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45 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 ...
ProfRob's user avatar
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23 votes
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How bright will Alpha Centauri A be from Earth when it becomes a red giant?

First of all, by the time Alpha Centauri A becomes a red giant, it will no longer be this close to the Sun due to the orbit of the stars around the galaxy so it probably wouldn't be visible. But let's ...
WarpPrime's user avatar
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16 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 ...
AtmosphericPrisonEscape's user avatar
15 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 ...
ProfRob's user avatar
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15 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'...
Ken G's user avatar
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9 votes

Can a brown dwarf accrete enough mass from red giant to become a star?

One scenario that can work is if the wind from the red giant is accreted by the brown dwarf. The brown dwarf can be in quite a wide orbit and still accrete mass because the wind from the red giant, ...
ProfRob's user avatar
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9 votes

When a star enters a red giant phase, does the pressure in its core increase or decrease?

If you heat up a balloon, the balloon will expand. Is the balloon's internal pressure higher? Yes. Is the balloon less dense? Also yes. Is this a contradiction? I think what you aren't seeing is that ...
Darth Pseudonym's user avatar
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 ...
Steve Linton's user avatar
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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 ...
Steve Linton's user avatar
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8 votes
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When a star enters a red giant phase, does the pressure in its core increase or decrease?

Newton's shell theorem tells you that what is outside the core has no gravitational influence on what is at the core (as long as the envelope is spherically symmetric). The equation of hydrostatic ...
ProfRob's user avatar
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7 votes
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What are Thorne-┼╗ytkow 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 ...
Warrick's user avatar
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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 ...
ProfRob's user avatar
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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 ...
ProfRob's user avatar
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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,...
Ken G's user avatar
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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) ...
Ken G's user avatar
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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 ...
Ken G's user avatar
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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 ...
James K's user avatar
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5 votes

Could the Earth survive a red giant Sun?

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 ...
J. Chomel's user avatar
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5 votes
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How can white dwarf form Oxygen ? (Temperature problem)

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. ...
ProfRob's user avatar
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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 ...
ProfRob's user avatar
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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 ...
ProfRob's user avatar
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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. ...
Warrick's user avatar
  • 2,847
5 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 ...
ProfRob's user avatar
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5 votes

How bright will Alpha Centauri A be from Earth when it becomes a red giant?

In 1 billion years Sol and Alpha Centauri will have travelled about 4 times around the galaxy on different paths, so it could be 100k light years away. It's moving at 22km/s relative to Sol.
bandybabboon's user avatar
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5 votes

How did we figure out that Stars become red giants and when did we find out that they will?

In the 1940s and early 1950s the understanding of the evolution of stars was radically reformed. A key person was George Gamow, but others such as Edward Teller and Fred Hoyle and Geffory Keller also ...
James K's user avatar
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4 votes
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What would be the apparent magnitude of Betelgeuse if it were as close to Earth as Sirius?

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 ...
James K's user avatar
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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 ...
ProfRob's user avatar
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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 ...
PM 2Ring's user avatar
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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 ...
Mike G's user avatar
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