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A guide to the Stars

The Stars
What are they
Types and classifications
What they can tell us
We are made of Star-Stuff
Nucleosynthesis
Life of Stars
Birth
Youth
Middle-Age
Death

An introduction to Stars for the Layperson

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What are they?

Spectrocoscopy is the science of looking at the light given off by hot gases and materials to dtermine what they are made of. If you burn pure gas or metal then specific wavelengths of light are given off, its spectra. The strength of the light, its luminosity, depends on how hot the material is. The wavelengths given off are determined by the allowed and forbidden transitions of electrons in orbit about atoms.

By measuring the spectra of stars it was discovered that they are made of gas, very hot gas. Mostly they are made of hydgrogen (H) and Helium (He), relative abundances 75% and 25% respectively. Two stars orbiting each other, Binary Stars, obey keplers laws. So measuring the periods of the orbits allows us to measure the mass of the stars, if we know how far apart they are. Through this it was found that stars are massive, millions of times more massive than the Earth.

The Laws of Gravity say that anything that big should collapse under its own `weight'. The heat of the star generated at the core stops this happening. We say the Star is in Thermal Equiilibrium. This allows us to use Newtons Laws of Gravity and the Gas Laws to understand how stars are formed, live and die.

Working backwards it can be worked out that the center of a star must be incredibly dense and yet still made of Hydrogen and Helium. So, dense in fact that Nuclear Fusion is the most likely cause of the stars energy.

It is easy to see that Stars come in many colours and of different brightness. Using the principle that things further away are dimmer we can guess that dim looking stars are further away than brighter ones. Or, that dim stars are really dim but are closer. This leads us to the ideas of Apparent and Absolute Magnitude. The Apparent Magnitude of a star is how bright it looks to us. Quantifying this we use numbers where the larger the number the dimmer the are. A magnitude 1 star is brighter than a magnitude 2 star. The Human eye actually repsonds to light logarithmically so a star that is twice as bright as another is actually 2.718. times as bright. The brightest star in the sky is Sirius A about magnitude -1. The absolute magnitude of a star is how bright it really is. Brightness (called Luminosity) is the measure of how much light the star puts out at a given wavelength of light through a known area, called a solid angle, and then summed over the surface area of the stars surface.

Also, assuming that `bright' things are hotter than dim things we can understand why some stars are different colours. After all a hot piece of metal is red, a really hot piece is white. The relation between heat and colour (wavelength of light) is given by the Boltzmann Equation. This shows how, for a given temperature, a body will emit light radiation in a given way. The Boltzmann equation only works for perfect heat radiators, called Black Body radiators so we must assume Stars are Black Body Radiators. Common sense also dictates that the larger something is the brighter it appears, there is more surface area so more light. The Bolzmann-Saha equation relates the Luminosity, L of a star to its Temperature T, and Radius, R. Using Spectroscopy we can measure the wavelengths and strengths (amplitude) of light from a star and using the last few equations work out how far away they are based on how hot they are at the surface. Also using Spectroscopy we know the relative chemical abundances of elements in a Star, this allows us to guess how new elements are made.

By measuring the orbits of stars that rotate around each other (binaries) we can measure their masses. Using the above Boltzamann-Saha equation and the distance relation we can begin to understand all the different types of star there are. Once we know the mass and surface temperature to a star, plus the elements it is made of we can beginn to classify the types of star.

When this was done the stars where categorized by letters of the alphabet A-Z. The system has been refined since and now stars are categorized as either O,B,A,F,G,K,M,C,N (remebered by the mnemic Oh Be A Fine Girl, Kiss Me Cutely Now). Where O stars are the hottest and N stars the coolest. These are further split into sub-categories by numbers 0-9. Where a type G9 is almost a type F0. These categories are used to quickly say somehting about the temperature, mass and makeup of a star.

Another classification is used to say how big a star is. We use the Roman Letters I-V where type V is a small dwarf and type I is a huge Giant star. The Giants are further sub-divided into Ia and Ib. The majority of stars are type IV dwarfs. By far the majority of stars are cool red M dwarfs. A very few stars are type O, the hottest and largest.

Amazingly when this data was plotted, called the Hertzsprung-Russel Diagram, it was found that most stars fit on a line, the Main Sequence This shows that the hottest stars are the largest, normally. Surprisingly it was found that some of the biggest stars where quite cool, called Red-Giants. Yet other Stars have been found that are small and hot. This all gives a clue about the life history of a star

The Main Sequence or Hertzsprung Russel Diagram

Stars come in many shapes and sizes. Using Interferometry and Spectrometry it is possible to measure what elements are in stars, how hot they are and how fast they are moving. Using these techniques it was found that stars that look similar also have similar charateristics. These have been put together into a classification scheme as follows

O
B
A
F
G
K
M
K
C

This can be remembered by the mnemonic Oh Be A Fine Girl Kiss Mee Kwick Carefully. This classification is further broken down into classes 0-9 where a M0 star is followed by class K1. The size of stars is given by their type, broken into 5 types

Ia } Super -
Ib } Giants
II
III }Giants
VI }Dwarfs
V }Sub-Dwarfs

This scheme also follows the size and temperature of a star. Class M stars are the smallest, ligtest and coolest stars. Class O stars are the largest, hottest and most massive stars. If the data for a large number of stars are plotted onto a graph we get a Hertzsprung-Russell diagram as below, (to be included)

A table also shows the realtionship between all this,

Spectral Class Mv Mbol Teff(K) Mass Diameter Luminosity Mean Density Kg m-3
Main Sequence (V)
O5 -5.8 -10 40000 40 18 500000 0.01
B0 -4.1 -6.8 28000 18 7.4 20000 0.06
B5 -1.1 -2.6 15500 6.5 3.8 800 0.17
A0 0.7 0.1 9900 3.2 2.5 80 0.28
A5 2.0 1.7 8500 2.1 1.7 20 0.55
F0 2.6 2.6 7400 1.7 1.3 6.3 0.98
F5 3.4 3.4 6580 1.3 1.2 2.5 1.07
G0 4.4 4.3 6030 1.10 1.05 1.26 1.3
G5 5.1 5.0 5520 0.93 0.93 0.79 1.6
K0 5.9 5.8 4900 0.78 0.85 0.40 1.8
K5 7.3 6.7 4130 0.69 0.74 0.16 2.4
M0 9.0 7.8 3480 0.47 0.63 0.063 2.5
M5 11.8 9.8 2800 0.21 0.32 0.008 10
M8 16 - 2400 0.10 0.13 0.0008 63
Giants (III)
G0 1.1 1.1 5600 2.5 6.3 32 0.016
G5 0.7 0.5 5000 3.2 10 50 0.004
K0 0.5 0.2 4500 4.0 16 80 0.0013
K5 -0.2 -1.0 3800 5.0 25 200 0.004
M0 -0.4 -1.8 3200 6.3 - 400 .0001
M5 -0.8 -3 - - - 1000 -
Supergiants (I)
B0 -6.4 -9 30000 50 20 250000 0.008
A0 -6.2 -7 12000 16 40 20000 0.0003
F0 -6 -6.0 7000 12.5 63 8000 0.00006
G0 -6 -5.2 5700 10 100 6300 0.000013
G5 -6 -5.2 4850 12.5 125 6300 0.000006
K0 -5 -5.4 4100 12.5 200 8000 0.000002
K5 -5 -6 3500 16 400 16000 0.0000004
M0 -5 -7 - 16 500 32000 0.0000002

Looking at the nearst stars that can be imaged some 70% of the universe is made from

What powers a Star

For a very long time no one knew what Stars where made of or how they got the energy to keep going. From human History we know the Stars have changed little over thousnands of years. As we learned more about them it became obvious that stars change little over many millions of years. So, their fuel source must be tremendous. It was once thought that our Sun must be made of Coal, the only thing at the time known to burn continually. Some one then worked out that if this was true the Sun would burn out in a few hundred years.

Only after the laws of Quantum Mecahnics where discovered around the 1900's was it realised that Nuclear fusion must power stars. Several possible types of Fusion power stars, the Proton-Proton chain, the Triple-Alpha chain and the Carbon-Nitrogen-Oxygen cycle. This all shows that stars convert, Hydrogen and Helium, the two simplest and mos abundant elements into more complex elements through Nucleosynthesis. This is how the Universe has turned a small amount of itself into Carbon and Oxygen that we are all made of.

Nucleosynthesis

This fancy sounding name simply means the creation of new elements through nuclear processes. In the Proton-Proton reaction and triple-alpha reaction Hydrogen is turned slowly into Helium. Some helium is then turned into Beryllium but this is unstable and decays back into Helium. If there is Carbon and Nitrogen present, left over from a supernovae further reactions can take place that create Lithium, Beryllium, Boron etc. This is called the slow-process (s-process).

As a star evolves on the Main Sequence more Hydrogen is turned into heavier elements. These require more energy to fuse so the core looses heat and the star cools. This allows the star to collpase a little, that heats the core and starts the fusion process going but using Helium nad heavier elements. This process continues as more and more heavier elements are created. Eventually Iron is produced, at this point it tales more energy for fusion to continue than is gained in the reaction, this is when a supernovae occurs. When a supernovae occurs the heat of the explosion causes nuclear fusion to occur throughout the whole star. This allows for heavier elements than iron to be created, the energy input of the explosion is great enough for Uranium and the likes to be produced, this is the Rapid-Porcess (r-process) . It is through supernovae explosions that heavy elements are seeded throughout the Universe.

Life of Star

For some time it was thought that the Main Sequence described the life history of a star. Basically stars where thought to start at the bottom right of the sequence and evolve up the diagram. It is know thought that when stars are created they start life as a Zero Age Main sequence (ZAMS) star. They then spend all their life in this state unless part of a <\ href="#binary">Binary system. Depending on the mass of the star there are several things that may happen

Before the main sequence

Stars are thought to be created from large clouds of gas and dust. Typically a Galaxy is a dense clump of gas. Inside this body some bits may be denser than others. This will allow the clump to start collapsing under gravity. As the cloud collapses conservation of (angular) momentum allows the cloud to spin faster and a thin disk to form with a dense inner ball. This is analagous to a Pizza chef taking a lump of dough and spining it faster and faster so that a pizza is made. As the material falls inward faster and rotates faster the core gets denser. Eventually the core gets dense enough that Fusion starts. At this point the photons produced are absorbed by the surrounding gas and emitted as lower energy Infra-Red photons. At this point the `star' is called a T-Tauri star, named after the object T Tauri. These are readily detected as they are strong emmitters of infra-red radiation.

As Fusion gets going the core starts putting out large amounts of highly ionised gas, photons and neutrinos. This is a Stellar Wind and it blows away the obscuring dust. Observations of Nebulae and Dark Dus clouds sho `blimps' where circular holes exist, in the center of which are new stars.

In the early universe Only large amounts of Hydrogen and Helium exist. These early stars where behemoths that probabbly only lasted a few million years, they are called Population II stars. Due to Nucleosynthesis these stars quickly convert light elements into heavy elements. When they go Supernovae the heavy elements are seeded throughout space. New stars made from this material are called Population I stars. These are the stars we see each night. Some of these, in the past, where type O stars and so made even heavier elements that seeded space when they went supernovae. So it is that the Earth and our Sun are the byproducts from material made from at least two earlier stars.

When a stars first starts fusion and becomes visible they are called Zero Age Main sequence (ZAMS). They then evolve onto the Main Sequence and terminate there lives depending on there mass.

Binary systems

The evolution of Binary stars is complex to say the least. Typically two stars of differenet masses orbit each other about their common center of gravity. IF one star is more massive than the other it evolves more rapidly. As it ages the atmosphere of the star can expand as it turns into a Red Giant. If the radius of the red giant exceeds the Roche Limit for the star the star looses this mass. The companion can then gravitationally attract this material and begin to increase its mass, assuming they are close enough. As the primary star evolves rapidly it will typically end its life cataclysmically as a supernovae. As explained below many possible end results can happen.

At this point the companion suddenly gains a lot of mass and starts to evolve even more rapidly. If the this stars expands beyond its roche limit it looses mass onto the compact companion. Either way the material falling onto the compact object foorms an `accretion disk'. This can be detected as it is hot and the material is accelerating rapidly giving off strong X-Rays as a result. The companion then evloves and ends its life cataclysmically ending a s compact object with two or more shells of gas surrounding two objects still orbiting each other.

If the stars are very close the larger one may actually expand to include the other star. This is thought to happen in some variable stars where a massive star has subsumed a much smaller companion. You have two fusion cores and a common envelope.

If the material falling from the `companion' onto the orbiting compact object does so irregularly then occasionally a large `clump' of matter hits the neutron star, or whatever. When tis happens a huge burst of energy is given off and detected as a Novae

Black hole

These are the most peculiar of all astrophysical objects. They are formed only if the mass of the stars core exceeds 1.44 dolar masses (Chandrasekhar Limit). Black Holes are not understanble by modern physics. They are made after the all the atomic particles are crushed neyond the limits fo the Strong-Nuclear force to withstand graviational collpase of a cores collpase. This means the individual protons and neutrons have been stripped down into their constituent quarks and gluons. This is intermixed with electrons and any leftover neutrinos into a some unknown material. It is generally accetped that quarks can not exist as free particles though gluons can. Whatever makes up a black hole is certainly strange.

The laws of gravity say that the force increases as the radius decreases, if mass is not lost. As the radius approches zero the force approaches infinity. It may be that a black hole actually contains nothing only a single point containing all the mass of the core. It mat be it is made of a totally new substance, a form of huge single atom.

What we do know is the effects of a black hole on space around it.

Neutron Star

If the enrgy of the gravitaional collpase is great enough the electrons around the atoms are first stripped from their orbits abd than forced to co-exist with the nuclei as the density of the core increases. When this happens the electrons combine with the protons and form Neutrons. Very quickly, all the material of the core turns into pure Neutrons. The collapse is stopped now because the strong-nuclear forces between the neutrons is greater than the gravitational force of the collpase. If the star is big enough graviational energy could exceed the Strong-Nuclear forces. In this case the neutrons are stripped down into their constituent quarks and gluons. A Black Hole is now formed.

The core is now only a minute fraction of its original size, about 10Km diameter, and has a denity of millions of Kg per cubic meter. Free neutrons are unstable and decay back to electrons and protons with an addtional photin and neutrino.

So, the Neutron Star is heated by a continual process and atomic decay and recombination of sub-atomic particles. Also, as the core is collapsing angular momentum is conserved and the core starts spinning rapidly. Simple calculations show that a 1 solar mass core, 10 Km in diameter, spins initally at over a 1000 times a second. This is a Pulsar

Novae

Planetary Nebulae

Pulsar

When neutron stars are formed they rotate very rapidly. Even though neutrons are electrically neutral there is a magnetic field about the neutron star. Any material caught in the field is accelerated to near relativistic velocities and ejected near the magnetic poles. As the poles rotate with the star we see a beam of high energy particles and radio waves `pulsing' as it sweeps past us.

As the neutron star grows older the spin decreases slowoly until it stops totally. A famous exmple of a pulsar is in the Crab Nebulae. The chinese witnessed and recorded the Crab Supernovae about 2,000 years ago. Though we can not se tha actual pulsar, its pulse is readily detectable on even a crude radio telescope.

Red Giant

Shell Stars

These are interesting stars when the star is in `middle age'. Hydrogen burning fuoon takes place in a shell around the core of the star. This occurs becuase the core has converted most of its Hydrogen to Helium and more energy is needed to fuse the Helium. This cools the core and the star collapses slightly, this generates more heat that allows new nuclear fusion to occur outside the core. As the core produces more and more heavier elements it cools more and further collapse takes place. As this happens the shell of Hydrogen fuion moves away from the core and, depending on mass, shells of Helium and even Lithium fusion can take place.

Eventualy the core cools enough that the heatof the core can not stop total gravitational collapse. Depending on the mass of the Star several things may happen. The core collapses gravitationally under the mass of the star and either a White Dwarf, Neutron Star or Black Hole is formed.

Supernovae

T-Tauri

White-Dwarf

As the core of the star starts fusing elements into iron it rapidly looses heat. It now takes more energy to fuse elements than is gained in the process. The core is now so cool that the heat prodiuced is insufficient to counteract the gravitational collapse of the star. At this point the mass of the star cuases it to collpase rapidly, typically in around a few minutes. This creates a huge amount of heat and new fusion starts rapidly throughout the atmosphere. This causes the stellar envoleope t be blown off as a Planetary Nebulae. In the core the atoms that where squashed together are now squashed closer. Initally the electro-static repulsion of the electrons stops the core collpasing further but as the gravitational energy increasing this is overcome and the electrons are stripped from their orbits. This creates an object of tighly packed atomic nuclei surrounded by free electrons in a `sea'. The more powerfull electro-static repulsion of the nuclei now exceeds the energy of the collpase and the star stabilises as a compact, hot, White-Dwarf star surrounded by a nebulae. If the energy of the collpase exceeds the electro-Static repulsion a Neutron Star is produced.

Nulcear fusion continues as the nuclei are so close and energetic that any remaining light elements are still fusing. The White-Dwarf eventually dies many billions of later as the energy from fusion reactions slowly dies out.

ZAMS