If you just want to scan or watch the sky or milkway, any binoculars will do for astronomy.  However, if you want to explore seriously the little cornors of universe, you need a binocular with higher magnification than 15x.  And, at night, for low-light viewing, the bigger the objective size of the binoculars, the more light gathering power and the bright objects you'll see. 

For serious astronomy, it's important to see all the way to zenith and so a 45 deg. ocular is preferred to a straight-through one. 

The flexiblity of magnification is also of great importance - you can lock down on the objects at low power with wider field of view, then switch to the high power to see more details of the objects.  So, the giant binoculars with 1.25" focuser or the dual power rotating turrets are more advantageous for astronomy.

Based on the above consideration, we think the binoculars for astronomy are those with at least 15x magnification and the objetive size not less than 70mm.  They are ranked by suitability for astronomy with 1-5 stars:

5 star: BT 100 45, BT 100 45 DL, BT 70 45 DL
4 star: BT 100, 25/40x100 standard, 25/40x100 painted
3 star: 25x100 HK, 25x100 FB, 20x80 HK, 20x80 triplet and light weight, 15x70

Besides suitability, You should probably consider the cost performance of each model using the cost performance formula.