<p><img src="https://i.sstatic.net/lpZDP.jpg" alt="pre main sequence evolution"></p>

<p>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.</p>

<p>Plus, the problem with gravitational contraction is not the amount of energy that can be generated per second. That can actually be larger than that produced from fusion if you insist on really massive objects. The logic, when we ruled out gravitational contraction as a source of energy for Sun, was that gravitational contraction cannot sustain it for long. Geologists knew from their dating science that Sun had to be older than grav. contraction accounts for.</p>

<p>Thus, admit it. Gravitational contraction makes pre-main sequence stars radiate more energy than main sequence starts, but that's just luminousity and not really energy.
Luminousity is energy per second.</p>

CC BY-SA 3.0
2014-05-25T01:10:42.817Z
2014-05-25T01:10:42.817Z
Cheeku
<p>The luminosity of a PMS star is provided by the rate of change of gravitational potential energy as it contracts.</p>
<p>Since PMS stars contract along Hayashi tracks at almost constant effective temperature, their luminosities decrease considerably with time as their surface area diminishes. However, the virial theorem tells us, that the core temperature of the PMS star must <em>increase</em>, roughly as the reciprocal of the radius. Ultimately, this leads to the ignition of hydrogen, but the rate of nuclear burning only ramps up to be equivalent to the luminosity at that point. Therefore the luminosity of a main sequence star is generally<span class="math-container">$^1$</span> lower than during the majority of the PMS phase for a star of the same mass.</p>
<p><span class="math-container">$^1$</span> This is not always true. There can be a brief phase of constant or slightly increasing luminosity just prior to hydrogen ignition that is caused by the development of a radiative core - called the Henyey track.</p>

CC BY-SA 4.0
2022-04-28T22:20:47.377Z
2022-04-30T13:05:26.503Z
2022-04-30T13:05:26.503Z
ProfRob
<p>How can a pre-main sequence star radiate more energy by gravitational contraction than a main-sequence star can by hydrogen fusion?</p>

<star><star-formation><protostar><main-sequence>
How can pre-main sequence stars radiate more energy than main-sequence stars?
analogyschema
# How can pre-main sequence stars radiate more energy than main-sequence stars?

(Asked by: username_0 on 2014-05-24)

How can a pre-main sequence star radiate more energy by gravitational contraction than a main-sequence star can by hydrogen fusion?

**Tags:** star, star-formation, protostar, main-sequence


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## 2 answers

### Answer by username_1 on 2014-05-25. Score: 2 | Accepted answer

![pre main sequence evolution](https://i.sstatic.net/lpZDP.jpg)

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 actually be larger than that produced from fusion if you insist on really massive objects. The logic, when we ruled out gravitational contraction as a source of energy for Sun, was that gravitational contraction cannot sustain it for long. Geologists knew from their dating science that Sun had to be older than grav. contraction accounts for.

Thus, admit it. Gravitational contraction makes pre-main sequence stars radiate more energy than main sequence starts, but that's just luminousity and not really energy.
Luminousity is energy per second.


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### Answer by username_2 on 2022-04-28. Score: 2

The luminosity of a PMS star is provided by the rate of change of gravitational potential energy as it contracts.

Since PMS stars contract along Hayashi tracks at almost constant effective temperature, their luminosities decrease considerably with time as their surface area diminishes. However, the virial theorem tells us, that the core temperature of the PMS star must *increase*, roughly as the reciprocal of the radius. Ultimately, this leads to the ignition of hydrogen, but the rate of nuclear burning only ramps up to be equivalent to the luminosity at that point. Therefore the luminosity of a main sequence star is generally$^1$ lower than during the majority of the PMS phase for a star of the same mass.

$^1$ This is not always true. There can be a brief phase of constant or slightly increasing luminosity just prior to hydrogen ignition that is caused by the development of a radiative core - called the Henyey track.


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