Stellar mass

Stars come in many sizes. Some are very big. Some are very small. Our Sun is a star too.

A star's size can change. It can lose weight over time. It can also gain weight. This happens if it takes in matter from another star.

Very big stars can end in a huge blast. This is called a supernova. These blasts can make black holes.

Some stars are tiny. They are smaller than our Sun. They are even smaller than the planet Jupiter.

It is fun to wonder about the stars.

Astronomers use a term called stellar mass. This describes how much a star weighs. They often compare stars to our Sun. For example, Sirius is about 2 times the mass of the Sun.

A star's mass can change over time. It can lose mass through stellar wind. This is a stream of matter moving away. A star can also gain mass. This happens if it takes matter from a companion star.

Stars are grouped by how they live and die. Massive stars are very big. They have at least 5 to 10 times the mass of the Sun. These stars end in a supernova. A supernova is a huge explosion. This can create a black hole.

Some stars are very small. AB Doradus C is a very tiny star. It is only 0.09 times the mass of the Sun. Objects smaller than stars are called brown dwarfs. These sit in a grey area between stars and gas giants. The Sun will lose mass as it grows older. It will lose 46% of its mass by the time it becomes a white dwarf.

Astronomers use the term stellar mass to describe a star. It tells us how much matter a star holds. Most of the time, scientists compare stars to our Sun. They call the Sun's mass one solar mass. For example, the bright star Sirius has 2 solar masses. This number is important for understanding how a star lives. A star's mass can change over its long life. It might lose mass through stellar wind. It can also lose mass through pulsational behavior. Sometimes, a star gains mass from a companion star.

Mass also decides how a star will end its life. Very-low-mass stars stay below 0.5 solar masses. These stars do not enter the asymptotic giant branch. Instead, they evolve directly into white dwarfs. Low-mass stars below 2.2 solar masses do enter that stage. They develop a core made of degenerate helium. Intermediate-mass stars undergo helium fusion. These stars develop a core of carbon and oxygen. Massive stars have at least 5 to 10 solar masses. These huge stars undergo carbon fusion. Their lives end in a core-collapse supernova explosion. This can create a stellar-mass black hole.

Scientists have found stars that are truly huge. One of the most massive known stars is Eta Carinae. It has 100 solar masses. Its life is very short, lasting only a few million years. Some studies suggest 150 solar masses is a limit. This limit may relate to Eddington luminosity. This is the maximum light that can pass through a star. If it is too bright, it ejects gas into space. However, the star R136a1 is measured at 215 solar masses. This makes the 150 limit a question. Stars in the R136 cluster might grow by colliding. They can merge in close binary systems to get bigger.

There are also very tiny stars in space. AB Doradus C is a very small star. It has only 0.09 solar masses. It is the smallest known star with nuclear fusion. For stars like our Sun, the minimum mass is 75 solar masses. But some very faint stars are even smaller. They can be about 8.3% of the Sun's mass. These are about 87 Jupiter masses. Objects smaller than this are called brown dwarfs. They sit in a grey area between stars and gas giants.

Even our own Sun is changing its mass. The Sun loses mass through electromagnetic energy. It also loses mass through the solar wind. The Sun loses about 10^9 kilograms every year. This loss will increase when the Sun becomes a red giant. It will peak when the Sun creates a planetary nebula. By the end, the Sun becomes a white dwarf. It will have lost 46% of its starting mass. This shows how mass shapes everything in the universe.

Astronomers use the term stellar mass to describe the total amount of matter in a star. Because stars vary so much in size, scientists usually measure mass in relation to our Sun. They call the mass of the Sun one solar mass. For instance, the bright star Sirius has about 2 solar masses. This measurement is vital because mass dictates how a star lives and dies. A star's mass is not always a constant number. It can change over time through several different processes. A star might lose mass through stellar wind or pulsational behavior. Conversely, a star can gain mass if it accretes matter from a companion star.

Mass is the primary factor that determines a star's evolutionary path. As stars reach the end of their nuclear fusion lifetimes, they can be grouped by mass. Very-low-mass stars, those below 0.5 solar masses, follow a unique path. They do not enter the asymptotic giant branch, which is a specific stage of stellar evolution. Instead, they evolve directly into white dwarfs. In theory, these stars live so long that none have had time to reach this stage in the history of the universe. Low-mass stars with masses below 1.8 to 2.2 solar masses do enter the asymptotic giant branch. During this stage, they develop a degenerate helium core.

Stars with more mass follow even more complex paths. Intermediate-mass stars undergo helium fusion. This process results in the development of a degenerate carbon-oxygen core. Massive stars are much larger, with a minimum mass of 5 to 10 solar masses. These giants undergo carbon fusion during their lives. Their existence ends violently in a core-collapse supernova explosion. This massive explosion can leave behind a stellar-mass black hole. The mass of a star also works with its radius to determine surface gravity. Giant stars have much lower surface gravity than main sequence stars. In contrast, compact stars like white dwarfs have much higher surface gravity. This gravity can even change how a star's spectrum looks by causing a broadening of absorption lines.

Some stars reach incredible sizes that challenge our understanding of physics. Eta Carinae is one of the most massive stars ever known. It possesses 100 solar masses, but its life is very short. It lasts only a few million years at most. Some studies of the Arches Cluster suggest that 150 solar masses might be the upper limit for stars today. This limit may be linked to Eddington luminosity. This is the maximum amount of light that can pass through a star's atmosphere without pushing its gases into space. However, the star R136a1 in the RMC 136a cluster was measured at 215 solar masses. This discovery questions the 150 solar mass limit. Scientists believe stars larger than 150 solar masses in the R136 cluster may form through collisions. These stars merge in close binary systems to bypass the limit.

History and theory also suggest that the first stars were much larger than those we see now. These are called Population III stars. They formed after the Big Bang and may have reached masses of 300 solar masses or more. They were so large because they lacked elements heavier than lithium. While these supermassive stars are now long extinct, they remain a key theoretical concept. On the other end of the scale, we find extremely small stars. AB Doradus C is the smallest known star undergoing nuclear fusion. It has a mass of only 0.09 solar masses, which is about 93 times the mass of Jupiter. For stars with a composition similar to the Sun, the theoretical minimum mass for fusion is about 75 Jupiter masses. However, in stars with very low metallicity, the minimum mass can be as low as 8.3% of the Sun's mass, or 87 Jupiter masses.

Objects that fall below these minimum mass limits are known as brown dwarfs. These bodies occupy a grey area between true stars and gas giants. It is also important to note that even stable stars like our Sun are constantly changing. The Sun loses mass through the emission of electromagnetic energy and the solar wind. Currently, the Sun expels about 10^9 kilograms of mass every year. This rate will increase as the Sun enters its red giant stage. It will climb to 10^11 kilograms per year at the tip of the red-giant branch. Later, on the asymptotic giant branch, it will reach 10^-13 solar masses per year. Finally, as it creates a planetary nebula, the rate peaks between 10^-5 and 10^-4 solar masses per year. By the time the Sun becomes a white dwarf, it will have lost 46% of its original mass.