Space - Our Universe
The Totality of Everything that Exists

Space - Our Universe
The Totality of Everything that Exists

Outer space is the closest natural approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along their ideal orbits.

However, even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter.

Outer space is the void that exists beyond any celestial body, including the Earth.

It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos.

Theoretically, it also contains dark matter and dark energy.

Outer space is the closest natural approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along their ideal orbits.

However, even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter.

In recorded history, various cosmologies have been proposed to account for what people saw in the sky.

Most early models thought the Earth was the centre of the Universe. Some ancient Greeks thought that the Universe has infinite space and has existed forever.

They thought it had a set of spheres which corresponded to the fixed stars, the Sun and various planets.

The spheres circled about a spherical but unmoving Earth. Over the centuries, more precise observations and better ideas of gravity led to Copernicus's Sun-centred model.

This was hugely controversial at the time, and was fought long and hard by authorities of the Christian church (see Giordano Bruno and Galileo). The invention of the telescope in the Netherlands, 1608, was a milestone in astronomy.

By the mid-19th century they were good enough for other galaxies to be distinguished. The modern optical (uses visible light) telescope is still more advanced. Meanwhile, the Newtonian dynamics (equations) showed how the Solar System worked.

The improvement of telescopes led astronomers to realise that the Solar System is in a galaxy made of millions of stars, the Milky Way, and that other galaxies exist outside it, as far as machines can reach.

Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the red shift showed that the Universe is expanding.


The nearest star to our Solar System, and the second nearest star to Earth after the Sun, is Proxima Centauri. It is 39.9 trillion kilometres away.

This is 4.2 light years away, meaning that light from Proxima Centauri takes 4.2 years to reach Earth.Astronomers think there are a very large number of stars in the Universe.

They estimate (guess) that there are at least 70 sextillion stars.

That is 70,000,000,000,000,000,000,000, which is about 230 billion times the number of stars in the Milky Way (our galaxy). Most stars are very old. They are usually thought to be between 1 and 10 billion years old.

The oldest stars are thought to be around 13.7 billion years old. Scientists think that is close to the age of the Universe.Stars vary greatly in size. The smallest neutron stars (which are actually dead stars) are no bigger than a city. The neutron star is incredibly dense. If you were to take a layer a micron thick and apply it onto a tank, it would be a very tough armor. The tank would be so heavy, it would sink into the center of the Earth.

Supergiant stars are the largest objects in the Universe. They have a diameter about 1,500 times bigger than the Sun. If you changed the sun into one of these supergiant stars down where the sun is, its outer surface would reach between the orbits of Jupiter and Saturn and the earth would be inside the star.

Asteroid Impacts

Small objects frequently collide with the Earth. There is an inverse relationship between the size of the object and the frequency that such objects hit the earth.

The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater's diameter, which is on average proportional to the diameter of the impactor.

Asteroids with a 1 km (0.62 miles) diameter strike the Earth every 500,000 years on average. Large collisions – with 5 km (3 miles) objects – happen approximately once every ten million years. The last known impact of an object of 10 km (6 miles) or more in diameter was at the Cretaceous–Tertiary extinction event 65 million years ago.

Asteroids with diameters of 5 to 10 m (16 to 33 ft) enter the Earth's atmosphere approximately once per year, with as much energy as Little Boy, the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized.

Objects with diameters over 50 m (164 ft) strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908.

At least one known asteroid with a diameter of over 1 km (0.62 miles), (29075) 1950 DA, has a possibility of colliding with Earth on March 16, 2880, but the Torino scale only works for impact possibilities within 100 years, and thus cannot apply to this asteroid.

Objects with diameters smaller than 10 m (33 ft) are called meteoroids (or meteorites if they strike the ground). An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these are typically recovered and made known to scientists.

Black Holes

According to the general theory of relativity, a black hole is a region of space from which nothing, including light, can escape.

It is the result of the denting of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon.

It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics black holes possess a temperature and emit Hawking radiation through slow dissipation by anti-protons.

Despite its undetectable interior, a black hole can be observed through its interaction with matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space.

Alternatively, when gas falls into a stellar black hole from a companion star or nebula, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.

Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of every galaxy. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.

Are We Alone?

Alien life, such as bacteria, has been theorized to exist in the Solar System and quite possibly throughout the Universe. This theory relies on the vast size and consistent physical laws of the observable Universe.

According to this argument, supported by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.

This argument is embodied in the Copernican principle, which states that the Earth does not occupy a favored position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth. Life may have emerged independently at many places throughout the Universe.

Alternatively life may form less frequently, then spread between habitable planets through panspermia or exogenesis. Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars, Jupiter's moon Europa, and Saturn's moons Titan and Enceladus.

In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it". Life may appear on extrasolar planets, such as Gliese 581 c, g and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.

No samples of extraterrestrial life have been found. However, various controversial claims have been made for evidence of extraterrestrial life. Beliefs that some unidentified flying objects are of extraterrestrial origin, along with claims of alien abduction, are dismissed by most scientists.

Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.

New Worlds

An extrasolar planet, is a planet outside the Solar System. As of August 10, 2011, 573 extra-solar planets have been identified. The first published and confirmed discovery was made in 1988.

It was finally confirmed in 1992. In early 1992, radio astronomers announced the discovery of planets around another pulsar.

These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery.

Technological advances, most notably in high-resolution spectroscopy, led to the detection of many new exoplanets at a rapid rate. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Additional extrasolar planets were eventually detected by observing the variation in a star's apparent luminosity as an orbiting planet passed in front of it.

Boldly Go

The realistic proposal of space travel goes back to Konstantin Tsiolkovsky. His most famous work, "The Exploration of Cosmic Space by Means of Reaction Devices", was published in 1903, but this theoretical work was not widely influential outside of Russia.

Spaceflight became an engineering possibility with the work of Robert H. Goddard's publication in 1919 of his paper 'A Method of Reaching Extreme Altitudes'; where his application of the de Laval nozzle to liquid fuel rockets gave sufficient power that interplanetary travel became possible.

He also proved in the laboratory that rockets would work in the vacuum of space; not all scientists of that day believed they would. This paper was highly influential on Hermann Oberth and Wernher Von Braun, later key players in spaceflight.

The first rocket to reach space, an altitude of 100 km, was the German V-2 Rocket, on a test flight in June, 1944. On the 4th of October, 1957, the Soviet Union launched Sputnik 1, which became the first artificial satellite to orbit the Earth. The first human spaceflight was Vostok 1 on April 12, 1961, aboard which Soviet cosmonaut Yuri Gagarin made one orbit around the Earth.

The lead architects behind the Soviet space program's Vostok 1 mission were the rocket scientists Sergey Korolyov and Kerim Kerimov. Rockets remain the only currently practical means of reaching space. Other non-rocket spacelaunch technologies such as scramjets still fall far short of orbital speed.