I Zwicky 18

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Observation data (J2000 epoch)
I Zw 18
Hubble Space Telescope (HST) image of galaxy I Zwicky 18. Credit: HST/NASA/ESA.
Constellation	Ursa Major
Right ascension	09^h 34^m 02.0^s^[1]
Declination	+55° 14′ 28″^[1]
Redshift	751 ± 2 km/s^[1]
Distance	0.59 Gly
Type	I^[1]
Apparent dimensions (V)	0′.3 × 0′.2 ^[1]
Apparent magnitude (V)	16.0^[1]
Other designations
UGCA 166,^[1] Mrk 116^[1]
See also: Galaxy, List of galaxies

I Zwicky 18 is a dwarf irregular galaxy located about 59 million light years away.^[2] The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies.^{[citation needed]} Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted with star formation billions of years after its galactic neighbors. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. The Hubble Space Telescope, however, found faint, older stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.

Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of hydrogen and helium, the main ingredients created in the Big Bang. The galaxy's primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA's telescopes, including the Spitzer Space Telescope, the Chandra X-ray Observatory, and the Far Ultraviolet Spectroscopic Explorer (FUSE). However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now.

The Hubble data suggest that I Zwicky 18 is 59 million light-years from Earth. The galaxy's larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy. The faint, old stars in I Zwicky 18 are almost at the limit of Hubble's resolution and sensitivity.

Astronomers at the Space Telescope Science Institute and the European Space Agency in Baltimore, Maryland discerned the distance by observing Cepheid variable stars within I Zwicky 18. These massive flashing stars pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. The team determined the observed brightness of three Cepheids and compared it with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18's deficiency in heavy elements, indicating the galaxy's stars formed before these elements were abundant in the universe. The Cepheid distance also was validated through the observed brightness of the brightest red stars older than one billion years.

I Zwicky 18 is classified as a dwarf irregular galaxy and is much smaller than our Milky Way. I Zwicky 18 may still be creating Population III stars—spectroscopy shows that its stars are composed almost entirely of hydrogen and helium, with heavier elements almost completely absent. The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions are bubbles of gas that have been blown away by stellar winds and supernovae explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a new generation of hot, young stars.