The stars in the sky

Stars are the most obvious topic imaginable when it comes to astronomy. Studied since time immemorial, twinkling every night in the heavens and visible from even the most light polluted areas, these pinpricks of light have fascinated and delighted for many a year.

Although it may seem initially that when you’ve seen one star you’ve seen them all, there is a surprising variety of stars out there – stars of different colours, double stars, stars that vary in their brightness. The first part of this post will deal with the evolution of stars. Then I will talk about a few different types of stars from the evolutionary point of view. Next, naked eye observationals and finally a few telescopic stellar sights.

Stellar Evolution

Stars can be said to be the result of raging war between the constant forces of gravity on the one hand and the short term but powerful heating and expansion of plasma on the other. When I say short term, I mean several billion years, which is a blink in the eye of the Universe.

The story of a star begins in the cold reaches of interstellar space, usually in a dark nebula or some other high concentration of gas. Anywhere where gas is dense and able to fall in on itself to form a clump. As the gas does this, the entire mass begins to collapse under gravity, still taking more gas from the surrounding nebula. As particles of gas fall to the core, they pick up speed and energy, transferring it as they bash into other particles of gas. This causes the protostar to start to heat up in a process known as Jean’s collapse, once thought to be responsible for the burning of all stars.

For low mass (lower than 0.8 solar masses), a small core develops that warms up under Jean’s collapse. The outer layers transfer this heat upwards through convection. Eventually the core reaches a temperature and pressure suitable for nuclear fusion of the main hydrogen gas to helium to occur, which then provides the star’s energy. In the case of stars from 0.8 to 4 masses of the Sun, a similar thing happens, but the core warms up faster and to a greater temperature. The greater radius provides a bit of a shield against the cold of space, meaning the fall off in temperature between the core and the next layer isn’t great enough for convection. Instead, radiation transports heat upwards to a second layer, where convection then occurs. For stars greater than around four solar masses, the core heats up faster and to a greater temperature again. It ionises the inner part of the star, making it difficult for radiation to travel, and the core also warms up a lot faster than the rest of the star, reversing the situation from moderate mass stars – there is a convective layer outside the core and a second radiative layer transferring energy to the surface. For truly enormous stars (greater than around 100 solar masses), the core warm up is so fast and to such a great temperature that the protostar blows itself apart, placing an upper limit on the size of stars. Stars below around 0.08 solar masses are unable to compress the core enough to produce temperatures and pressures required for significant self sustaining nuclear fusion. These failed stars are known as brown dwarfs. At some point even lower down the mass scale, these become similar objects to gas giant planets such as our own dear Jupiter.

Once the fusion process is underway, the star immediately begins to clear the region around it. Heat and radiation pressure begins to drive the smaller particles away from the star. T Tauri stars are stars that heat up the gas they push away from them, causing the gas to emit spectral lines. Young stars can also shoot out gas at either pole in an effect known as bipolar outflows. More gas ends up being pushed away from the star than exists in the star itself, and colliding gas pushed away from several stars is capable of producing the densities required for the birth of new stars, assisting in the development of Stellar Nurseries, where much star formation is seen.

When protostars reach the stage where the inward gravitational force balances the outward pressure caused by the high temperatures in the core, the star is said to be a Main Sequence star, like our Sun. In the case of stars of the sun’s mass, over the course of the next five billion or so years, the core will get compressed further and further and heat more and more, causing the outer layers to expand a bit. Eventually, a stage will be reached when there is no more hydrogen to burn through fusion in the core. The core begins to cool, which causes the layers outside of it to collapse onto it, which causes heating again due to Jean’s collapse once more. This new heating causes the outer layers to expand massively (our sun will increase to a diameter 100 times its present one, vapourising Mercury, stripping the atmospheres of the gas giants). The tiny core becomes surrounded by a layer of hydrogen from the main shell that now begins to burn, using the heat from the core to start off. This new source of temperature eventually allows the helium in the core of the star to start its own nuclear fusion reactions. The star has become a red giant – a massive outer envelope of mainly hydrogen gas surrounding a smaller envelope where some hydrogen is fusing into helium, surrounding a tiny core where helium is now fusing.

In stars above 2-3 solar masses, the transfer of power from hydrogen to helium occurs gradually as the star warms up. In the case of lower mass stars it happens after the core has been compressed into a degenerate state – crushed together so much the only way for the particles to get closer together would be for the particles to change into other particles. Because they’re crushed as far as they’ll go, altering the temperature doesn’t alter the pressure – the core won’t expand to cool off like a normal gas. As a result, when the core just about hits the point where helium burning begins in one part, the burning heats up the rest of it incredibly quickly, causing an explosive start to fusion called the Helium flash. Eventually, the heat generated is enough to push back against the outer layers and the core ceases to be degenerate.

Eventually, even the helium in the core gets used up. When this happens, the core becomes degenerate again and rather inert. The layer outside it starts burning Helium, fusing it to carbon and oxygen, whilst the layer outside that starts hydrogen burning, having repeated the helium flash with the previous hydrogen burning shell. During these times of core instability, events known as dredge-ups occur, where the convection layer extends to the edge of the core and brings up some of the newly created material. The first dredge up occurs when hydrogen fusion ceases in the core, the second when helium fusing ceases and in stars greater than twice the mass of the Sun, a third one can occur, which brings significant amounts of carbon to the surface. The carbon signature from these stars gives them the name Carbon Stars.

At this point, stars beneath four solar masses reach the end of their life. Throughout their time shining in the sky, the stars emit to some extent a stellar wind, where the outer layers simply come off as hot gas expands. As the star ages, the entire star expands and the mass of the star decreases, weakening the gravitational hold. After the red giant phase, these layers simply float off to create Planetary Nebulae. The layers don’t head away all at once, of course. The star ends its life in a more and more rapid series of pulsations with the outer envelope collapsing to a density suitable for hydrogen burning, expanding as soon as the burning begins, loses a bit of gas, falls in again as the lack of pressure switches off fusion, starts H-burning again and so on until all the shells of gas have been thrown off. What is left behind is the inert shell of degenerate material, known as a white dwarf star. This is then left to simply cool to a blackened blob.

Stars greater than 4 solar masses continue on into new burning phases. They have traded in lifespan for a more interesting life, with their temperatures and pressures responsible for both burning hydrogen faster and allowing elements beyond helium to be burnt. After the Helium burning phases, the star will arrive at burning phases for Carbon, Neon, Oxygen and Silicon (with higher masses able to reach further phases). Each time requires a new version of the helium flash, with new shells of fusion of the previous stage developing outside of the current shells. Eventually, silicon burning produces iron. Iron requires energy to be fused into even heavier elements and as a result burning iron reduces the temperature of the core.

Below a mass of 8 solar masses, the star will die as a planetary nebula, in common with a lower than 4 solar mass star. Any larger than that and the sudden switching off of the iron core and the cooling effect can cause the core and entire star to contract. The outer layers slam into the core and bounce off in an explosive supernova.

There are four types of supernova, known as types 1a, 1b, 1c and, just to be different, 2. As you might expect, three of these types are closely related and one is not. The related types are 1b, 1c and 2. Naturally. These are supernova as described above, a type two supernova happens when a pristine supermassive star explodes. Type 1c occurs when a star that has previously thrown off its hydrogen layer explodes and type 1b occurs when a star has thrown off both hydrogen and helium. Type 1a however is totally different. This occurs when a small, dense companion star such as a white dwarf star (which has a high gravitational attraction at its surface) takes matter away from a bloated giant star, which is less able to hold onto the layers. The gas builds up on the surface of the dwarf until it reaches the density required to star hydrogen burning. Once that occurs, the heat build up blows the accreted gas off the surface of the white dwarf.

After the supernova, there is the supernova remnant. Massive stars collapse their cores further than less massive stars. As a result, electrons and protons are forced to fuse together into neutrons. These neutrons then pack themselves together as much as they can, leading to Neutron Stars. As with electrons, there is a limit to how far neutrons can be compressed – known as neutron degeneracy. Above a certain mass limit and even this can be overcome, leaving behind not a star at all, but a black hole.

Stellar Types

In the above description, I introduced main sequence, red giant, proto-, white dwarf, brown dwarf, neutron and T Tauri stars. These are a few, but not all the available types. Differing sizes and temperatures can add extra features – larger brown dwarfs are hotter and so glow as red dwarfs. Larger red giants are red supergiants, hotter red giants or supergiants are called blue giants or supergiants, due to the change in their colour. But some stars are different due to what they do.

Stars over 20 stellar masses with strong solar winds are known as Wolf-Rayet stars. The Doppler shift of the spectral lines of gas in their extremely fast stellar winds broadens them to an unusual extent, giving them their classification. They are generally red supergiants leading up to a type 1b or 1c supernova.

Cepheid Variable stars are stars that are undergoing core instability, usually due to switching between hydrogen and helium burning. All stars are thought to have some level of pulsation in their outer layers (that of the sun is small, but measurable). For Cepheids, the switching off of the core causing the envelope to collapse and heat up causing a bit of fusion and expansion settles into a regular rhythm.

Neutron stars also have different versions. Electrons surrounding a neutron star can be held by the star’s magnetic field. The electrons are able to travel up and down the field lines, from north to south and back, and drift round the star, but find that is all they can do. The fast spinning of the neutron star can force the electrons, held at a large distance from the centre of the star, to approach the speed of light. Unable to do this, they shoot down the field lines at great velocities, emitting synchrotron radiation at the poles. If we happen to be viewing a neutron star spinning so the magnetic pole faces towards and then away from us, we can record a radio signal that peaks when the pole faces us. As this happens in pulses, the object is termed a Pulsar. Magnetars are thought to be neutron stars with very strong magnetic fields that are decaying away, creating bursts of gamma and x-rays.

Naked eye visible stars

Objects in the sky up to magnitudes of 5 or 6 are visible to the naked eye in good dark skies and conditions. For binoculars, this rises to magnitude 10 and moderate telescopes will reach around 14, with professionals breaching 27 or more. This provides around 4,000 stars to the naked eye.

Hipparchus started the magnitude system as a way of classifying stars. Assigning them numbers from 1 to 6 (with 1 including all objects brighter than one, including the Sun and Moon), he sort to create a manageable list of the stars. Modern adjustments mean the magnitude system has been extended in both directions, with every magnitude step being a 2.5 times change in brightness. The system is reversed so a magnitude 6 object is dimmer than a magnitude 5 one.

Stars have also been classified in a rather more unusual way – someone played join the dots in the heavens to create the constellations using the brighter stars. These provide helpful pointers to where things are in the sky, and give newspaper sky charters something to sketch.

Using the naked eye, it is possible to see many variable stars change in brightness by comparing them to other nearby stars. These may be Cepheids such as Delta Cepheus, whose outer layers are expanding and contracting. They may also be eclipsing binary stars where one star moves in front of the other, reducing the light shining towards Earth (indeed the light curves from these were very helpful in determining what happens when exoplanets pass in front of their host stars).

Telescopic stars

Of course, turning a telescope to the heavens opens a whole new world of stars. Field stars is the general name given to the tiny myriad dots in the heavens behind all the better studied stars. The colours of stars start to become better defined (although some can see this in naked eyes stars). Double stars such as the one pictured below become better resolved. Mizar and Alcor in the plough are the most famous approximately naked eye resolvable double stars. Although many stars are in binary systems (where two stars orbit each other) many of these double stars are actually tricks of perspective where a brighter but further star appears to be next to a similar star. One use of double stars is that stars of different colours can benefit from the contrast and look quite stunning through the eyepiece.

A double star

A double star

If you’re not here for the beauty, then there’s always some good science to do for the dedicated observer. A great challenge of the late Victorian era was to obtain the distances to stars through the parallax method. This used the fact that as the Earth goes round its orbit, closer stars will move more in the sky than farther away stars (the same as trees by the roadside move faster than mountains in the distance when looking out of a car window). By measuring the parallactic angle the star ships by, the distance can be found using nothing more than trigonometry. The greater the parallax, the smaller the distance away.

Once the distance of a star is known, its actual brightness (rather than just how bright it appears to us) can be determined. Brightness drops off with distance and knowing how far a star is and how bright it appears gives its Absolute Magnitude.

Further work can be done using a bit of physics and a bit of spectroscopy. A blackbody is a hypothetical construct that represents the most efficient absorber and hence emitter of radiation. When absorption and emission due to gases in the solar atmosphere are taken into account, the Sun follows a fairly faithful blackbody curve. The use of this is that the peak emission colour of a blackbody depends on its temperature. If we make an estimate of the colour of a star (or measure it photographically or with a spectrometer) the star can be assigned a temperature. An archaic way of classifying stars – O B A F G K M – exists where O is in the blue region and M in the red. They are subdivided further into 1-8, with 8 being the redwards number. This ranges from 3,000 in the red t0 greater than 25,000 Kelvin in the blue.

The spectrum of a star is then able to provide us with quite a few pieces of information. From earlier, we’ve seen how Wolf-Rayet stars are identified through spectral lines, and indeed the Doppler shift can be used to identify slower winds around stars too, even using amateur equipment. Chemical composition can be determined from spectral lines (absorption lines tell us there’s cold gas between us and the star, emission that there’s hot gas – also Carbon stars mentioned earlier). Now we have a way of getting temperature using the peak colour.

Having obtained temperature of the star, distance and worked out luminosity, we can use blackbody equations to give us the radius of the star.

The Hertzsprung-Russell or H-R diagram is a way of plotting star colour, or spectral type or temperature against Luminosity or Magnitude (pick the pairing that best suites you and use it). By building up more and more stars on the diagram, it is possible to trace the evolution of stars of different masses and types using their positions on the diagram. Most stars lie on the main sequence line. Red giant stars are brighter (move up in magnitude due to their increased radius) but cooler (move to the right on the diagram) and so follow their own branches.

Further studies of stars have allowed a new way of determining parallax entirely using spectroscopy. The apparent brightness is measured, the peak colour and temperature taken, comparisons to the HR diagram made to obtain absolute luminosity and the distance and radius worked out from there. Much faster and can be applied to any star in the sky.

The Internal Constitution of Stars

As the founder of the Eddington Society, I might as well add this extra little bit. Eddington used stars a lot during his career, the name of this section is the title of one of his books on the subject. It was Eddington who decided that stars must have a fusion source for their power, disbelieving the Jean’s collapse theory, which held that stars were only 4,000 or so years old.

Eddington also lay the foundations for how radiation pressure was able to make Cepheids variable. He suggested Helium in the outer part of the star could be responsible, later confirmed by others who discovered ionised Helium’s opacity was enough to trap radiation from the newly restarted fusion processes in sufficient quantities for the star to expand.

Eddington also came up with the Mass-Luminosity relationship for stars, whereby stars on the bluer, brighter part of the H-R diagram held greater mass.

Eddington also investigated the hydrodynamics of stars and how gas inside the stars holds up against the pressure outside, coming up with the equation of state for stars (the subject of the book named above).

Even Eddington’s achievements in measuring Mercury’s position when close to the Sun to help confirm the predictions of general Relativity required our closest star – The Sun – as a testbed, the gravity of that enormous object being the only nearby source of the kind of conditions that differentiate between Einstein and Newton’s gravitational predictions.

Of course, even Eddington’s mistakes involved stars – such as his denial of Chandrasekhar’s work on the mass limit of white dwarfs. Eddington disbelieved the predictive power of quantum physics in this case.

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