When Does Shabbat End?

The Source in the Gemara for Bein Hashemashot

1. The Gemara discusses what is considered to be "Bein Hashemashot" / בין השמשות, or literally, "between the suns"--i.e. the time period that is after sunset, but before nightfall. It is uncertain whether or not this time is day or night, so it is treated with the stringency of both days.
2. R. Nechemya holds it is the time it takes to walk 1/2 of a "Mil", or 1,000 amos, while R. Yehuda holds it is the time to: -walk 2/3 of a "Mil" (1,334 Amos), according to Rav Yosef, or -3/4 of a "Mil" (1,500 Amos), according to Rav Yosef, after sunset. Rabbi Yose holds it is a few seconds (the time it takes to blink an eye softly) after the end of R. Yehuda's time; before then, it is still day.
3. The Gemara rules that, since it is doubtful who is correct, we treat all of R. Yehuda's period on Friday afternoon as Shabbos. Rashi implies that following Shabbos, we treat R. Yose's period stringently.
4. However, the Gemara rules on Shabbos 35b that when one star is visible, it is still day; when two stars are visible, it is Bein Hashemashos; when three stars are visible, it is night-time. These are only stars of medium size, neither so big as to only be visible during the day, nor too small to only be visible during the night.
5. The Gemara rules that if one does a Melacha on Shabbos for the duration of Bein Hashemashos on Friday evening and Saturday evening, one has certainly violated Shabbos, and must bring a sin-offering
6. Therefore, one can do no Melacha during Bein Hashemashot .

Rabbeinu Tam

1. According to Sephardim, a Yireh Shamayim should be strict to refrain from Melacha until 72 minutes after sunset like Rabbenu Tam.
2. According to Sephardim, it is permitted for someone who keeps Rabbenu Tam time to ask someone who doesn't keep Rabbenu Tam time to do Melacha for him.