Our sunlight will ultimately exhaust its main point hydrogen and also evolve off the key sequence right into a red giant. ~ the helium flash initiates helium-burning that will move onto the horizontal branch then increase to come to be an AGB star. This page now explores the fate of stars of 8 solar masses or less. These stars room destined come eject planetary nebulae and also end up together white dwarfs.

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Planetary Nebulae

In an AGB star, if the helium fuel in the He-burning covering runs low, the external radiation push drops off. As this was previously holding the end the covering of hydrogen gas this shell now contracts, heats up and also ignites, convert hydrogen to helium. This helium "ash" in turn falls onto the helium shell, heater it increase till that is hot sufficient to re-ignite in a helium-shell flash, producing a thermal pulse. Enhanced radiation push now causes the hydrogen covering to expand and also cool, shutting down H-shell burning.

The interval in between successive thermal pulses decreases as the AGB star ages. For solar-mass stars together pulses drastically increase the luminosity for number of decades. With time the outer layers the the AGB star are nearly totally ejected and also may initially appear as a circumstellar shell. The ejected cloud consists of dust seed of silicates and also graphite in enhancement to hydrogen and elements created via nucleosynthesis reactions in ~ the parent star. The cloud has typical expansion velocities of 10s of kilometres per second.Our sunlight will shed about 40% that its mass throughout this phase. Much more massive stars shed a higher percentage of their initial mass.

With the ejection that the external layers that the star, that is hot, thick core is left exposed. The is initially so hot that the intense ultraviolet radiation that emits ionises the expanding, ejected shell. This results in the cloud glowing, comparable to an emission nebula. Such objects are referred to as planetary nebulae after your initial description by Herschel in the 18th century. Through tiny telescopes they show up as faint discs, favor a dim planet though they room not related. Planetary nebulae commonly contain 0.1 - 0.2 solar masses in ~ densities identical to a vacuum top top Earth. Spectacular photos by contemporary telescopes consisting of the HST disclose a wide range of forms that pose interesting problems for philosophers to explain.


The bipolar nature of numerous planetary nebulae may be as result of the parental star gift in a binary system. Strong magnetic fields of remnant cores may additionally influence the form of the nebulae. Colour of the nebulae expose information about their composition. The properties blue-green colour is indigenous the doubly-ionised oxygen emissions, OIII. Oxygen, carbon and some s-process aspects ejected by AGBs and also found in planetary nebulae may ultimately seed the ISM for the following generation of star formation. Few of the carbon and oxygen in our bodies may have actually come from such nebulae, the rest most likely came indigenous supernovae explosions.

Planetary nebulae carry out not exist because that long. An expanding shell of dust and also gas might only it is in visible because that a couple of 20,000 year or so prior to dispersing right into the ISM. Over there are, however, over 1,500 known in our galaxy and others room visible in adjacent galaxies. Castle are helpful as one method for determining distances to these galaxies.


Credit: Left:NASA, ESA, HEIC, and also The Hubble legacy Team (STScI/AURA), Right: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO).

Although a planetary nebula is only short-lived, the exposed core remains. We shall currently see what happens to it.

White Dwarfs

The exposed, remnant core that ionised the planetary nebula product is basically very hot, thick sphere of carbon and oxygen. Any hydrogen no ejected easily fuses via shell-burning. When at first exposed, the main star, v a temperature higher than 30,000 K may have a luminosity 100,000 × that of ours Sun but it conveniently fades by as much as 90% in ~ a century. The stellar remnant becomes a white dwarf or wd v a surface ar temperature of around 104 K.


White dwarfs have actually unusual properties. Firstly, they space very little but the an ext massive white dwarfs are actually smaller than less huge ones. Through their fuel offered up no combination takes ar so over there is no outward radiation push to resist gravitational collapse. Much more massive stellar cores experience more powerful gravitational force so in reality compress more. A 0.5 solar-mass white dwarf has actually a radius 1.9 × that of Earth, a 1.0 solar-mass one is just 1.5 planet radii whilst a 1.3 solar-mass white dwarf 1.4 planet radii. A white dwarf is written of carbon and also oxygen ions mixed in through a sea of degenerate electrons. That is the degeneracy pressure listed by the electrons that avoids further collapse.

A white dwarf, through a mass around that the the sunlight packed into a volume no much higher than the planet must have an extremely high density. At 109 kg m-3 its thickness is one million times greater than the of water. Although its surface temperature is around 10,000 K, the core temperature might be as high together 107 K. The warmth trapped in ~ a white dwarf will gradually be radiated away by it however with its little radius, a white dwarf has actually only a little surface area. Heat as such cannot escape quickly. In reality it will certainly take 10s to numerous billions the years because that a white dwarf come radiate away its heat and cool down to a black, inert clump of carbon and degenerate electrons. Together the universe is no yet old enough for this to have happened, all the white dwarfs the have ever formed in single-star systems space still white dwarfs.

Interestingly, not only are the more massive dwarfs smaller sized than less enormous ones, lock are also less luminous because that the reason defined above. Usual luminosities are much less than 10-3 the of our Sun. Much more massive white dwarfs, having actually smaller surface areas but much more trapped heat, take much longer to cool down than lower-mass ones. As white dwarfs space so pass out they are also hard come detect. Us are only able to observe fairly close ones. However white dwarfs room thought come comprise around 10% of the stars in our galaxy. Adjacent examples space Sirius B and Procyon B, both of i m sorry are found in binary systems.


White dwarfs in the globular swarm M4. The white dwarfs (circled) are about 12 billion year old, the oldest yet observed and at 30th magnitude, also the faintest. For an ext details review the press release.

The Chandrasekhar Limit

What is the upper-mass limit for a white dwarf? We may imagine that if we boost the massive of a degenerate core it just becomes smaller. Whilst this is true over there is a limit. If the massive of the stellar remnant over 1.4 solar masses then the electron degenerate pressure is poor to stand up to the force of gravity. The core will thus proceed to please and type either a ghost star or a black color hole. Both of these are questioned in an ext detail top top the following page. The worth of 1.4 solar masses is referred to as the Chandrasekhar border after the Indian astrophysicist, Subrahmanyan Chandrasekhar (1910-1995). He winner the 1993 Nobel prize in Physics because that this discovery and now has NASA"s X-ray observatory, Chandra, named in his honour. His theoretical calculations have so much held the observational check - no white dwarfs have been found with a mass greater than 1.4 solar masses.

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One suggest to remember around this Chandrasekhar border is that it refers to the fixed of the material in the remnant main point after all other mass loss. Stars loose a the majority of their mass together they evolve off the main sequence and become an AGB. The upper mass-limit for a key sequence star that will go ~ above to type a white dwarf quite than a neutron star is not precisely known however is believed to be about 8 solar masses. A 2 solar-mass star will certainly probably finish up together a 0.7 solar-mass white dwarf. At present the lower-mass border for any kind of white dwarf is about 0.6 solar masses. These dwarfs type from main sequence stars slightly less than 1 solar-mass. Main sequence stars much less than this have actually not yet had time come evolve turn off the key sequence and kind white dwarfs as the world is not yet old enough.