New Mars Forums

Tom Kalbfus

Banned

Registered: 2006-08-16

Posts: 4,401

Lets have some fun with Gamma Crucis "Gacrux"

This is a massive red giant, I didn't know there was one so close to Sol, at 88 light years distance, this is a star which may someday be visited by humans. At 1,500 times as bright as Sol, the habitable zone is around 38 AU, but the mass of this star is only 3 times as much as our Sun, this gives me some ideas of what a Type II civilization might make out of this. You want to mine stars and make huge structures out of stellar masses? Well this red giant mines itself, it expels its own mass outward, as its puny surface gravity is insufficient to contain its atmosphere. One could build a Dyson Bubble around this, a shell supported by radiation pressure, like a Solar Sail statlite, some allowances have to be made for this red giant's pulsations. I think a 40 AU Dyson bubble could be possible, and we can make it using vapor deposition around a framework surrounding this star. When the bubble is complete, we can capture most of the material expelled from this star as well as harvest its 1500 Suns worth of radiative output. Its a dying star, but that doesn't mean we can't profit from it! One project, we could build a ringworld, not for this star, but for some other star. We'd build it on the inside of a 2 AU hole in the side of this Dyson bubble, which would gather the material expelled from the star and accelerate it, and fuse this material using the power of 1500 Sun into proper building material, one Jupiter's worth of mass, Should we make carbon, or iron, or maybe something stronger than normal matter? Perhaps particle accelerators that are 40 AU in radius could make magnetic monopoles, it could make antimatter and microscopic black holes as well, certain to be worth exporting over interstellar distances.

Below is a Summary of information about this star:

The Star

Gacrux is a cool red giant star of spectral and luminosity type M3.5 III (but was once thought to be a "bright giant" of luminosity type II). The star may have as much as three times Sol's mass, as much as 113 times its diameter, and about 140 times Sol's visual luminosity (based on a HIPPARCOS Johnson Vmag of 1.59 at 87.9 ly). It may have around 1,500 times Sol's luminosity including infrared radiation (according to calculations by Professor Jim Kaler -- see his Stars page on Gacrux). Given Gacrux's high mass, the star would have been a spectral type B dwarf before it evolved out of the main sequence.

NASA Observatorium

Gacrux is redder and
bigger than giant stars:
Aldebaran, Arcturus,
Capella, and Pollux.

See a discussion of
the "main sequence"
as part of stellar
evolution and death.

Gacrux has been found to be variable by a few tenths of a magnitude. It is defined to be a "semi-regular variable" of less than 90 days, but its variability is not quite predictable (Murdoch et al, 1992). The star has been given the New Suspected Variable designation of NSV 5672. Useful catalogue numbers and designations for Gacrux include: Gam Cru A, HR 4763*, Gl 470, Hip 61084, HD 108903, CD-56 4504, CP(D)-56 5272, SAO 240019, FK5 468, LTT 4752, WDS 12312-5707 A, Dunlop 124 A, and ADS 5672 A.

Although it is now considered to be a single star, Gacrux may actually have a binary companion. It has long been known that the outer layer of Gacrux is contaminated with barium and other elements which are formed by the slow capture of neutrons. Such "barium stars" may be binaries, where a more massive companion has already thrown off its outer gas envelopes as a planetary nebula in becoming a white dwarf (see HD 147513 AB). In casting off its outer layers, however, such an evolved companion star would have contaminated Gacrux with the later by-products of its more advanced nuclear processes. Hence, as a "mild" barium star, Gacrux is believed to have an unseen white dwarf companion, like Alphard (Alpha Hydrae) (Mennessier et al, 1997).

Given the short life of massive B-type stars, it is highly unlikely that an Earth-type planet with advanced multi-cellular life could have developed in Gacrux's water zone before it left the main sequence. By now, moreover, any Earth-type planets that orbited Aldebaran A during its youth would have been burnt to a cinder, and possibly fallen into the star from frictional drag with the giant star's gaseous envelope. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.

As a star that has evolved out of the "main sequence," Gacrux has shifted fully from the fusion of hydrogen to helium at its core to the fusion of helium to carbon and oxygen, with trace activity of other nuclear processes. This helium-burning, orange-red giant stage is relatively brief, lasting tens to hundreds of million years (e.g., lasting around 700 million years for a star of one Solar mass).

Gacrux, however, appears to be more highly evolved than nearby giant stars: Aldebaran, Arcturus, Capella, and Pollux. The star is blowing a strong but variable wind (Mullen et al, 1998). As a result, Gacrux may have passed the helium-burning "clump" giant stage. It may now be entering the "asymptotic" or "second-ascent" giant stage, where it is becoming brighter for the second time.

qraal

Member

From: Brisbane, Australia

Registered: 2008-01-02

Posts: 65

Re: Lets have some fun with Gamma Crucis "Gacrux"

Hi Tom

Giant stars pump out a lot of carbon so fusing its emissions may not be needed. Just separate out the light stuff, storing it on some suitable smaller mass, and gather the carbon for construction. Diamondoid is the material of the "far future". Unfortunately you won't be making rotating Dyson spheres out of the stuff.

Tom Kalbfus

Banned

Registered: 2006-08-16

Posts: 4,401

Re: Lets have some fun with Gamma Crucis "Gacrux"

Hi Tom

Giant stars pump out a lot of carbon so fusing its emissions may not be needed. Just separate out the light stuff, storing it on some suitable smaller mass, and gather the carbon for construction. Diamondoid is the material of the "far future". Unfortunately you won't be making rotating Dyson spheres out of the stuff.

Not rotating, but you could make one supported by radiation pressure against very weak gravity at this distance. Gacrux had 3 Sol Masses or 990,000 Earth masses, might as well call it an even million. 40 AU is 6000 million km, the Earth's radius is 6400 km, so the radius of this sphere is 1 million times larger than the Earth, since gravity follows the inverse square law the gravity experienced at the surface of this Dyson would be 1/(1,000,000^2) of Earth's surface gravity or about 0.000000000001g , it says here http://en.wikipedia.org/wiki/Radiation_pressure that radiation pressure of sunlight at 1 AU is 9 micronewtons since were talking about an equivalent pressure at this radius the Dyson would experience 8.1 micronewtons per square meter at 40 AU. 1 g exerts 9.8 newtons on a kg so 0.000000000001g would exert 0.0000098 micronewtons on that same kg, if a square meter weighs 1 kg, then it would experience 8.1 micronewtons of outward force and 0.0000098 micronewtons of inward force due to he pull of gravity to balance that out by mass alone 8.1/0.0000098 = 826,531 kg, the density of diamond is 3,500 kg/m^3 this would imply the radiation pressure alone could support a shell that was 236.15 m thick made out of diamond. The diamond shell would have to hold itself together against the outward force of radiation pressure if you didn't make it that thick!

Last edited by Tom Kalbfus (2014-11-12 06:22:16)

Void

Member

Registered: 2011-12-29

Posts: 7,169

Re: Lets have some fun with Gamma Crucis "Gacrux"

No criticism, just curiosity.

If a sphere is made, then to some degree it is a concentrating mirror with the star at it's focus.  What does that do?  Or did you already address that?

---

Done.

Tom Kalbfus

Banned

Registered: 2006-08-16

Posts: 4,401

Re: Lets have some fun with Gamma Crucis "Gacrux"

No criticism, just curiosity.

If a sphere is made, then to some degree it is a concentrating mirror with the star at it's focus.  What does that do?  Or did you already address that?

I think to harness the star's energy, the surface must absorb it, this halves the radiation pressure as most of the light is not reflected back at the star but absorbed by solar collectors, the heat can then be radiated from the outside of the sphere. Radiation pressure would still support it. The thing to remember is when a star becomes a red giant, it becomes thousands of times brighter, but the mass stays the same. You get a whole lot more radiation pressure in relation to gravitational pull, this means you can make a thicker Dyson Shell without worrying about how it would support itself against the star's gravity. This thing is 40 AU in radius, just think about it, whereas a 1 AU radius Dyson Shell has the surface area of 1 billion Earths, this Dyson has a surface area 1500 time greater or 1.5 trillion Earths. Your could make a Matrioshka brain out of it. http://en.wikipedia.org/wiki/Matrioshka_brain, I'm not interested in creating a supreme being with godlike intelligence, but in simulations of virtual worlds. With a Matrioshka brain, one could simulate the living worlds of an entire galaxy, one could upload into it, and have a whole galaxy of Earthlike worlds to explore, with convenient physics, no pesky light speed that you can't get around for instance, just press the magic button and poof, you are their in virtual space, the simulation can be as real as one desires. The Star is dying anyway, might as well make some use of its tremendous energy output before it goes.