All things are possible: one model of the universe we know sees it, and so us, as being contained within the event horizon of an extremely massive black hole.
Stars are separated by large distances, and the gaps beyond and between are known as interstellar space. The stars have almost no effect on interstellar space.
Around each star, there is a boundary where the stellar wind slows down and eventually stops as it comes in contact with material in interstellar space.
The edge of the Sun's sphere of influence, the heliosphere, lies three times as far away as Pluto. Voyager I will take 40 years to get to the heliosphere.
Around 150 BC, Hipparchus estimated that the Moon is 30 Earth diameters away from our planet, by taking sightings of the Moon at zenith from two places.
Around 1450, Nicolas Cusanus asserted that the world was round, and moved, but nobody reacted to this as they would later react to Giordano Bruno and Galileo.
In 1543, Nicolaus Copernicus suggested a heliocentric model of the solar system, but there was no great reaction, possibly because he died as the book appeared.
Bruno of Nola, Giordano Bruno, was burned at the stake in 1600 for, among other things, saying that the Earth went around the Sun, not the Sun around the Earth.
In 1671 Giovanni Cassini completed an accurate measurement of distance to Mars and used that, combined with a known scale of the solar system to measure it all.
In 1796 Pierre-Simon de Laplace stated his nebular hypothesis, that the solar system was formed from a nebula of gas and dust as it became organized.
In 1992 the Catholic Church formally acknowledged its error over Galileo Galilei, after his book was taken off the Vatican's banned list in 1835.
Cosmology is the study of how and why the universe is as it is. These studies rely on inference based on the theories of physics, and careful observation.
Understanding how the universe developed requires careful measurement and thought, but without the right observations and measures, can still be in error.
The universe started with the Big Bang, a point in time when all of the material we know about was in a very small volume, after which the material spread out.
The Big Bang happened about 15 billion years ago, and while most of the interesting changes happened fast, forming galaxies took several billion years.
Once upon a time, there was no time. That was when the cosmic egg, the super-atom, or the Big Bang happened. Then once that had happened, there was time.
Three minutes after the Big Bang, the temperature had dropped to 1 trillion degrees, and protons and neutrons were able to begin forming nuclei of atoms.
Since the Big Bang, the universe has been expanding outwards. So far, there is no evidence to suggest that it will slow down and collapse in a Big Crunch.
A few hundred thousand years after the Big Bang, the universe had cooled enough so electrons could link to nuclei and form atoms of hydrogen and helium
Stars can change over time, and as they develop through standard sequences, types of star can be recognized, and their future development can be predicted.
In 1910, Ejnar Hertzsprung and Henry Norris Russell studied the link between magnitudes and spectral types of stars, leading to the Hertzsprung-Russell diagram.
Annie Jump Cannon sorted Edward Pickering's arbitrary star classes to an order that made sense: she put the spectral classes in the order O, B, A, F, G, K, M.
Blue-white, helium-rich stars like Rigel, are known by the letter B, while G is used for the Sun and other yellow stars, and M for red stars like Betelgeuse.
We now know that a G star like the sun is at the cool end of the range, hotter only than K and M stars, while all of the rest are bluer and hotter.
A class A star is a white star like Sirius or Vega, in whose spectra we see a very strong series of dark lines caused by hydrogen in its atmosphere.
Between A and G are the F stars; between G and M, the K stars. The letters B, A, F, G, K, M, stand for six divisions, including a great majority of the stars.
Stars beyond a certain size must inevitably collapse to form black holes, dense objects which exert enough gravitational force to stop even light escaping.
Space can be thought of as curved, the curvature being caused by the influence of gravity on space. This helps explain many otherwise puzzling observations.
The heavier elements in the universe have been formed as the result of fusion reactions in early stars, and later stellar explosions forcing nuclei together.
Today, the universe is thought to be made up of about 74 percent hydrogen and 25 percent helium, the other elements amounting to only about 1 percent in total.
Edwin Hubble made his first measurement of the Hubble constant in 1929, relating redshift to distance, leading to the conclusion that the Universe is expanding.
Stars can orbit each other in binary systems, orbiting about the joint centre of gravity, which lies between, at a place determined by their respective masses.