Final answer:
The sky could have a red border in the distant future when our Sun expands into a red giant phase; this will fundamentally change the sun's appearance and potentially cast a reddish hue over the sky.
Step-by-step explanation:
The question refers to the future astronomical event when our Sun will transition into a red giant phase. During this stage, the sky could appear to have a red border due to the sun's drastically changed appearance and its effect on the Earth's atmosphere. As the sun expands, its outer layers will cool and turn red, which could cast a reddish hue over the sky. Furthermore, the sun's increased size and luminosity would also affect the sky's appearance from Earth. However, before the sky could exhibit a red border, it's probable that the Earth's atmosphere and surface conditions could be severely altered, potentially making it uninhabitable.