Sagittarius A: Cosmic Monster Lurking at the Center of Our Galaxy

At the Heart of the Milky Way's Supermassive Black Hole

In the infrared image, a number of Emission Nebulae (nebulae that emit their own light) are visible thanks to the massive young stars illuminating them from within.

Mysterious Object: Gas and Dust Cloud?

The Milky Way, like almost all galaxies, harbors a black hole at its center. This black hole, with a mass of several million suns, is named Sagittarius A (Sgr A). Sagittarius A is located 27,000 light-years away from us.

Astronomers from the Max Planck Institute in Germany used the Very Large Telescope (European Southern Observatory in Chile) to observe the center of the Milky Way in infrared. The astronomers detected an object at a wavelength of 3.76 microns, indicating that it is a gas and dust cloud, rather than a star. A star would have been detected at a shorter wavelength of around 2.16 microns.

The cloud is about three times more massive than Earth, emits five times more energy than the Sun, and extends over 250 times the Earth-Sun distance. Its temperature is approximately 550 Kelvin. The immense gravity of Sgr A significantly accelerates the cloud. In 2004, the cloud was rushing toward the black hole at 1200 km/s; by 2011, its speed had reached 2350 km/s. In the summer of 2013, the cloud will plunge toward Sagittarius A when it is at 260 AU.

Scientists will be able to observe this event for more than a decade and see the cloud stretch and heat up to temperatures of millions of degrees. Astronomers hope that once in the black hole, the cloud will emit X-rays.

Mysterious Object: Star?

Andrea Ghez (an astronomer at the University of California) saw the same object in her data and has a much more likely interpretation. It would be a star, noting that the dust surrounding a star can absorb visible light and re-emit it in the infrared. If the object is a star, it will not fall into the black hole but will pass around Sagittarius A every 140 years.

What is a Black Hole?

A black hole is a massive object whose gravitational field is so intense that it prevents any form of matter or radiation from escaping. They are described by the theory of general relativity. When the core of a dead star is too massive to become a neutron star, it contracts inexorably until it forms this mysterious astronomical object, the black hole.

Stellar black holes have a mass ranging from a few solar masses to billions of solar masses. They are born from the gravitational collapse of the remnants of massive stars. A man predicted the existence of dark stars as early as the 18th century. The British physicist, astronomer, and geologist John Michell (1724-1793). In his notes, he writes that when a star becomes too massive, it attracts light under the influence of its gravitational force. However, as his calculations give a star density corresponding to 18 billion tons in a cm3, he concludes that this cannot exist.

Today, the theory of black holes states that they are objects so dense that their escape velocity exceeds the speed of light. Light, therefore, cannot overcome the surface gravitational force and remains trapped. The theory also precisely defines the intensity of a black hole's gravitational field. It is such that no particle crossing its horizon (theoretical boundary) can escape.