Asked by a student in Mrs. Carson’s class at Sunnyside Elementary
Answered by Brian Tonks, Professor of Physics – BYU-Idaho
Answer: Our Sun is a typical normal star, like 80-90% of other stars in the sky. Astronomers call these stars “Main Sequence” stars. So, what does that mean? What makes a normal star different than other types of stars?
Stars form out of great clouds of gas and dust, like the Orion nebula. Our galaxy contains many of these nebulae. Nebulae are made mostly from hydrogen gas (about 92% of all the atoms in a nebula are hydrogen—most of the remaining 8% is helium). The gravity of the individual atoms pulling on each other causes the nebula to collapse and forms stars. Stars have the same original composition as the nebula from which they are formed. As the cloud collapses, the gas becomes compressed, heating it. As the gas becomes hotter and hotter, the gas atoms in the deepest interior become hot enough to undergo the process of nuclear fusion, the power source of normal stars.
Nuclear fusion in the Sun (and all normal stars) takes four hydrogen nuclei and binds them together to make 1 helium nucleus. This process liberates a significant amount of energy, which is released into the environment. Because hydrogen is so abundant, all stars begin their lives by undergoing hydrogen to helium fusion. A normal, Main Sequence star like our Sun is a star undergoing hydrogen to helium fusion in their deepest interior, the star’s core.
The term “Main Sequence” comes from a famous diagram produced around 1913 that plots a star’s light output versus its surface temperature. Normal stars plot along a specific trendline on this diagram, which astronomers call the “Main Sequence”. In addition to Main Sequence stars, astronomers discovered that other interesting stars plot away from the Main Sequence, like red giants, white dwarfs, red supergiants, etc. This discovery eventually led to our current understanding of how stars change as they age.
Our Sun converts approximately 600 million tons of hydrogen into 596 million tons of helium every second. The 4 million ton difference is the amount of mass converted into energy by the nuclear fusion process and is radiated from the Sun. Comparing the rate that the Sun is now “burning” its fuel to how much hydrogen it contained in its core when it first formed gives us an estimate of how long the Sun will last. A similar calculation shows that the Sun will last about 10 billion years as a normal star.
The Sun has been around for a long time. As astronomers observe the clouds that make stars, they have discovered that disks of gas form around them. Scientists theorize that planets form within these disks. To find the age of the disk that originally surrounded the Sun, we need to find material that has not been melted or vaporized since the Solar System was young. We have discovered that virtually all meteorites that fall to the Earth were formed from the Sun’s disk about 4.5 billion years ago. That means that the Sun is about halfway through the “Main Sequence” phase of its life cycle.
Even after the Main Sequence phase, a star is not yet finished. As hydrogen begins to run out in its core, it becomes larger, expanding 50-100 times its original diameter, and cooling off. It becomes a “red giant” star, hundreds to thousands of times brighter than during its Main Sequence stage. The Sun will last about 1 billion additional years as a red giant star.
Our understanding of what our Sun is and how it will change as it ages has come as scientists carefully observed the stars, developed an understanding of how nuclear fusion works, learned how to model the processes on computers, and compared these models to the observations. This effort has deepened our understanding of and appreciation for the beauties of our universe and how it allows us to live on our beautiful planet.