How Will The Composition Of The Sun Change Over The Next Billion Years?

The Sun'south Evolution

The Hertzsprung-Russell Diagram (aka the Master Sequence)
About stars are rather simple things. They come in a diverseness of sizes and temperatures, but the bully majority can exist characterized by just two parameters: their mass and their age. (Chemical composition likewise has some effect, only not enough to change the overall picture of what we will exist discussing here. All stars are about 3-quarters hydrogen and one-quarter helium when they are born.)

The dependence on mass comes well-nigh considering the sheer weight of the star's mass determines its central pressure, which in turn determines its rate of nuclear burning (higher pressure level = more collisions = more energy), and the resulting fusion energy is what drives the star'southward temperature. In full general, the more than massive a star is, the brighter and hotter it must be. It is also the case that the gas force per unit area at any depth in the star (which also depends on the temperature at that depth) must balance the weight of the gas above it. And finally, of course, the total energy generated in the core must equal the full energy radiated at the surface.

This last fact generates yet another constraint, because the free energy radiation of a sphere suspended in a vacuum obeys a law known every bit the Stefan-Boltzmann Equation:

L = C R² T^iv (Full luminosity of a hot sphere)

Here L is the luminosity of the star, C is a constant ¹ , R is the radius of the star in meters, and T is the surface temperature of the star in K°. Notation how swiftly the energy radiated by a star rises with T: doubling the temperature causes its energy output to increment by 16 times.

A star which meets all these constraints is said to be in hydrostatic equilibrium. Hydrostatic equilibrium has the fortunate upshot that it tends to make stars stable. Should a star'southward cadre exist compressed, the compression causes nuclear burning to increment, which generates more heat, which forces up the pressure and makes the star aggrandize. It goes back to equilibrium. Likewise, if a star's core should be decompressed, then nuclear burning decreases, which cools the star and brings the pressure down, and thus the star contracts and once more returns to equilibrium. The free energy output of the Dominicus has not fluctuated by more than than possibly 0.1% to 0.2% in human history – not bad for a nuclear reactor that has no regulatory committee, no engineers, and hasn't had a safety check in nearly five billion years.

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Introduction
Matter Under Force per unit area
The Birth Of The Sun
The Sun's Development
The Stop Of The Sun
How Big Stars Evolve
Type 2 – The Other Supernova
Later on The Supernova

1 – Very well, if you must know, the abiding is equal to 5.67 x x^-8 West m^-2 Yard^-4.

This equation is important because it demonstrates how even small changes in the surface temperature of a star tin can lead to large variations in free energy output. If the Sunday's temperature was just raised from 5780 K° to 5900 K°, its luminosity would rise past well-nigh ix%.

The tight interrelation of temperature, pressure, mass, and rate of nuclear called-for means that a star of a given mass and age tin only achieve hydrostatic equilibrium at ane set up of values. That is, every star in our milky way of the aforementioned mass and age as the Sun too has the same bore, temperature, and energy output. There is no other way for everything to residue. If one generates a very hard-cadre astrophysics graph known equally a Hertzsprung-Russell Diagram (H-R diagram for short), the human relationship between a star's mass and its other properties becomes more clear. An H-R diagram is shown in Figure 1.

An H-R diagram takes a set up of stars and plots their luminosities (relative to the Sun) versus their surface temperatures. Note that the temperature scale on the H-R diagram in Effigy 1 runs backwards, right to left, and that the luminosity axis is highly compressed. (Historically, this was how the first H-R diagram was synthetic, then now they all are.) When done for a large sample of stars, we find that the overwhelming bulk of the stars fall along a single, remarkably narrow ring that runs from the lesser-right to the superlative-left: that is, from dim and red to bright and white-hot. Astronomers call this band the Master Sequence, and hence any star forth the band is called a main-sequence star. ²

The main sequence exists precisely because of the inflexible nature of hydrostatic equilibrium. Stars with very low masses (as piddling equally 7.5% that of the Sun) lie at the lower right of the H-R diagram. They must lie at the lower right. This office of the H-R diagram corresponds to extremely low luminosity – as picayune equally a ten thousandth that of the Sunday – and low surface temperature, equivalent to the dull orangish-yellow glow of molten metal. These stars do non have enough mass to create the pressure necessary to make the nuclear burning in their cores go any faster. High-mass stars (upwards of 40 solar masses) reside at the upper left, equally they must. Contrary to the depression-mass stars, their immense masses and loftier central pressures requite rise to giants that tin be 160,000 times more luminous than the Sun, and and so hot that they give off more energy in the ultraviolet than they practice as visible lite. The Sun lies near exactly halfway between these extremes, and thus it is neither extremely dim nor extremely bright every bit stars go. It shines with a brilliant yellowish-white color.

The one-to-one nature betwixt mass and hydrostatic equilibrium means that equally you vary the mass of a star, all you tin can do is slide forth a single, predetermined track with respect to all its other physical properties. This track is exactly the primary sequence. But now that I've said that, a second look at the H-R diagram reveals that there is a smattering of stars well off the main sequence: they are full-bodied in "islands" at the upper right and lower left. Since the stars at the upper right are very luminous nevertheless nonetheless have cool, reddish surfaces, astronomers call them red giants. Similarly, since the stars at the lower left are very dim nonetheless also white-hot, they are chosen white dwarfs. We have met the white dwarfs already, in a theoretical mode. Now let'southward see where the real ones come up from.

two – Astronomers traditionally classify principal-sequence stars with messages, like so:
O - 30,000 to 40,000 Thousand°
B - x,800 to thirty,000 K°
A - 7240 to ten,800 Yard°
F - 6000 to 7240 K°
G - 5150 to 6000 G°
Thou - 3920 to 5150 K°
M - 2700 to 3920 K°

Within each grade, numbers from 0 to ix provide subclasses, with nothing being the highest bracket (highest temperature). The Lord's day is classified as a G2 star.

Ruby Giants And White Dwarfs
Red giants and white dwarfs come about because stars, like people, change with historic period and eventually dice. For people, the cause of aging is the deterioration of biological functions. For a star, the cause is the inevitable energy crunch every bit information technology begins to run out of nuclear fuel.

Since its nativity 4.5 billion years ago, the Lord's day'due south luminosity has very gently increased by about xxx%.

³ This is an inevitable development which comes most because, as the billions of years roll by, the Sun is burning up the hydrogen in its core. The helium "ashes" left behind are denser than hydrogen, so the hydrogen/helium mix in the Sunday's core is very slowly becoming denser, thus raising the pressure. This causes the nuclear reactions to run a little hotter. The Sun brightens.

This brightening process moves along very slowly at first, when there is notwithstanding ample hydrogen remaining to be burnt at the center of the star. Only eventually, the core becomes so severely depleted of fuel that its free energy production starts to fall regardless of the increasing density. When this happens, the density of the core begins to increase even more than, because without a oestrus source to help it resist gravity, the but possible way the core tin reply is by contracting until its internal pressure level is high enough to hold upward the weight of the entire star. Bizarrely, this emptying of the central fuel tank makes the star brighter, non dimmer, because the intense pressure at the surface of the core causes the hydrogen there to burn down fifty-fifty faster. This more takes upward the slack from the fuel-exhausted center. The star's brightening not just continues, it accelerates.

iii – One of the outstanding questions in geology is how the Dominicus could have steadily get brighter even every bit the overall temperature of the Earth has remained more than-or-less constant. We practice not know exactly, but in two words or less, the reply is: greenhouse effect. The Earth's atmosphere evidently had a much college greenhouse gas content iv billion years ago, which kept it warm. (In fact, very warm. Average global temperatures may have been equally high equally 140 F°.) Various circuitous bio-geological feedback loops have steadily decreased the greenhouse issue precisely because the Lord's day is getting brighter.
The Sun is about half-way through a very long process of shifting from a mode where hydrogen is burned in a kernel at its heart to a style where hydrogen will be burned in a spherical shell wrapped effectually an intensely hot, very dense, simply quite inert, helium core. Once information technology makes the transition from core burning to crush burning, it will be entering its twilight years. As the helium core grows, so does the hydrogen-burning shell above it, thus making the Sun ever brighter even while ominously increasing the rate at which helium is accreted onto the core. The growing cadre burns the Dominicus's hydrogen all the same more rapidly, which in plow but enlarges the core more speedily. . . .

In short, in the end, the nuclear furnace at the center of every star begins to overheat. To put numbers on this, when the Sun was formed 4.v billion years ago it was almost thirty% dimmer than at nowadays. At the cease of the adjacent 4.8 billion years, the Sunday will be about 67% brighter than it is now. In the ane.6 billion years following that, the Sun's luminosity will rise to a lethal ii.2 L_o. (L_o = nowadays Sun.) The Earth by then volition accept been roasted to bare rock, its oceans and all its life boiled abroad by a looming Sunday that will be some 60% larger than at present.

^iv The surface temperature on the Earth will be in excess of 600 F°. But even this version of the Lord's day is notwithstanding stable and golden compared to what is to come.

4 – Alas, the feedback loops mentioned in footnote 3 cannot protect the Earth forever. Once its greenhouse consequence has dropped to zero, the Earth cannot do annihilation more to cool itself.

Effectually the twelvemonth 7.1 billion AD, the Sunday will begin evolving so rapidly that information technology will cease to exist a main-sequence star. Its position on the H-R diagram will begin to shift from where it is now, near the eye, towards the upper right where the cerise giants live. This is because the Sun'south helium core will eventually reach a disquisitional point where the pressure from normal gasses cannot hold up the crushing weight existence piled on it (not fifty-fifty gasses heated to tens of millions of degrees). A tiny seed of electron-degenerate matter will begin to grow at the middle of the Sun. The details of this transition are subject area to debate, but theoretical calculations indicate that it volition brainstorm when the Sun's inert helium core reaches near 13% of a solar mass, or about 140 Jupiters.
At this point in its life, the Dominicus volition become unruly. The mechanism that has been slowly making it brighter for the past eleven billion years – more than core force per unit area, yielding hotter nuclear burning, yielding more helium to enlarge the core – is now accelerated to disastrous levels by the steadily increasing electron-degeneracy. 500 meg years after it hits the critical point, the Sun'south luminosity will balloon to 34 L_o, peppery enough to create glowing lakes of molten aluminum and copper on the Globe's surface. In only 45 million years more it volition reach 105 Fifty_o, and 40 one thousand thousand years after that it will leap to an incredible 2,300 L_o.
By this time the enormous energy output of the Lord's day will have caused its outer layers to inflate into a vast but very tenuous temper at least the size of the orbit of Mercury, and possibly as large as the orbit of Venus. (Think of how violently the h2o behaves in a pot of quickly humid water as compared to that in a gently simmering pot. This is coordinating to why the Lord's day's atmosphere "boils" outward as its core becomes hotter.) ^five The huge size of the solar temper and the enormous heat output of the Sun mean that: #ane) the Earth volition have been burnt downwardly to nothing but a seared iron core by this point, if not vaporized altogether – calculations show that information technology could get either way – and #2) the solar atmosphere will exist relatively cool despite the Sun'southward tremendous energy output. Thus, the Sun will be both red in color and extraordinarily luminous. Information technology will have joined the red giants. (Come across Figure 2.)

5 – But information technology is non a very adept analogy. Click here to read the total story, or click the icon.

The number of stars in the red giant part of the H-R diagram is only a fraction of a per centum of that on the primary sequence, because no star tin remain a giant for long. When the Sunday reaches its maximum luminosity every bit a cerise giant, it will be burning more nuclear fuel every half dozen million years than it did during its entire eleven-billion-year lifetime on the chief sequence. This is not sustainable. Also, at to the lowest degree as importantly, red giant stars are never really stable in the same sense as the Sun is at present. They are always growing and burning their fuel ever faster, until something stops them. There is no long-term equilibrium for a red giant. Side by side: The End Of The Sun

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