The concept of the "undying stars" (also known as the "imperishable stars") was important to ancient Egyptian cosmology and religion.

In the Pyramid Texts from the tomb of the Pharaoh Unas (reigned circa 2353 BC - 2323 BC), some of the oldest texts in existence, we find this expression in the Sarcophagus Chamber's south wall, utterance 214:
The (glorified) humanity bewail you after the Imperishable Stars have carried you. Enter then into the place where your father is, where Geb is! He gives you that which was on the brow of Horus, so that you become powerful and full of glory through it, so that you become the One-at-the-Head-of-the-Westerners through it.
Evidence suggests that the undying or imperishable stars were those stars which never set but which can be seen all through the night, every night of the year.

This concept can be easily understood if you imagine the earth's path around the sun each year. You might imagine the edge of a large round dinner plate or platter as the earth's path (it travels around the edge of the plate -- of course we know that its path is slightly elliptical, but for this illustration you can imagine your large platter to be round).

If you placed a burning candle or a bright yellow golf ball in the middle of this plate and think of it as the sun, you can then think of night and day for the observers on earth as they travel around the edge of the plate throughout the year. Perhaps you could imagine another golf ball (or a marble) as the earth rolling along the edge of the plate, and remember that it spins on its axis once each day as it proceeds. When your place on the spinning earth is facing towards the sun, it is day -- you can't see any stars other than the sun, which drowns out all the others with its light. When your place on the spinning earth is facing away from the sun (the candle in the middle of the plate, in this case), then it is night.

If you imagine night time on our little model, you can imagine an observer on the ball of the earth looking "outward" away from the sun along the plane of the table that the plate is resting on. This plane of the table is the "ecliptic plane" -- the plane on which the planets are orbiting. Constellations along this plane will rise and set during the night as the earth turns.

However, if you imagine night time on our little model and imagine an observer looking "straight up" (as it were) from the surface of the table, then he will be looking up and seeing stars and constellations that are always visible, because even as the earth spins he can still see the stars located in that direction, while the stars located in the direction of "straight out" (along the table's surface) will sometimes be obscured by the turning of the earth.

If the observer on our model were located right at the north pole (latitude 90o north) then the star that was "straight up" (around which the entire sky seems to turn due to the earth's spinning on its axis) would be the north star, Polaris. It would be at an elevation of 90o from the horizon (the horizon being "straight out"). However, if the observer were to walk south for one entire degree of the earth's surface, then the north star would sink behind him by one degree in the sky.

The further south he walked, the lower in the sky the north star would sink. By the time he had walked sixty degrees along the full circle of the earth (to latitude 30o north, near the latitude of the Great Pyramid), the elevation of the north star above the horizon would have sank a corresponding sixty degrees, so that it was at an elevation of 30o above a "straight out" horizon.

Each night, however, Polaris would still be visible from this latitude: it is an undying star and never sets below the horizon. The rest of the sky and all the other stars appear to turn around this point in the sky due to the rotation of the earth. Stars very close to the north star would also stay in the sky the entire evening, but as we looked further out from the north star (more towards the plane of the "table" in our mental exercise and less "upwards" towards the ceiling of the room) we would begin to see stars whose paths through the sky would eventually take them below the horizon as the earth turns.

How far down from Polaris the north star would we have to look before we started encountering stars that dip below the horizon as the "sky turns"? Well, at latitude 30o north (where the north star is located at an elevation of 30o from the straight-out horizon) we would have to go only 30o "down" from the pole star. All the stars within 30o "down" from that point would be undying stars or imperishable stars -- they never set during the night. All the stars further "down" than 30o from that pole star would dip below the horizon at some point. We can now see why these stars are often referred to as the "circumpolar stars" in modern terminology -- they are those stars arranged close to the celestial north pole.

The convention for locating a star's elevation in the sky is called "declination," and by this convention the celestial north pole has a declination of +90o . At a latitude of 30o north, then, all stars with declinations of +60o or more will be circumpolar or undying stars. This includes the stars of the Big Dipper (pictured above).

Note that those stars which are not undying stars not only dip below the horizon at some point during the earth's rotation daily, but are also invisible during some months of the year. This is because for any star not within that circumpolar circle, there will be some times during the earth's orbit when they are only above the horizon during the day.

Thinking back to our dinner plate model, we can understand that an observer on the earth ball looking across the dinner plate towards the sun candle in the middle will not be able to see the stars or constellations arranged along the "far wall" on the other side of the candle, because those stars and constellations will be "up" during the day. However, when the earth ball progresses around to the other side of the plate, those stars and constellations on that wall will be visible during the night, because the candle will now be "behind" the earth at that point.

This is why Orion is absent from the night sky during certain parts of the year, and makes his reappearance later in the year. The same holds true for all the constellations not within that circle of imperishable stars, and it is especially noticeable for constellations along the plane of the ecliptic (the direction of "looking out along the table top" in our mental construct) or close to that plane.

Now you understand the concept of the undying stars!