The Real Age Of Celestial Bodies

[Likekinds]

My nemesis was math. I have no idea how the age of a given object is calculated. But if I am correct, when it is determine that the light we observe from an object has been traveling for 2 billion years, we can say the object is 2 billion years old. I have trouble understanding this.

If the light we observe at a given time has been traveling for 2 billion years, this should (I think) tell us only that the object was in place 2 billion years, ago. What I can't grasp is how this gives us the overall age of the object.

We can only see the object at an instant in time. Who can say that the object wasn't emitting/reflecting light for another billion years before it emitted/reflected the light we can now observe? What I'm getting at is, the object didn't materialize instantly and began shedding light. First, there had to be formative years, maybe billions of them.

It takes about 8 minutes for sunlight to reach Earth, but we know the sun is more than 8 minutes old. What am I misunderstanding or failing to connect, here?

[vlaiv]

If we see that light has been traveling for 2By from an object - we can only conclude that object is at least 2By old - but that can't be its actual age - it has to be more than that - your logic is sound, it can't just pop into existence for us to see after 2By.

There are different ways to determine age of things.

If we look at a galaxy - we can see estimate what sort of stars are in galaxy based on spectrum - stellar classes. This is in turn related to mass of those stars and mass of a star determines how long a star can live. There are different populations of stars - population I, II and III - that differ in chemical make up (again spectroscopy / color of stars - examining their light).

In early universe - there was no heavier elements that could be present in stars - those are made either in stars themselves as byproduct of fusion - or in supernovae explosions. More complex chemistry of a star is - higher likelihood that it was created from remnants of older stars that exploded.

We can therefore look at a galaxy and determine how much population I and II stars there is - and from that ratio and statistics determine likely age of that particular galaxy. Similarly - we can observe galaxies in clusters that are of similar age (like stellar clusters contain stars of similar age).

There are bunch of different methods to estimate age of something.

We know an upper bound of about 13.8By - that is the age of universe. That we know from something else. We measured CMB - which is cosmic microwave background radiation.

Every body that has some temperature emits radiation and from shape of spectrum of that radiation we can measure temperature of that body. CMB shows that universe at the time of last scattering is now at about 2.7K. It has cooled down from initial temperature to that temperature due to expansion of universe. We can also know what temperature at the time of last scattering was - because we know temperature at which plasma forms.

From these and other indicators and fact that space is expanding - we can calculate age of universe.

Hope this answers your question a bit?

[Sabalias]

I don’t think you are missing anything. Your logic is sound for most celestial bodies. If we see a star 500 light years away go nova then it went nova 500 years ago by our reckoning or time. The problem is the concept of time and space. If we see something sufficiently far away then, in theory, we might be looking at the light from an object that just came into being (possibly at the point of the Big Bang though that brings up even more questions, particularly when you consider that as you get farther away the objects are travelling faster). The age of a star is estimated by looking at its motion, luminosity and spectrum. 

Stu

[Xilman]

If you see now something which is two billion light years away then, without further information, all you can conclude is that it was in existence two billion years ago. It might have exploded/collapsed/evolved into something else/whatever since then. Likewise, you can not say how old it was when the light left it which you see it now (again absent further information).

Of course, we generally do have further information and can predict how old it was when the light we see left it, and we can predict its likely evolution since then.

[Likekinds]

On 19/06/2021 at 10:35, vlaiv said:

If we see that light has been traveling for 2By from an object - we can only conclude that object is at least 2By old - but that can't be its actual age - it has to be more than that - your logic is sound, it can't just pop into existence for us to see after 2By.

There are different ways to determine age of things.

If we look at a galaxy - we can see estimate what sort of stars are in galaxy based on spectrum - stellar classes. This is in turn related to mass of those stars and mass of a star determines how long a star can live. There are different populations of stars - population I, II and III - that differ in chemical make up (again spectroscopy / color of stars - examining their light).

In early universe - there was no heavier elements that could be present in stars - those are made either in stars themselves as byproduct of fusion - or in supernovae explosions. More complex chemistry of a star is - higher likelihood that it was created from remnants of older stars that exploded.

We can therefore look at a galaxy and determine how much population I and II stars there is - and from that ratio and statistics determine likely age of that particular galaxy. Similarly - we can observe galaxies in clusters that are of similar age (like stellar clusters contain stars of similar age).

There are bunch of different methods to estimate age of something.

We know an upper bound of about 13.8By - that is the age of universe. That we know from something else. We measured CMB - which is cosmic microwave background radiation.

Every body that has some temperature emits radiation and from shape of spectrum of that radiation we can measure temperature of that body. CMB shows that universe at the time of last scattering is now at about 2.7K. It has cooled down from initial temperature to that temperature due to expansion of universe. We can also know what temperature at the time of last scattering was - because we know temperature at which plasma forms.

From these and other indicators and fact that space is expanding - we can calculate age of universe.

Hope this answers your question a bit?

 

 

 

 

 

 

 

 

 

 

 

 

 

Very much so.

Thanks!

 

 

 

 

 

 

 

 

 

 

 

[Likekinds]

On 19/06/2021 at 10:43, Sabalias said:

The age of a star is estimated by looking at its motion, luminosity and spectrum. 

Great. Thanks.