What is on Your Mind?

So the Sun will never explode (even though more massive stars can and do). The difference is that the Sun isn't massive enough to ignite anything past helium in its core. More massive stars continue nuclear burning until they start making iron. This creates an unstable core which will then explode in a supernova explosion.

When the Sun runs out of hydrogen in its core completely (which won't be for another 5 billion years or so) nuclear reactions will stop there, but they will continue in a shell around the core. The core will contract (since it is not generating energy) and as it contracts it will heat up. Eventually it will get hot enough to start burning helium into carbon (a different nuclear reaction). While the core is contracting the hydrogen burning around it heats will heat up the outer layers which will expand, and while they do that they will cool. The Sun will then become what is called a Red Giant and its radius will be large enough to envelop the Earth.

If the Sun became a red giant will the Earth still be able to support life here?

No, the Earth will not be able to support life if the Sun becomes a giant star. Giant stars have large radii as their name implies. When the Sun becomes a giant star, it may become so large as to engulf Earth, in which case the planet will be destroyed. Even if this does not happen, the sun will expand so far out that the temperatures on Earth will become extremely high so that all oceans will evaporate away, and there will be no water left on Earth. So, no life which depends on water will be able to survive.

How long will it take from the process starts til the earth is engulfed, or at least uninhabitable?

Timescales are difficult in evolutionary models of stars. It's not clear quite what'll happen to the Earth either. It could be engulfed by the Sun, or it might get pushed out into a larger orbit and freeze as the Sun expands. The Sun will be a Red Giant for a few million years. By then I think it's safe to say that the Earth will be uninhabitable.

Originally Posted by Perderabo

However, if you would like something to worry about, how about Eta Carinae. It could hypernova in our lifetime. Since it is only 8,000 light years away, the resulting gamma ray burst would completely sterilize the Earth. Or to look at it another way, because of the distance, it may have already gone hypernova and we are just waiting for the GRB to arrive.

How close to Earth would a supernova need to be before it caused significant damage to our ecosystem?

The conclusion seems to be that a supernova would need to be within tens or hundreds of light-years from us to cause significant damage to the Earth and life on our planet.

The end of a short life

Eta Carinae is destined to die young. Most stars live for billions of years, but stars as massive and active as Eta Carinae burn through their fuel in an extremely short time -- as short as one million years or so, very quick for a star. They almost always end the same way: With a supernova explosion, a massive detonation that blows the star apart and scatters its remains for trillions of miles (kilometers) around.

That's how most supermassive stars end, but Eta Carinae is such an extreme case that another possibility exists: It could end as a hypernova, a super-supernova that at its peak will outshine the entire galaxy.

Chandra's X-ray image of Eta Carinae reveals a hot inner core and three distinct structures racing outward at high speeds.

The blazing violence of such an event is difficult to describe. Were it much closer it could even wipe out all life on Earth, eradicating our thin biosphere just like an ultraviolet lamp kills microbes. Fortunately it's not that close, but at 7,500 light-years it's still close enough to do some damage.

However, the likely damage is not to humans directly, but to satellites and the upper atmosphere. That's because an explosion of this type generates huge amounts of high-energy radiation such as gamma rays. We on Earth are well shielded from gamma rays by our atmosphere, but satellites in space would be vulnerable and some of their electronics could be damaged by such an event.

Some have speculated that a huge blast of gamma rays could also affect the upper atmosphere, including the ozone layer. But that remains only speculation, and any such effect is likely to be very transient because the blast of gamma rays would be fairly brief.

The only humans who might suffer directly from Eta Carinae's violent demise would be astronauts in space. Outside of the Earth's protective atmosphere they would be subject to the same powerful radiation as satellites, with conceivably lethal effect. While our own sun is also capable of lethal emissions, such as coronal mass ejections that could be harmful to astronauts, the difference is that our sun's eruptions usually give us some warning, whereas Eta Carinae would not.

If a hypernova came to burst in our galaxy, and if the Earth is in the axis of the radiations, the burst of X-rays and gamma rays would destroy the upper ozone layer, so destroying the shield which protects us from the particles from the Sun. The life on Earth would then be compromised. Fortunately, we know only 3 stars in the galaxy which could end in hypernova by less than a million years, the closest to which is Eta Carina, at the distance of 8000 light years. But we also know that in case of explosion of this star, the Earth would not be in the axis of the GRB.

Originally Posted by Perderabo

Other pages seem to support your view that a few thousand light years is completely safe except for astronauts in outer space.