Hubble catches a cosmic 'bubble'

The latest image of the Bubble Nebula

January 18, 2000
Web posted at: 9:49 AM EST (1449 GMT)

(CNN) -- NASA's Hubble Space Telescope has taken pictures of the Bubble Nebula with unprecedented clarity, helping astronomers understand for the first time the geometry and dynamics of the enigmatic system, the Space Telescope Science Institute said in a statement.

The remarkably spherical "bubble" marks the boundary between an intense wind of particles from a massive central star and the more quiescent interior of the nebula, astronomers said.

The star of the nebula has 40 times more mass than the sun and is responsible for a stellar wind moving at 4 million mph (7 million km/h), which propels particles off the surface of the star.

The bubble surface actually marks the leading edge of this wind's gust front, which is slowing as it plows into the denser surrounding material. The surface of the bubble is not uniform because as the shell expands outward it encounters regions of the cold gas, which with varying densities slow the expansion by differing amounts, resulting in the rippled appearance.

It is this gradient of background material that the wind is encountering that places the central star off center in the bubble. There is more material to the northeast of the nebula than to the southwest, so that the wind advances less in that direction, offsetting the central star from the geometric center of the bubble.

The Bubble Nebula is located 7,100 light-years from Earth in the constellation Cassiopeia. It has a diameter of 6 light-years. Scientists refer to the nebula as NGC 7635.

The region between the star and ridge reveals several loops and arcs that have never been seen before. The high-resolution capabilities of Hubble make it possible to examine these features in detail in a way that is not possible from the ground.

A lower ridge appears to lie within the sphere of the bubble but is not actually inside the gaseous region. Instead, it is being pushed up against the bubble like a hand being pushed against the outside of a party balloon, according to Hubble astronomers.

As the bubble moves up but not through the ridge, bright blue arcs form where the supersonic wind strikes the ridge.

Scientists say the origin of this "bubble-within-a-bubble" remains unknown. It may be due to a collision of two distinct winds. The stellar wind may be colliding with material streaming off the ridge.