Questions tagged [red-giant]

Questions about red giants, large luminous low-mass stars in a late phase of stellar evolution.

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7 votes
1 answer
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Are red giants still primarily made of hydrogen? What is their exact composition?

How much lower is the ratio of hydrogen to helium in a red giant vs a 'normal' main sequence star like our Sun? Also, how low does the ratio get at the end of a red giant's life? (Just before it ...
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1 vote
1 answer
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Which star has a higher effective temperature, a red dwarf or a red giant?

According to the Hertzsprung-Russell diagram, they would roughly have the same temperature, but I've read in bibliography that the surface temperature of the red giant is about 5000K and the one of ...
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8 votes
2 answers
233 views

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

Let's assume a brown dwarf is on orbit around a main sequence star. Than the star becomes a red giant. Let's assume the brown dwarf has "the right" orbit and can syphon matter from the red ...
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3 votes
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How can I calculate evolutionary timescales of low mass stars?

How can I calculate how long a star of a given mass will spend on an evolutionary branch before evolving off it? I'm thinking about the evolution of low mass stars from the subgiant branch to the red ...
4 votes
2 answers
151 views

How hot would the core of the Red Giant Sun be?

After the Sun becomes a red giant, it will heat up its core to 100 million kelvins, which initiates the Triple-Alpha fusion process, which heats up the star's core even further and cause it to undergo ...
3 votes
0 answers
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Are there any binary red supergiants?

I wonder if we ever have identified or observed a pair of binary stars (red supergiants). And I also wonder what would happen if they exploded, (theoretically) as we haven’t observed it. Also, would ...
9 votes
2 answers
2k views

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

As of right now, both stars from Alpha Centauri are in their main sequence stages, but eventually Alpha Centauri A is going to quickly expand in a matter of time, and I’m pretty sure its luminosity is ...
6 votes
0 answers
57 views

Source of information on density profile of a red giant star?

Is there a good source of information of the expected density profile inside a red giant star? I have read that when a star such as the sun has converted much or most of the hydrogen in its core into ...
1 vote
2 answers
163 views

How much of the earth would remain intact if the sun expanded into a red giant encompassing its orbit?

It seems to me that part of the earth might survive being absorbed by a red giant. How long would it take the heat, plasma, magnetic field, etc. of the sun to eat away at the earth? How long could it ...
2 votes
1 answer
106 views

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

Do red giant stars produce more stellar wind than when they were ordinary yellow stars? If so, how big a difference are we talking about?
4 votes
1 answer
246 views

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

The red giant phase of stars are relatively fast. How will this impact a star's total mass? Are there significant mass losses taking place when this happens? If so, do we know how much? The sizes of ...
0 votes
1 answer
147 views

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

When the sun goes red giant will the planets be pushed outwards towards the periphery of the solar system? If so, will this happen simultaneously to all planets or could, for example, a planet like ...
1 vote
1 answer
245 views

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? [duplicate]

I've seen numerous claims surrounding this, with some claiming the expansion of the Sun from its end-main sequence radius to its peak red giant radius will take over 1 billion years, while others ...
2 votes
0 answers
41 views

Dynamics of red giant hydrogen shell

From what I understood from this answer to How can a red giant grow so big? But this is not at all the way the shell fusing in a red giant self-regulates its fusion rate-- it cannot regulate its ...
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4 votes
1 answer
281 views

Where will the frost line be when the Sun becomes a red giant and what effect will it have on the solar system?

I understand the frost line is currently about 5.2 AU and earlier in the solar systems formation was 2.7 AU. But when the Sun becomes a red giant the frost line should move outward. I understand the ...
1 vote
0 answers
34 views

Why red giants have cooler surface than MS stars?

I know that with increasing age and depletion of hydrogen in the core the radiation inside a main sequence (MS) star increases pushing the envelope/surface of the star outwards. What I don't ...
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3 votes
0 answers
44 views

What is the theoretical maximum variability a pulsating red giant can have such that a habitable planet can stay habitable for long periods of time?

I was reading about red giants and came across this statement: Some research suggests that, during the evolution of a 1 M☉ star along the red-giant branch, it could harbor a habitable zone for ...
4 votes
1 answer
93 views

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

It turns out that red giant stars are not defined by uniform mass. Some can be only one-third as massive as our sun, whereas others can be eight times as massive. So I'm calling the both sides "...
1 vote
0 answers
45 views

Is a star with 2/3 solar masses and 9/10 solar radii still in the main sequence?

As far as I know, K-type and G-type stars (among others) grow in radius as they age, eventually leading to red giant phases. My question is, would a star that is about 0.66 solar masses and 0.94 solar ...
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2 votes
1 answer
283 views

How does the stellar evolution for low and intermediate mass stars differ?

I'm trying to understand stellar evolutionary tracks for different masses. So far I have the general understanding from a cloud of gas and dust to a white dwarf, neutron star or black hole. What I don'...
7 votes
1 answer
105 views

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?

OK, I didn't know how I should word this question. But the basic point is that most white dwarfs that we have classified fall in temperature ranges from ~50,000 K to 6000 K. However, at the end of a ...
1 vote
1 answer
218 views

How large will our Sun get during the red giant phase (ie which planets will it engulf) before the helium flash?

I have read that our Sun will increase in size to a red giant. But after it goes through the helium flash it will contract somewhat. Then after that grow again to be even larger before finally ...
1 vote
2 answers
253 views

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

As of now, Proxima b is the only confirmed planet (or dwarf planet if it didn't clear its orbit - sorry, couldn't resist) around Proxima Centauri, the nearest-known star to the Sun about 4.2 ly away. ...
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10 votes
1 answer
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If Sunlike stars become a red giant and eventually a white dwarf, what do red dwarfs become?

The Sun is said to become a red giant at the end of its life (before that it will become an orange subgiant first and then an orange giant or so) and after ejecting its outer layers it should become a ...
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0 votes
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Could the Sun's planetary nebula be dangerous for the planets?

Since the Sun's mass isn't high enough it won't go supernova being a red giant but instead release a planetary nebula. So that seems to be kind of a substitute for a supernova. Would ejecting the ...
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22 votes
2 answers
5k views

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

In about 5 billion years the Sun is predicted to become a red giant and have more than 200 times its current size, reaching a radius of about 5 AU when largest. I wonder what spectral class the Sun ...
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16 votes
3 answers
2k views

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

Let's say that instead of the sun, we have a red giant, but are orbiting it at a safe distance, within the goldilocks zone. Would the sky actually look more red? Or would it be closer to white/...
1 vote
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90 views

What if the Sun turned into a red giant tomorrow? [closed]

Let's say that, hypothetically, our sun uses up all of it's hydrogen tonight, and tomorrow morning is the first day of its red giant phase. How long would it take for the star to turn red? How long ...
1 vote
0 answers
45 views

Relation between initial mass and evolutionary phases lifetimes

Consider the following image (Source: An Introduction to Modern Astrophysics): There seems to be a relation between the initial mass/mass at the beginning of the MS and the time spent during ...
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2 votes
1 answer
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How Long Will Earth's Year be When Our Sun Goes Red?

We live in a planet that orbits 93 million miles from a G-type main-sequence star, or "yellow dwarf". That is far enough for a revolution of 365 days. Such is the case of Kepler's Third Law of ...
1 vote
1 answer
93 views

How Wide Would the Habitable Zone Be in a Singular Red Giant System?

Astrobiologists looking for signs of life outside Earth usually take excitement over planets orbiting red giant stars because a larger, brighter star usually means a farther-away but still wider ...
2 votes
1 answer
94 views

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

I was going to ask if it's physically possible for one red giant to orbit another, but fortunately, I have just found such a thing--a red giant binary system known only as KIC 9246715. In this ...
0 votes
1 answer
732 views

Is there a theoretical minimum mass main sequence star that can become a red giant?

Wikipedia's Red Giant page says they range in mass from 0.3 to 0.8 solar masses. Which corresponds to an initial mass of a bit less than 1 solar mass, about 0.8 or so, if I was to estimate and a few ...
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3 votes
1 answer
158 views

What will be the temperature on Earth when Sun finishes its main sequence?

We know that presently Sun is 4.5 billion years into its main sequence. It has another 5 billion years before it enters the Red Giant phase. We also know that Sun's luminosity increases by 10% every ...
3 votes
1 answer
2k views

How long does it take a star to transition from main sequence to red giant?

I have read that it takes billions of years for stars of approximately the size of the Sun to get through the main sequence, after which it will enter the red giant phase. When that happens, how long ...
9 votes
2 answers
2k views

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

We all know that our sun will become a red giant in a couple of billion years? But how fast will the process be when it starts? Would it be theoretically possible (disregarding the deadly effects of ...
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18 votes
2 answers
2k views

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

Please refer to this answer from Quora: ... a star will become a red giant before it begins burning helium. In fact, it will bloat into a red giant while still burning hydrogen in a shell on the ...
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11 votes
2 answers
756 views

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

It is stated that: After the main sequence, as fusion weakens or stops in the core, outward radiation weakens. The helium core contracts and heats up. Gravitational energy is converted to thermal ...
15 votes
4 answers
2k views

How can a red giant grow so big?

Supposedly when the sun becomes a red giant, it will grow big enough to swallow Earth. However, this requires the radius of the sun to expand by a factor of approximately 215×, meaning its volume ...
4 votes
0 answers
271 views

How is Phosphorus made in in stars? [duplicate]

What elements are required to fuse into phosphorus in a star. Can someone give me a step by step process please e.g 4 hydrogen nuclei fuse into one helium nuclei and then 3 helium nuclei fuse ...
2 votes
0 answers
242 views

Jupiter/Saturn's fate after Sun goes red giant and planetary nebula?

The solar wind contributes a very small amount of matter to the gas giants. As the sun approaches the red giant phase, the solar wind increases significantly. 10% of the sun's mass lost as solar ...
4 votes
2 answers
321 views

Origin of terms "white dwarfs" and "red giants"?

Can someone explain my why "white dwarfs" and "red giants" were so named? And maybe that's a silly question but why not in the opposite way? @EDIT Is this somehow co-related with Stefan–Boltzmann law?...
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1 answer
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What are Thorne-Żytkow objects?

Last night I was randomly surfing astronomy pages and came across this.I searched up Wikipedia and other sites,but it doesn't exactly give me enough information about their characteristics.
7 votes
2 answers
1k views

Could the Earth survive a red giant Sun?

So my question is that, is not a lie that we are flying away from the sun slowly, that is increasing by time because the sun loss mass, so when the scientist usually talk that we will get swallow by ...
6 votes
3 answers
2k views

When exactly does a sub giant become a red giant?

A simple, but interesting question I think. I understand that after the hydrogen exhaustion on the main sequence the post main sequence starts with the SGB where they burn hydrogen in a shell around ...
6 votes
2 answers
175 views

How can white dwarf form Oxygen ? (Temperature problem)

I’ve got a question about white dwarfs and oxygen. I read in a book that a temperature of 100 million degrees is required to fuse Helium in the core of a red giant. The Helium fuses into Carbon by ...
4 votes
2 answers
550 views

Why do post main sequence stars enter the red giants branch?

I am an early graduate student in astronomy and have hard time understanding why do post-MS stars move up the RGB. Here is what I understand about post main sequence evolution of stars. As their ...
1 vote
2 answers
782 views

What would be the apparent magnitude of Betelgeuse if it were as close to Earth as Sirius?

How large and bright would Betelgeuse appear if it were as close to Earth as Sirius, before and after it goes supernova?
10 votes
1 answer
665 views

Will the Sun rotate slower in red giant form?

When the Sun runs out of hydrogen to fuse it will grow bigger in size and my question is does it rotate more slowly, like a spinning ice skater extending their arms to reduce angular velocity? What ...
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18 votes
6 answers
10k views

Why do stars become red giants?

Disclaimer: I’m not a career astronomer. I don’t own a telescope. I have no professional credentials. But I do find this stuff fascinating, and I consume all the astronomy documentaries I can. ...
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