Astro-Lesson: The Star Life Cycle

This week's topic kinda picks up from last weeks topic, the Sun. From our short and limited view of the cosmos, we have been able to discern something amazing, the life cycles of stars. It turns out stars have a birth, then long lives of constant work converting hydrogen to helium and other elements, and then they die, either by withering away or massive violent explosion.

Art of the life of a Sun-like star.

 There is a significant reason why knowing this is important. A star can last for BILLIONS -- or even TRILLIONS!-- of years, and yet humans have only been around for about 200,000 years, but a brief moment of any star's life. But if it isn't possible for us to observe the entire life span of a star, how have we figured it out? Well, there are lots of stars out there, of all different ages that show features that relate to other stars, this allows us to work with something to make a model out of.

The best analogy I can provide is this: Imagine being in the woods, deep in a seemingly infinite forest that stretches to the horizon. You are surrounded by trees, while you are stuck to a tree yourself, the only one that you can see up close. Your tree appears to be a sturdy adult tree, analogous to our Sun. Looking out into the forest you see a variety of trees; without knowing the life cycle of the tree, they may not seem otherwise connected. You have seeds on the ground, saplings sprouting up, small trees, big trees, really big trees, and rotting logs on the ground.

Sagittarius Star Field - Our Forest.

This analogy can go further; those rotting logs provide subsistence for the next generations of trees, much like supernovae provide matter for new stars. And occasionally you might hear the crash in the distance of a tree falling, much like a supernovae or gamma ray burst going off. And what lies beyond the horizon remains a mystery since you are tied to one tree, just like massive size of the Universe.

Now the trick is, you have only a minute to gaze out into this forest to try discern what is what. And that brings the analogy home with what humanity has been able to accomplish. In our brief glimpse, we have been able to identify saplings, healthy full-grown trees, various species of trees, and dying or dead trees, of course in stellar terms. So lets now look at a few of these stages of the life of a star.

Concept art of the birth of a star in a molecular cloud. (NASA)

Protostar: This is where a star begins to form. A molecular cloud or nebula of dust and gas that starts "clumping" together. As the atoms gather, their gravitational attraction pulls in more atoms, making a larger "clump". This process is known as accretion. Lots of reactions happen inside an unstable protostar, which can have influences on a possible planetary system. Once the protostar achieves and maintains equilibrium, balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center, it becomes a star.

Not all stars are born equal! Stars are born into a variety of sizes and colors, depending on their composition, mass of formation nebula, and temperature. These stars lead vary different lives, of varying length, and die in dramatically different ways.

Small-Medium Stars: The overwhelming majority of stars, at least in the Milky Way, fall into this category, including the Sun, up to 1.5 times the mass of the Sun. These are amongst the longest living stars, the Sun is predicted to last 14 billion years, and it is roughly 4.6 Billion years old now. Smaller stars than the Sun can last billions of years longer. The smaller a star is, the longer it will live.

Hertzsprung–Russell diagram, commonly used to classify stars.

The majority of the life of a star will take place during a phase called the main sequence. This is when the star fuses hydrogen to produce helium in high-temperature and high-pressure reactions near the core.

After a small star like the Sun burns up all of its hydrogen, it leaves the main sequence. It begins to burn helium instead, converting helium atoms into carbon. The star loses its previous equilibrium, so in order to maintain it and keep cool, the star 'puffs out.' The star is now called a red giant, and is the first step in old age. The Sun may expand out to roughly the distance of Earth, 250 times its current size!

A red giant is very unstable, and may expand and contract, these stars are known as variable stars. This period if very short, only lasting a couple million years over the billions of years in a star's life. Soon the helium will burn up and and the star will again change as it switches to the last phase of fusion -- carbon burning.

The Cat's Eye Nebula is a planetary nebula. (NASA)

When a star switches to fusing carbon, the core contracts inward, down to about the size of the Earth. The outer layers of a star are blown off into space into a planetary nebula. Often beautiful to see from Earth, these are the dying throws of a star. The star becomes a white dwarf, densely packed, but not massive enough to become an neutron star or black hole. A white dwarf burns slowly and will gradually fade into a black dwarf, a cold dark mass. However, the Universe is not old enough for any black dwarf stars to exist yet.

A VERY Giant Star (the biggest known)

Huge and Giant Stars: These are stars much more massive then the Sun, with drastically shorter lifetimes. There are two classifications here based on the predicted end results of the star. Here we will call a star that is between 1.5 to 3 times the mass of the Sun a huge star; and a star greater than 3 times the mass of the Sun will be a giant star.

These stars spend a relatively brief period on the main sequence, since they are larger and burn hotter, they burn their hydrogen quickly. When they begin helium burning, they enter a special sort of phase, they become red supergiants, the largest known stars in the Universe by volume.

The onion-like layers of a massive star just before core collapse. (Not to scale.)

These stars are so large, they begin to fuse elements in an onion like structure near the end of their lives. Fusion of elements all the way up to iron on the periodic table is possible before a core collapse. The core of the star essentially is unable to support its weight against gravity. A massive explosion then occurs, a supernova, which violently blasts the upper layers of the star into space, leaving behind the core.

One of two things happens to the core: If it was a huge star, the core becomes a neutron star. If it was a giant star, it becomes a black hole. You can read more about black holes and neutron stars in one of my older articles.

The Crab Nebula, the remains of a supernova first observed around 1050 AD.

Tiny Stars: A special case in stellar physics, these are stars smaller then 0.5 times the mass of the Sun. These stars never fuse helium into higher elements, they do not have the mass to exert the pressure on the core. They are known as red dwarfs, like Proxima Centuri. These stars can exist on the main sequence for up to trillions of years and have life spans longer then when the Universe is predicted to end. They may eventually fade to white dwarfs, but there is no real way of knowing.

That accounts for an overview of star life cycles. I hope this helps you to understand some of the differences between stars and their end results. Astronomers were able to piece this together looking at the clues that the Universe has given us, from our brief glimpse. We have been able to determine the amazing course of existence for the very things that gave us life.

Posted in Astro-Lesson,Black Holes,Main Sequence,Neutron Stars,Planetary nebula,Red Giant,Star Life Cycle,Stars,Sun,Supernova,White Dwarf 34 comments