1. Stars
Dr Katy Lancaster

2. Overview Introduction to stars The Hertzsprung-Russell Diagram
What they are
What we can measure
The Hertzsprung-Russell Diagram
Star life cycles
Evolution of stars across the HR diagram

3. The Sun and all the stars we can see at night are part of the Milky Way galaxy
It contains about 1011 stars, plus gas and dust between the stars (the intersellar medium)
The Galaxy is basically disk shaped with a spheroidal bulge at the centre

4. You are here!

5. A very ordinary star!

6. What is a Star? In lay terms, a star is a big ball of burning gas
Violation of this condition
leads to the death of the
star in that it will explode
violently, scattering its
constituent matter far and
wide
What is a Star?
In lay terms, a star is a big ball of burning gas
More technically, a star is a body which satisfies two conditions:
It is bound by self-gravity
Spherical due to the symmetric nature of gravity
It radiates energy from an internal source
Nuclear energy released by fusion reactions in its interior
Violation of this condition (i.e. if the fuel
source runs out) also leads to the death of
the star in that it will simply fade away

7. Observing Stars
The information which we can gather from an individual star is quite restricted
‘Apparent Brightness’ - amount of radiation falling per unit time per unit area (of detector)
This is the ‘radiation flux’, F. This is not an intrinsic property of the star since it depends on its distance from us

8. Stellar Luminosity
We measure the star’s flux, but the intrinsic property is the Luminosity
This is the amount of power radiated per unit time
Related to the measureable flux by:
Inverse Square Law

9. Inverse Square Law
Applies to radiation, gravitational and electric fields

10. Measuring Distances: Parallax
Observe ‘nearby’ star at the extremes of the earth’s orbit
Measure the difference in its apparent position relative to ‘distant’ background stars
Use trigonometry to deduce the distance of the nearby star

11. Using the Inverse Square Law
Measure distance to star (using parallax), measure flux  luminosity
Star’s power output in Watts
Find other similar stars, assume luminosities are the same  distances
Use particular types of star as ‘standard candles’ for determing distances to e.g. stellar clusters

12. Stellar Emission
Stars emit their radiation thermally (rather than via atomic transitions)
In physics, a ‘black body’ is an object that absorbs all radiation that falls upon it, thus appearing black in colour
Practically, black bodies also radiate (in order to retain their thermal equilibrium)
Stars are approximately black body emitters

13. Blackbody Temperatures
This plot is the intensity of the radiation vs wavelength
The peak intensity shifts to longer wavelengths as temperature decreases
We can use this to derive stellar temperatures

14. Finding temperatures from real observations
We could use a spectrometer to measure a star’s spectrum
Flux vs wavelength
From the shape, we can determine its temperature
Which stars are the hottest here? Which are the coldest?

15. Filters and colours
Alternatively, we can compare the star’s flux in two different wavebands
range of wavelengths, eg
This can be done more easily than taking a spectrum of the star
The bands are defined by standard filters
U (ultra-violet): nm
B (blue): nm
V (visual): nm

16. Filters
No sharp cut-off

17. Colour Indices
Compare the ratio of the star’s flux in two filters, e.g. B and V, to find its ‘colour’
Blackbody peak shifts to shorter wavelength as temperature increases
See more flux in the B (short )filter relative to the V filter (longer ) for a hot star
This means that we can deduce temperatures from these measurements
F(B)/F(V) large for hot stars
F(B)/F(V) small for cooler stars

18. Filters vs blackbody spectra
Hot blue stars:
Comparatively more flux
in B filter
Cool red stars:
ratio is smaller

19. Spectral Classification
The Harvard classification system was developed in the 1890s by Annie Jump Cannon
Still in use today
The classes are based on features in the stars’ spectra….
….but actually it’s more useful to order the stars by their temperature or colour

20. Absorption Lines
The cooler outer regions of a star absorb photons from the hotter inner regions
Different elements absorb light at different frequencies
Atoms in different states absorb different frequencies

21. Titanium Oxide
‘Metals’
Helium lines

22. Stellar Classes
The spectral classes, ordered according to temperature:
O: > 25,000K
B: 11, K
A: 7, ,000K  Sirius
F: 6, ,500
K
G: 5, ,000K  The Sun!
K: 3, ,000K
M: < 3,500K  Betelguese

23. Very red (cool)
Very blue (hot)

24. Luminosity vs Temperature
We have just seen how ‘colour’ (derived from flux) reflects temperature
There is also a correlation between luminosity (the intrinsic property) and temperature
If we plot the luminosities and temperatures of a large, representative sample of stars, we produce a ‘Hertzsprung-Russell’ diagram
Stars of the same type all lie in the same area of the HR diagram

25. 80% of stars lie across this diagonal. This is the ‘Main sequence’

26. HR Diagram About 80% of stars lie on a diagonal line across the plot
Main sequence
These are ‘dwarf’ stars
Giants lie above the main sequence
Sub-types populate separate areas
White dwarfs lie below the main sequence
This is the general case. Now let’s look as some specifics.

27. Open clusters Found in disk of galaxy E.g. the Pleiades
Contain stars
HR diagrams may contain less red giants
Predominantly young stars

28. Predominantly main sequence stars
Pleiades - HR diagram
Few giants
Predominantly main sequence stars

29. Globular Clusters
Found well away from galactic plane, in ‘halo’ of galaxy
E.g. M80
Contain stars
Blue end of main sequence not present
Many more red giants
Older stellar population

30. HR diagram for M80
Many giants
No blue main sequence

31. HR Diagram Summary
In practice we could:
classify a star from its spectrum, thus estimating its temperature
use the HR diagram to find its luminosity
compare its luminosity with its measured flux to derive its distance from us
Or, for a star cluster at known distance:
Plot the luminosities and temperatures on an HR diagram
Deduce the cluster type, i.e. open or globular
The HR diagram is a plot of luminosity vs temperature for a population of stars
Stars of different types lie in different places on the HR diagram
80% of stars lie on the Main Sequence
HR diagrams will look different for different stellar populations
Stars ‘evolve’ and move around the HR diagram. To understand this we need to study the life cycle of a star.

32. Evolution of Stars

33. Star Formation
In between the stars in a galaxy, there is a lot of gas which we call the interstellar medium (ISM)
The gas exists in clouds
Small clouds support themselves against gravity using their internal pressure
Large clouds( with masses greater than typical stellar masses) have gravity which exceeds the internal pressure, so are unstable to collapse
Clouds fragment, forming multiple stars and hence star clusters

34. Star Formation Regions
Young stars
Ionised gas
Rosette Nebula

35. Protostars
The initial ‘free-fall’ phase of collapse is dominated by gravity
Gas still cool, radiates in the infra-red
As collapse progresses, internal pressure builds up, process slows
Star stars to heat up, makes transition to ‘pre-main sequence’

37. Main Sequence: Processes
For stars with masses at least 0.08 Msun
Central temperature reaches 107K, stars start burning Hydrogen (fusion) in their cores
Net effect: four protons turn into Helium nucleus:
This releases significant amounts of energy
The energy is transported to the star’s surface by radiation or convection

38. Main Sequence: Timescales
This process of turning Hydrogen into Helium is the energy source for main sequence stars
It takes around 1010 years for a star to deplete the Hydrogen in its core
The star then moves off the main sequence
Massive stars evolve off the main sequence more quickly

39. Aside: Smaller ‘Stars’
Stars with masses less than 0.08 Msun never become hot enough to burn hydrogen
Smaller stars continue contracting, forming ‘brown dwarfs’ which are essentially failed stars
Jupiter is about 80 times less massive than a typical brown dwarf

40. Post Main Sequence
Hydrogen burning ceases and the core contracts, thus heating the star again
Helium now fusing in the core. Outside the core, a Hydrogen-burning shell forms
Star is now larger and cooler, but more luminous than before - Red Giant
When the Helium runs out, core collapses again, Carbon burning starts
This continues for all elements up to Iron
Evolution on HR diagram depends on mass

41. Shells of Fusion
No elements heavier than Iron (Fe) can be created in this way

44. Star Death
Earlier, we defined stars as bodies which fulfill two criteria:
They are bound by self-gravity
They have an internal fuel source
Violation of either results in star death
The actual endpoint of a star is governed by its mass

45. Massive Stars
A massive star (10-60Msun) will complete all stages of fusion shown on the ‘shell’ diagram
The Iron core rapidly loses energy and contracts again, forming an extremely dense neutron star
This leaves the envelope (mainly Hydrogen and Helium) unsupported so it collapses
The rapid heating leads to a thermonuclear explosion - a Supernova
Supernovae produce the heavier elements

46. Supernovae
Supernovae are extremely luminous, with fluxes similar to those of entire galaxies
Most are seen in external galaxies (e.g SN1987a in the Large Magellanic Cloud)
We expect 1 SN every 30 years in our galaxy, but most are obscurred by interstellar dust
They leave behind a neutron star (which may be a pulsar), plus a remnant shell
These remnants may be observed for centuries afterwards

47. Neutron Stars and Pulsars
Neutron stars are tiny, but very dense
E.g. radius ~10km, mass 1.5Msun!
Hard to detect unless they are pulsars
Discovered in 1967 by Jocelyn Bell. Her PhD supervisor won the Nobel prize….
Pulsars radiate beams from their magnetic poles (radio and optical)
These may sweep across the direction to the Earth as the star rotates
Incredibly accurate ‘clocks’

48. Supernova Remnant
Pulsar!
Crab nebula

49. Lower Mass Stars
Lower mass stars, such as the Sun, only form elements up to Helium via fusion
They undergo periods of instability while they evolve as giants
Eventually, pulsations in the star blow off the surface layers, revealing the hotter interior
The material which is blown off forms a ‘Planetary Nebula’
The central star, made mostly of Carbon, cools and contracts to become a White Dwarf
They have high temperature but low luminosity

50. Planetary Nebula

51. Comparative Sizes!

52. Summary of Star Lifecycles
The formation, evolution and death of stars is a cyclical process
Starts off with big cloud of gas
Cloud collapses under gravity until it becomes hot enough to burn and shine
When the fuel runs out the star dies
Massive stars end in supernova explosions which returns material to the interstellar medium
This is recycled into new stars!

53. THE END
Any questions?