NGC 604, an emission nebula in Messier 33

NGC 604, an emission nebula in Messier 33

Image Credit: H. Yang (UIUC), J. Hester (ASU), HST/NASA/ESA

NGC 604 is an emission nebula, a star-forming (H II) region of nearly 1,500 light-years across, located in a spiral arm of the Triangulum Galaxy (Messier 33), about 2.7 million light-years away from Earth in the northern constellation of Triangulum (the Triangle). It is approaching us at approximately 226 kilometers per second.

NGC 604 is one of the largest known stellar nurseries in the Local Group of galaxies. This expansive cloud of interstellar gas and dust is over 40 times the size of the visible portion of the Orion Nebula, and more than 6,300 times as luminous as the Orion Nebula. If it were at the same distance it would outshine Venus.

The gas of the nebula – around nine tenths of it hydrogen – is ionized by a young, loose cluster of more than 200 massive blue stars, much more massive than our Sun (15 to 60 solar masses). These bright stars are extremely young by astronomical standards, having formed a mere 3.5 million years ago.

The star cluster is located within a cavity near the center of the nebula. Stellar winds from these hot blue stars, along with supernova explosions, are responsible for carving out this hole at the center. Ultraviolet radiation floods out from these hot stars, making the surrounding nebular gas fluoresce, while their light also highlights the nebula’s three-dimensional shape.

The fierce ultraviolet radiation released by the stars is also the cause of their eventual disappearance. The radiation and winds blowing from the surface of these stars gradually erode the cloud they formed from, causing the gases to slowly disperse. The complex structure of NGC 604, with irregular bubbles and wispy filament-like structures alongside denser areas is due to the same forces that will eventually make the cloud disappear. The cavities show areas of stronger erosion of the cloud.

However, a massive wall of gas divides the east and west sides of NGC 604, and there is a difference between the two sides. The western (right) side is entirely powered by winds from the 200 hot massive stars, while the bubbles on the eastern (left) side appear to be much older and were likely created and powered by young stars and supernovas in the past. This implies that this massive wall of gas shields the relatively quiet region in the east from the active star formation in the west.

The image was taken on January 17, 1995 with Hubble’s Wide Field and Planetary Camera 2. Separate exposures were taken in different colors of light to study the physical properties of the hot gas (10,000 degrees Kelvin).

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