In the discussions of the Milky Way Project there were several collected items that are subclasses of main classifications. Here I present a list of donut bubbles, inverted bubbles, butterflies (bipolar bubbles) and green balls. Additionally there is a list of bubbles that are reflection nebulae (RN) found by matching bubbles of MWP DR1 (Simpson+, 2012) and the merged catalogue of reflection nebulae (Magakian, 2003), as well as checking new bubbles in the list above with SIMBAD.
A total of 177 objects are collected with the help of volunteer citizen scientists. Most objects are inverted bubbles (108 items). The subclasses can overlap:
Following images are from the Spitzer Space Telescope (RGB-images) and DSS2 (RB-images).
In the Milky Way Project blog a normal bubble is described with three rules:
- Sharp green or yellow-green inner rims (the single most important factor).
- Less bright green emission (in the 8 micron band) within the bubble—but not a corresponding lack of stars inside the bubble (see “Dark Clouds” below).
- Usually: Some evidence of ‘red emission’ (in the 24 micron band) within the bubble. If this emission is lacking, the bubble had better be very sharp (i.e. if you’re going to violate rule 3, make sure rule 1 holds).
For most inverted bubbles all rules are violated, but they still appear in many bubble catalogs. Inverted Bubbles can be described with the rule: They have a bright green inside and a red outside, but usually no sharp rim. Here is a selection of inverted bubbles:
Butterflies (Bipolar Bubbles)
Something is called a butterfly or a bipolar bubble when two bubbles are attached to a yellow ball or a point source:
An inverted bubble is sometimes a butterfly:
Donut bubbles are between inverted bubbles and normal bubbles. Their thick rim is similar to inverted bubbles, while the donut hole satisfy rule 1-3 of normal bubbles. Donut bubbles have two radii for outer and inner rim. The ratio between both is for a number of large bubbles either close to r1=1.8xr2 or close to r1=2.3xr2:
Donut bubbles with a 2.3 ratio:
Donut bubbles with a 1.8 ratio:
Bubbles with Reflection Nebula Nature
Some bubbles are at the same time reflection nebulae:
These bubbles can also be inverted bubbles: (Here: Group close to M8)
Or they can be donut bubbles:
Green balls appear to be green, compared to yellow balls. They show some emission at 24 μm and some might be distant inverted bubbles or green looking yellow balls.
The complete list of subclasses of bubbles and yellow balls can be found here: subclasses