<p>This answer may be a bit speculative but still I thought to give it try. </p>

<p>A.S.Eddington <em>The Internal Constitution of the Stars</em> (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.</p>

<p>On page 302 ff.  of the book Eddington discusses the energy generation of the stars and its dependency on physical properties, like temperature and pressure, but on page 304 also on the <em>age of the material</em>. There he writes, referring to the consumption of material, and thus aging of the star:</p>

<blockquote>
  <p><em>The question what zero the age is to be reckoned from remains unanswered.</em></p>
</blockquote>

<p>I’m not a native speaker and this sounds quite old style English to me, so  I may be misinterpreting something here. But since every person working in the field of stellar models will have read this book, it seems quite plausible to me that Eddington here may have coined the term <em>zero age</em>. </p>

<p>Here’s the link to the page: <a href="https://archive.org/details/TheInternalConstitutionOfTheStars/page/n311" rel="nofollow noreferrer">https://archive.org/details/TheInternalConstitutionOfTheStars/page/n311</a></p>

CC BY-SA 4.0
2019-11-26T21:08:25.220Z
Nice find! Thank you Hartmut!
Gabriel
CC BY-SA 4.0
2019-11-30T08:17:42.473Z
@Gabriel my pleasure. Even today this book is, quite generally, a very good read. The way he comes to conclusion about stellar properties with much less information than today is fascinating. At that time not even the source of energy for the stars was exactly known, and it was Eddington who proposed nuclear fusion as a possible source.
Hartmut Braun
CC BY-SA 4.0
2019-11-26T20:36:02.803Z
2019-11-26T20:36:02.803Z
Hartmut Braun
<p>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 for eliminating confusion about what the rising helium content is doing, it's not intended to be a statement about the actual age of the star.  When astronomers talk about the age of a star, it's the age before the ZAMS, plus the age after the ZAMS.  Usually the distinction doesn't matter, because main-sequence lifetimes tend to be much longer than pre-main-sequence lifetimes.  This is convenient, because it is not possible to identify an exact moment when a star "first forms."</p>

<p>The situation is made even more unclear when you look at the formal definition of a "star," which is almost never used in practice but requires that the object be undergoing fusion.  So if you believe that definition, the ZAMS is also the birth of the star, and its age is the age on the main-sequence.  No one actually uses that poor language, because we do actually want to talk about the pre-main-sequence lifetime of a star!  I suppose it would be a bit like counting the 9 months we are in the womb as part of our "lifetime," but this is the standard for what is done in professional astronomy (though not for human lifetimes).</p>

CC BY-SA 3.0
2018-04-14T12:06:30.507Z
This answer is more misleading than the myriad things that Ken G thinks are misleading. Ken G is of the opinion that fusion in a star is a trivial, unimportant factor. This is a rather unorthodox view. When most astronomers talk about the age of a star, they talk about the time since it enters the main sequence. A protostar is not a star, just as a dwarf planet is not a planet.
David Hammen
CC BY-SA 3.0
2018-04-14T12:33:53.447Z
@DavidHammen If age meant age from the ZAMS, then the ages of stars in the Pleiades (which were all born at the same time and so have the same age) would range from 100 million years to *minus* 100 million years (since the lowest mass stars have not reached the ZAMS yet). Obviously what matters is not  the time since an arbitrary, mass-dependent event, but the time since birth and that is what the "age" of a star means.  It is sometimes convenient to ignore the PMS lifetime, because it may be much less than the total age or because it is actually age-since-ZAMS that is measured.
ProfRob
CC BY-SA 3.0
2018-04-14T12:37:20.640Z
For example, asteroseismological determination works for 1-2 solar mass stars  by probing how much H has been turned to He. But here, the PMS lifetime of millions of years is negligible compared with the billions of years the stars have been in the main sequence.
ProfRob
CC BY-SA 3.0
2018-04-14T12:38:10.073Z
These days "protostar" is used almost exclusively to describe young stellar objects that are still surrounded by envelopes and are generally younger than a few million years.
ProfRob
CC BY-SA 3.0
2018-04-14T19:28:53.033Z
@DavidHammen: I fail to see where I implied fusion is "trivial," when I actually said that astronomers don't date the age of a star starting at fusion.  Which they don't, as Rob Jeffries corroborates.  Astronomers talk about "pre-main-sequence lifetime" in time units that are not negative ages, all the while understanding that fusion is a significant physical effect.  The real problem is that stars are often officially defined as being stars when they fuse, so if astronomers actually used that definition, the ZAMS would be the start of the stellar "lifetime."  That is not the convention.
Ken G
CC BY-SA 3.0
2018-04-13T16:34:20.960Z
2018-04-13T16:34:20.960Z
Ken G
<p>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.</p>

<p>Prior to this, the objects are known as pre-main sequence stars and most of their luminosity arises from the release of gravitational potential energy as they contract.</p>

<p>The <em>exact</em> definition (whether it is 99% or 90% of the luminosity) does vary, but since the nuclear reactions ramp up quite quickly this doesn't materially affect any quantitative analysis.</p>

<p>In any case, the age of a star is not measured from the ZAMS. The reason for this is that what is of physical interest is the time since the star formed and that must include the pre-main sequence lifetime. Whilst this is very short (maybe 1 million years) for stars more massive than the Sun, and certainly shorter than the total main sequence lifetime, it can be much longer (100 million years) for a low-mass M dwarf. This is also the reason that the ZAMS is a theoretical construct. Stars born at the same time will never be all on the ZAMS at the same time. By the time low-mass stars reach the ZAMS, more massive stars will have evolved away from the main sequence!</p>

<p>As to the origin of the term, I suspect it comes from the early computer/analytic models of stellar evolution. There you would likely miss out the pre main-sequence phase and <em>start</em> your model with "zero age" at that point where it achieves equilibrium with the luminosity supplied by fusion and with a pristine He/H ratio in its core. Thus in reality, stars of different mass would have different ages at this point, but from the point of view of a theoretical modeller, it would be their starting condition and hence a "zero age".</p>

CC BY-SA 3.0
2018-04-14T22:50:54.527Z
Although this answer is entirely accurate, my question was more about the *origin* of the term "zero age".
Gabriel
CC BY-SA 3.0
2018-04-14T23:03:40.453Z
@Gabriel Yes. I misread your question.
ProfRob
CC BY-SA 3.0
2018-04-14T23:14:30.247Z
@Gabriel but see my edit. I am searching for evidence to back my suspicion.
ProfRob
CC BY-SA 3.0
2018-04-14T10:53:32.213Z
2018-04-14T23:13:50.373Z
2018-04-14T23:13:50.373Z
ProfRob
<p>The <a href="https://en.wikipedia.org/wiki/Main_sequence" rel="nofollow noreferrer">main sequence</a> stage for a star begins after it stars burning hydrogen into helium. This is often referred to as the <em>zero age</em> main sequence.</p>

<p>This is confusing since stars have already lived several million years in their pre-main sequence stage.</p>

<p>Where does this come from? Is it a historical designation that just stuck?</p>

<star><stellar-evolution><age><main-sequence>
Why is the main sequence often called 'zero age' main sequence?
Gabriel
# Why is the main sequence often called 'zero age' main sequence?

(Asked by: username_0 on 2018-04-13)

The [main sequence](https://en.wikipedia.org/wiki/Main_sequence) stage for a star begins after it stars burning hydrogen into helium. This is often referred to as the *zero age* main sequence.

This is confusing since stars have already lived several million years in their pre-main sequence stage.

Where does this come from? Is it a historical designation that just stuck?

**Tags:** star, stellar-evolution, age, main-sequence

---

## 3 answers

### Answer by username_4 on 2018-04-14. Score: 8 | Accepted answer

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 of gravitational potential energy as they contract.

The *exact* definition (whether it is 99% or 90% of the luminosity) does vary, but since the nuclear reactions ramp up quite quickly this doesn't materially affect any quantitative analysis.

In any case, the age of a star is not measured from the ZAMS. The reason for this is that what is of physical interest is the time since the star formed and that must include the pre-main sequence lifetime. Whilst this is very short (maybe 1 million years) for stars more massive than the Sun, and certainly shorter than the total main sequence lifetime, it can be much longer (100 million years) for a low-mass M dwarf. This is also the reason that the ZAMS is a theoretical construct. Stars born at the same time will never be all on the ZAMS at the same time. By the time low-mass stars reach the ZAMS, more massive stars will have evolved away from the main sequence!

As to the origin of the term, I suspect it comes from the early computer/analytic models of stellar evolution. There you would likely miss out the pre main-sequence phase and *start* your model with "zero age" at that point where it achieves equilibrium with the luminosity supplied by fusion and with a pristine He/H ratio in its core. Thus in reality, stars of different mass would have different ages at this point, but from the point of view of a theoretical modeller, it would be their starting condition and hence a "zero age".

> **Comments:**
> **username_0**: Although this answer is entirely accurate, my question was more about the *origin* of the term "zero age".  
> **username_4**: @Gabriel Yes. I misread your question.  
> **username_4**: @Gabriel but see my edit. I am searching for evidence to back my suspicion.  

---

### Answer by username_1 on 2019-11-26. Score: 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 discusses the energy generation of the stars and its dependency on physical properties, like temperature and pressure, but on page 304 also on the *age of the material*. There he writes, referring to the consumption of material, and thus aging of the star:

> *The question what zero the age is to be reckoned from remains unanswered.*

I’m not a native speaker and this sounds quite old style English to me, so I may be misinterpreting something here. But since every person working in the field of stellar models will have read this book, it seems quite plausible to me that Eddington here may have coined the term *zero age*.

Here’s the link to the page: <https://archive.org/details/TheInternalConstitutionOfTheStars/page/n311>

> **Comments:**
> **username_0**: Nice find! Thank you Hartmut!  
> **username_1**: @Gabriel my pleasure. Even today this book is, quite generally, a very good read. The way he comes to conclusion about stellar properties with much less information than today is fascinating. At that time not even the source of energy for the stars was exactly known, and it was Eddington who proposed nuclear fusion as a possible source.  

---

### Answer by username_2 on 2018-04-13. Score: 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 for eliminating confusion about what the rising helium content is doing, it's not intended to be a statement about the actual age of the star. When astronomers talk about the age of a star, it's the age before the ZAMS, plus the age after the ZAMS. Usually the distinction doesn't matter, because main-sequence lifetimes tend to be much longer than pre-main-sequence lifetimes. This is convenient, because it is not possible to identify an exact moment when a star "first forms."

The situation is made even more unclear when you look at the formal definition of a "star," which is almost never used in practice but requires that the object be undergoing fusion. So if you believe that definition, the ZAMS is also the birth of the star, and its age is the age on the main-sequence. No one actually uses that poor language, because we do actually want to talk about the pre-main-sequence lifetime of a star! I suppose it would be a bit like counting the 9 months we are in the womb as part of our "lifetime," but this is the standard for what is done in professional astronomy (though not for human lifetimes).

> **Comments:**
> **username_3**: This answer is more misleading than the myriad things that Ken G thinks are misleading. Ken G is of the opinion that fusion in a star is a trivial, unimportant factor. This is a rather unorthodox view. When most astronomers talk about the age of a star, they talk about the time since it enters the main sequence. A protostar is not a star, just as a dwarf planet is not a planet.  
> **username_4**: @DavidHammen If age meant age from the ZAMS, then the ages of stars in the Pleiades (which were all born at the same time and so have the same age) would range from 100 million years to *minus* 100 million years (since the lowest mass stars have not reached the ZAMS yet). Obviously what matters is not  the time since an arbitrary, mass-dependent event, but the time since birth and that is what the "age" of a star means.  It is sometimes convenient to ignore the PMS lifetime, because it may be much less than the total age or because it is actually age-since-ZAMS that is measured.  
> **username_4**: For example, asteroseismological determination works for 1-2 solar mass stars  by probing how much H has been turned to He. But here, the PMS lifetime of millions of years is negligible compared with the billions of years the stars have been in the main sequence.  
> **username_4**: These days "protostar" is used almost exclusively to describe young stellar objects that are still surrounded by envelopes and are generally younger than a few million years.  
> **username_2**: @DavidHammen: I fail to see where I implied fusion is "trivial," when I actually said that astronomers don't date the age of a star starting at fusion.  Which they don't, as Rob Jeffries corroborates.  Astronomers talk about "pre-main-sequence lifetime" in time units that are not negative ages, all the while understanding that fusion is a significant physical effect.  The real problem is that stars are often officially defined as being stars when they fuse, so if astronomers actually used that definition, the ZAMS would be the start of the stellar "lifetime."  That is not the convention.  

---

