Cosmic timeline 16
Formation and evolution of the Solar System
The Solar System formed from the gravitational collapse of a giant molecular cloud 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen. Within a few million years the light from bright stars will have boiled away this molecular cloud of gas and dust that has broken off from the Carina Nebula. This image spans about two light-years and was taken by the orbiting Hubble Space Telescope in 1999.
The formation and evolution of the Solar System began 4.55 to 4.56 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.
Hubble image of protoplanetary discs in the Orion nebula, a light-years-wide “stellar nursery” likely very similar to the primordial nebula from which our Sun formed. The earlier theory was that the Sun formed in relative isolation, but studies of ancient meteorites reveal traces of short-lived isotopes, such as iron-60, that only form in exploding, short-lived stars. This indicates that one or more supernovae occurred near the Sun while it was forming.
SN 1054 (Crab Supernova) was a supernova, or stellar explosion, that was widely seen on Earth in the year 1054. It was recorded by Chinese, Japanese, Native Americans, and Persian/Arab astronomers as being bright enough to see in daylight for 23 days and was visible in the night sky for 653 days.
A shock wave from a supernova may have triggered the formation of the Sun by creating regions of over-density within the cloud, causing these regions to collapse. Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region that produced massive stars, possibly similar to the Orion nebula.
When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. The initial collapse takes about 100,000 years. After that time the star reaches a surface temperature similar to that of a main sequence star of the same mass and becomes visible.
A main sequence star is one that falls within the continuous and distinctive band of stars that appear on plots of stellar colour versus brightness. These color-magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell.
In Potsdam in 1906, the Danish astronomer Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. These stars are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them “giant” and “dwarf” stars. The following year he began studying star clusters; large groupings of stars that are co-located at approximately the same distance. He published the first plots of color versus luminosity for these stars. These plots showed a prominent and continuous sequence of stars, which he named the main sequence.
At Princeton University, Henry Norris Russell was following a similar course of research. In 1933, Bengt Strömgren introduced the term Hertzsprung-Russell diagram to denote a luminosity-spectral class diagram. This name reflected the parallel development of this technique by both Hertzsprung and Russell earlier in the century
The Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the sun fuses 430–600 million tons of hydrogen each second. Once regarded by astronomers as a small and relatively insignificant star, the Sun is now presumed to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs.
Theoretical models of the Sun’s development suggest that 3.8 to 2.5 billion years ago the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth’s surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today. The consensus among scientists is that the young Earth’s atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide, methane and/or ammonia) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching the planet.
Over about 100,000 years, the competing forces of gravity, gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning protoplanetary disc. At this point in its evolution, the Sun is believed to have been a T Tauri star, a class of variable stars named after their prototype – T Tauri.
This is an artists impression of a protoplanetary disk (or proplyd) that is a rotating disk of dense gas surrounding a young newly formed star.
The various planets of our solar system are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun’s formation. The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the Sun as the central protostar. Through direct contact, these grains formed into clumps up to 200 metres in diameter, which in turn collided to form larger bodies of 10 kilometres (km) or so in size. These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years.