Earth).
Masses ~ 1 Msun (million * denser).

24. Stellar radii without a ruler!

25. Radii exercise
Practise exercise: Convert to Earth radii.

26. Review
To get the radii of the stars in this image we measure the diameter of the star on the image in arcsec. We then find the distance using the parallax method. This allows us to convert arcseconds into linear units such as kilometres.
True
False

27. Stars: Luminosity, Temperature and Radius on H-R diagram
position on the H-R diagram depends on mass, composition, and stage of evolution.
lifetime on the MS depends on mass.

28. The majority of stars are in pairs – binary systems.
Stars: Mass
The majority of stars are in pairs – binary systems.
Forbit = Finertia
(r = radius of of the orbit).
Recall we did this to find the mass of planets using their moons. 

29. v = circumference of ellipse/Period of orbit.
Stars: Mass
This can be done for any set of stars – they do not have to be on the main sequence.
Visual binary  r and v
v = distance/time
v = circumference of ellipse/Period of orbit.
Also Kepler’s III Law (book), Doppler shifts & light curves constrain masses.

30. Example for mass of 2 stars:

31. Example for mass of 2 stars:

32. Example for mass of 2 stars: