42

Models for the future behaviour of the Sun do vary, mainly as a result of uncertainty of mass loss during the red giant (H shell burning) and asymptotic red giant (H+He shell burning) phases. A highly cited paper by Schroeder & Smith 2008 claims that the Sun will reach its maximum size of about $256 R_{\odot}$ (1.18 au) at the very tip of the red giant ...


17

Arcturus is a RGB star, probably fairly similar how the sun will look when it becomes a red giant. Arcturus is slightly more massive than the sun ($m_{\rm Arc}=1.08 m_{\odot}$), but the main difference is the lower metallicity of $[Fe/H]\approx-0.5$. This low metallicity reduces the opacity in the stellar radiative zone (which fills a significant portion of ...


11

It will become a planetary nebula like e.g. the Cat's Eye nebula that was formed by the death of a star with about the same mass as the Sun ~ $1\ M_\odot $:                              Composite image using optical ...


10

(This is somewhat simplified but I hope it gets the idea across.) The reactions stop in the core because it runs out of fuel. During the main sequence, the star is supported by the fusion of hydrogen into helium. Eventually, the hydrogen runs out at the centre, so hydrogen fusion is no longer possible there. Why doesn't it start fusing helium into carbon ...


9

Indeed conservation of angular momentum dictates that in a single star like the the Sun, rotation should be much slower when it becomes a red giant. This is because at the present time the Sun does not rotate at vastly different rates with depth, thus when it expands, the moment of inertia increases drastically and convection in the outer envelope will ...


9

In my mind, none of these explanations really cover the actual reason that red giants expand. Indeed, this subject seems like an area where people just make up anything that sounds plausible, but it's often quite wrong (Fraser Cain mentions both light pressure and a higher volume in the fusing shell, but light pressure plays no role at all, and the volume ...


8

There are some answers in this lecture and this one although even there the author admits that the whole story is complicated, and not perfectly known. It seems that shell burning and contraction of the no-longer-burning core of the star causes some expansion of the outer layers (this is described as the "mirror principle" and partly explained in lecture ...


8

The answer is yes, there are many authors who do take mass loss from stars into account when trying to work out the fate of their planetary systems. Examples include Schroder & Smith 2008; Adams et al. (2013); Adams & Bloch (2013). It is thought that (e.g. Kalirai et al. 2008) that the Sun will lose half its mass by the time it ends up as a white ...


6

There's a nice description here. Remember that a star is made of gas (well plasma if you want to be picky), so it doesn't have a fixed volume. Once fusion starts a star will expand until it reaches a size where it can balance the amount of energy being produced by fusion with the amount radiated away from the surface. If it's too small, it will heat up, ...


6

The difference is that your analysis is assuming that the albedo stays fixed, so the surface temperature simply scales like luminosity to the 1/4 power. The Wiki entry is including feedback from the greenhouse effect, which will tend to further increase the surface temperature. Note that an analysis that just looks at solar irradiation would get way too ...


6

The real reason stars bloat into red giants is not due to opacity changes, it is due to the creation of a degenerate core of non-fusing helium at the center. This degenerate core has a strong gravity, which dictates a high temperature to the hydrogen fusing shell that sits atop it. This is quite important, because when a star is undergoing core fusion like ...


6

Yes, you can see one tonight. Arctaurus is a red giant star with a mass of about 1.1 times the solar mass, so rather similar to the sun. It currently has a spectral type of K0 III. It is ascending the red giant branch, so it's luminosity and spectrum are not stable in the longer term. The sun will pass through this phase, and following a hydrogen flash ...


5

The size of star in equilibrium is a balance of forces, the pressure produced by the hot plasma, heated by the nuclear reactions in the core, balanced by gravity. Fusion rates are strongly affected by temperature. Increase the temperature a little and you get a lot more energy coming out. As the core runs out of hydrogen, it begins to collapse and heat up, ...


5

Yes, the time is very long, but exactly how long depends on when you "start the clock" for the expansion to a red giant. If you take the end of the "main sequence" (when the core hydrogen runs out) as the start of the expansion phase, it's more like a billion years to complete the process of becoming a maximally expanded red giant. But the answer that gets ...


4

The AAS Nova article cites Rawls et al. 2016, who analyze light curves and spectra of the eclipsing binary KIC 9246715 to estimate its physical properties. Besides the stellar masses and radii in the article, their Table 2 lists orbital semimajor axis a = 211 R☉ = 147 million km = 0.98 au, and orbital eccentricity e = 0.356. During the 171-day period, ...


4

The year length depends on the distance between the planet's centre & the Sun's centre, not the Sun's surface. So if the Sun merely expands, Earth at 1 AU will still take a standard year to perform 1 orbit. However, when the Sun becomes a red giant, it won't just expand. As Wikipedia mentions, red giants shed a considerable amount of mass in the form of ...


4

Rayleigh scattering happens at all wavelengths, but the scattering cross section goes as $\lambda^{-4}$. On Earth, the atmospheric optical depth to Rayleigh scattering is very small at red wavelengths, so hardly any red light is scattered, even at sunset when the Sun is viewed through a thick atmospheric layer. On the contrary, there is sufficient optical ...


3

I will replace the previous answer to focus on the "subgiant" branch prior to red giant, rather than the pre-main-sequence or the "horizontal branch" of core helium fusion. Those are other times that the luminosity is constant, but this question is about the subgiant branch, which I missed before. The reason the luminosity is nearly constant on the ...


3

The straightforward definition is in terms of where a star lies on its evolutionary track in the HR diagram (see below). The subgiant branch stars are those which have exhausted their hydrogen core and which are burning hydrogen in a shell but their He cores have not begun to contract significantly. The distinct upturn in luminosity marks the beginning of ...


3

The destiny of a star basically depends upon its mass. All its activities variety depends upon its mass. If a star's core has a mass that is below the Chandraseckhar limit ($M\sim1.4M_{sun}$), then is destined to die as a white dwarf (or, actually, as a black dwarf in the end). The composition of the white dwarf, also depends upon the original mass of the ...


2

The distance to betelgeuse is poorly known, so we don't actually know how bright it is with much accuracy measurements of its parallax by satellite give a distance of 197 parsecs +/- 45 parsecs (1 parsec is 3.26 light years). The absolute magnitude (the brightness if it were 10 parsecs distant) is estimated to be -5.85. This is based on both the distance and ...


2

Given a distance of $d_1 = 640~\mathrm{ly}$ for Betelgeuze and $d_2 = 8.6~\mathrm{ly}$ for Sirius the difference in the apparent magnitude for Betelgeuze at distances $d_1$ and $d_2$ that is given by $$m_1 - m_2 = 5~\mathrm{mag} \cdot log_{10}(\frac{d_1}{d_2})$$ is $9.4~\mathrm{mag}$. With magnitude $m_1 = 0.45~\mathrm{mag}$ we get $m_2 = -8.9~\mathrm{mag}$...


2

For a more fundamental understanding, it is helpful to realize the difficulties of fusing He-4 into C-12. This is called the Triple-Alpha process. When two He-4 nuclei (alpha particles) have sufficient energy to overcome to the Coulomb barrier and have their cross sections align, it produces Be-8. The Be-8 nucleus is so unstable (due to it being ...


2

Some oxygen is produced during CNO cycle processing of hydrogen, starting with carbon nuclei. Oxygen is also produced by alpha capture onto carbon nuclei at temperatures well below 350 million K. Both of these occur in and around the cores of low mass stars before they become white dwarfs. Neon production is not required.


2

The time frames on these two phenomenon are quite distinct. It will take approximately 5 billion years for the sun to reach this red giant phase. Now at the speed of earth drifts away, and taken into account the mass loss: By fusion, the sun "burns" about 564 million tons hydrogen per second, resulting in 559.7 million tons of helium. The loss of mass, ...


2

Perhaps you are interested in the lower mass limit (given in the comments as 0.3 solar masses), and why that lower limit exists. This is similar to the question of why stars become red giants in the first place. You are right that given the age of the universe stars less massive than 0.8 solar masses won't have had time, so the lower mass limit is ...


1

Habitable zones have a width that scales as the square root of the luminosity. So a 10000 solar luminosity star will have a zone about 100 times wider. However, the temporal width is another thing: the luminosity changes relatively fast, so planets will not be in the zone over long timescales. (The reason for the square root dependence is that a piece of ...


1

It takes a star like the Sun about a billion years to go from the end of core hydrogen burning to the beginning of helium core burning. One might not call that entire phase a red giant phase, however, because the puffing out process takes quite a while to get going. In the mean time, the star is what is called a "subgiant," rather than a red giant, and ...


1

According to a Physics.org article: When the sun does begin to expand, it will do so quickly, sweeping through the inner solar system in just 5 million years. So, no. No human alive could possibly see the sun change from its more-or-less present size to a red giant in their life time, unless extreme advances in human longevity approaching science ...


1

The intuitive way to think about it is to understand that there are multiple changes that, in essence, amplify each other. Amplification in astronomy isn't all that uncommon. It explains why gravity can make massive objects so small, because as the massive object gets smaller, the gravitation and weight of the object grows exponentially. In a sense, ...


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