44 votes
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Do we know a star that is similar to the Sun when it would be a red giant?

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 ...
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17 votes

Do we know a star that is similar to the Sun when it would be a red giant?

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 ...
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12 votes
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How can a red giant grow so big?

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'...
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11 votes
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Why do stars become red giants?

(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 ...
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10 votes
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Will the Sun rotate slower in red giant form?

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 ...
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8 votes
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When a star reaches the red giant phase, why does it become more opaque?

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 ...
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8 votes
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Could the Earth survive a red giant Sun?

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 ...
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8 votes

How can a red giant grow so big?

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 ...
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7 votes
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What are Thorne-Żytkow objects?

I'll try to add a bit of context to the Wikipedia article, though the major references are all available there. The article covers things like the potential formation mechanisms but I guess doesn't ...
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7 votes
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Exactly how long does it take for the exposed core of a star to cool from its starting temperature (several billion K) to ~50,000 K?

The answer is of order 1 million years to cool from a standard end of He burning temperature of just over $10^8$ K to the top end of the white dwarf temperature range you give in your question. The ...
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6 votes

When a star reaches the red giant phase, why does it become more opaque?

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,...
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6 votes

How can a red giant grow so big?

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 ...
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6 votes

How fast will the sun grow when it becomes a red giant?

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) ...
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6 votes
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What will be the temperature on Earth when Sun finishes its main sequence?

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 ...
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6 votes
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How would the sky look if Earth orbited a red giant at a safe distance?

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 ...
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5 votes

What would a red giant Sun look like from Proxima b?

The Sun will not become a red giant for approximately 7 billion years, so there is little chance that Proxima Cen will be anywhere near it. [EDIT: To answer a comment, the current helocentric radial ...
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5 votes
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How large will our Sun get during the red giant phase (ie which planets will it engulf) before the helium flash?

Low mass stars like the Sun do become very large prior to He ignition in the core. The exact value depends a bit on models for mass loss from the extended atmosphere (e.g. Guo et al. 2016), but ...
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5 votes
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What exactly will happen when the Sun leaves the main sequence? I know it expands to a red giant twice, but what happens exactly? What's the timeline?

I've used a research-grade tool to calculate a very simplified and definitely not research-grade run of the evolution of a roughly Sun-like star roughly from birth to its white-dwarf cooling sequence. ...
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4 votes

When exactly does a sub giant become a red giant?

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 ...
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4 votes
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In System KIC 9246715, How Far Does One Star Orbit the Other?

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 ...
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4 votes

How Long Will Earth's Year be When Our Sun Goes Red?

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 ...
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4 votes

Does a red giant star produce more stellar wind than a yellow star?

The Sun's current mass loss rate - a combination of the solar wind and radiation from the Sun - amounts to something like $10^{-13}$ solar masses per year. The mass loss rates from red giant stars are ...
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3 votes

Why is the track of the subgiant stage almost horizontal on the HR diagram?

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 ...
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3 votes

Why do stars become red giants?

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 ...
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3 votes

When exactly does a sub giant become a red giant?

Let's walk through the stages of post-main sequence evolution. For reference, the images, and much of the content is being pulled from An Introduction to Modern Astrophysics by Carroll and Ostlie. I'...
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3 votes
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How does the stellar evolution for low and intermediate mass stars differ?

Where does the evolution actually stop? Is the Helium Core Flash the last thing a Sun-like stars experiences or does it in fact follow the rest of the tractory? Stars with masses below about 2 solar ...
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3 votes

Do low-mass red giants last longer than high-mass red giants?

Higher mass stars will have shorter lives. Even though they have more fuel for nucleosynthesis, they burn this fuel much quicker than lower mass stars. Generally, you should think of "red-giant&...
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3 votes

Will the planets be pushed outwards when the sun goes red giant?

The Sun loses mass all the time. That mass loss will accelerate as it ascends the red giant branch and ultimately the Sun will end up as a white dwarf of about half a solar mass. Since the solar wind ...
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3 votes

How much mass is lost in the red giant phase of a sun?

The exact amount is not known - because the details of mass-loss in the later stages of a star's life are not well understood - but about $0.47M_\odot$. Details We can still attempt to answer the ...
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