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First, the star will appear to cool slowly and will undergo a modest increase in luminosity. During this phase, the path the star will follow in the HR diagram is almost horizontal to the right of its position on the Main Sequence.

Stars in this phase are usually referred to as subgiants. Next, the star will grow to as much as, or even more than, times its original size, which will cause a significant increase in luminosity with only a small decrease in temperature, so the star will move almost vertically in the HR diagram. Stars in this area of the HR diagram are usually referred to as red giants. The evolutionary track for the star as it undergoes the transition to a red giant is shown below:.

If you look at the dashed lines in this HR diagram, they represent lines of constant radius. When a star has reached the tip of the red giant branch the highest point in luminosity on the track above , it has a radius of approximately solar radii. There are several well known red giant stars even larger than this, which have radii of several hundred solar radii.

The immense growth expected in the Sun when it becomes a red giant will cause its radius to swell from roughly 1 AU to perhaps 2 AU or so. This means that Mercury and Venus will definitely be engulfed by the Sun, and the Earth and Mars are likely to be engulfed as well. The core of the Sun when its envelope is 1 AU will only be of order 10 Earth radii, or a factor of more than 2, times smaller than the radius of the envelope.

Compare the illustrations below: the first shows the Sun as a red giant and compares it to the Sun at its current size Fig. Your naked eye is not usually capable of making out the color of stars. The white dwarf will be surrounded by an expanding shell of gas in an object known as a planetary nebula.

They are called this because early observers thought they looked like the planets Uranus and Neptune. There are some planetary nebulae that can be viewed through a backyard telescope. In about half of them, the central white dwarf can be seen using a moderate sized telescope. Planetary nebulae seem to mark the transition of a medium mass star from red giant to white dwarf. Stars that are comparable in mass to our Sun will become white dwarfs within 75, years of blowing off their envelopes.

Eventually they, like our Sun, will cool down, radiating heat into space and fading into black lumps of carbon. It may take 10 billion years, but our Sun will someday reach the end of the line and quietly become a black dwarf. White dwarfs can tell us about the age of the Universe. If we can estimate the time it takes for a white dwarf to cool into a black dwarf, that would give us a lower limit on the age of the Universe and our galaxy.

But because it takes billions of years for white dwarfs to cool, we don't think the universe is old enough yet for many, if any, white dwarfs to have become black dwarfs. Finding black dwarfs would certainly alter our understanding of the cooling process in white dwarfs. There are several ways to observe white dwarf stars. The first white dwarf to be discovered was found because it is a companion star to Sirius, a bright star in the constellation Canis Major.

In , astronomer Friedrich Bessel noticed that Sirius had a slight back and forth motion, as if it was orbiting an unseen object. In , the optician and telescope maker Alvan Clark spotted this mysterious object.

This companion star was later determined to be a white dwarf. Eventually elements such as chromium, manganese, iron, cobalt and nickel may be produced. The core region of a supergiant thus resembles the layers of an onion with a dense iron core surrounding by shells of silicon and sulfur, oxygen and carbon, helium and an outer shell of hydrogen as shown in the diagram below. The onion-like layers inside a supergiant in the final stages of its life.

Successive layers correspond to the different elements produced by fusion, with a dense core of iron at the centre. Nucleosynthesis of elements above helium is less efficient so that each successive reaction produces less energy per unit mass of fuel. This means that the reactions occur at greater rates so that radiation pressure balances gravity.

Whilst a massive star may spend a few million years on the main sequence, its helium core-burning phase may be a few hundred thousand years. The carbon burning phase lasts a few hundred years, neon-burning phase a year, oxygen-burning half a year and the silicon-burning only a day. These massive stars evolve extremely rapidly once they move off the main sequence. Statistically they are very low in numbers as they are less likely to form than lower-mass stars and their lifetimes are so short anyway.

As we shall see in a later section , they also make dramatic exits. As discussed previously, low mass stars consume their core hydrogen at much lower rates than stars such as our Sun. Their main sequence lifespans are tens to hundreds of billions of years. Once they have consumed their core hydrogen, gravitational core collapse causes the core to heat up. For stars with less than 0. A brief period of hydrogen-shell burning sees its luminosity rise as with higher-mass stars.

Unable to release energy from helium fusion, H-shell burning does not last long. The star's luminosity quickly decreases and the star cools down. Its evolutionary track crosses back across the main sequence and down. Skip to main content. Australia Telescope National Facility. Accessibility menu. Interface Adjust the interface to make it easier to use for different conditions. Interface Size. High contrast mode This renders the document in high contrast mode.

Invert colors This renders the document as white on black. Disable interface animations This can help those with trouble processing rapid screen movements. Optimize fonts for dyslexia This loads a font easier to read for people with dyslexia. Post-Main Sequence Stars What happens when a main sequence star runs out of hydrogen in its core? An artist's impression of a red supergiant engulfing a Jupiter-like planet as it expands. When the Sun becomes a red giant its radius will be approximately 0.

The triple alpha process for post-main sequence stars. Two helium nuclei alpha-particles fuse to form a beryllium-8 nucleus. This is unstable and normally decays back into two H-4e nuclei within a fraction of a second but given the high number of He-4 nuclei in the core will sometimes collide with one before it has had a chance to decay. Eventually the core exhausts its supply of hydrogen and the star begins to evolve off of the main sequence.

Without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity the core contracts until either electron degeneracy becomes sufficient to oppose gravity or the core becomes hot enough around MK for helium fusion to begin. What happens after a low-mass star ceases to produce energy through fusion has not been directly observed; the universe is thought to be around Recent astrophysical models suggest that red dwarfs of 0.

Slightly more massive stars do expand into red giants, but their helium cores are not massive enough to ever reach the temperatures required for helium fusion so they never reach the tip of the red giant branch. When hydrogen shell burning finishes, these stars move directly off the red giant branch like a post AGB star, but at lower luminosity, to become a white dwarf. Internal structures of main-sequence stars, convection zones with arrowed cycles and radiative zones with red flashes. To the left a low-mass red dwarf, in the center a mid-sized yellow dwarf and at the right a massive blue-white main-sequence star.

Mid-sized stars are red giants during two different phases of their post-main-sequence evolution: red-giant-branch stars, whose inert cores are made of helium, and asymptotic-giant-branch stars, whose inert cores are made of carbon. Asymptotic-giant-branch stars have helium-burning shells inside the hydrogen-burning shells, whereas red-giant-branch stars have hydrogen-burning shells only. This lifts the outer layers away from the core, reducing the gravitational pull on them, and they expand faster than the energy production increases.

This causes the outer layers of the star to cool, which causes the star to become redder than it was on the main sequence.

Initially, the cores of red-giant-branch stars collapse, as the internal pressure of the core is insufficient to balance gravity. This gravitational collapse releases energy, heating concentric shells immediately outside the inert helium core so that hydrogen fusion continues in these shells. The core of a red-giant-branch star of up to a few solar masses stops collapsing when it is dense enough to be supported by electron degeneracy pressure.

Once this occurs, the core reaches hydrostatic equilibrium: the electron degeneracy pressure is sufficient to balance gravitational pressure.

This in turn causes the star to become more luminous from 1,—10, times brighter and expand; the degree of expansion outstrips the increase in luminosity, causing the effective temperature to decrease. The expanding outer layers of the star are convective, with the material being mixed by turbulence from near the fusing regions up to the surface of the star.

At this stage of evolution, the results are subtle, with the largest effects, alterations to the isotopes of hydrogen and helium, being unobservable. These are detectable with spectroscopy and have been measured for many evolved stars. As the hydrogen around the core is consumed, the core absorbs the resulting helium, causing it to contract further, which in turn causes the remaining hydrogen to fuse even faster. This eventually leads to ignition of helium fusion which includes the triple-alpha process in the core.

In stars of more than approximately solar mass, it can take a billion years or more for the core to reach helium ignition temperatures. When the temperature and pressure in the core become sufficient to ignite helium fusion, a helium flash will occur if the core is largely supported by electron degeneracy pressure stars under 1.

In more massive stars, the ignition of helium fusion occurs relatively quietly. Even if a helium flash does occur, the time of very rapid energy release on the order of 10 8 Suns is brief, so that the visible outer layers of the star are relatively undisturbed.

The star contracts, although not all the way to the main sequence, and it migrates to the horizontal branch on the Hertzsprung—Russell diagram, gradually shrinking in radius and increasing its surface temperature.

Core helium flash stars evolve to the red end of the horizontal branch but do not migrate to higher temperatures before they gain a degenerate carbon-oxygen core and start helium shell burning. These stars are often observed as a red clump of stars in the colour-magnitude diagram of a cluster, hotter and less luminous than the red giants. Higher-mass stars with larger helium cores move along the horizontal branch to higher temperatures, some becoming unstable pulsating stars in the yellow instability strip RR Lyrae variables , whereas some become even hotter and can form a blue tail or blue hook to the horizontal branch.

The exact morphology of the horizontal branch depends on parameters such as metallicity, age, and helium content, but the exact details are still being modelled. After a star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star follows the asymptotic giant branch on the Hertzsprung—Russell diagram, paralleling the original red giant evolution, but with even faster energy generation which lasts for a shorter time.

Helium from these hydrogen burning shells drops towards the center of the star and periodically the energy output from the helium shell increases dramatically. This is known as a thermal pulse and they occur towards the end of the asymptotic-giant-branch phase, sometimes even into the post-asymptotic-giant-branch phase. Depending on mass and composition, there may be several to hundreds of thermal pulses.

There is a phase on the ascent of the asymptotic-giant-branch where a deep convective zone forms and can bring carbon from the core to the surface, This is known as the second dredge up, and in some stars there may even be a third dredge up.

In this way a carbon star is formed, very cool and strongly reddened stars showing strong carbon lines in their spectra. A process known as hot bottom burning may convert carbon into oxygen and nitrogen before it can be dredged to the surface, and the interaction between these processes determines the observed luminosities and spectra of carbon stars in particular clusters. Another well known class of asymptotic-giant-branch stars are the Mira variables, which pulsate with well-defined periods of tens to hundreds of days and large amplitudes up to about 10 magnitudes in the visual, total luminosity changes by a much smaller amount.

In more massive stars the stars become more luminous and the pulsation period is longer, leading to enhanced mass loss, and the stars become heavily obscured at visual wavelengths.

These stars are clearly oxygen rich, in contrast to the carbon stars, but both must be produced by dredge ups. These mid-range stars ultimately reach the tip of the asymptotic-giant-branch and run out of fuel for shell burning.

They are not sufficiently massive to start full-scale carbon fusion, so they contract again, going through a period of post-asymptotic-giant-branch superwind to produce a planetary nebula with an extremely hot central star. The central star then cools to a white dwarf. The expelled gas is relatively rich in heavy elements created within the star and may be particularly oxygen or carbon enriched, depending on the type of the star. The gas builds up in an expanding shell called a circumstellar envelope and cools as it moves away from the star, allowing dust particles and molecules to form.

With the high infrared energy input from the central star, ideal conditions are formed in these circumstellar envelopes for maser excitation.

It is possible for thermal pulses to be produced once post-asymptotic-giant-branch evolution has begun, producing a variety of unusual and poorly understood stars known as born-again asymptotic-giant-branch stars.



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