How to remove the square border from a image button

I have an image button shape like a 'star' and i what the clickable area to be only the 'star' but instead of that i have a transparent square clickable area around the 'star', can anyone please tell me how to remove the square bcoz the next 'star' it's close and the square overlaps the first 'star'. thank you so much

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:orientation="vertical"
    android:layout_weight="1"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:layout_marginRight="190dp"
    android:layout_marginBottom="180dp">

    <ImageButton
        android:id="@+id/star"
        android:layout_width="120dp"
        android:layout_height="120dp"
        android:background="@drawable/star"
        />