The Cloud
Stars all come from interstellar gas and dust: a perfect concoction of different elements and molecules made as a result of the death of a star. This is made in events such as planetary nebulae: clouds of dust and gas spewing in a circular pattern away from a white eye in the middle (the white dwarf) as a result of the outer layers of a red giant ripping apart because it has become highly unstable in its old age [1].
Events like these, along with other events such as supernovae (explosions at the end of lives of massive stars, greater than 10 times the mass of our Sun) then led to all the dust and gas that’s floating about in the Universe today, visible as clouds which either emit or absorb light, producing characteristic dark holes and glowing regions in the sky [2].
Evidence of this lies in the star that results from these clouds, emitting specific wavelengths of light, revealing what elements lie within: elements only obtainable due to what goes on at the end of a star’s life, when it run out of hydrogen and helium fuel to fuse to carry on churning out the immense amounts of energy it emit throughout its lives [3][15].
Collapse
However, to get the cloud into the concentrated form needed for a star, the clouds then need to coalesce. The process starts by a trigger, leading to the cloud beginning to fragment.
This trigger can be many things, including clouds colliding into each other (a possible explanation for spontaneous star formation within clouds found in Serpens: a constellation containing the Pillars of Creation, among others); or even the shockwaves from a supernova explosion. Indeed, this particular theory is thought to be a possible explanation for the Solar System’s birth, since short lived isotopes formed by Supernovae have been found to be trapped within meteorites formed at the start of the Solar System (also called ‘primitive meteorites’).
Then, as the gas and dust in the cloud collapses, it heats up, radiating this heat into the surroundings to carry on collapsing and increasing the gravitational pull and pressure within the now dense gaseous area. However, as this area gets hotter and hotter because of the ever increasing pressure, the gas and dust stops collapsing since the immense thermal energy pushing out wins over the weaker gravitational energy pushing in: hydrostatic equilibrium [4][5][6][7][8][15].
The Pillars of Creation, with high levels of star formation visible as the glowing tips of the pillars
Credit: ESO, CC BY 4.0
Protostar
A star is then born, glinting out of the darkness (created due to the high density of surrounding material) like an eye lifting its lid on the world. However, the story is not over yet! This new star keeps on shrinking and shrinking like Alice in Wonderland, until its core reaches the key temperature of 10 million Kelvin (around 10 million ºC). However, whilst it’s shrinking, unlike Alice, it also has a spin [4].
And as the radius of the cloud shrinks, it begins to spin faster and faster due to the centripetal force (like when you feel a stronger pull the closer you are to the middle of the roundabout when it’s spinning), leading to a circumstellar accretion disc around the equator (a disc made up of the gas and dust from the cloud it came from), whilst the remaining matter is sucked into the star to be ejected at a later date in jets created by a newly formed magnetic field (like the relativistic jets of black holes, only much weaker) [7][11].
Looking at the temperature now, it may seem a random number but it’s the temperature that coulombic repulsion (the force which repels two particles of the same charge) between hydrogen ions (protons) becomes less than the attraction between the ions, leading to hydrogen fusion to make helium and energy: the way most stars, such as our Sun, make energy. This temperature marks the second time where a star reaches hydrostatic equilibrium [9][10][4][15].
Jets streaming from a protostar HH 212 (Herbig-Haro object)
Credit: ESO/M. McCaughrean, CC BY 4.0
Stellar Adolescence of Low-Mass Protostars
If a protostar is 3 times the mass of our Sun or less and is still collapsing (since low-mass stars collapse much slower than high-mass stars), it’s known as a T Tauri star (TT for short). They eject material at up to 500,000 kilometres per hour, ejecting a mass equal to the Sun in its 10 million year lifetime.
And this evolution through the emission of matter marks the transition from a baby protostar into a fully fledged star: a timescale also known as the Kelvin-Helmholz timescale. It is characterised by how the initial source of energy during this time is not nuclear fusion (since the high temperature seen in high-mass protostars hasn’t been achieved yet) but gravitational binding energy: the energy it takes to bind everything together into a body.
The jets discussed earlier are also still present, leading to the star emitting variable energy, meaning that the star pulsates: a variable star (where the star visibly changes in brightness) [12][13][14][15].
Hubble images of binary system XZ Tauri, at a scale much larger than the Solar System (the
two stars seen as circles in the bottom left are at a similar distance to Pluto and the Sun
Credit: John Krist (STScI), Karl Stapelfeldt (NASA Jet Propulsion Laboratory), Jeff Hester (Arizona State University), Chris Burrows (ESA/STScI)
Smaller Still
Protostars can only become real stars if they have enough material though. Anything less than that and it’s a non-starter; a brown dwarf.
At masses less than two twenty-fifths of the Sun’s mass (0.08 solar masses), the internal temperature of the star can never reach the heights of 10 million Kelvin due to the matter’s low gravitational attraction. This leads to an absence of nuclear fusion, though not the absence of any emission, with infra-red radiation (light at the low energy, redder, part of the spectrum) still being emitted.
However, brown dwarfs are nothing to be laughed at, with enough energy to emit infra-red radiation for more than 15 million years (part of the Kelvin-Helmholz timescale), and they are a similar size to that of Jupiter (but much hotter). Plus, due to their low emission of radiation, and therefore their hidden nature in the night sky (being much harder to find than their bright stellar counterparts) they could be part of the missing mass that we’re looking for in the Universe, labeled mostly as ‘dark matter’, but could in fact partly consist of brown dwarfs [15][16].
Images of the Gliese 229 system, consisting of Gliese 229A (the big one) and Gliese 229B (the much smaller one, which is a brown dwarf, 20-50 times Jupiter's mass).
Credit: T. Nakajima/S. Kulkarni (CalTech), S. Durrance/D. Golimowski (JHU), NASA
Scanning the Skies
To learn more about these relatively dark events, a team of researchers using MUSE (the Multi-Unit Spectroscopic Explorer) on the VLT (Very Large Telescope), along with ALMA (the Atacama Large Millimetre/sub-millimetre Array) both at ESO (the European Southern Observatory), have recently released images of galaxies.
Known as PHANGS (Physics at High Angular Resolution in Nearby GalaxieS), the team has put together pictures, described by ESO as “galactic fireworks”, which will help shed new light on the triggers of star formation (with the current science described earlier). These images provide astronomers with detailed information about the gases that later lead to star formation, and helping them create an ‘atlas’ of stellar nurseries (interstellar gas and dust which lead to star formation) in the nearby Universe. These include around 100,000 cold-gas regions and 30,000 nebulae of warm gas [17][18][19].
Five of the galaxies observed by MUSE (which is on ESO's VLT), shown as stacks of observations at several wavelengths of light. The golden glows reveal clouds of ionised hydrogen, oxygen and sulphur gas which are home to stars just coming into existence.
Hopefully with this new data scientists will understand, in more detail, the formation of stars, possibly leading to a better understanding of exactly how our own Sun, along with the planets that surround it, came into being.
by George Abraham, ADAS member.
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Click here to watch a computer simulation of stellar formation in MACS1149-JD1 (one of the furthest known galaxies from Earth)
Click here to Watch a simulation of the collapse of a cloud and the formation of a brown dwarf
Click here to watch the formation of a massive protostar
Click here to watch ESO's video about the 'atlas' of stellar nurseries compiled by PHANGS
Click here to look at the maths behind stellar formation
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