What is the Age of Our Sun?

Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Stars, just like humans, have a life cycle: they are born, they mature, they grow old and die. So when was our Sun born? And when will it die? Will we be around to see it? Read on to find out!

The Birth of Our Solar System

Our solar system began as a nebula, or a vast cloud of dust and gas, happily floating in the universe.

A nebula is a cloud of dust and gas in outer space.
Photograph of the Lagoon Nebula.

Then, around 4.6 billion years ago, something happened (maybe the explosion of a nearby star) that caused this mass of dust and gas to collapse in on itself. Gravity took over, pulling the cloud together into its center. The cloud formed a spinning disc that became increasingly hot and dense in the middle. The outer, cooler edges of the disc began to come together and form planets. The center eventually became so hot that a star, our Sun, was born.

Dating our Sun

So, according to the story in the introduction, our Sun and the rest of our solar system is about 4.6 billion years old. But how in the world do we know this? Where did scientists come up with that number?

Since, obviously, no one was around to witness the birth of our Sun, astronomers estimate the Sun's age based on information they do have. This section discusses three separate ways scientists accomplish this, and since they all give pretty much the same answer, it is fair to assume that their estimate is correct.

As we discussed in the introduction, astronomers believe that our entire solar system was created at nearly the same time. Thus, if we can figure out the age of other objects in our solar system, we can also know the age of the Sun. Luckily, some of those objects have crashed into our planet in the form of meteorites, outer space debris that survived their impact with the Earth's surface. Using radioactive dating, scientists have estimated the age of the oldest meteorites to be around 4.6 billion years old.

Meteorites are outer space debris that have survived the collision into our planet.
Photograph of a meteorite.

Radioactive elements were infused into the rocks and planets when the solar system was formed. In the process of radioactive dating, scientists take advantage of the fact that these radioactive elements decay. In other words, the radioactive atoms are unstable. So, the nuclei of the radioactive parent atoms decay, or breakdown, until they form stable, non-radioactive, daughter atoms. Uranium, for example, decays into lead. Additionally, this decay occurs at a predictable rate. Scientists can thus take the ratio of uranium atoms to lead atoms and calculate the approximate age of that rock. If there are still a lot of uranium atoms left, we know that the rock is relatively young. If most of the uranium has already decayed into lead, we know that the rock is much older.

If we continue with the idea that our entire solar system was created at the same time, then we could use the age of our own Earth to estimate the Sun's age. Again, through radioactive dating, we have found that the oldest rocks on Earth are also around 4.6 billion years old.

And, finally, we know that the oldest fossils are around 3.5 billion years old. This means that the Earth has been a suitable environment for life for at least that long. This is important because we know that stars have a life cycle. They are born, they mature, they grow old and then they die. If there was life on our planet 3.5 billion years ago, then we know our sun was, at that point, already in its main sequence. In a star's life cycle, the main sequence is basically like the grown up, adult star, in the prime of its life. It is no longer a baby (a protostar), and it has not yet hit old age (a red giant). And our sun is still currently in its main sequence, happily burning hydrogen and producing energy so we can live.

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