The initial mass function (IMF) calculates the range of masses for stars formed from gas condensation in space, with low-mass stars being more common.
The IMF predicts that most stars are 0.9 solar masses or less, while less than 1% are between 8 and 120 solar masses. Despite varying conditions, the same distribution of stellar intervals occurs.
As small stars age and lose mass, they skew the IMF towards the low-mass end. The Sun will expand and retain 50% of its mass as a red giant in about 5 billion years.
The initial mass function (IMF) was first derived in 1955 by Edwin Saltpeter, an Austrian astrophysicist, and is a method for calculating the range of different masses for stars that will form from the condensation of gases in space. It is a form of probability distribution using complex mathematical and physical equations with a baseline value of a solar mass, representing the mass of Earth’s sun as a starting point for the range of other stars that will form.
The premise of the initial mass function in stellar astronomy is that low-mass stars are much more common and likely to form in space than high-mass stars, with stars having about 0.5 solar masses being the most common in the la Milky Way as of 2011. Despite this fact, the rarest stars, with a mass of about 60 solar masses or more, contribute most of the visible light to the Milky Way galaxy.
Despite widely varying conditions in the substellar regions of space where stars form, the power laws of the initial mass function have been shown to be true. This means that whether star formation occurs in small molecular clouds of gas or in dense star clusters, the same distribution of stellar intervals occurs.
These observations conflict with theories of star formation as of 2011 due to conditions such as that, in a metal-dense region of space, the stellar distribution should include a higher percentage of large stars.
According to most astronomical estimates as of 2011, there are somewhere between 200,000,000,000 and 400,000,000,000 stars in the Milky Way galaxy. The initial mass function predicts that the probability for most of these stars is 0.9 solar masses or less, while less than 1% of them are between 8 and 120 solar masses in size.
The IMF calculates masses based on when each star first formed, and most stars start out as dwarf stars of only 0.085-0.8 solar masses. As these main-sequence stars age, they tend to lose mass and increase in volume.
It is estimated that, in about 5,000,000,000 years, the Sun itself will expand as it burns its hydrogen fuel and begins fusing helium into heavier elements. In this phase, the Sun will fill a volume of space reaching Earth’s orbit for about 20% of its total lifetime and retain 50% of its former mass as a red giant.
As small stars like the Sun age and lose mass in the process, they skew the initial mass function more and more towards the low-mass end of the spectrum, in large part because there are so many more small stars.
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