Well yes. The sun is about 5 billion years old. That is 5,000,000,000 years old! It’s pretty old … even older than your mom and dad together!! Ha ha.

So, the sun has hydrogen and the sun uses the hydrogen as its fuel. The reaction is:

Hydrogen = Helium plus lots of energy and light

Now, the sun only has enough fuel to “burn” for another 3 or 4 billion years. So 3 or 4 billion years from now the SUN will run out of hydrogen. This will make the sun swell up and become a RED GIANT star. It will be so big that the sun will swallow the EARTH and our planet will be toast…

So, if there are any life forms on the planet about 3 billion years from now they will have to migrate to another planet perhaps in another star system.

NOTE: Do not worry, 3 billion years is a lot, lot of time.

In terms of the sun's life cycle, no, it will never be harmful to humans because humans will likely not exist still when the sun reaches a state that will undermine Earth's current habitable condition. But let's go through that life cycle anyway.

According to the nebular hypothesis, the most widely accepted model to explain the formation and evolution of the solar system, our solar system began as a giant cloud of molecular dust and gas. Approximately 4.567 billion years ago, that cloud collapsed and pockets of denser material began to collect, begin rotating, and heat up. The ball in the center became a collapsing protostar (the beginnings of the sun we have today) and the remaining debris flattened into a disk around the protostar (making up the planets around the sun today). In just a few million years, the protostar settled into the star we see today.

As the sun continues to convert hydrogen into helium atoms, the core becomes smaller and denser. This means its gravitational force becomes stronger and its brightness becomes brighter. In about 1.1 billion years from today the sun will be about 10% brighter than today (and hotter as well), and the result will be wet greenhouse conditions on Earth that are similar to conditions that pushed Venus into runaway heating.

In approximately 3.5 billion years, the sun will be about 40% brighter than today (and again even warmer). This will lead to the ice caps melting, the oceans boiling, and all water on Earth will turn to vapor and be lost to space. Essentially Earth will likely turn into a similar state as Venus.

Finally, in about 5.5 billion years, the sun will burn up the last of the hydrogen and grow into a Red Giant. It will grow so large that it will possibly encompass the orbit of Earth. At this point the sun only has about 150 million years of life left. It will go through a series of expansions, explosions, and compactions as it burns up all the helium then loses mass by ejecting its outer envelope. It will eventually cool to a white dwarf. The white dwarf will survive for trillions of years before fading completely.

Outside of its life cycle, the sun can affect humans today through solar flares and coronal mass ejections. Solar flares are gigantic explosions that release radiation, and coronal mass ejections are huge explosions that release gas and magnetized plasma. Earth's atmosphere protects humans from both events. However, they can affect Earth's magnetic field and disrupt technology that humans rely on, such as damaging communications satellites above Earth's atmosphere, damaging electrical grids and causing black outs, and disrupting high-frequency radio communications. Though those effects are not directly harmful, they could impact humans greatly.

The sun is already harmful to humans if you are out in it for too long and get sunburned. Also, as the sun ages, it becomes larger and brighter, and it will eventually make the earth unlivable. This will probably happen in a billion years or so. It is all but certain that human beings like us will be extinct by then, but if we are the ancestors to any species alive then, those species will either have to escape the earth or be destroyed along with the rest of life when this happens.

This is an interesting question. The sun can already be harmful to humans in a variety of ways under conditions of extensive exposure.

I'm sure you know many people who have been sunburned from being out in the sun for too long without sunscreen. Furthermore, depending on how long they were out and how easily they burn (which is often correlated with the amount of melanin, a skin pigment, people have), they may have been burned pretty badly! Sunburns are one example of ways the sun can be damaging/harmful to people. Sunburns are caused by damage to skin cells that is caused by prolonged exposure to the UV radiation the sun perpetually emits. Wearing sunscreen, or even having naturally larger amounts of melanin can help mitigate the damage, but even that has its limits (that's why they tell you to re-apply sunscreen, etc!)

The sun can also be harmful to people who have spent too long by causing heat stroke, or overheating. People can become faint, feel unwell, or even vomit if they have been too hot for too long.

There are also indirect, or longer term ways prolonged or consistent exposure can be harmful to people. People who have been exposed to the sun consistently for extended periods of time without proper skin protection are more likely to age more quickly (think of older, leathery-skinned people). We don't often think of aging as "harmful," and it is certainly a natural process that all living organisms experience, but I think the acceleration of aging can be harmful in a sense.

Nevertheless, the sun is also directly and indirectly imperative for our livelihood. Without the sun, we would freeze and not have plants/vegetables to eat, or animals in the food chain. The vast majority of life as we know it would not exist (plants and many bacteria carry out photosynthesis to generate energy).

So where does that leave us? I think, as with many things, the lesson is that we must take things in moderation, including exposure to the sun. Too much of anything can often be harmful. I hope that helps!