It is common knowledge, for instance, that the sun has increased in brightness by roughly 40% since its formation... (The apparently early appearance of life has led to the 'Faint young Sun paradox', e.g...)

Everything I have read (book, textbook, article, etc.) says that main sequence stars generally get brighter (and larger) over their lifetimes (not sure about increased temperature....)

However, I recently came across articles about the discovery that TRAPPIST-1's planet called TRAPPIST-1b does not seem to have an atmosphere, and that this may be due to red dwarf stars' being brighter, NOT dimmer, very early in their lifetimes....

Is this true? Do red dwarfs age like Benjamin Button, in reverse, so to speak, in comparison to all other stars?

  • 1
    $\begingroup$ Can you reference the articles you came accross with that claim? $\endgroup$ Apr 6, 2023 at 13:54
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    $\begingroup$ Can you check the references, Red dwarfs may be very unstable and produce significant flares, especially when young. It is these flares that can strip an atmosphere of a planet. $\endgroup$
    – James K
    Apr 6, 2023 at 14:28

1 Answer 1


This image from Red Dwarfs and the End of the Main Sequence shows the evolution of a $0.1\,M_\odot$ star:

enter image description here

The x-axis is the effective temperature of the star (cooler on the right as usual), and the y-axis is its brightness.

Trappist-1 is a $0.09\,M_\odot$ star and this graph is applicable to its evolution.

The star begins life near the top right, following the Hayashi track (fairly constant temperature, but becoming less bright as the ball of gas (that isn't yet fusing Hydrogen) shrinks. This track is brought to an abrupt halt when hydrogen fusion starts in the core. This is the Zero Age Main Sequence "ZAMS"

Then follows a very very slow climb, the star slowly becomes hotter and brighter over a period of 5 trillion 742 billion years (I assume that the four figure accuracy here is overstated!) Trappist-1 would be somewhere near that ZAMS point, slowly slowly getting brighter, but it will live for so long that it is still very close to that corner, it's still really just a baby!

At this point an inert radiative core forms, and the star does a little turn on its track. It develops into a "blue dwarf", as it more rapidly burns out its remaining hydrogen, before cooling as a Helium white dwarf. They never enter a giant star phase, as the sun will.

It is not that young red dwarfs are brighter that imperils planets, it is the flares that they can produce:

Young red dwarfs are active stars, producing ultraviolet flares that blast out so much energy that they could influence atmospheric chemistry and possibly strip off the atmospheres of these fledgling planets. -Source


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