Interstellar bubbles are made when stellar winds caused by massive stars or supernovae push the interstellar gas around them outwards in a bubble shape. Stars clustered close enough together form giant bubbles when their bubbles merge. These giant bubbles are known as superbubbles.
Earth's solar system is in the midst of a superbubble called the Local Bubble. Formed in the past 10 to 20 million years by supernovae, it is about 300 million light years across. The Local Bubble is roughly egg-shaped and is adjacent to many other superbubbles in and around the Orion Arm of the galaxy. The Local Bubble is adjacent to the Loop I Bubble, which contains the star Antares, the Loop II Bubble and the Loop III Bubble.
Generally, when interstellar gas around a superbubble cools, it forms a shell around the bubble cavity. These are observed by optic line emission, X-ray emission and infrared continuum emission. When many superbubbles combine, they are known as supershells. Some are so large that they can blow completely outside the galaxy and into the intergalactic medium. Astronomers using the National Science Foundation's Green Bank telescope discovered a massive intergalactic superbubble in 2006. In 2010, NASA announced the discovery of enormous twin bubbles in the center of the Milky Way that may be the result of the eruption of a massive black hole in the galaxy's center.