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Hd140283 subgiant star

by bobie
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bobie
#1
Jul2-14, 02:26 AM
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HD 140283, informally nicknamed Methuselah star is a ...subgiant star about 190 light years away from the Earth
...estimate an age for the star of 14.46 0.8 billion years.[2] Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe , 13.798 0.037.[2] ...., the star must have formed soon after the Big Bang

SMSS J031300.36-670839.3 (abbreviated to SM0313[3]) is a star in the Milky Way at the distance of 6000 light years from Earth. With an age of approximately 13.6 billion years,[4] it is the oldest known star in the Universe.[2][5][6] The star formed only about 100 million years after BB

found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old.

Here http://en.wikipedia.org/wiki/Chronol...00_no_WMAP.jpg it is said that first stars appeared 400,000 year after BB,

- is HD (and SMSS) an exception, how can HD have formed soon after (if not before!) BB, and most of all
- how can it be only 190 ly away, when star formed after 500,000 y are 13.2 G ly away?
- why is its redshift not known?
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Bandersnatch
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Jul2-14, 05:08 AM
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Quote Quote by bobie View Post
- is HD (and SMSS) an exception, how can HD have formed soon after (if not before!) BB
I'm not sure why you find it surprising that stars had begun forming after the BB. They had to start sometime, don't you think? All that prevented the gas from collapsing before about 400 000 years after BB was its high temperature. Once it's cooled down, there was nothing to stop the process.
There's nothing exceptional about these stars, apart from low metal content. Any low-mass red dwarf will burn slowly for as much as trillions of years. As with all red dwarf stars, it's hard to find them due to their faintness.

- how can it be only 190 ly away, when star formed after 500,000 y are 13.2 G ly away?
If it were 13.2 Gly away, it'll look like a very young star that's just formed. If you want to see the oldest objects, you need to look close around you. The farther you look, the younger stuff looks. Our Milky Way looks like a ~13 Gy galaxy, while Andromeda looks some 2 million years younger.


- why is its redshift not known?
Why do you think so? I'm pretty sure that's how they calculated the radial velocity.
Unless you mean the cosmological redshift, due to the expansion of the universe? This effect doesn't exist on galactic scales, which is where these stars are.

(By the way, can you refrain from using different-sized text? It's hard to read)
mfb
#3
Jul2-14, 05:37 PM
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- is HD (and SMSS) an exception, how can HD have formed soon after (if not before!) BB, and most of all
14.46 0.8 billion years means a value of 13.60 (~200 million years after the BB) is well in agreement with the uncertainty, and even an age of just 13 billion years (~800 million years after the BB) is possible. The latter value is nothing special.
- how can it be only 190 ly away, when star formed after 500,000 y are 13.2 G ly away?
There are objects of all ages everywhere. We just can't measure the age of the galaxies nearby reliably and we cannot see individual stars very far away. To see old galaxies, we have to look far away, which is also a look into the past - we see the galaxies as they looked 13 billion years ago. This also implies they have to be at least 13 billion years old.

Here http://en.wikipedia.org/wiki/Chronol...00_no_WMAP.jpg it is said that first stars appeared 400,000 year after BB
400 million years, not thousand.

Bandersnatch
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Jul2-14, 11:37 PM
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Hd140283 subgiant star

Quote Quote by mfb View Post
400 million years, not thousand.
And, of course, I repeated the mistake without thinking. So let me clarify: 400 000 is about the time when the universe cooled off enough to stop being opaque to light, leading to the emission of CMBR. That is, the gas was about 3000K back then, way too high to form stars. It took another few hundred million years to cool down enough to allow the formation of stars.
bobie
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Jul3-14, 02:55 AM
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Quote Quote by Bandersnatch View Post
I'm not sure why you find it surprising that stars had begun forming after the BB.
I did 13.798- (14.46 -0.8) and found 138My.
So the galaxy started forming some 500My after HD, and the Sun much later. Do we know when Sagittarius A was formed?
We see HD because it is near, we see it because light was emitted only 190 years ago, if it were far its light would be too faint to be detected.

the cosmological redshift, due to the expansion of the universe? This effect doesn't exist on galactic scales,
What is the minimun scale, when can it be detected? redshift can be as small as 0.003 or even smaller?

Thanks
Chronos
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Jul3-14, 03:32 AM
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Expansion is really not detectable at less than about 30 megaparsecs [~100 million light years]. Peculiar motion tends to dominate at lesser distances.
bobie
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Jul3-14, 04:46 AM
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Quote Quote by Chronos View Post
Expansion is really not detectable at less than about 30 megaparsecs [~100 million light years]..
Thanks,
does that mean we cannot get information about H more recent than 100My? or we can , indirectly?
Chronos
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Jul3-14, 05:04 AM
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Expansion is only noticeable over vast distances. Keep in mind the typical galaxy has a peculiar velocity around 600 Km/sec. The Hubble constant is only about 70 Km/sec per megaparsec [about 3-1/4 million light years]. So, it is basically impossible to distinguish the peculiar velocity component from the Hubble component for any galaxy less than about 30 Mpc distant.
bobie
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Jul3-14, 05:12 AM
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Quote Quote by Chronos View Post
Gravity dominates at lesser distances.
you mean gravitational redshift?
So, we can get information on the current expansion rate only in 100 M years from now?
Chronos
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Jul3-14, 04:01 PM
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This in not related to gravitational redshift. The force of gravity is vastly stronger than the force of expansion over 'short' distances. This is why gravitationally bound systems [like galaxies and solar systems] are not measurably affected by expansion. Light travel time also confuses more than helps. Cosmic entities were not at that 'distance' when their photons were emitted, or received. The expansion of space keeps distances dynamic. They were closer when their photons were emitted and more distant by the time they are detected here on earth. Think of a police radar. A speeder flies by a radar unit. The radar gun fires when the car is 100 feet past the radar unit. The vehicle travels another 2 feet by the time the reflected signal is detected by the radar unit [yes, we are talking a serious fine here].
D H
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Jul3-14, 05:09 PM
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Quote Quote by bobie View Post
Quote Quote by Bandersnatch View Post
I'm not sure why you find it surprising that stars had begun forming after the BB.
I did 13.798- (14.46 -0.8) and found 138My.
What made you do that? I hope this aspect of your confusion has been cleared up.

Just in case it hasn't, we should see a mix of old and young stars in our vicinity. Many (most!) stars live 13.78 billion years or longer. It's only the big stars that die young, and most stars aren't big at all. However, assessing the age of a main sequence star is a non-trivial task, particularly those whose mass is over 0.5 solar masses. The signs of aging (helium that results from fusion) stays in the core in stars such as HD 140283, and our Sun. The main reason astronomers could establish an age for HD 140283 is because it just recently went off the main sequence.


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