Hmmm... let's see if I can make a useful first post in the lore forum...
Well, we know that stars are believed to be holes in the veil / fabric of Oblivion, caused by the Magna Ge (followers of Magna) abandoning the creation of Mundus and fleeing to Aetherius (as Hellmouth stated above).
And, iirc, the stars of the Serpent move about in the sky.
So, how can holes in Oblivion move about?
Here are some pasages from an http://www.imperial-library.info/content/cosmology posted by former Developer Michael Kirkbride (found at the Imperial Library site):
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What is space?Space is the interpretation of Oblivion, which is black and empty and surrounds the mortal plane. Space is infinite, but it acts just like a planet, in that Oblivion is 'surrounded' by Aetherius. You can see Aetherius by the stars.
What are stars?The stars are the bridges to Aetherius, the magic plane. They are perceived as holes on the inside surface of space. Because they are on the inside of a sphere, all stars are equidistant from Nirn. Larger stars, therefore, are not closer to the mortal plane, they are just larger tears in Oblivion. The largest tear in Oblivion is Magnus, the sun.
What are shooting stars, then?A misnomer. Shooting stars are bits of matter and magic, either from Oblivion or Aetherius, that sometimes move through the cosmos. The largest shooting stars are really planets with independent orbits, like Baan Dar the Rogue Plane.
What are constellations?Constellations are collections of stars. Since each star is a bridge to magic, constellations are very powerful phenomena, and are revered. There are generally accepted to be thirteen constellations. Nine of these are made up completely of stars. Three others are called guardian constellations, as they are each governed by a Dominion Planet. The Dominion Planets are Akatosh (eye of the Warrior), Julianos (eye of the Sage), and Arkay (eye of the Thief). The last constellation is made up of unstars, and is called the Snake.
What is the sun?Magnus is the sun, the largest hole in Oblivion, and the gateway to magic. Magnus was present at the creation of the mortal plane, and, in fact, was its architect (Lorkhan was its advocate and inspiration). Prehistoric (before ME2500, startyear) Nirn was a magical place, and highly unstable to the first mortals. Magnus then left, some say in disgust, and Oblivion filled in the void with the Void. His escape was not easy, and tatters of Magnus remain in the firmament as stars.
What is the sky?The sky is another visual phenomenon caused by mortal mental stress, the night sky in particular. The sky is as impossible as planets; in essence, when you look into the sky, 'you look outside the material plane'. At night, Nirn is surrounded by Oblivion. The day sky is the multicolored elemental cloak of Magnus the sun. It changes colors as elemental influences rise and fall. Thus, when one looks at the day sky, they see into the raiments of Aetherius, and stare at magic.
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I don't think there's a clear-cut answer here...
But maybe 'unstars' are somewhat like the 'shooting stars' mentioned above. "Shooting stars are bits of matter and magic, either from Oblivion or Aetherius, that sometimes move through the cosmos."
Anyway, I hope this somewhat helps... and that I'm not making a complete fool out of myself...
Cheers!