Three Largest Stars Identified

mOoZik writes "BBC News is reporting that astronomers have identified the three biggest stars known to science, having diameters of more than 1.5 billion km. If they were located in the same place as our own Sun - at the centre of the Solar System - the stars would stretch out further than the orbit of Jupiter!"

Comments

The three widest stars?

[gbulmash]

The three largest stars with huge diameters? That's easy... Louie Anderson, Bruce Bruce, and Roseanne.

Thanks folks, I'll be here all week. Try the veal.

- Greg

just wondering

[adamruck]

Why wouldn't these huge starts turn into black holes?

Re:just wondering

[roseblood]

Large dosen't mean heavy. LARGE RED stars are going to be very thin, not much density. All of their material will be spread out over quite a large area. A LARGE BLUE star on the other hand, would be quite dense (and short lived...they burn their fuel much faster and die in billiant novas, or if they are TOO heavy, as blackholes.)

Re:just wondering

[albn]

Why wouldn't these huge starts turn into black holes? This URL may help you

According to the web site: A star of 15 solar masses exhausts its hydrogen in about one-thousandth the lifetime of our sun. It proceeds through the red giant phase, but when it reaches the triple-alpha process of nuclear fusion , it continues to burn for a time and expands to an even larger volume. The much brighter, but still reddened star is called a red supergiant. Betelgeuse , at the shoulder of Orion, is the best-known example. Absolute luminosities may reach -10 magnitude compared to +5 for our sun.

Some of these supergiants are unstable and form the very important Cepheid variables. In their final stages, supergiants may explode into supernovae . The collapse of these massive stars may produce a neutron star or a black hole .

Re:just wondering

[Nogami_Saeko]

And for some more black-hole info:

Black Hole FAQ

And on a side note, it's been a long time since I've watched my DVD of "The Black Hole", so I may have to do that now :). The last time I watched it, I was surprised how dark it was (no pun intended) for a "Disney Movie". May also have explained why I liked it so much as a kid...

N.

Re:just wondering

[roseblood]

YES, they can be that thin.

If I may be lazy and just give you a URL:
http://www.astronomynotes.com/evolutn/s5.htm

Re:just wondering

[techno-vampire]

Alas, there are no green stars. Even if their temperature is such that their radiation peaks there, green has such a narrow band of frequencies that either yellow or blue will always predominate.

Re:just wondering

[physicsphairy]

This is a bit reverse logic, but the reason they don't collapse is because they're still burning.

You can think of the fusion reaction in a sun as it's 'defense' against collapse. The force driving the future collapse, gravity, is what's sustaining the fusion reaction, which creates internal photonic pressure, which in turn pushes the mass of the star outward, counteracting the force of gravity.

The reason these stars are so large is in fact directly related to the photonic pressure produced by this reaction. If the gases are very hot it prevents the gas from codensing, i.e., you need a lot of it (a big star) to combat gravity. Once these go supernovae and leave clouds of elements that burn at a lower temperature, smaller stars will be able to form.

Re:just wondering

[SandmanWAIX]

supernovae . The collapse of these massive stars may produce a neutron star or a black hole .

For a detailed example of supernovae -> black hole, click here.

Re:just wondering

[coyote-san]

Stars don't become black holes until they burn up their fuel, collapsing (and perhaps exploding, perhaps even multiple times) in on themselves until they are much more dense than any visible stars.

You might want to check university pages, not just some guy's geocities page.

Stars collapse once the core has exhausted its available fuel. This is only a minute fraction of the star's total mass, but it's critical. When the core goes dark the rest of the star falls on it.

According to an article in Discover magazine a few years ago, parts of the star will fall towards the center with a speed as high as a third of the speed of light! This causes enormous pressure, during the "big crunch" the density of the star may be 5-6 higher than the density of a neutron star. IIRC the massive neutrino flux is produced at this time. BTW this "core" is substantially far larger than the core mentioned earlier.

Matter can't be compressed this hard for long and the core "bounces" back. That is what flings the outer layers of the star into space. But force goes both ways - what throws stellar masses into space also increases the pressure on the remaining core. If the density gets too high a black hole is created and it quickly consumes the core, but the outer layers have already been ejected. Otherwise the core eventually bounces back entirely and you have a neutron star. A neutron star is a core of degenerate matter covered by a layer of normal matter.

You do not get cycles of explosions.

(I seem to recall hearing about flares on neutron stars after enough normal mass has fallen to trigger fusion, but those flares are fall smaller than supernovas.)

I'd like to announce the official...

[The Ultimate Fartkno]

..."Name Three Fat Women In Entertainment" thread right here. Skill points will be deducted for all mentions of Delta Burke, Oprah, and Anna Nicole Smith. You have thirty seconds from the time you read the headline and pounced on the "reply" button.

Go.

Solist!

[IcEMaN252]

Don't be so ethnocentric. There are such things as trinary systems.

Re:My only wish

[daeg]

Dear nihilogos: We're sorry, but all intelligent designers are currently assisting other beings. Your request will be tended to in the the order that was receieved. The current wait time is: /pause/ 3 billion years. Please hold.

Re:Non Red Giants

[mOoZik]

Actually, it will engulf the first three planets, but not extend to Jupiter.

Re:Visible black holes?

[cavetroll]

no. The mass of the stars is big, but they are very far away.

Consider:

density of sun = ~1400 kg/m^3

let us assume these stars have the same density (they don't, it will be lower, but that is ok for our purposes here)
diameter 1.5 billion km = 1.5E12 m

volume (assume a perfect sphere) = 4/3 pi r^3 ~ 1.8E36 cubic metres
giving a mass of 2.5E39Kg (about 1 billion times that of the sun)

the gravitational field strength on an object obeys an inverse square relationship
F=GM/r^2
The nearest of these stars is 5200 light years away, or 5E19 metres
G is the universal gravitational constant, about 7E-11

so
F=7E-11 * 2.5E39 / (5E19*5E19)
F~ 1E-10 N/kg

for comparison, the gravitational field strength on earth is about 10 N/kg, ie 100 billion times larger. /me waits for someone to point out an error in my arithmatic

No worry -- the world will not end

[helioquake]

Don't worry about it. These giants are big, but not necessarily massive enough to go supernova at the end of their lives.

Besides, hypothetically, even if it were to explode like a supernova, it won't affect us much. Here is the number:

d = distance to the closest giant (5200light-yr)
E = total energy arising from supernova (1e51erg or something like that)

The energy receied at the Earth is

E / (4 *pi *d*d).

Now compare this number with the energy we receive every second from the Sun:

E_sun / (4 * pi * r*r)

where r is the distance between the Earth and the Sun (1.5e13 cm). You do the math, then the ratio of these two quantities comes out to be:

[E/(4*pi*d*d)] / [E_sun/(4*pi*r*r)] ~ 2.4

So all we get from this supernova is about 2 seconds worth of energy received from the Sun. And I'll tell you that the actual energy received from the supernova is much, much smaller.

Largest?

[marevan]

Correct me if I'm wrong, but isn't red giants dencity pretty low? So when a star transforms into a red giant, it's bound to get much larger. So wouldn't it be cooler to find actually non-dying star of this magnitude?

(Well definetly not cooler)

If they were located in the same place

[frovingslosh]

If they were located in the same place as our own Sun - at the centre of the Solar System - the stars would stretch out further than the orbit of Jupiter!"

Shows what little they know. If they were located in the same place as our own Sun, Jupiter would burn up and not have an orbit!

More info for the non-physics folk...

[TiggertheMad]

Several other posts have danced around the question a little bit, without answering it directly. It's a good question.

While these stars are big, filling a large volume of space, the article doesn't mention their mass. This is the ultimate determinant of what becomes a black hole and what doesn't.

Stars have gravity trying to pull everything into the center off it's mass. In physics pressure is basically equal to temapture, so as all the mass is squezed together, it heats up and begins nuclear fission. This creates a lot of heat, and the star's mass tries to expand. Gravity and pressure find a happy meidum and that is how the star ends up a particular size.

As the star burns it's fuel, it has to get hotter or it will stop 'burning', due to the way nuclear fusion works. Eventually it will burn up its fuel and prssure will not balance gravity, and the whole star will collapse. If it is really heavy, say several times the mass of the sun, it will probably collapse into a black hole. If it is slightly heavier than our sun, it might end up as a very dense neutron star. Otherwise, it will end up as a white dwarf, a small star that is somewhat like a ember left over after a campfire. If a star is really massive it can also explode in a supernova to lose some weight and avoid becomming a black whole.

As I mentioned, the article doesn't say what the mass of the star is, but it's probably a safe bet that is above the black hole limit. When it finishes burining its fuel, it will likely go supernova and/or become a black hole.

Re:More info for the non-physics folk...

[Kugelfang]

You're wrong in two points:
a) Turning into a black hole is determined by
mass/radius ratio. You could even turn out sun
into a black hole by "somehow" ( :-) ) pushing
its radius below the Schwarzschild radius.
b) "it heats up and begins nuclear fission."
-> You mean "fusion", as fusion needs heated-up
gas (plasma) to start. Fission is what happens
in nuclear power plants ;-)

Wrong Units

[willpall]

"1.5 billion km across" means nothing to me. How many Libraries of Congress could the star hold?