Click on the left marker of the top slider and drag it to the right until the noise begins to disappear. Don't go too far. The middle marker will also move when you slide the left marker. This is normal. Just let it do its "thing". At this stage, leave just a little noise visible. Don't worry, we will fix it later. You don't want to hide faint stars. Leaving a little noise will lessen the chance of this.

The noise in the background has been greatly reduced but there is still a lot of faint noise left. We will use curves to eliminate this remaining noise.