# Tag Info

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Does a star fuse helium to beryllium on the main sequence? Stars don't fuse helium to beryllium except as a very, very short intermediate step toward carbon. Helium-helium fusion to form beryllium is endothermic: It consumes energy. To make matters worse, the beryllium-8 that results has an extremely short half-life, less than $10^{-16}$ seconds. Helium ...

10

What defines the main sequence? Main sequence stars are characterized by hydrogen fusion in their cores, either through the proton-proton chain (for lower-mass stars) or the CNO cycle (for stars more than about 1.5 times the Sun's mass). Outside the core, no significant fusion takes place; the outer layers are involved in radiative or convective energy ...

8

I have mulled over this a couple of times (it's a really interesting question!), and hopefully come up with a somewhat enlightening answer. I haven't been able to find a good, modern reference for these details (perhaps I just suck at literature searches...) so there's a little mucking around in the history books The total timescale of evolution onto the ...

8

No, the Main Sequence is more like a starting line. Most stars spend a long time on one point of it (10 billion years for the sun) while they fuse hydrogen into helium. Then they wander off it. In this diagram the black line is the Main Sequence. The colored lines show temporal sequences. This progress through time of an individual star is called its ...

8

Is this in any sense a temporal sequence? Not really. At least not in the sense of a star sliding along the main sequence. That doesn't happen. Instead, a star remains more or less parked at one spot on the main sequence during it's lifetime as a main sequence star. A protostar is more luminous and cooler than the zero age main sequence star it will become....

8

The main sequence is mostly a plateau that a star reaches after it is fully formed but before it begins to run low on hydrogen to fuel normal fusion reactions. And yes, the sequence is mostly an ordering - by mass, not age. I say mostly because age does have some effect (see the section from the Wikipedia article on the main sequence regarding temperature-...

7

This is a well-studied problem. The effect of rotation on the structure of a low-ish mass star (like the Sun) is summarised by Eggenberger (2013). Such stars are never observed to rotate so fast that the rotation plays any significant role in their hydrostatic equilibrium, however rotation does play a role by causing additional mixing in the star. This is ...

7

Here is another plot of a Hertzsprung Russell diagram (luminosity versus temperature), but this time based on theoretical models. (The plot is from D. Prialnik 2000, An Introduction to the theory of stellar structure and evolution). Note that the zero age main sequence is well behaved in this plot. Luminosity and temperature are related by smoothly changing ...

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

I assume by largest, you mean largest radius. Well it won't be VV Cep B since this is merely a B-type main sequence star. O-type main sequence stars are known and these have both larger masses and larger radii on the main sequence (when they are burning hydrogen in their cores). A selection of the most massive objects can be found in the R136 star forming ...

6

Astronomers distinguish a prototstar from a star based on whether the object is visible. A protostar is hidden by the gas cloud that surrounds it. Protostars aren't visible. At some point in their evolution (and where this occurs depends on mass and metallicity), a protostar will start clearing the surrounding cloud of gas. This process happens very quickly ...

5

Main sequence stars are defined by being hot enough in the core to fuse hydrogen, so their core is at least about 10 million K, and can get up to 20 million K for the more massive ones (because they are more luminous, so their nuclear fusion has to crank itself up a bit more). The way they get their cores hot enough to fuse hydrogen is via gravitational ...

5

The zero age main sequence (ZAMS) is the theoretical locus of points in the Hertzsprung-Russell diagram where the luminosities of young stars (at a range of masses) are mostly supplied by the nuclear reactions that have begun in their cores. Prior to this, the objects are known as pre-main sequence stars and most of their luminosity arises from the release ...

4

My kneejerk reaction is that your only option is to remove a chunk of mass from the outer part of the Sun. The Sun will respond (on a Kelvin-Helmholtz timescale), by contracting and becoming less luminous because the core temperature is lower in a less massive star. This will extend its main sequence lifetime, because only the central parts of the star are ...

4

Whether convection exists depends on whether the interior radiative temperature gradient reaches the adiabatic temperature gradient. The interior radiative temperature gradient is proportional to the opacity and the outward energy flux, and inversely proportional to $T^4$. As the star evolves on the main sequence, the central temperature goes up and the ...

3

The luminosity of a star remains approximately constant during its time on the main sequence. There is a slight increase over time, which is, as you guessed, related to the changing composition of the star due to fusion. The so-called Vogt-Russell theorem tells us that a star's mass and composition are the only two factors that affect its structure and, by ...

3

This answer may be a bit speculative but still I thought to give it try. A.S.Eddington The Internal Constitution of the Stars (Cambridge University Press, 1926) has probably been one of the most influential publications on stellar models and almost every physicist of that time working in that field will have read it. On page 302 ff. of the book Eddington ...

3

The term "zero age Main sequence" is there only because during main-sequence evolution, hydrogen is turned to helium, changing the stellar structure a little (electrons are eaten up, light escapes more easily, so the luminosity rises for example). Yet all stars along that process are called "main-sequence stars." So the "zero-age" subset is simply a term ...

3

Although pre-main sequence stars have lower temperature, they are essentially huge clouds of gas, often as large as 1 pc wide. The luminousity being proportional to square of the radius is essentially large for pre-main sequence stars. Plus, the problem with gravitational contraction is not the amount of energy that can be generated per second. That can ...

2

At the end of the protostar phase a vigorous outflow from the star develops called the T-Tauri Wind and this could cut off accretion. Eventually, it develops into a normal star and the strong wind dies down. Material that was not totally blown away could then continue to fall down and be accreted

2

TL;DR: Turn Off point (TO) means the location of the "knee" in the HRD of clusters of stars. TAMS is an imaginary line in the HRD that is defined by the location of stars of different mass at the end of hydrogen burning. It constitutes the upper boundary of the main sequence. More details: There maybe some confusion because the terms are not completely ...

1

The most important thing that happens is hydrogen is turned into helium, which essentially means that electrons and protons are getting merged into neutrons (the neutrons inside the helium). That reduces the number of particles. For example, if you turn a pure hydrogen gas into a pure helium gas, you now have half the electrons you had before. Also, you ...

1

The isochrones are not straight lines in the CMD. Extinction moves the stars both redward (right) as well as fainter (down) in the CMD. The vertical shift (the extinction $A$) is calculated from the horizontal shift (the reddening) using an extinction law. (e.g. $A_V = 3.1 E(B-V)$). The effects of extinction and distance (which only moves the stars ...

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

This has been considered before. The phrase you want is star lifting (aka stellar husbandry), which is the (theoretical) practice of removing mass from a star to extend its lifetime. Since the rate of fusion reactions in the core of star increases faster than increasing math, smaller stars have much longer lifetimes, up to several trillion years for a 0.1 M☉ ...

1

If your question is why the star starts moving up the red giant branch, it's in essence because of the behaviour of the surface opacity and the development of a substantial convective envelope in order to meet the surface boundary condition. It's basically the Hayashi track in reverse. We can say this because if you create a model of a star and ...

1

To get an order of magnitude estimate you can just use the total mass $M$ and luminosity $L$ of the star and an assumption of your fusion process. Main sequence stars fuse Hydrogen in to Helium through the proton-proton chain, which converts 0.7% of mass into energy. So the estimated lifetime of the star would just be: $0.007\frac{Mc^2}{L}$ ($c$ is the speed ...

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