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

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 the HR diagram

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)

You are here!

A very ordinary star!

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 conditionleads to the death of the star in that it will explode

violently, scattering itsconstituent matter far and

wide

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 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

Inverse Square Law

Applies to radiation, gravitational and electric fields

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

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

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

• 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

Blackbody Temperatures

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?

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

Filters

No sharp cut-off

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

Filters vs blackbody spectra

Hot blue stars:Comparatively more flux

in B filter

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’ spectra….• ….but actually it’s 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 frequencies

•Atoms in different states absorb different frequencies

Helium lines

‘Metals’

Titanium Oxide

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 - 7,500K – G: 5,000 - 6,000K The Sun!– K: 3,500 - 5,000K– M: < 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 let’s look as

some specifics.

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

Predominantly main sequence

stars

Few giants

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

HR diagram for M80

No blue main sequence

Many giants

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 diagram

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 (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

Star Formation Regions

Rosette Nebula

Young stars

Ionised gas

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’

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

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

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

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

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

• 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

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

Pulsar!

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!

THE END

Any questions?