Stars form when cool, relatively dense clouds (molecular clouds) of interstellar gas and dust shrink upon themselves as a result of gravitational collapse. In a spiral galaxy, such as the Milky Way, star formation is usually triggered when gas clouds are compressed by shock waves from nearby supernovae.
The Birthplace of Stars
Stars are born in regions of unstable clouds of dust and gas which are scattered throughout the outer spiral arms of our Milky Way. Hundreds of newborn stars are nursed in the Orion Nebula, which is just visible to the naked eye as a fuzzy speck of light in the constellation of Orion. However, the lit up regions of the nebula only represent a small portion, as much of the nebula is actually full of dense molecular clouds which absorb visible light and can only be probed with radio waves.
The Formation of Stars
Once the clouds start to collapse, the material breaks down into massive lumps, and as these continue to collapse, gravitational compression causes the lumps of cloud to warm up as gravitational potential energy is converted into heat. It is these lumps that will eventually form a single star, two stars or even a star with its own planetary system.
As the pressure and temperature increases in these lumps, a sphere made up of superhot gas called a proto-star (a potential star) is formed. This proto-star will continue to collapse until its core approaches close to 10 million kelvins as that is the required temperature to undergo nuclear fusion.
The entire formation process for a star like our sun might take about 50 million years. Once a star has begun the conversion of hydrogen into helium, the remainder of its life will be determined exclusively by its mass. This nuclear reaction releases heat and produces an outward pressure which supports and holds up the star against further gravitational collapse for as long as there is enough nuclear fuel to burn.
The Composition of Stars
All the stars that are born at this moment will start out with roughly the same mix of raw materials:
- hydrogen, which accounts for just under 75 percent
- helium at just over 25 percent
- a small amount of heavy elements such as zinc
Stars from earlier generations contained slightly more hydrogen and less helium with very few, if any, heavy elements. It is fortunate for us that our sun was formed some 4.6 billion years ago as opposed to 10 billion years ago as the lack of heavy elements in early stars would have made it difficult for life to form on any of the planets.
The End of an Era
After exhausting all readily available hydrogen in its core, nuclear fusion comes to a halt and the core begins to contract due to gravity. Stars with masses between 0.5 and 8 solar masses (1 solar mass is the mass of the sun) expand to become red giants.
These massive stars have a radius several hundred times the size of our sun and each will ultimately expel its outer shell. The expelled material is called a planetary nebula as it resembles a giant planet when observed through an optical telescope. This ring of expelled material will remain visible for about 10 thousand years before gradually dispersing into interstellar space. The star is now dead as it has no fuel. All that is left is a white dwarf star.
A Violent Death
The evolution of stars with masses greater than 8 solar masses isn’t as pleasant but much more dramatic. As a result of their sheer size, these stars burn through their hydrogen supply at a faster rate but at the cost of a considerably shortened lifespan. Once their hydrogen to heliumconversion phase is over, they expand to become red supergiants.
These are the largest stars in existence in terms of radii. Once all its energy is spent, the red supergiant collapses and explodes as a supernova, shining briefly with the light of a billion suns. Most of this material is expelled into space. All that remains is a spherical body of incredible density which will either collapse into a neutron star or possibly, a black hole.
Facts about Stars For Kids
- The nearest star to Earth is the Sun, which is the source of most of the energy on Earth.
- Other stars are visible from Earth during the night, when they are not obscured by atmospheric phenomena, appearing as a multitude of fixed luminous points because of their immense distance.
- At the end of its lifetime, a star can also contain a proportion of degenerate matter.
- Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names.
- For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen in its core releasing energy that traverses the star’s interior and then radiates into outer space.
- Almost all naturally occurring elements heavier than helium were created by stars, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when stars explode.
- Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space.
- A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
- Once the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core.
- The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.
- The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period.
- By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere.
- Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.
- These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements.
- Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars.
- However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no stars under about 85% of solar mass, including all red dwarfs, are expected to have moved off of the main sequence.
- Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature.
- This process continues, with the successive stages being fueled by neon, oxygen, and silicon.
- The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma.
- These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster.