In astrophysics and physical cosmology, Olbers’ paradox, named after the German astronomer Heinrich Wilhelm Olbers and also called the “dark night sky paradox”, is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. The darkness of the night sky is one of the pieces of evidence for a non-static universe such as the Big Bang model. If the universe is static, homogeneous at a large scale, and populated by an infinite number of stars, any sight line from Earth must end at the (very bright) surface of a star, so the night sky should be completely bright. This contradicts the observed darkness of the night.
The paradox is that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark. To show this, we divide the universe into a series of concentric shells, 1 light year thick. Thus, a certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell between 2,000,000,000 to 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear four times dimmer than the first shell. The total light received from the second shell is the same as the total light received from the first shell, so each shell of a given thickness will produce the same net amount of light regardless of how far away it is. That is, the light of each shell adds to the total amount. Thus the more shells, the more light. And with infinitely many shells there would be a bright night sky. Dark clouds could obstruct the light. But in that case the clouds would heat up, until they were as hot as stars, and then radiate the same amount of light. Kepler saw this as an argument for a finite observable universe, or at least for a finite number of stars. In general relativity theory, it is still possible for the paradox to hold in a finite universe: though the sky would not be infinitely bright, every point in the sky would still be like the surface of a star.
Ridges of glowing interstellar gas and dark dust clouds inhabit the turbulent, cosmic depths of the Lagoon Nebula. Also known as M8, the bright star forming region is about 5,000 light-years distant. But it still makes for a popular stop on telescopic tours of the constellation Sagittarius, toward the center of our Milky Way Galaxy. Dominated by the telltale red emission of ionized hydrogen atoms recombining with stripped electrons, this stunning, deep view of the Lagoon’s central reaches is about 40 light-years across. Near the center of the frame, the bright hourglass shape is gas ionized and sculpted by energetic radiation and extreme stellar winds from a massive young star.
“Astronomer Vera Rubin’s pioneering work with the idea of spinning galaxies and dark matter deeply impacted the scientific community at a time when she was rejected from Princeton on the grounds that she was a woman. Rubin’s dedication and devotion to science extends to mentoring women in the field and being “available twenty-four hours a day to women astronomers.” (Photo via American Institute of Physics)
Circinus X-1 is a freak of the Milky Way. Located in the plane of the galaxy, Circinus X-1 is the glowing husk of a binary star system that exploded a mere 2,500 years ago. The system consists of a nebula and a neutron star, the incredibly dense collapsed core of the exploded star, still in the orbital embrace of its companion star.
The rings are light echoes from Circinus X-1’s X-ray burst. Each of the four rings indicates a dense cloud of dust between us and the supernova remnant. When X-rays encounter grains of dust in interstellar space they can be deflected, and if the dust clouds are dense they can scatter a noticeable fraction of the X-rays away from their original trajectory, putting them on a triangular path.