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Offline flr

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confusing article?
« on: 16/01/2013 03:32:02 »

I was a bit confused by this article:
http://www.dailymail.co.uk/sciencetech/article-2262091/A-glimpse-universe-soon-Big-Bang-astronomers-identify-star-dated-13-2-billion-years-old.html

They mentioned that the oldest star formed about 13.2 billions years ago is just 186 light years away from us.

186 light years away from us? How this could be?

This star exploded long ago and it must be at least 13.2 billions years away from us + extra distance due to space dilation, right?


 

Offline Pmb

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Re: confusing article?
« Reply #1 on: 16/01/2013 06:55:57 »
Quote from: flr
I was a bit confused by this article:
I read the article and don't see what could confuse you about it. Coud you explain why you find it confusing in more detail pleaae?

Quote from: flr
They mentioned that the oldest star formed about 13.2 billions years ago is just 186 light years away from us.

186 light years away from us? How this could be?
Why couldn't it be that close?

Quote from: flr
This star exploded long ago and it must be at least 13.2 billions years away from us + extra distance due to space dilation, right?
Why must it be so far away? Perhaps you're confusing the fact that the further away objects are from us the younger they are because we see them as they were long ago.
 

Offline evan_au

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Re: confusing article?
« Reply #2 on: 16/01/2013 09:00:50 »
It is true that when you peer deep into space, you are looking back in time. So often when astronomers talk about "the oldest", it usually is a severely red-shifted, distant object. But because of this distance, you can't make out individual stars, but only the heart of an active galaxy, or "Quasar": http://en.wikipedia.org/wiki/Quasar

However, nearby matter can also be old - cosmologists think that most of the hydrogen atoms in your body would have formed in the big bang. And a nearby star can also be old, if it is formed of matter with a composition that would have appeared in the second generation of stars, as is claimed for this star.

The larger a star grows, the faster it burns its fuel, and the more quickly it dies. The Sun is thought to have enough fuel to last about 10 billion years and is now middle-aged; a smaller star will burn its fuel more slowly, and last a lot longer.

Small stars are miserly in their use of fuel, and so they are not visible over long distances. To analyse a star's chemical composition, you need a good spectrum, which is easier if the star is closer. To confidently estimate the age of an old star using our current telescopes, the star must be fairly close. Stars are spinning around our galaxy in their own individual orbits, so this star would probably have been more distant from us just half an orbit ago.
 

Offline CliffordK

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Re: confusing article?
« Reply #3 on: 16/01/2013 10:45:07 »
Like Evan says, our sun is supposed to have a lifespan of about 10 billion years as a "main sequence" star, after which it will likely become a white dwarf, at which time, it could endure for billions of years, slowly cooling.

HD 140283is considered to be a Population II, Metal Poor star, which is in the Subgiant phase.  I think that puts it at near the end of its lifespan.

I suppose, however, there are two ways to look at the age of a star.
  • The difference between when the star originally formed, and today.
  • The total amount of time between when the star formed, and the light that we see was formed.
If one is considering the lifespan of a star to when the light was generated, then one would have to consider close stars.
 

Offline flr

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Re: confusing article?
« Reply #4 on: 16/01/2013 22:01:34 »

I read the article and don't see what could confuse you about it. Coud you explain why you find it confusing in more detail pleaae?


I though stars born more than 13 billions years ago were huge and short lived (only tens of millions of years life-span).
 

Offline Pmb

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Re: confusing article?
« Reply #5 on: 16/01/2013 22:34:52 »

I read the article and don't see what could confuse you about it. Coud you explain why you find it confusing in more detail pleaae?


I though stars born more than 13 billions years ago were huge and short lived (only tens of millions of years life-span).
I did some reading about this subject today. The larger a star is the shorter its lifespan. This seems to be because the larger stars have more hydrogen to burn but they’re much more luminous. The more luminous a star is the faster it burns up its fuel. A 30-solar-mass star has about 30 times more hydrogen but burns with a luminosity about 300,000 times greater. Thus a 30-solar-mass star lives only 30/300,000 = 1/10,000 as long as the sun. On the other hand a star which as a mass of 0.3-solar-masses emits luminosity just 0.01 times that of the sun and thus lives about 0.3 /0.01 = 30 times longer than the sun. So a star that has lived 13.2 billion years doesn’t seem that outrageous if its mass was much less than that of the sun.

I'm glad this subject was raised. I've always wondered about this but always had something else I wanted to learn. Seeing this made me want to finally get an answer to it which I've now got. So thanks for asking! :)
 

Offline evan_au

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Re: confusing article?
« Reply #6 on: 17/01/2013 09:33:21 »
Quote
I thought stars born more than 13 billions years ago were huge and short lived

It is the first-generation "Population III" stars that had almost no "metals", and are thought to have grown into very large balls of hydrogen and helium before fusion could start.

Astronomers are unique in describing atoms like carbon, oxygen and nitrogen as "metals", but these atoms are thought to play an important role in the collapse of a protostar, and as catalysts for nuclear fusion. 

The star mentioned in the original post is classed as a "Population II" star, formed from original hydrogen & helium which has been enriched by the injection of matter from supernovae of the earlier "Population III" stars. This gives them a small but significant metal content, allowing fusion to start despite their low mass.
 
See http://en.wikipedia.org/wiki/Metallicity#Population_III_stars
 

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Re: confusing article?
« Reply #6 on: 17/01/2013 09:33:21 »

 

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