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Stars Dr Katy Lancaster Overview Introduction to stars What they are What we can measure The Hertzsprung-Russell Diagram Star life cycles Evolution of stars across ... – PowerPoint PPT presentation

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Title: Stars

  • Dr Katy Lancaster

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

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
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A very ordinary star!
What is a Star?
Violation of this condition leads to the death of
the star in that it will explode violently,
scattering its constituent matter far and wide
  • 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
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

Stellar Luminosity
  • We measure the stars flux, but the intrinsic
    property is the Luminosity
  • This is the amount of power radiated per unit
  • Related to the measureable flux by

Inverse Square Law
Inverse Square Law
Applies to radiation, gravitational and electric
Measuring Distances Parallax
  • Observe nearby star at the extremes of the
    earths orbit
  • Measure the difference in its apparent position
    relative to distant background stars
  • Use trigonometry to deduce the distance of the
    nearby star

Using the Inverse Square Law
  • Measure distance to star (using parallax),
    measure flux ? luminosity
  • Stars 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

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

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

Finding temperatures from real observations
  • We could use a spectrometer to measure a stars
  • Flux vs wavelength
  • From the shape, we can determine its temperature
  • Which stars are the hottest here? Which are the

Filters and colours
  • Alternatively, we can compare the stars 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

No sharp cut-off
Colour Indices
  • Compare the ratio of the stars 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

Filters vs blackbody spectra
Hot blue stars Comparatively more flux in B
Cool red stars ratio is smaller
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
  • .but actually its more useful to order the
    stars by their temperature or colour

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

Titanium Oxide
Helium lines
Stellar Classes
  • The spectral classes, ordered according to
  • O gt 25,000K
  • B 11,000 - 25000K
  • A 7,500 - 11,000K ? Sirius
  • F 6,000 - 7,500K
  • G 5,000 - 6,000K ? The Sun!
  • K 3,500 - 5,000K
  • M lt 3,500K ? Betelguese

Very red (cool)
Very blue (hot)
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

80 of stars lie across this diagonal. This is
the Main sequence
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 lets look as some

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

Pleiades - HR diagram
Few giants
Predominantly main sequence stars
Globular Clusters
  • Found well away from galactic plane, in halo of
  • E.g. M80
  • Contain 105 - 106 stars
  • Blue end of main sequence not present
  • Many more red giants
  • Older stellar population

HR diagram for M80
Many giants
No blue main sequence
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.
  • 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
  • Deduce the cluster type, i.e. open or globular

Evolution of Stars
Star Formation
  • In between the stars in a galaxy, there is a lot
    of gas which we call the interstellar medium
  • 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

Star Formation Regions
Young stars
Ionised gas
Rosette Nebula
  • 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

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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
  • This releases significant amounts of energy
  • The energy is transported to the stars surface
    by radiation or convection

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

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

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

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

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

  • 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

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

Supernova Remnant
Crab nebula
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

Planetary Nebula
Comparative Sizes!
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!

  • Any questions?