Winter on Helliconia lasts for around 600 years. This is because the planet has not one sun, but two.

The planet circles around a small, yellow-orange star called Batalix that is rather like our sun. The problem is that Batalix doesn’t give out enough heat to thaw the wintry ice on Helliconia. In fact, it seems to almost permanently remain in a state of winter.

Just like Earth, the planet circles around the small sun (Batalix) at around the same speed as an Earth year. But Batalix is circling around a larger star, Freyr. Freyr is the second sun.

Batalix’ orbit is highly elliptical, so that the difference between the closest point to Freyr and the farthest point from Freyr is huge. Freyr emits vast amounts of heat, which means that at the closest point, the ice melts, spring and summer arrive, and the planet becomes scorchingly hot. At the farthest point, the planet is  icebound again.

This can all be seen in the picture below.

Of course, the two stars and the planet are so far apart that they are not drawn to scale. Furthermore, the orbit of Helliconia seems to pass right over Freyr, which is impossible.

Still, I hope this gives you the general idea of the effect of the two stars on Helliconia: Batalix provides day and night, whereas Freyr provides the seasons.

You might also want to read the physics of Hothouse.