[bugmenot]

I was having a conversation with a physics teacher a few years ago about the sun and if it would explode nobody on earth would see it for as much as 15min because light from the explosion would travel at the speed of light and nobody would see it as it happened because of the distance between the earth and sun and how fast the light from the explosion would travel.

I than asked if that was true of the stars in the sky. Many of the stars we see today might not be there at all because they could have blown up million of years ago but we still see them because of the delay of the light from the explosion that takes too long to travel for us to see it when it happened. He said that was not true. Am I right or was my physics teacher wrong?

[infamous]

Originally Posted by bugmenot

I was having a conversation with a physics teacher a few years ago about the sun and if it would explode nobody on earth would see it for as much as 15min because light from the explosion would travel at the speed of light and nobody would see it as it happened because of the distance between the earth and sun and how fast the light from the explosion would travel.

Actually bugmenot, it would take only about eight minutes for the light of our sun to reach earth.

Originally Posted by bugmenot

I than asked if that was true of the stars in the sky. Many of the stars we see today might not be there at all because they could have blown up million of years ago but we still see them because of the delay of the light from the explosion that takes too long to travel for us to see it when it happened. He said that was not true. Am I right or was my physics teacher wrong?

It's difficult for me to believe that your physics teacher would not agree with this statement. If the star in question is a million light years away, it would take a million years for the light to reach us. One could safely say that you are correct in your reasoning. However, the nearest star is only about 4 light years away so the light we see coming from Alpha Centauri is only 4 years old.

[bugmenot]

So if the sun just goes dim right now with no light, everyone would still see the light from the sun for the next 8min even though the sun is already dim? and that goes for any other star in the sky?

Than how do scientist know if a star is actually there or not?

[infamous]

We can only know within reason by the light we recieve from the objects. Some scientists believe that gravitation propagates instantaeously which would mean that if we could measure gravitational waves we could observe the present condition of various objects. However, the majority of scientists believe that gravity propagates at the same speed as light. This latter view of things would have our ability to know at the mercy of the speed of light.

[Turtle]

Originally Posted by bugmenot

So if the sun just goes dim right now with no light, everyone would still see the light from the sun for the next 8min even though the sun is already dim? and that goes for any other star in the sky?

Than how do scientist know if a star is actually there or not?

___Stars end their lives in various ways depending on their size. A super nova is an exploding star that gives a spectacular albeit short-lived sky show & leaving behind an enormous field of gas & debris . Some stars die a little less dramatically but leave their mark nonetheless such as neutron stars which emit detectable pulses of X rays & radio waves. No doubt a web search of "star formation" will secure you at least one introductory article. I note we currently have started finding planets around stars not by seeing their reflected light, but by recording the host stars wiggles which arise from the orbiting planets gravitational effect on the star (Sun).

[bugmenot]

I understand how stars form and how they dye. I was using the dimming as an metaphor.

Originally Posted by Turtle

I note we currently have started finding planets around stars not by seeing their reflected light, but by recording the host stars wiggles which arise from the orbiting planets gravitational effect on the star (Sun).

I heard of that but nothing travels faster than the speed of light that we know of, maybe gravity but even the wiggle that is detected is delayed just like everything so it might not even be there.

The question still remains. How does anyone know that what they are looking at is still there and not dead. There might not be as many stars in the sky as scientists think there are. When you look at the sky you don't see the present, you see the past.

[Tormod]

Originally Posted by bugmenot

The question still remains. How does anyone know that what they are looking at is still there and not dead.

We don't. We don't know how many stars are yet to be born, either.

[Tormod]

Well, to be correct we can detect at what stage in it's life a star is at by measuring different things like the light spectrum etc. We can also spot stars that are about to go supernova etc. Of the few thousands stars that we see with the naked eye, and the trillions and trillions of stars in the universe, there will be stars of almost any age (albeit younger than the universe) and any stage in their development.

[Qfwfq]

I believe he meant about looking at a specific star Tormod.

Originally Posted by bugmenot

I than asked if that was true of the stars in the sky. Many of the stars we see today might not be there at all because they could have blown up million of years ago but we still see them because of the delay of the light from the explosion that takes too long to travel for us to see it when it happened. He said that was not true. Am I right or was my physics teacher wrong?

I agree the teacher's anwer was wrong, unless there was some misunderstanding.

It is an established fact that whatever happens some distance d away cannot be observed until at least a time d/c has elapsed. This includes a star exploding or whatever.