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image produced by Cherie Miskey

HST/STIS ULTRAVIOLET SPECTROSCOPY OF A "BURST" OF STAR FORMATION IN THE NEARBY GALAXY M33 -This spectral image demonstrates the capability of using the Space Telescope Imaging Spectrograph (STIS) to probe young star-forming regions in nearby galaxies. At left shown in red, one sees the bright nebula, NGC 604, in the nearby galaxy M33 at a distance of 2.4 million light years. The stars of NGC 604 represent one of the most active regions of star formation in the local Universe. The very young, hot, luminous stars embedded and born in the nebula excite and ionize (remove electrons from atoms) the surrounding gas, making it visible. The image at the left was obtained previously with the Wide Field Planetary Camera 2 aboard the Hubble Space Telescope (HST) in the red light of hydrogen (emitted at 6563 Angstroms wavelength). The skinny rectangle, superposed upon the image, corresponds to the aperture of the Space Telescope Imaging Spectrograph  (STIS). At right shown in green, one sees the ultraviolet (UV) spectral image obtained with STIS of the very young, hot, bright stars inside the aperture. 

A spectrograph separates light into its component colors, which correspond to different wavelengths. This is similar to the way a prism separates white light into a rainbow of distinct colors. By analyzing light this way, astronomers learn a great deal about the object emitting the light, such as its temperature, chemical composition, and motion. The UV light used by STIS is actually invisible to the human eye. 

The image shows the UV spectra of approximately 40 hot stars. These stars are very young O, B and Wolf-Rayet stars with ages of only a few million years. These stars are the hottest and intrinsically brightest that can be found in any galaxy, about 100,000 times brighter than the Sun with surface temperatures between 50,000 and 90,000 degrees Fahrenheit. For comparison, the surface temperature of the Sun is about 10,000 degrees Fahrenheit. Atoms that have been energized and lost electrons are called ions and give off light at specific wavelengths. Ions of each element have a specific pattern of emitted light, like an optical "fingerprint". The bright and dark features in the spectra clearly show ions of triply ionized (three electrons removed) carbon and silicon, and four times ionized (four electrons removed) nitrogen. The UV spectra cover the wavelength range from 1170 to 1730 Angstroms. 

The UV spectra of hot O and B stars in nearby galaxies can be used to refine the cosmic distance scale by allowing scientists to determine how intrinsically bright the stars are. Knowing this, they can calculate the distance to the star and the galaxy it is embedded in by measuring how bright the star appears to us on Earth. In addition, the spectra will help astronomers understand how galaxies and the Universe are enriched in heavy elements such as nitrogen and iron. These elements are created inside large stars like these and ejected into space when they explode at the end of their lives - an event called a supernova.

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