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The Sun is the star at the center of the Solar System. ... because the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses. ...
The estimated end of the Sun's current phase of development, after which it will swell into a red giant, either scorching or swallowing Earth, will occur around five billion years from now. However, as the Sun grows gradually hotter (over millions of years), Earth may become too hot for life as early as one billion years from now. [212] [213] [214]
Thereafter, the Sun will continue to burn hydrogen in a shell surrounding its core until the luminosity reaches 121% above the present value. This marks the end of the Sun's main-sequence lifetime, and thereafter it will pass through the subgiant stage and evolve into a red giant. [1]
The third phase is astronomical twilight, which is the period when the Sun is between 12 and 18 degrees below the horizon. [2] Dusk is at the very end of astronomical twilight, and is the darkest moment of twilight just before night. [3] Finally, night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky. [4]
The current Sun compared to its peak size in the red-giant phase. The Sun's main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun's pre-remnant life combined. [30]
When the Sun leaves the red-giant branch and enters the asymptotic giant branch, the habitable zone will abruptly shrink to roughly the space between Jupiter and Saturn's present-day orbits, but toward the end of the 200 million-year duration of the asymptotic giant phase, it will expand outward to about the same distance as before.
Numbers in brackets for cycle 25 indicate the minimum possible value for that month, assuming there are no more sunspots between now (Jan 3, 2024) and six months after the end of the month in question. The table shows averages for each hemisphere and the average for the entire Sun.
For the Sun and stars of less than about 2 M ☉ [13] the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate , it will continue to heat until it reaches a temperature of roughly 1 × 10 8 K , hot enough to begin fusing helium to carbon via the triple-alpha process .