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Watsisname
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02 Feb 2017 02:08

Wow.  0.63% the speed of light.  By a strange coincidence, that's very close to the rotational speed of the electron in a hydrogen atom (in the Bohr model of the atom, anyway).
 
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05 Feb 2017 07:44

I remember seeing a gigantic star with a radius of 23 AU somewhere, but I don't remember where it is.
 
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Xoran
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05 Feb 2017 08:42

jasperhb wrote:
Source of the post I remember seeing a gigantic star with a radius of 23 AU somewhere, but I don't remember where it is.

Radius of 23 AU is diameter of 46 AU, since radius is average distance from an objects surface to its center, and diameter is average distance between a objects surface to the other side of the object.
Someone found a star with a diameter of 143 AU earlier in this thread i think.
But an object at a size of 46 AU is a gigantic object :)
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Andromeda
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05 Feb 2017 15:35

Xoran wrote:
Source of the post Radius of 23 AU is diameter of 46 AU, since radius is average distance from an objects surface to its center, and diameter is average distance between a objects surface to the other side of the object.
Someone found a star with a diameter of 143 AU earlier in this thread i think.
But an object at a size of 46 AU is a gigantic object

But, is this possible in real life? Is there a maximum size of stars?
 
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08 Feb 2017 03:16

The Pistol star in the Quintuplet Cluster is the largest known star coming in at a whopping 115 solar masses.
The first order theoretical limit on stellar size is from the Eddington Limit. Calculations by Sir Arthur Eddington and Paul Ledoux.
A purely Hydrogen star could possibly reach 200 solar masses or if stars combine to form one large mass. As the star collapses it is balanced by radiation pressure from fusion. However, the fusion rate scales strongly with density (which is why the most massive stars have extremely short lifetime) so if the star was massive enough, the radiation pressure would probably blow it apart. In fact, this could lead to a pair-instability supernova and there wouldn't even be a black hole remnant even though the star is so massive. Here is a whole bunch of math about it:
http://astronomy.stackexchange.com/ques ... for-a-star
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08 Feb 2017 12:12

There are some big uncertainties in our knowledge of the properties of the most massive stars, but Gnargenox pretty much got the gist of it.  It seems to be in the realm of about 150-200 solar masses, and is borne out of both theoretical considerations and by observations.

We can also relate this to a star's maximum size through stellar models, where we look at the conditions throughout the star (density, temperature, etc) as a function of initial mass and composition, and evolve the star through time.  Here's an interesting study which evolves models of population III stars as a function of initial mass and rotation.  

If I were home right now I'd post snapshots of the relevant figures, but for now the link will have to suffice and you can browse at your pleasure.  Figures 1 and 2 are what I'll be drawing upon here.  These figures show the log (base 10) of the luminosity and effective temperature of the stars as they evolve (essentially these are their HR diagrams).  We can use this info to derive the radius of the star by using the Stefan Boltzmann law,

[math]

or solving for radius and expressing as ratios to solar units,

[math]

From figures 1 and 2, it looks like in their late evolution these stars can reach log(L) ~6.2 to 6.6, or about 2 to 4 million solar luminosities, and the surface temperatures drop to about log(T)~3.6 = 4000K or so.  

That means their sizes could grow to as much as 3000 to 4000 times that of the Sun!  That's nearly 20AU!  Stars can become very big, indeed.  Compare to Betelgeuse, which is about 900 (plus or minus 200) solar radii, and the largest known star according to wikipedia is UY Scuti at about 1700 [math]200 solar radii.
 
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10 Feb 2017 16:08

Found a 15 star system with a black hole.

RSC 8485-2360-0-0-0
Just someone.
 
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10 Feb 2017 17:30

scruffygamer wrote:
Found a 15 star system with a black hole.

RSC 8485-2360-0-0-0

Like in every other globular cluster. That's pretty average
 
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14 Feb 2017 03:39

Is over 1000 atm high for a terra planet?
scr02050.jpg
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14 Feb 2017 04:07

Gnargenox wrote:
Source of the post Is over 1000 atm high for a terra planet?

Yes
Gnargenox wrote:
Source of the post The Pistol star in the Quintuplet Cluster is the largest known star coming in at a whopping 115 solar masses.

Isn't R136a1 more massive? Wikipedia says it's mass is 315 solar masses.
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14 Feb 2017 05:41

The universe's first stars, objects whose masses are estimated to have ranged from 300 to 1,000 times the mass of the sun were short lived, maybe 1 to 2 million years. None of them are around today for us to see, or at least close enough to be seen by us.

However, in a region of intense star formation called R136a, some 165,000 light-years from Earth and deep in the Tarantula Nebula, a brightly glowing region of hydrogen gas in the Large Magellanic Cloud is spewing large amounts of radiation at very high temperatures. Four of the stars in this stellar nursery account for more than half of the radiation emitted by the cluster's full complement of 100,000 stars.

In the early 1980s, astronomers thought that the entire star cluster, of which the new object is a part, was one star. If so, it tipped the cosmic scale at 2,500 solar masses. But improved observations showed it to be a bright cluster of massive young stars, rather than one object.

R136a1 is what’s known as a Wolf–Rayet star. Its surface temperature is over 100,000 degrees F. It’s also the most luminous star known at more than 7 million times the luminosity of our sun. It might be possible that it is a merger of multiple stars. Since it is so short lived it will go supernova with relatively short notice. Perhaps that is also why we don't see others this size.

The observing technology the team of astrophysicist used can't resolve closely spaced individual stars 165,000 light-years away. It's still possible that any object they interpret as a single star could instead be close binary stars with orientations that make them look as though they are a single object.
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14 Feb 2017 05:49

Wow, 1,030 atm is equal to over 34,900 feet deep in Earth's Oceans. Just 1,100 feet shy of the bottom of Challenger Deep in the Mariana Trench! That's just walking around on the beach on that planet LOL

http://www.kylesconverter.com/pressure/ ... t-of-water
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18 Feb 2017 19:46

Wouldn't it still be possible to view extremely far away stars formed during the beginning of the universe? With sufficiently powerful tools, of course. Even if the stars were very short-lived, we would be able to see into the window of time in which they existed.
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19 Feb 2017 01:31

We do receive light from the first generation of stars (and studying this light helps us understand early periods of the universe, and is a big motivation for the James Webb Space Telescope), but to actually distinguish individual stars from that far back is a very difficult problem.  Let's see what "sufficiently powerful" would actually be for that task:

It is possible to distinguish individual stars in M31 (~2.5 million LY away) with a half-meter telescope (maybe even a bit smaller for the blue giants) under good observing conditions.  A half meter telescope has a diffraction-limited resolution of about 1 parsec at that distance, and a limiting magnitude of about +17.  With a bigger telescope (like Hubble: 2.4m diameter), the stars really stand out.

Now consider resolving stars in a galaxy a thousand times farther away (~2.5 billion light years distant).  Then to achieve the necessary resolution and limiting magnitude, the telescope's size must be at least 500 meters!  Such a large telescope diameter is of course completely implausible, and the scope would also need to be either in space, or use adaptive optics if on the ground.  We could also achieve this resolution with interferometry, but not the limiting magnitude (since that is also dependent on the individual telescope diameter and the atmospheric seeing.)

Finally, consider the first generation of stars, when they first formed.  Let's take this to be when the universe was 500 million years old (we actually think stars began even earlier than that, but let's be generous).

For such a great look-back time, we must account for cosmic expansion.  The universe at 500 million years of age corresponds to a co-moving distance of almost 10Gpc (~32Gly), or a redshift of z=10.  This means the light is redshifted by a factor of z+1=11, so the wavelength we see is 11 times longer than what was emitted.  That puts most of the star's spectrum in the infrared.  (This is also why JWST focuses in the infrared instead of the optical spectrum.)

The expansion also affects the image's apparent size and luminosity.  Fortunately, that actually ends up helping us!  Beyond a lookback time of about 10 billion years (z~1.7), images actually look bigger with increasing distance.  At z=10, the image scale is 4.2kpc per arcsecond on the sky, which is comparable to how big something would look 3 billion light years away if space was not expanding.  But 3 billion light years is still very far away, and as we saw above, requires a stupidly big telescope to actually resolve individual stars.  So even though the expansion helps us, it doesn't help us nearly enough.  Having to look in the infrared also hinders us, since our resolution isn't as good at longer wavelengths.

So with JWST, we won't literally see the first stars, but we will see the first galaxies, and analyze their spectra in much greater detail than ever before.  This will teach us a lot about what the first population of stars was like, how and when they formed, and the interplay between star and galaxy formation.  Should be really fascinating. :)
 
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19 Feb 2017 03:21

Banana wrote:
Source of the post Wouldn't it still be possible to view extremely far away stars formed during the beginning of the universe? With sufficiently powerful tools, of course. Even if the stars were very short-lived, we would be able to see into the window of time in which they existed.

In the beginning of the universe, light would not have very much time to get to where you are, so no.
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