|Host galaxy||Milky Way|
|Preceded by||SN 1604 (observed), Cassiopeia A (unobserved, c. 1680)|
|Succeeded by||SN 1885A|
Supernova remnant G1.9+0.3 in the constellation Sagittarius is the youngest known supernova remnant (SNR) in the Milky Way Galaxy. The remnant's young age was established by combining data from NASA's Chandra X-ray Observatory and the VLA radio observatory. It was a type Ia supernova believed to have exploded about 25,000 years ago, and the signal began reaching Earth around 1868. The light from the supernova would have been visible to 19th century astronomers, had it not been obscured by the dense gas and dust of the Galactic Center. Prior to this discovery, the youngest-known Milky Way supernova remnant was Cassiopeia A, at about 330 years. The remnant has a radius of over 1.3 light years.
G1.9+0.3 was first identified as a SNR in 1984 from observations made with the VLA radio telescope. Because of its unusually small angular size, it was thought to be young—less than about one thousand years old. In 2007, X-ray observations made with the Chandra X-ray Observatory revealed that the object was about 15% larger than in the earlier VLA observations. Further observations made with the VLA in 2008 verified increase in size, implying it is no more than 150 years old. A more recent estimate put its age at 110 years. 
The coordinates of G1.9+0.3 are right ascension 17 hours 48 minutes 45.4 seconds, declination –27 degrees 10 minutes 06 seconds, which places it in the constellation Sagittarius, near its border with Ophiuchus.
The discovery that G1.9+0.3 had been identified as the youngest known Galactic SNR was announced on May 14, 2008 at a NASA press conference. In the days leading up to the announcement, NASA would only hint that they were going "to announce the discovery of an object in our Galaxy astronomers have been hunting for more than 50 years."
- "G1.9+0.3: Discovery of Most Recent Supernova in Our Galaxy.". NASA. 2008-05-14. Retrieved 2008-05-14.
- Green, D.A.; S.F. Gull (December 1984). "Two new young galactic supernova remnants". Nature 312 (5994): 527–529. Bibcode:1984Natur.312..527G. doi:10.1038/312527a0.
- Reynolds, S. P.; K. J. Borkowski; D. A. Green; U. Hwang; I. Harrus; R. Petre (June 2008). "The Youngest Galactic Supernova Remnant: G1.9+0.3". Astrophysical Journal Letters (American Astronomical Society) 680 (1): L41–L44. arXiv:0803.1487. Bibcode:2008ApJ...680L..41R. doi:10.1086/589570.
- Green, D. A.; S. P. Reynolds; K. J. Borkowski; U. Hwang; I. Harrus; R. Petre (June 2008). "The radio expansion and brightening of the very young supernova remnant G1.9+0.3". Monthly Notices of the Royal Astronomical Society Letters (Monthly Notices of the Royal Astronomical Society) 387 (1): L54–L58. arXiv:0804.2317. Bibcode:2008MNRAS.387L..54G. doi:10.1111/j.1745-3933.2008.00484.x.
- "Sagittarius Constellation charts". The Constellations. International Astronomical Union. 2008. Retrieved 2008-05-20.
- "NASA to Announce Success of Long Galactic Hunt". NASA. Retrieved 2008-05-14.