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Star Smaller Than Some Planets Found
Abhishek writes "Astronomers have found the tiniest full-fledged star known, an object just 16 percent bigger than Jupiter. It is smaller than some known planets that orbit other stars.
The star is a companion to a Sun-like star toward the center of our Milky Way Galaxy. It was found and measured by observing changes in the light output of the system when the smaller star passes in front of the larger star from our vantagepoint. This would give a better idea of brown dwarfs or failed stars. The star has been named OGLE-TR-122b. This discovery also marks the possibility of stars that look strikingly like planets."
The article is misleading confusing "size" with "mass". This new star has 95 times the mass of Jupiter. However, it's density is so great that its physical size is only slightly larger. Density is the trigger key for fusion.
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Neutron stars were stars at one point, but they're not stars in the same sense that the Sun is. When a Sun-like star ends its lifecycle with a (super)nova burst, it gets rid of most of its mass, and the rest collapses into a small neutron 'star', or a black hole if the mass was sufficiently large.
IANAP, but I think no thermonuclear reactions take place in neutron 'stars' (or maybe just not enough to sustain the emission of light, so they're not easily visible). So basically neutron stars are just dead stars.
Before anyone starts panicing about Juptier collapsing into a companion star to the sun, and screwing over our whole solar system pretty royally - please note that while this star is only 16% larger than Jupiter in volume, it contains 95 times as much mass.
The Chandrasekhar limit is 1.4 solar masses, and is quite irrelevant to the lower limit on the mass of hydrogen which will form a star.
This star has no bearing on the Chandrasekhar limit.
The Chandrasekhar has to do with the _death_ of stars, not their birth. To be considered a star all you need is sustained hydrogen fusion at the core, not electron degeneracy. While it's interesting that we are seeing a dense low mass star the value of L C is in no danger of being rewritten.
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Furthermore, neutron stars aren't dead -- they often radiate a hell of a lot of energy. Those that do are called *Pulsars* -- that's where all those directional radio/x-ray waves come from in deep space -- they spin like lighthouses, you see.
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You are a lucky man, Lord Pillage; you now have a golden opportunity to expand your horizons by learning all about white dwarfs and neutron stars, and about the stark and dramatic physical differences between them.
Enjoy!
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Been a long time since I was I was into nuclear phys, but how can it maintain that density with such (relatively) small mass? The process of fusion, which tends to expand a star, equally balances gravity which tends to contract a star. Seems to me a normal star would expand due to fusion.
Basically, it doesn't make sense that it can maintain being 1/10 the mass of the sun and 50x as dense. This means its fusion output must be tiny (little to balance gravity), but why? Is it mainly made of non-hydrogen mass? They should be able to tell the elemental composition from the spectrum. And how could it have such little fusion and not be a brown dwarf?
Wish this press release had some science in it.
Actually, you got the right answer! The star has expanded due to nuclear fusion taking place it just hasn't expanded dramatically because it doesn't take a dramatic amount of fusion to support a star with so little mass.
The reason fusion is needed to support a star is because the heat generated through contraction is radiated in to space. The energy lost through this process needs to be replaced to keep the volume of the star constant - and fusion provides this energy.
There are two reasons why a star this small can exist. The first is the low mass of the star. The smaller the mass of the star the less heat it takes less energy to raise the tempreture of the entire star. This means it takes less energy to maintain the tempreture of the star and this means less fusion.
The second reason is surface area. The Sun has a surface area of approximately 6 x 10^20 meters compared with Jupiter's 6.4 x 10^18 meters. This star is only slightly larger than Jupiter in terms of volume and so will have a comparable surface area. This means that the radiation of heat in the star will not be as efficient as in the sun and that means less fusion is required to keep the tempreture of the star constant.
Since the tempreture, among other things, determines the size of a star both of these factors allow the star to remain balanced and still stay fairly compact. So while it suprising that stars this small exist it is not inconsistent with theory.
Simon.