Crab Nebula.
Credit: NASA, ESA and Allison Loll/Jeff Hester (Arizona State University)

The brightest astronomical event in historic times was the supernova of 1054, which produced the Crab Nebula. The supernova was far brighter than Venus. It was bright enough to be visible in daylight and to cast a shadow at night. We know of it through the astronomical records of China, Japan, and the Middle East. (source)

Since the invention of the telescope, no supernovae have been observed within our galaxy. Supernovae were recorded in 1572 and 1604, while Hans Lippershey invented the telescope in 1608 and Galileo was the first to turn his telescope skyward in 1609. (source)

The telescope was invented in 1608 when spectacle-maker Hans Lippershey's apprentice was playing games. The apprentice was amusing himself with lenses and found a combination that made things seem closer. When Lippershey was shown this combination, he enclosed the lenses at two ends of a tube. (source)

The philosopher Immanuel Kant (1724–1804) was the first person to propose that what we now call galaxies lay outside the Milky Way and were indeed galaxies (or "island universes", as Kant called them) in their own right.

As late as 1820, the universe was thought by European scientists to be 6,000 years old. It is now thought to be about 13,700,000,000 years old. (source)

The Earth is rotating on its axis at a rate of 460 metres per second at the equator, and is orbiting the sun at a rate of about 30 kilometres per second. The sun is orbiting the centre of the Milky Way at a rate of about 220 kilometres per second. The Milky Way is moving at a speed of about 1000 kilometres per second towards a region of space 150 million light years away called the Great Attractor. (source)

Globular cluster M22.
Credit: NASA, ESA, and Kailash Sahu (STScI)

It is possible that many planets in the galaxy may not orbit around stars. Recent work by Kailash Sahu found six gravitational lenses in the star cluster M22 from objects smaller than brown dwarfs, the smallest type of star. Only one gravitational lensing event by a star was found in the same work.

The Large Zenith Telescope (LZT), located near Vancouver, has a mirror made out of mercury. The telescope features 28 litres of mercury in a pan which spins, causing the mercury to assume a parabolic shape. The telescope cost only $500,000, about ¹/₁₀₀^th as much as a similarly-sized telescope with a glass mirror would cost. Its main disadvantage is that it can only look straight up—otherwise the poisonous mercury would spill.

A "light year" is a measure of distance, not time. It is defined as the distance light travels in one year. Light moves at a velocity of about 300,000 kilometres each second, so in one year, it travels about 9,500,000,000,000 kilometres. (source)

While astronomers used to believe that galaxies were distributed more or less evenly through space, they have now found regions where galaxies are rare or absent. The largest of these regions is located in the direction of the constellation Bootes, and measures more than 300 million light years across.

In the universe, matter is so thinly dispersed that space is as empty as a building twenty miles long, twenty miles wide, and twenty miles high that contains only a single grain of sand. (source)

The term "Big Bang" started as a putdown. In the 1940's, there were many competing theories about the nature of the universe. British astrophysicist Fred Hoyle coined the term "Big Bang" as a snide putdown of his competitors, only to have the term find its way into the general consciousness as the description of the correct theory. (source)

A massive star has a shorter lifetime than a less massive star. The more massive a star, the more tightly its gravity pulls it together, the hotter it must be to keep it from collapsing, and the more rapidly it uses up its hydrogen fuel. The reason there are so few really massive stars is that they do not live very long, as little as a million years. For comparison, our sun has an expected lifetime of about 11,000 million years.

According to string theory, the universe has not just three or four dimensions, but eleven dimensions, ten of space and one of time. We do not observe the extra spatial dimensions because they are curled tightly around each other.

It is not possible to hear in space. Because there is no atmosphere in space to conduct the sound, it would not carry. So, the object would make a noise, but it would not carry to any receiver, and no one would hear it. (source)

About 25% of the universe consists of "dark matter", and about 70% consists of "dark energy", leaving only about 5% of the universe visible to us. (source)

The word arctic is derived from the ancient Greek word for bear, arktos. The reason is that the constellation of Ursa Major, the Great Bear, lies in the northern sky. (source)

There is a giant supercluster of galaxies in the direction of the constellations Perseus and Pegasus that is over a thousand million light-years long, and is the largest supercluster known. In 1989, astronomers found another structure that they dubbed the "Great Wall". It is a collection of galaxies some 500 million light-years long, 200 million light-years wide, and 15 million light-years thick.

There are an estimated 50 thousand million galaxies in the universe, with the typical galaxy containing 50 thousand million to 100 thousand million stars. It is estimated that there are 10²² stars in total in the universe. (source)

The star Betelgeuse, a bright star in the constellation of Orion, is estimated to have a diameter of around 700 million miles. If it were placed at the centre of our solar system, it would extend beyond the orbit of Jupiter.

Alpha Centauri, one of the stars in the system closest to the sun, is never visible in the sky north of about 30° Northern Latitude. Though it is the third-brightest star in the sky, it was not seen by the ancient Greeks nor the chief observatories of the mediaeval Arabs at Cordoba, Baghdad, and Damascus, all located north of the 30° line. (source)

Information about what has fallen into a black hole is stored on the black hole's event horizon. Recent calculations by those who study quantum gravity theory and superstrings have confirmed what Stephen Hawking and his collaborators proposed a decade or more ago. Evidently, the information contained in matter that falls into a black hole is by some curious means encoded in the pattern of frozen quantum fields at the horizon. This raises some interesting possibilities that we could resurrect clocks, humans, spacecraft, and whole planets into something like their pristine form if we could magically reverse the in-fall and collapse process. Many believe that this mathematical result means that we have reached a watershed moment in history in understanding the connection between quantum mechanics and gravitation theory. Quantum mechanics deals with statements about the information that we can extract about a quantum mechanical process involving observation. Now this same information can be applied to configurations of the gravitational field and space-time itself. (source)

For black holes, distant observers will see only the outside of the event horizon, while individual observers falling into the black hole will experience quite another "reality." General relativity pre­dicts that for distant observers outside the horizon, they will experience the three space-like coordinates and one time-like coordinate, as they always have. For someone falling into a black hole and crossing the horizon, this crossing is mathematically pre­dicted to involve the transformation of your single time-like coordinate into a space-like coordinate, and your three space-like coordinates into three time-like coordinates. Along any of these three former space-like coordinates, they now all terminate on the singularity; you're experiencing them as time-like now. All choices always terminate on the singularity—at least in the case of a non-rotating black hole. The coordinate which used to measure external time now has a space-like character which affords you some wiggle room, but dynamically, in terms of these new reversed space and time coordinates, you find that no stable orbits about the singularity are possible no matter what you try to do. Without any stable orbits, and the inexorable freefall into the singularity, relativists often refer to this as the collapse of space-time geometry. (source)

There is sound in space. Sound is a pressure wave, and as long as there is some kind of gaseous medium, there is the possibility of forming pressure waves in it. In space, the interplanetary medium is a very dilute gas at a density of about 10 atoms per cubic centimeter, and the speed of sound in this medium is about 300 kilometers per second. Typical disturbances due to solar storms and "magneto-sonic turbulence" at the Earth's magnetopause have scales of hundreds of kilometers, so the acoustic wavelengths are enormous. Human ears would never hear them, but we can technologically detect these pressure changes and play them back for our ears to hear by electronically compressing them. (source)

Abell 2218. Two lensed images of the same galaxy, the most distant one observed, are circled.
Credit: European Space Agency, NASA, J.-P. Kneib (Observatoire Midi-Pyrénées) and R. Ellis (Caltech)

The most distant galaxy ever observed is estimated to be around 13,000,000,000 light-years away. Discovered by the Hubble Space Telescope in 2004, it is located behind the galactic cluster Abell 2218, which bends the object's light. It is a small, energetic galaxy whose light that is seen on Earth now would have set out when the universe was just 750 million years old. This is the most distant object that can be observed consistently; some ephemeral gamma-ray bursts have been observed that are slightly more distant than this object. (source)

In 1974, Stephen Hawking showed that black holes evaporate. According to quantum mechanics, pairs of virtual particles are constantly being created and then annihilating each other near black holes (as well as everywhere else in the universe). On occasion, one of the pair of particles ends up inside the black hole's event horizon, and so cannot annihilate its pair, which is forced to become a real particle. This results in a slight increase in the total mass-energy of the outside universe, and that mass-energy has to come from the black hole, whose mass-energy is slightly decreased. Eventually (it would take a very, very long time for normal-sized black holes) the black hole would disappear in an explosion of particles and energy. (source)

Most of the elements found in the human body were created in a supernova; we are literally made of stardust. (source)

The Milky Way has a radius of about 50,000 light years. (source)

The oldest galaxies we can see are right next door to us in the present universe. Looking out into space, we see images of galaxies that become younger and younger the further out we look. We actually see images of the youngest galaxies in the universe the farther out we look. (source)

If space is infinite, there is nothing on the other side. If space is finite because it has been bent around upon itself because of gravity, then again there is nothing on the other side of it because there is no seam. It looks like the surface of a smooth ball which represents a piece of flat paper bent upon itself. (source)