# Density of a star as a function of time **URL:** \ **Category:** Factual Questions\ **Created:** [February 28, 2017, 11:38pm UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076 "2017-02-28T23:38:53Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [February 28, 2017, 11:38pm UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/1 "2017-02-28T23:38:53Z")
For sake of simplicity, please assume a star of 2 solar mass. If one were to plot the density (bulk density) over its lifetime - will the curve be linear, parabolic, exponential, etc ? Also, how does the curve look when the star transitions to a black hole ? Is it sudden, like a step functions, does it pulsate, linear or some other form ? Thanks in advance ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [February 28, 2017, 11:59pm UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/2 "2017-02-28T23:59:16Z")
Nitpick: a star of 2 solar masses will become a white dwarf, not a black hole. You may have meant a more massive star’s curve… ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [March 1, 2017, 12:51am UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/3 "2017-03-01T00:51:28Z")
> [@am77494](#): > > For sake of simplicity, please assume a star of 2 solar mass. If one were to plot the density (bulk density) over its lifetime - will the curve be linear, parabolic, exponential, etc ? It’s going to be a complicated curve, changing wildly when fusion first intermittently begins in the star, stabilizing for a long period during the star’s middle age, then decreasing as the star gets hotter, before jumping sharply when the star collapses (all based on my memories of stellar physics lectures) This article talks about stellar evolution [Stellar evolution - Wikipedia](https://en.wikipedia.org/wiki/Stellar_evolution) and this has a little more info [Lecture 11: The Internal Structure of Stars](http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit2/structure.html) ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [March 1, 2017, 12:54am UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/4 "2017-03-01T00:54:15Z")
Thanks John for the correction. Please assume 50 solar masses. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 1, 2017, 2:07am UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/5 "2017-03-01T02:07:15Z")
It’ll depend on what you mean by “the star”. Suppose, for instance, that a star’s core collapses, while its outer layers get puffed out into a planetary nebula. Do you count the density of the core remnant, or of the nebula as a whole? ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [March 1, 2017, 2:16am UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/6 "2017-03-01T02:16:10Z")
Let’s count all the matter that ends up in the final black hole. I am not an expert -so please make all the assumptions to simplify the question. All I am looking for is the shape of the density versus time curve during the life of the star. Especially the shape of the curve around the time the star transitions to a black hole. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [March 1, 2017, 2:45am UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/7 "2017-03-01T02:45:45Z")
Lets assume that what the OP means here is, when hydrogen fuses into helium, then helium into carbon, etc-etc until the dead end at iron, where the star goes supernova, yes? Taking into account the mass loss of photons and solar wind, is it possible to calculate a 2 solar mass stellar diameter over it’s lifetime? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [March 3, 2017, 1:10am UTC](https://boards.straightdope.com/t/density-of-a-star-as-a-function-of-time/781076/8 "2017-03-03T01:10:40Z")
The problem with defining density as a function of time is that when the star gets very dense the choice of global time coordinate will greatly influence the answer. For example Schwarzschild coordinates will become singular at the event horizon.