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Sagittarius A*
1) What is Sagittarius A*?
Sagittarius A* (pronounced “Sagittarius A-star”) is the supermassive black hole at the center of the Milky Way galaxy.
Mass: about 4 million times the mass of the Sun
Distance from Earth: about 26,000 light-years
Location: the constellation Sagittarius (toward the galactic center)
#SMBH
#SuperMassiveBlackHole
#Sagittarius_A_Star
2) Why the name “A*”?
Sagittarius A” refers to a strong radio source in the galactic center.
A*” identifies the most compact and energetic object within that region.
The star (*) indicates something exceptional in astrophysics.
3) Size and Structure
Event horizon radius: about 12 million kilometers
Diameter: about 24 million kilometers
Despite its huge mass, its physical size is relatively small.
#MilkyWayGalaxy
4) Evidence for its existence
Astronomers observed stars orbiting an invisible massive object near the galactic center.
The star S2 orbits Sagittarius A* in about 16 years.
Its orbit proves the presence of an extremely dense, massive object.
This discovery led to the 2020 Nobel Prize in Physics (Reinhard Genzel and Andrea Ghez).
#2020NobelPrizeinPhysics
5) First image (2022)
Captured by the Event Horizon Telescope (EHT).
Shows a bright ring of light surrounding a dark shadow.
Matches predictions of Einstein’s general relativity.
6) Activity level
Sagittarius A* is relatively quiet compared to other galactic black holes.
Occasionally emits X-ray and infrared flares.
Evidence suggests it was more active in the past.
Possibly responsible for the Fermi Bubbles (giant gamma-ray structures above and below the galactic plane).
7) Scientific significance
Sagittarius A* is crucial for:
• Testing general relativity
• Understanding black hole physics
• Studying galaxy evolution
• Exploring high-energy astrophysics
One-sentence summary
Sagittarius A* is the supermassive black hole at the center of the Milky Way, with a mass about four million times that of the Sun.
A more technical explanation (astrophysics level),
A simple version (ELI5),
Or a comparison with M87*.