Galaxy

That looks very nice! I like blue hue, the stars and their slight colour variation too.

If you want to draw more spacescapes, my best advice would be to get lots of references. You can never have too many references. But piece-specific, I'd say:
- The blue surrounding the galaxy is spread too wide. From a far away perspective, light appears to concentrate around the galaxy.
- The white areas of the galaxy are transparent/thin. As you reach the main parts of the galaxy, the light from the stars cause a very bright glow, much like a lightsaber with a less harsh gradient transition. Normally you won't be able to see any stars near the middle of the galaxy's thickness.
- There's no galaxy core visible. Galaxies have a central core that's often the main attraction.

Although, with a bit more tweaking, you could actually take this technique and use it to draw Nebulae as well.