The event which is believed to mark the origin of time and space. Consequences of the Big Bang include the fact that space is expanding, that the temperature of the Universe is falling and that elements or isotopes such as helium, lithium and deuterium have certain abundances in the Universe.
An hypothesis that, if there were enough matter in the Universe, the expansion of the Universe would eventually become a contraction, leading to a big crunch. This hypothesis is no longer believed to be valid.
A region of the Universe in which gravity is so strong that not even light can escape.
The shift in the spectrum of a source of electromagnetic radiation that’s approaching the observer. Equal to the shift in wavelength of a particular feature in the spectrum divided by the original (or rest) wavelength of the same feature. A blueshift corresponds to a shortening of the emitted wavelength.
Brown dwarfs are cool, faint objects, with a temperature too low for hydrogen burning to commence. They are inherently difficult to observe, but a number have been discovered.
A structural component of the Milky Way and other spiral galaxies, consisting of a thick, dense concentration of stars around the centre of the disc. Bulges are a common feature of spiral and lenticular galaxies. In the case of the Milky Way (and many other galaxies), the bulge is elongated to form a bar.