Stars are born, they are not made. Stars form within clouds of gas and dust in space called nebulae. Gravity causes the gas and dust in these nebulae to collapse inward, and when enough material comes together, nuclear fusion begins and a star is born.
How Do Stars Form?
Stars form from large clouds of gas and dust in space called nebulae. These nebulae are made up mainly of hydrogen and helium left over from the creation of the universe. Gravity causes areas within these nebulae to collapse inward forming dense regions called molecular clouds. Within these molecular clouds, gravity continues to draw material in and the pressure and temperature increases. Eventually, the temperature and pressure becomes great enough to initiate nuclear fusion at the core of the collapsing cloud. When this happens, a protostar is formed. As the protostar continues to acquire mass from the surrounding molecular cloud, its core temperature eventually reaches the point where sustained hydrogen fusion can occur and a star is fully formed.
Stages of Star Formation
Star formation occurs in several stages:
- Giant molecular cloud – Large cloud of gas and dust from which stars form.
- Dense cores – Regions within the cloud that gravity causes to collapse inward forming dense clumps.
- Protostar – A dense core becomes so compressed it forms a rotating ball of gas that continues to draw in surrounding material.
- T Tauri star – A newly formed star still surrounded by its protoplanetary disk accretion continuing.
- Main sequence star – Nuclear fusion in the core is stabilized and the star joins the main sequence.
Giant Molecular Clouds
Giant molecular clouds consist mainly of molecular hydrogen and measure hundreds of light years across. They can contain thousands of solar masses of material. These clouds exhibit areas of clumping where density fluctuations lead to localized regions of enhanced density. It is within these clumps that gravity can gain hold and initiate a collapse.
Dense Cores
As clouds continue to clump under gravity, the pressure in the center of the clumps increases causing contraction. When a clump reaches a density of around 10,000 molecules per cubic centimeter, it becomes opaque and is considered a dense core. These dense cores range from .3 to 3 light years in diameter and further contraction occurs from gravitational instability.
Protostars
As the dense core continues to contract and spin faster, it flattens into a disk shape forming a rotating protoplanetary disk surrounding the protostar. Mass continues to fall onto the protostar from the disk and surrounding envelope. When mass infall decreases and the protostar reaches hydrostatic equilibrium stabilizing against further gravitational collapse, fusion begins and a star is born.
T Tauri Stars
Newly formed protostars that have reached the main sequence but are still surrounded by their protoplanetary disk are known as T Tauri stars. These young stars continue to accumulate mass from their disk and exhibit variable luminosity as bursts of material fall onto the star.
Main Sequence Stars
Once a protostar has accumulated enough mass and its core reaches about 10 million K, the sustained proton-proton chain reaction of hydrogen fusion can begin. At this point, the star enters the main sequence and will stably fuse hydrogen into helium for a significant portion of its life, maintaining equilibrium between the thermal pressure of the fusion reactions pushing outward and the weight of the star’s material pushing inward.
What Factors Determine the Mass of a Star?
There are several key factors that determine the final mass a star will have:
- Mass of the original giant molecular cloud
- Efficiency of the star formation process
- How much matter is ejected in stellar winds
- Presence of nearby massive stars
Mass of Molecular Cloud
The upper limit for star mass is determined by the amount of matter available in the parent molecular cloud. The most massive stars form from the largest molecular clouds. Typical molecular clouds range from thousands to millions of solar masses, enabling a wide range of stellar masses.
Star Formation Efficiency
Not all the matter in a molecular cloud will end up incorporated into a star. Star formation is actually quite inefficient with only around 1-10% of a cloud’s mass ending up in stars. The rest of the material remains in the interstellar medium or becomes part of planets. The efficiency depends on conditions in the cloud and affects final stellar masses.
Stellar Winds
As protostars form, some fraction of material can be blown away from the accreting matter through stellar winds. More massive stars have stronger stellar winds that can remove larger amounts of mass and limit growth. Less massive protostars have weaker winds and retain more mass from the protoplanetary disk.
Nearby Massive Stars
Radiation and stellar winds from nearby massive stars can also limit mass growth by evaporating away the gas and dust around protostars before it can be accreted. This external photoevaporation effect puts an upper limit on protostellar masses in stellar nurseries with large O and B type stars.
How Common Are Different Mass Stars?
Smaller mass main sequence stars are far more common than high mass stars. This is due to the physics of star formation favoring lower masses and the faster evolution of high mass stars. The following table shows rough estimates for the frequencies of stars by mass:
| Mass Range | Spectral Type | Frequency |
|---|---|---|
| 0.08 to 0.5 M☉ | M dwarfs | 76% |
| 0.5 to 1.04 M☉ | K dwarfs | 13% |
| 0.6 to 1.04 M☉ | G dwarfs | 7% |
| 1.04 to 2.1 M☉ | F, A, B dwarfs | 3% |
| 2.1 to 16 M☉ | B, O stars | 1% or less |
As seen, lower mass red dwarfs make up the great majority of stars, while hot high mass O and B type stars are quite rare in comparison.
How Long Do Stars Live?
The lifespan of a star depends critically on its mass. Higher mass stars burn through their fuel much faster than low mass stars, as summarized below:
- Low mass red dwarfs – Trillions to quadrillions of years
- Intermediate mass Sun-like stars – Billions to tens of billions of years
- High mass blue stars – Millions to tens of millions of years
Because high mass stars have such short lives compared to lower mass stars, there are far fewer of them around. Low mass red and orange dwarfs have barely even started nuclear fusion, while the highest mass stars have already exhausted their fuel and expired after just a few million years.
Can Stars Form Today?
Yes, star formation is an ongoing process in our universe. Some estimates indicate several hundred thousand stars are still forming every year in our galaxy. However, the peak era of star birth in the universe was billions of years ago. The rate of star formation was much higher early in the universe when more giant molecular clouds were around to spawn new stars.
Where is Star Formation Still Happening?
Star formation still takes place today in molecular clouds found primarily in the spiral arms of galaxies. The Orion Nebula and Taurus Molecular Cloud are two nearby regions where active star birth is still occurring. Star forming regions can be identified by the presence of dense molecular hydrogen gas and emission nebulae showing where hot young stars are firing up the surrounding gas.
Can Stars Form in Globular Clusters?
Stars do not form in globular clusters. Globular clusters formed early in the universe before galaxies had fully come together. Their stars were all formed at around the same time when the universe was young. Today, globular clusters lack the gas and dust needed for star formation.
Conclusion
Stars are born through the process of gravitational collapse within large clouds of interstellar gas and dust. Their final masses are dependent on the mass available in their parent cloud and how much material gets ejected or cut off during formation. Smaller mass stars are far more common than high mass stars. Star formation was more active early in the universe’s history, but still continues today in molecular clouds found primarily in spiral galaxies where new stars are coming to life every day.