Segue 2: Most Lightweight Galaxy in Universe

According to the findings of a study published online in the Astrophysical Journal, a dwarf galaxy named Segue 2 is the least massive galaxy in the known Universe.

The galaxy Segue 2: boxes are ten member stars identified so far by University of California Irvine astronomers and their colleagues (Sci-News.com / Centre de Données astronomiques de Strasbourg / SIMBAD)

The galaxy Segue 2: boxes are ten member stars identified so far by University of California Irvine astronomers and their colleagues (Sci-News.com / Centre de Données astronomiques de Strasbourg / SIMBAD)

Segue 2 is located in the constellation of Aries about 114,000 light-years away. It was discovered in 2009 as part of the massive Sloan Digital Sky Survey.

The galaxy consists of just 1,000 or so stars with a bit of dark matter holding them together. It is also one of the faintest known galaxies with light output just 900 times that of the Sun. That’s miniscule compared to the Milky Way, which shines 20 billion times brighter.

“Finding a galaxy as tiny as Segue 2 is like discovering an elephant smaller than a mouse,” said co-author Dr James Bullock from the University of California Irvine.

“Astronomers have been searching for years for this type of dwarf galaxy, long predicted to be swarming around the Milky Way. Their inability to find any has been a major puzzle, suggesting that perhaps our theoretical understanding of structure formation in the universe was flawed in a serious way.”

“Segue 2’s presence as a satellite of our home galaxy could be a tip-of-the-iceberg observation, with perhaps thousands more very low-mass systems orbiting just beyond our ability to detect them.”

Segue 2 is located in the constellation of Aries (Roger Sinnott / Rick Fienberg / IAU / Sky & Telescope magazine / Sci-News.com)

Segue 2 is located in the constellation of Aries (Roger Sinnott / Rick Fienberg / IAU / Sky & Telescope magazine / Sci-News.com)

The team determined the upper weight range of 25 of the major stars in the galaxy and found that it weighs at least 10 times less than previously estimated.

Lead author Dr Evan Kirby of the University of California Irvine said: “it’s definitely a galaxy, not a star cluster. The stars are held together by a globule called a dark matter halo. Without this acting as galactic glue, the star body wouldn’t qualify as a galaxy.”

The study also offers tantalizing clues about how iron, carbon and other elements key to human life originally formed.

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Bibliographic information: Evan N. Kirby et al. 2013. Segue 2: The Least Massive Galaxy. ApJ 770, 16; doi: 10.1088/0004-637X/770/1/16