This is an introduction to stars, including the basics of observing and classifying stars as well as their evolution and life cycle. This is a modification of a presentation I found online.

1. Stars
Anna Quider
Institute of Astronomy
University of Cambridge
13 July 2009

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

3. It contains about 1011 stars, plus
gas and dust between the stars
(the intersellar medium)

4. 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)

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

7. You are here!

8. A very ordinary star!

9. 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

10. 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

11. Violation of this condition
What is a Star? leads to the death of the
star in that it will explode
violently, scattering its
• In lay terms, a star is a big ball of burning far and
constituent matter gas
wide
• 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

12. 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

13. 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

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Observing Stars
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Image Spectrum

15. Observing Stars
• ‘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

16. 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 measurable flux by:
Inverse Square Law

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

18. Think of a star as a torch...
1. The apparent brightness of the torch
changes with distance from your eyes
2. Torches have different intrinsic strengths

19. 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

20. 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 determining distances to e.g.
stellar clusters

21. 01%2.-31'$14
Stellar Emission
• Stars are emitting light. We can study this
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light by putting it through a spectrograph
(prism) and splitting the light into its
component wavelengths.

22. 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

23. 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

24. 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?

25. But...
• It takes a lot of time to get a spectrum so is
there a way to determine a star’s peak
wavelength (and therefore it’s temperature)
without taking a spectrum?

26. 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): 300-400nm
– B (blue): 400-500nm
– V (visual): 500-600nm

27. Filters
No sharp cut-off

28. The Sun in Different Filters

29. 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 λ) filter relative
to the V filter (longer λ) 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

30. 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

31. Absorption Lines

32. 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

33. Titanium Oxide
‘Metals’
Helium lines

34. Stellar Classes
• The spectral classes, ordered according to
temperature:
– O: > 25,000K
– B: 11,000 - 25000K
– A: 7,500 - 11,000K  Sirius
– F: 6,000 - 7,500
– G: 5,000 - 6,000K  The Sun!
– K: 3,500 - 5,000K
– M: < 3,500K  Betelguese

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

36. 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

38. 80% of stars lie across
this diagonal. This is
the ‘Main sequence’

39. 32

40. 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.

41. Open clusters
• Found in disk of
galaxy
• E.g. the Pleiades
• Contain 10 - 1000
stars
• HR diagrams may
contain less red
giants
• Predominantly
young stars

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

43. Globular Clusters
• Found well away from
galactic plane, in ‘halo’
of galaxy
• E.g. M80
• Contain 105 - 106 stars
• Blue end of main
sequence not present
• Many more red giants
• Older stellar population

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

45. HR Diagram Summary
• 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.

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

47. Evolution of Stars

48. 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 and collapse
• Clouds fragment, forming multiple stars and
hence star clusters

49. Star Formation Regions
Young stars
Ionised gas
Rosette
Nebula

50. 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 starts to heat up, makes transition
to ‘pre-main sequence’

52. 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 from p-p chain:
• This releases significant amounts of energy
• The energy is transported to the star’s
surface by radiation (light) or convection

53. 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

54. Layers in the Sun

55. The Proton-Proton Chain

56. The CNO Cycle
• Main core
reaction in
stars greater
than 1.5Msun

57. 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

58. 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

59. Relative Sizes

60. Shells of Fusion

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

63. 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

64. 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 elements heavier
than iron

65. 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 obscured 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

66. Supernovae

67. 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’

68. Supernova Remnant
Pulsar!
Crab
nebula

69. Supernova Remnant
Crab
nebula in
x-rays

70. Supernova Remnant
Crab
nebula

71. 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

72. Planetary Nebula

73. More Planetary Nebulae

74. 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!

75. THE END
Any questions?