Stars

What Are Stars Made Of?

Planets – Solids (Rock, Ice) or Gas

Stars are made from the fourth state of matter, plasma.

Heat up a gas such as the hydrogen / helium mixture in the Sun, and it's molecules will break apart into individual atoms. Heat up the gas further and the atoms will separate into positively charged ions and negatively charged electrons. This is the plasma state.

The Sun – A Summary

Sunspots are cool regions on the Sun's photosphere

The frequency of sunspots appearing on the Sun's surface follows an 11 year cycle. But occasionally maxima are separated by 9 or 14 years.

In 1645 there began a 70 yearspell of persistent sunspotminima – the Maunder minimum that was associated with a mini ice-age on the Earth. Sunspots can affect the Earth's climate.

Sunspot maxima are associated with an increase in the solar magnetic field strength. At the end of each solar cycle the magnetic field briefly disappears before reappearing with the north and sole poles reversed.

Sunspots always appear in pairs near to the equator at maxima, but appear closer to the poles at minima. They rotate around the Sun, following the Sun's rotation. The Sun's equator rotates faster than its poles.

In 1908 George Hale discovered that sunspots were regions of intense magnetic field. We now know that they areas of the sun's surface where magnetic field loops emerge – leading to sunspot pairs.

Magnetic fields are created by themotion of the charged particles inthe sun's plasma.

The solar wind continually bombards the Earth with charged particles from the Sun's plasma, that give rise to the Aurora. But at solar maxima large groups of sunspots at the equator can generate enormous flares that eject charged particles away from the Sun. If the flare is directed towards the Earth it can cause massive damage to electrical systems, orbiting satellites, and astronauts.

Stars are very massive. The Sun, a typical star, has a mass of2.0×1030 kg

that's 2 000 000 000 000 000 000 000 000 000 000 kg!

Therefore stars have an extremely large gravitational attraction that keeps their plasma held together. As gravity acts equally in all directions the plasma that forms the star is moulded into a sphere.

But there must be some force keeping the star from collapsing inon itself. Because stars are so massive, their cores are very hot and at very high pressure. When the temperature exceeds a fewmillion degrees fusion occurs inside the core, that generates a lot of heat that radiates outwards from the centre of the star. This radiation pressure keeps the star from collapsing.

Nuclear Fusion – Burning Hydrogen

In the core of our Sun and stars like it hydrogen atoms (the lightest element) are fused together to form helium. Without nuclear fusion to keep them hot stars would cool down only after a few million years.

Einstein showed us that matter can be converted into energy at atomic scales. Mass from the hydrogen is converted into gamma rays!

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The hot plasma inside the Sun emits radiation (i.e. light) that passes through the 1.4 million km wide solar interior. After 10 million years this reprocessed radiations passes through the Sun's outer layers – the photosphere, where it has now cooled to just 5700 K.

T (Kelvin) = T (Celsius) + 2730K = -273 C (Absolute Zero)

The light emitted by a hot plasma always has acharacteristic spectrum – it's intensity (or brightness) is different at different energies (or frequencies, wavelengths, colours).

Stars are almost blackbodies (i.e. they emit and absorb light equally) and so have a characteristic blackbody spectrum dependent on their surface temperature.

Spectra

Analysing spectra gave rise to the subject of Astrophysics. Previously astronomy was just about categorising objects based on their appearance (morphology). But by studying their spectra, we can usephysical models to understand their nature. Many of these astronomical objects just appear as dots in the sky, but by studying their spectra we can determine their shape, temperature, and a whole load of other useful information.

The first simple way of measuring spectra is by observing the colour ofan object – a redder object emits more low-energy (long-wavelength) photons, and a bluer object emits more high-energy (short-wavelength) pgotons.

Intensity

Energy

Intensity

Energy

Red RedBlue Blue

Telescope Spectrometers: Dispersion Gratings and Prisms

But measuring the spectra accurately, determining exactly how the lightintensity depends on wavelength, gives far more information.

Fraunhofer Lines

Using a spectrometer Fraunhofer observed dark lines in the spectrum of light from the sun.

Cool elements in the sun's photosphere absorb some of the light before it escapes from the sun. Elements only absorb at certain wavelengths – black lines in the spectrum : absorption lines or Fraunhofer lines.

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Each element has a characteristic pattern of lines in the spectrum -> discovery of Helium in the sun.

How lines affect the blackbody spectrum

What else can spectral lines tell us?

Due to the Doppler shift effect, sometimes the position of lines of a given element will in the spectrum will appear shifted in wavelength.

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This implies that the absorbing material is moving either towards or away from us. Spectral observations of the sun reveal the shift in linewavelength away from the rest wavelength is greater at the equator than at the poles. Therefore the equator is moving more rapidly than the poles.

Close binary star systems cannot be resolved into two separate stars, however due to Kepler's third law they must be orbiting each other quickly.

If they are inclined so that they orbit roughly in our line of sight, then one of the stars will be moving towards us, the other away from us.

Spectroscopic Binary Star Systems

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Therefore the spectral lines of one star will be shifted to longer wavelengths and the spectral lines of another star will be shifted to shorter wavelengths.

By measuring the change in this shift over time we can see the orbital period,and can calculate the separation, and masses of the two stars.

Stars – Spectral Types

Since the development of spectrometry the spectra of many stars have been measured, and from these measurements stars have been grouped into different types.

The Hertzsprung-Russell Diagram

In 1912 Hertzsprung andRussell determined the temperature and brightnessof all of the stars within a given star cluster. Since all of these stars will lie at roughly the same distance away their relative absolute brightnesses are the same as their relative apparent brightnesses.

So they could plot the absolute brightnesses of the stars against their temperature.

They found most stars lie along a line called the Main Sequence

Main sequence stars obey a Mass-Luminosity law

The luminosity of a star, L, increases as the mass increases by M3.5

L M3.5

Mass-Luminosity Law

Mass

Radius

OB type starsare up to 100 solar masses

M type starsare around 0.1 solar masses

Stellar Lifetimes

When a star runs out of nuclear fuel to burn, it will no longer be able to maintain its balance against the force of gravity, and will collapse.

We can calculate the lifetime of a star as being the amount nuclear fuel it contains divided by the rate at which it burns nuclear fuel. This is roughly equal to the star's mass divided by its luminosity.

Therefore, T M / Lusing the mass luminosity relation: T M / M3.5

or T 1 / M2.5

Therefore, bigger, brighter main sequence stars die younger

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Stellar Evolution

Stars are formed in nebulae, such as the Orion Nebula, when a cloud of hydrogen gas collapses under its own gravity

When the centre of this protostar becomes hot enough it will ignite the nuclear fusion process, and the star becomes a main sequence star with a temperature/spectral type dependent upon its final mass.

The ignition of new star was seen for the first time this year in the Orion Nebula.

Stars spend most of their lives on the main sequence, when their hydrogen fuel runs out, the core of the star collapses, and the outerenvelope expands and cools. As the core collapses it heats up andeventually becomes hot enough to ignite the fusion of helium atoms,which enables the star to maintain an equilibrium. At this stage ithas grown so large that it is now a cool red giant star like Betelgeuse(Alpha Orionis).

What happens next, after the helium fuel is exhausted depends upon the star's mass.

In solar mass stars the core collapses further until it forms a dense white dwarf star (the size of the Earth). The outer layers of the red giant star are blown away by the intense heat of the core collapse, and form a planetary nebula.

In more massive stars further core collapse leads to the fusion of heavierand heavier elements – in these stars all of the metals were formed.

Eventually the star runs out of efficient fusion reactions (once iron is formed), and the core collapses in a violent explosion – a supernova. Leaving behindan extremely dense core, containing the mass of the sun within a 20 km radius – the size of a small asteroid!

These neutron stars rotate very rapidly due to the conservation of angularmomentum, and some that emit radio waves are known as pulsars for theirrapidly pulsed radio emission.