Our Sun has been "burning" for 4.6 billion years; converting hydrogen to helium through nuclear fusion it provides the warmth we require to sustain us. It should continue to do so for many more billions of years since it has only used up an estimated 4 percent of its hydrogen fuel or about 0.03 percent of its total mass. It is after all about 110 times the size of the earth, roughly 1,400,000 km in diameter. It is composed of hydrogen (75%), helium (25%) and a scattering of other elements including carbon, nitrogen, oxygen, silicon, magnesium and iron that total less than 1%. (You may feel free to reduce either the hydrogen or helium percentage, or both, to make the math work.)
The sun is a "main sequence G class star" according to astronomers. Which means is that it is a yellow star (i.e. spectral class G) that gets its energy through nuclear fusion of hydrogen and helium. In short there is nothing special about it as stars go. (Except maybe to the rest of us who may think it is special since it is our star.)
Sol (our sun to the Romans, hence Sol-ar system) is comprised of three relatively distinct layers, the interior, the photosphere and the corona. The photosphere is the visible surface of the sun and has a temperature around 5,600 deg. C. and pressure is low, about 1000 times less than the pressure at the surface of the Earth. (We are talking about hydrogen and helium here.) As you move towards the center, however, it really starts to get warm due to the effects of gravitationally induced pressure and temperature and pressure increases rapidly with depth. At the center it can reach temperatures of 15 million degrees and 300,000 bars of pressure. (A bar is a unit of pressure approximately equivalent to standard atmospheric pressure Earth standard that is.) At those high temperatures the conversion of hydrogen to helium (fusion) occurs. The process provides the thermal energy required to prevent the sun from succumbing to the pull of gravity and collapsing into a neutron star. The energy is eventually radiated out to space and thus warms the inner planets, including Earth.
The third component of the sun's composition is the corona and these outer layers are visible to the naked eye only during eclipses. The extremely hot gases (plasma) thin out into space creating the solar wind which is made up of a continuous stream of protons and electrons and can create some spectacular visual effects as it is the solar wind that causes the tails of comets to glow. On the approach to Earth (and any other planetary body with a magnetic field) the solar wind is bent by the magnetosphere into a funnel-shape. The wide mouth of the funnel is the point of first contact and the trailing end is formed as the solar wind passes out of the magnetosphere influence, thus we are protected from the worst of the solar winds effects. It is the solar wind that will pose a great danger to future explorers of Mars since our neighbouring planet, like our Moon, has no magnetic field to protect them. The solar wind is not the greatest danger to astronauts and marsnauts. They are relatively easily protected from the effects of the relatively low energies involved.
It must be borne in mind that the surface of Sol is not a uniform distribution of matter as it would appear to the naked eye of anyone foolish enough to look directly at it. It is mottled with darker areas called "sunspots" that, for reasons only speculated at, have an eleven year half-cycle from low activity to maximum activity in which the number, latitude and magnetic polarity varies. Sunspots and their full 22 year cycle from maximum activity back to maximum are associated with solar flares and their ejection from the interior of the sun to thousands and hundreds of thousands of kilometres above the photosphere.
The greatest danger to space travelers of the past, present and future come from the solar flares. Many of us have seen pictures of giant solar flares arcing across the surface of the sun and that arc may cause complacency in the minds of some. But it is not the material returning to the sun's surface that is the problem. The problem lies with the increased fluxes of ultraviolet radiation and the release of high energy particles, some of which can penetrate the magnetosphere, reaching Earth's atmosphere to interact directly with the ionosphere and playing havoc with radio telecommunications. Mind you, there is a good side to this if you live in areas relatively close to the North and South poles the auroras durign solar flares are truly spectacular to see. Fortunately for our future travelers in space these high energy particles do not travel at the speed of light and they can be given about half an hour's warning to get into their bomb shelters.'
There is, however, another possible danger to those of us who live on this fair and pleasant planet and that concerns our weather. We know the total energy output from Sol may not be constant and some scientists feel that a drop of only 0.5 percent could drastically affect our weather. Some feel that a reduction of less than 2 percent would cover the planet into with ice. After all, our world has seen ice ages before. Well, maybe if that reduction in energy output from the sun should occur, global warming wouldn't be such a bad thing and help save us. Of course we should probably try to solve the problem we do have and not worry about one we might have in a million or so years.