In physical cosmology, the Big Bang is the scientific theory that the universe emerged from a tremendously dense and hot state about 13.7 billion years ago. The theory is based on the observations indicating the expansion of space (in accord with the Robertson–Walker model of general relativity) as indicated by the Hubble redshift of distant galaxies taken together with the cosmological principle.

Extrapolated into the past, these observations show that the universe has expanded from a state in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity (for reporting on some of the more notable speculation on this issue, see cosmogony).

The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the universe ( Hubble's law) began — calculated to be 13.7 billion ( 1.37 × 10¹⁰) years ago (±2%) — and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher–Bethe–Gamow theory.

From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB). The CMB was discovered in 1964 and further corroborated the Big Bang theory, giving it an additional advantage over its chief rival, the Steady State theory.

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A black hole is an object predicted by general relativity with a gravitational field so strong that nothing can escape it — not even light.

A black hole is defined to be a region of space-time where escape to the outside universe is impossible. The boundary of this region is a surface called the event horizon. This surface is not a physically tangible one, but merely a figurative concept of an imaginary boundary. Nothing can move from inside the event horizon to the outside, even briefly.

The existence of black holes in the universe is well supported by astronomical observation, particularly from studying X-ray emission from X-ray binaries and active galactic nuclei. It has also been hypothesized that black holes radiate energy due to quantum mechanical effects known as Hawking radiation.

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The Cat's Eye Nebula ( NGC 6543) is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets and sinewy arc-like features.

It was discovered by William Herschel on February 15, 1786, and was the first planetary nebula whose spectrum was investigated, by the English amateur astronomer William Huggins in 1864.

Modern studies reveal several mysteries. The intricacy of the structure may be caused in part by material ejected from a binary central star, but as yet, there is no direct evidence that the central star has a companion. Also, measurements of chemical abundances reveal a large discrepancy between measurements done by two different methods, the cause of which is uncertain.

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Comet Hale–Bopp ( formally designated C/1995 O1) was probably the most widely observed comet of the 20th century, and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.

Hale-Bopp was discovered on 23 July 1995 at a very large distance from the Sun, raising expectations that the comet could become very bright when it passed close to the Sun. Although comet brightnesses are very difficult to predic] with any degree of accuracy, Hale-Bopp met or exceeded most predictions for its brightness when it passed perihelion on April 1, 1997. The comet was dubbed the Great Comet of 1997.

The passage of Hale-Bopp was notable also for inciting a degree of panic about comets not seen for decades. Rumours that the comet was being followed by an alien spacecraft gained remarkable currency, and inspired a mass suicide among followers of the Heaven's Gate cult.

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Space exploration

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  Space Shuttle Columbia launches at Space Shuttle program, by NASA
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  Wernher von Braun and the Saturn V engines at Saturn V, by NASA
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  Buzz Aldrin on the Moon at Apollo 11, by Neil Armstrong
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  Apollo 17 on the launch pad at Apollo 17, by NASA
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  International Space Station, by NASA
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  Space Shuttle Challenger disaster, by NASA
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  Space Shuttle launch plume shadow at Space Shuttle program by NASA (edited by Fir0002)

Astronomy

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  Moon, by Mkfairdpm
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  Persian astrolabe at Astrolabe, by Andrew Dunn
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  Distance between Earth and Moon at the Moon's orbit by Nickshanks
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  Solar eclipse, by Luc Viatour (edited by Diliff)
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  433 Eros, by NEAR Shoemaker spacecraft
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  Io, by Galileo probe
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  Sombrero Galaxy M104, by Hubble Space Telescope
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  Kepler's Supernova (SN 1604), by Spitzer Space Telescope, Hubble Space Telescope and Chandra X-ray Observatory
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  Mars, by Viking 1 orbiter
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  Sunspot, by TRACE space telescope
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  Atmosphere of Jupiter at Jupiter, by Voyager 1 spacecraft
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  Dione and Saturn at Dione, by Cassini orbiter
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  Deep Impact Probe Collision at Deep Impact, by NASA
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  Eagle Nebula, by Hubble Space Telescope
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  Jupiter, by Cassini orbiter
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  Earthrise as seen by Apollo 8, by William Anders
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  Comet Hale–Bopp by Philipp Salzgeber
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  Orion Nebula, by Hubble Space Telescope
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  Crab Nebula, by Hubble Space Telescope
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  Victoria Crater, by Mars Reconnaissance Orbiter

Animations