Suggested Searches

 
Comic-style illustration showing a large star in the lower left, gaseous material coming out of it, and planets around it

2.2. How was the Sun different when it formed compared to now?

A core learning question from the Astrobiology Learning Progressions

Astrobiology Learning Progressions Navigation

Table of Contents
Left arrow

Previous

2.1. What was the Earth like right after it formed?

Right arrow

Next

2.3. Where could life have gotten started on Earth?

Grades K-2 or Adult Naive Learner

The young Sun was a little different than the Sun we see today. The young Sun was not as bright and didn’t give off as much heat as it does today. But the young Earth was also a very different place than it is now. Sometimes we talk about the air around Earth like it’s a blanket. But some blankets are better at making you warm, and the air around the young Earth was probably pretty good at keeping the planet warm.

Portrait photo of Comic-style illustration showing a large star in the lower left, gaseous material coming out of it, and planets around it
When the sun formed, 4.6 billion years ago, it was less luminous and cooler than it is today.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

Just as the young Earth was a very different place than it is now, the young Sun was different than the Sun we see today. Back then, the Sun gave off less light and heat than it does today. It wouldn’t have looked as bright then! The way that the Sun makes heat causes it to get brighter and hotter over time.

But, if the young Sun wasn’t as bright or as hot, that makes us wonder how the young Earth was able to stay warm enough for life to survive. For instance, all of the water on Earth could have frozen, and we know that life needs liquid water to grow and thrive.

Scientists think that the atmosphere on the young Earth was also pretty different. The atmosphere is made up of all of the air around us and above, the whole way up until we get to space. The atmosphere acts kind of like a blanket, covering our world and keeping us warm. On the young Earth, that blanket might have been pretty good at keeping Earth warm enough for water to be liquid and living things to survive, even though the Sun wasn’t as bright or as hot.

Portrait photo of Comic-style illustration showing a large star in the lower left, gaseous material coming out of it, and planets around it
When the sun formed, 4.6 billion years ago, it was less luminous and cooler than it is today.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

The young Sun, just like the young Earth, was very different than it is now. The Sun creates all of that light and heat through a process called “nuclear fusion”; that’s where atoms of one element are turned into atoms of another element (they’re fused together!). The Sun mostly does this by turning hydrogen into helium. Over time, as the Sun has made more helium, it has slowly gotten hotter and brighter. But that leads us to an interesting question – if the Sun was producing less light and heat back then than what it does today, wouldn’t there be freezing cold temperatures all over Earth, preventing life from getting started and surviving?

Scientists know that there was liquid water on the surface of the early Earth, so global freezing did not occur. So how did the water stay liquid? One explanation is that the atmosphere around the young Earth may have been very different than it is today. For instance, there might have been more gases like methane, carbon dioxide, and even water vapor in the atmosphere. These gases are what we call greenhouse gases. They can make the atmosphere act like a blanket which keeps the heat from Earth at the surface (rather than escaping into space). Such a blanket insulates Earth, keeping everything warm, and that’s pretty lucky for life as we know it!

Portrait photo of Comic-style illustration showing a large star in the lower left, gaseous material coming out of it, and planets around it
When the sun formed, 4.6 billion years ago, it was less luminous and cooler than it is today.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

The young Sun, just like the young Earth, was very different than it is now. To understand why, we need to know a little bit about how the Sun and other stars give off so much energy.

Stars creates all of their light and heat through the process of nuclear fusion. This is where atoms of one kind of element merge together to make atoms of another element! You may recall that the Law of the Conservation of Energy tells us that energy cannot be created nor destroyed, but it can change forms. We also know from the Special Theory of Relativity that energy and mass have a very special relationship. We call it the “mass-energy equivalence”, but you’ve probably heard it more often stated as the famous equation E=mc2\. This tells us that any amount of mass also has an equivalent amount of energy, and vice versa. Well, it turns out that the process of nuclear fusion is also one where mass is converted into energy. When stars are fusing together elements, they producing lots and lots of energy. But they’re also making bigger and bigger chemical elements in the process.

The most common nuclear fusion reactions in stars are ones that convert hydrogen into helium. This is actually how most of the energy coming from the Sun is produced. However, stars will also then fuse together helium atoms to make elements like carbon, oxygen, and nitrogen. Many of the chemical elements are made this way. It’s such an important process that is has a special name, “stellar nucleosynthesis”, and it’s responsible for our Sun giving off so much light and heat.

Over time, as stellar nucleosynthesis has gone on in the Sun, making more and more helium and carbon and such, the Sun has slowly gotten hotter and brighter. The main reason for this is that as these heavier elements form and then start undergoing stellar nucleosynthesis reactions, the core of the Sun starts to contract (it shrinks down and becomes more dense) and that increases the temperatures and pressures inside, which then in turn causes even more reactions to release even more energy.

The Sun was about 30% less luminous in its youth than it is now, but scientists also know that there was liquid water on Earth’s surface at that time (from studying the rock record). How could water on Earth’s surface remain liquid with that much less light and heat coming from the Sun? How could life – which requires liquid water – have gotten started on the young Earth in freezing conditions? Scientists call this the “faint young Sun paradox.” Resolving this question is an active area of research, and there are likely many contributing factors. One way to resolve the conflict is with the greenhouse hypothesis, which states that Earth’s early atmosphere likely contained higher levels of greenhouse gases – especially methane, carbon dioxide, and water vapor. Those and other gases were coming from degassing and volcanic processes following the accretion of rocky particles during Earth’s formation. On Earth today, the carbon dioxide in the air is converted into oxygen by photosynthetic organisms like plants and algae. On the early Earth, there were no such organisms to do that conversion, so carbon dioxide remained in the atmosphere and contributed to global warming, which kept the water liquid on the surface despite the fainter, less luminous Sun.

Portrait photo of Comic-style illustration showing a large star in the lower left, gaseous material coming out of it, and planets around it
When the sun formed, 4.6 billion years ago, it was less luminous and cooler than it is today.
NASA/Aaron Gronstal