APOD: Alpha Centauri: The Closest Star System (2011 Jul 03)

[APOD Robot]

Alpha Centauri: The Closest Star System

Explanation: The closest star system to the Sun is the Alpha Centauri system. Of the three stars in the system, the dimmest -- called Proxima Centauri -- is actually the nearest star. The bright stars Alpha Centauri A and B form a close binary as they are separated by only 23 times the Earth- Sun distance - slightly greater than the distance between Uranus and the Sun. In the above picture, the brightness of the stars overwhelm the photograph causing an illusion of great size, even though the stars are really just small points of light. The Alpha Centauri system is not visible in much of the northern hemisphere. Alpha Centauri A, also known as Rigil Kentaurus, is the brightest star in the constellation of Centaurus and is the fourth brightest star in the night sky. Sirius is the brightest even though it is more than twice as far away. By an exciting coincidence, Alpha Centauri A is the same type of star as our Sun, causing many to speculate that it might contain planets that harbor life.

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[neufer]

Possible life form on Alpha Centauri

Alpha Centauri: The Closest Star System

By an exciting coincidence, Alpha Centauri A is the same type of star as our Sun,
causing many to speculate that it might contain planets that harbor life.

[Beyond]

Get sirius neufer. IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

[Ann]

It is kind of fantastic that the very closest star to the Sun, Proxima Centauri, is so faint that you can't even see it with a normal pair of binoculars. According to my software, the luminosity of Proxima Centauri is 0.00005500 ± 0.000000340 times that of the Sun, which would make Proxima Centauri roughly one part out of 15,000 as bright as the Sun. Or, to put it differently, image that you were to divide the visible disk of the Sun into 15,000 equal parts. Then Proxima Centauri would shine as brightly as one of those 15,000 parts.

Admittedly the software I use measure the brightness of stars in relatively blue wavelengths. The V wavelength it measures corresponds to 505 nm, which is definitely bluer than the "normal V", which is closer to 550 nm (or so I think). That suggests that Proxima, a cool and therefore reddish star, would be brighter if we measured it in the "normal V" instead. It will definitely be still brighter if we measure it in infrared wavelengths, which is to say that its bolometric, total, luminosity is not as feeble as only one part out of 15,000 of the luminosity of the Sun.

Still, it's amazing that the very closest star to the Sun is so faint that you need a telescope to see it. It makes me wonder if perhaps there is something out there which is even fainter and even closer - something with a respectable mass, much, much more massive than Jupiter, but with such a feeble luminosity that we simply haven't discovered it yet. (Please note that I'm not talking about something like the infamous "Nemesis" or something like that. A large and massive planet or a brown dwarf that was really close to us would have been detected by now, I'm sure of that.)

And of course, it is also amazing that the second closest star to the Sun, Alpha Centauri A, is so very similar to the Sun. I don't know where you will find the next early G-type main sequence star in the sky. Undoubtedly that star is sufficiently far away to look faint enough that it does not in any way call attention to itself.



18 Scorpii, a near-solar twin, whose luminosity is 1.042 ± 0.026 times that of the Sun. Photo: ESO Online Digitized Sky Survey. 18 Scorpii is 45.74 ± 0.57 light-years away. Its "normal V" magnitude is 5.49, which means that you need good eyes to see it at all. Its B-V is 0.652 ± 0.009. This is almost exactly the same color as the Sun, which is to say that the very yellow color in the picture above isn't "true-color".

Ann

[neufer]

Get sirius neufer.

IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

Just kidding.


Actual life form on Alpha Centauri

[rstevenson]

This is my favourite image of Proxima Centauri. It's almost invisible and you need the gray arrowhead to have any hope of finding it (just to the left of center). I'm sure I got this image from the forum here (I've greatly reduced this one so I can post it here) but I can't find it as an APOD. Anyone have the right link to it so I can give credit where due?

Rob

[nstahl]

There's speculation in one or more of the links about inhabitable planets orbiting Alpha Centauri, but wouldn't Beta orbiting at the distance of Uranus from the sun make the chance of a stable elliptical orbit around either one nil? And I suppose there could be orbits about the center of mass of the two but my intuition (fwiw) says such an orbit would mean a lot of time spent too far from either to be comfortable. I suppose there could be an interesting accumulation of matter at a stable Lagrange point in such a system.

[nstahl]

Another question: could the 200 million year difference in ages between Alpha and Beta, and the sun, given in the ESO "above picture" link preclude them having all been formed in the same cluster, or would it make it pretty likely? I.e. are those neighbors also Sol's sibs?

[neufer]

There's speculation in one or more of the links about inhabitable planets orbiting Alpha Centauri, but wouldn't Beta orbiting at the distance of Uranus from the sun make the chance of a stable elliptical orbit around either one nil? And I suppose there could be orbits about the center of mass of the two but my intuition (fwiw) says such an orbit would mean a lot of time spent too far from either to be comfortable. I suppose there could be an interesting accumulation of matter at a stable Lagrange point in such a system.

<<Science fiction has often featured planets of binary or ternary stars as a setting, for example George Lucas' Tatooine from Star Wars, and one notable story, Nightfall, even takes this to a six-star system. In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. Planets that orbit just one star in a binary pair are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or "circumbinary" orbits. It is estimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges.

Simulations have shown that the presence of a binary companion can actually improve the rate of planet formation within stable orbital zones by "stirring up" the protoplanetary disk, increasing the accretion rate of the protoplanets within.

Detecting planets in multiple star systems introduces additional technical difficulties, which may be why they are only rarely found. Examples include the white dwarf-pulsar binary PSR B1620-26, the subgiant-red dwarf binary Gamma Cephei, and the white dwarf-red dwarf binary NN Serpentis. More planets around binaries are listed in THE PHASES DIFFERENTIAL ASTROMETRY DATA ARCHIVE. V. CANDIDATE SUBSTELLAR COMPANIONS TO BINARY SYSTEMS, Muterspaugh et al.

A study of fourteen previously known planetary systems found three of these systems to be binary systems. All planets were found to be in S-type orbits around the primary star. In these three cases the secondary star was much dimmer than the primary and so was not previously detected. This discovery resulted in a recalculation of parameters for both the planet and the primary star.>>

The Question:

Binary stars are popular in science fiction. Star Wars' Tatooine, for example.

I'm wondering about the orbit of a planet around binary stars. Is it possible?

Would the greater combined mass of two stars be more likely to pull in surrounding material, hence making the formation of planets less likely in a binary system?

Would the combined heat and radiation from binary stars mean that habitable planets would have to have a VERY large orbital radius?

The Answer:
David Palmer and Maggie Masetti
for Ask an Astrophysicist

<<There are stable orbits for planets in binary star systems. There are various stability criteria which say when an orbit is stable. One such criteria (and I don't know the actual numbers) says that if all orbits are circular and the stars are the same size, then the planet must orbit one of them at less than /some fraction/ of the inter-star distance, or must orbit both combined at more than /whatever/ times the inter-star distance. Figure-eight orbits are unstable, and can eject the planet from the system.

If you have two Sun-like stars at the center of the system, a planet would be the same temperature as Earth if it were at sqrt(2) = 1.4 astronomical units away, rather than Earth's 1 AU. This distance is closer than Mars's orbit (1.6 AU). Most stars are dimmer than our Sun, so the orbit could be even smaller.

The high energy astronomers at NASA don't know much about this subject, so we asked an expert: Eric Mamajek of Pennsylvania State University:

The solar-like stars 16 Cygni B and 55 Cancri A have been found to have Jupiter-size extrasolar planets orbiting them. So we do have indirect proof, through Doppler spectroscopy methods (Marcy & Butler, SFSU, Lick Observatory), that planets indeed form in binary systems.

The formation mechanisms for forming stars and planets are very different. Planets require accretion to form, specifically accretion in a protoplanetary disk around a young star. Stars can form from the collapse of a molecular cloud core on their own, however planets can only form in the disk around a star. (Pulsar planets are likely formed "posthumously" around pulsars, and are a different beast all together). The main problem with forming planets in multiple star systems is dynamic ejection... stars can simply toss planetesimals out of the system all together (or even accrete them). An example of this is the Kirkwood gaps in the asteroid belt where Jupiter doesn't allow asteroids to exist in certain orbits, and conversely it "shepherds" asteroids in to certain other orbits. A companion star would have a similar effect, except there would be a lot less "shepherding" orbits. The vast majority of binary stars have eccentric orbits. It is difficult for bodies to exist in a system with two very massive bodies in an eccentric orbit. They can only exist very close to each star, or very far from both stars.

An excellent example in the lines of the Tatooine example is the nearby solar-like stars Alpha Centauri A and B. They orbit each other at an average distance of 23 AU, however the eccentricities of each orbit bring them to as close to 11 AU and as far as 35 AU. Numerical simulations by Paul Weigert at University of Toronto have shown that each star has a "safe zone" about 3 AU in radius in which planets could safely survive for billions of years. Objects placed further out from each star than about 3 AU are dynamically ejected in a matter of millions of years or less. Alpha Cen A is about 1.5 times as luminous as our Sun, and Alpha Cen B is about .45 times as luminous as our Sun, and if you do the simple physics, one can see that a "habitable zone" exists around BOTH stars within the 3 AU dynamic "safe zone." Indeed, it could be possible that BOTH Alpha Cen A and B have planets conducive to life. Theoretical models age them anywhere from 3-8 Gyr... plenty of time for life to develop if the planets have the right conditions... >>

[orin stepanek]

It would be interesting to wonder if there were any planets around Alpha Centauri; would they orbit both A and B or just one of the stars. If orbiting only one; then it would have to pass between the two on occasion creating hot climate problem. On the other hand orbiting both would cause climate changes also! Doesn't sound too hospitable for life as we know it.

[neufer]

It would be interesting to wonder if there were any planets around Alpha Centauri; would they orbit both A and B or just one of the stars. If orbiting only one; then it would have to pass between the two on occasion creating hot climate problem. On the other hand orbiting both would cause climate changes also! Doesn't sound too hospitable for life as we know it.

At an orbital distance of 3AU from Alpha Centauri A a planet would receive only ~ 1/6 our own solar radiation from Alpha Centauri A and only up to a maximum of ~ 1/142 our own solar radiation from Alpha Centauri B so there probably wouldn't be a hot climate problem. Life would probably exist only underground or beneath a thick cloudy atmosphere both of which would help to damp out temperature fluctuations. In any event, penguins & polar bears undergo significant annual fluctuations of both temperature & light here on earth.

[Beyond]

Get sirius neufer.

IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

Just kidding.


Actual life form on Alpha Centauri

That's more like it
Notice that there is no bridge in the glasses and that the hands only have 4-digits. Definitely an alien life form.

[neufer]

That's more like it

Notice that there is no bridge in the glasses and that the hands only have 4-digits.

Definitely an alien life form.

All left-handers are from another solar system, IMHO.

[First Contact radio message: "okely dokely" ]

[aestival]

I love this discussion of improbability, given that the Earth-Moon system in which we all live is neither particularly probable nor stable. Also, while it's doubtless very unlikely that planets could have stable orbits outside of 3AU around either star, there are more chaotic systems in this universe, Horatio, than are dreamt of in any computer simulation.

[aestival]

All left-handers are from another solar system, IMHO.

Does that make Triton a visitor from the Alpha Centauri system?

[neufer]

All left-handers are from another solar system, IMHO.

Does that make Triton a visitor from the Alpha Centauri system?

Okely dokely.

[Ann]

Get sirius neufer.

IF that's a possible lifeform on Alpha Centauri, Alpha Centauri is in B-I-G trouble

Just kidding.


Actual life form on Alpha Centauri

That's more like it
Notice that there is no bridge in the glasses and that the hands only have 4-digits. Definitely an alien life form.

Beyond, that's priceless!!!

Ann

[bay area john]

A suggestion to those writing the text for APOD items... I'd think that an item about "the closest start system" would say how close that is, and how it compares with some others. Wouldn't you expect a story about the oldest living human to mention how old that is, and the average lifespan? Of course I can discover the distance with minimal research, but I've noted that APOD more than occasionally misses its own point in the short text. Love you guys, but but think through your story... THANKS.

[Beyond]

Beyond,that's priceless!!!

I can't take full credit for it,Ann. I've been exposed to neufer for tooo long

[nstahl]

Neufer thanks for your research. Again.

[saturn2]

"Proxima Centauri is actually the nearest star" The distance between Sun and Proxima Centauri is 4.22 light year.

[Chris Peterson]

A suggestion to those writing the text for APOD items... I'd think that an item about "the closest start system" would say how close that is, and how it compares with some others. Wouldn't you expect a story about the oldest living human to mention how old that is, and the average lifespan?

Well... maybe. I'd certainly agree if this caption were in print. But this is the Internet, built on a web of hyperlinks. In the case of this APOD, the research has already been done; all you need to do is click on one of the provided links- fully nine of them take you directly to pages giving the distance. Sure, that could have been included, but I don't see its absence as being significant given the available links. Indeed, leaving out a little info might encourage people to click through those links, and learn more about the topic than a short APOD caption can possibly provide.

[saturn2]

Object: Spaceship
Speed 30 Km/ seg

How long does it take from Earth to Proxima CentaUri ?
42200 years

[neufer]

Object: Spaceship
Speed 30 Km/ sec

How long does it take from Earth to Proxima Centauri ?

42200 years

Actually, we would only have to travel for 32,000 years (~3.2 light years)
since THAT is about where Proxima Centauri will be 32,000 years from now.

Alternatively, we could go to another red dwarf flare star:
Ross 248 in about 33,000 years ...like Voyager 2 is (sort of) attempting to do:

<<Ross 248 (HH Andromedae) is a red dwarf star located approximately 10.3 light years from Earth in the constellation Andromeda. This star was first catalogued by Frank Elmore Ross in 1926 with his second list of proper motion stars.

This star has about 12% of the Sun's mass and 16% of the Sun's radius, but only 0.2% of the Sun's luminosity. This is a flare star that occasionally increases in luminosity. With high probability there also appears to be a long-term variability with a period of 4.2 years. This variability ranged in visual magnitude from 12.23 to 12.34. In 1950, this became the first star to have a small variation in magnitude attributed to spots on the photosphere.

Long term observations of this star by the Sproul Observatory show no astrometric perturbations by an unseen companion. The proper motion of this star was examined for a brown dwarf or stellar companion orbiting at a wide separation (between 100–1400 AU) but none was found. A search for a faint companion using the Hubble Space Telescope Wide Field Planetary Camera revealed nothing, nor did a search with near infrared speckle interferometry. However, none of these searches rule out a companion that is smaller than the detection minima.

The space velocity components of this star are U = –32.9 ± 0.7, V = –74.3 ± 1.3 and W = 0.0 ± 1.4 km/s. The trajectory of Ross 248 will bring it closer to the Solar System in the future. In 1993 Matthews suggested that in about 33,000 years Ross 248 would be the closest star to the Sun, approaching within a minimum distance of 3.024 light-years in 36,000 years. However, it will recede thereafter and will again be further from the Sun than Proxima Centauri 42,000 years from now.

The spacecraft Voyager 2 is traveling on a path headed roughly in the direction of Ross 248, and is expected to come within 1.76 light-years of the star in 40,176 years. A spacecraft that escaped the Solar System with a velocity of 25.4 km/s would reach this star 37,000 years from now when the star is just past its nearest approach. By comparison, the Voyager 1 has an escape velocity of 16.6 km/s.>>

<<Proxima Centauri's mass is about an eighth of the Sun's, and its average density is about 40 times that of the Sun. Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity. The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun. The mixing of the fuel at Proxima Centauri's core through convection and the star's relatively low energy production rate suggest that it will be a main-sequence star for another four trillion years, or nearly 300 times the current age of the universe.

Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of brown dwarfs and supermassive planets. Precision radial velocity surveys have also ruled out the presence of super-Earths within the star's habitable zone. The detection of smaller objects will require the use of new instruments, such as the proposed James Webb Space Telescope. Since Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could support life is disputed. Nevertheless, because of the star's proximity to Earth, it has been proposed as a destination for interstellar travel.

Based on the parallax of 768.7 ± 0.3 milliarcseconds, measured using the Fine Guidance Sensors on the Hubble Space Telescope, Proxima Centauri is roughly 4.2 light years from Earth, or 270,000 times more distant than the Sun. From Earth's vantage point, Proxima is separated by 2.18° from Alpha Centauri, or four times the angular diameter of the full Moon. Proxima also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky. It has a radial velocity toward the Sun of 21.7 km/s.

Distances of the nearest stars from 20,000 years ago until 80,000 years in the future. Proxima Centauri is in yellow

---

Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 33,000 years, after which the closest star to the Sun will be Ross 248. In 2001, J. García-Sánchez et al. predicted that Proxima will make its closest approach to the Sun, coming within 3.11 light years of the latter, in approximately 26,700 years. A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly in about 27,400 years. Proxima Centauri is orbiting through the Milky Way at a distance from the galactic core that varies from 8.3 to 9.5 kpc, and with an orbital eccentricity of 0.07.

From the time of the discovery of Proxima, it was suspected to be a true companion of the Alpha Centauri binary star system. At a distance to Alpha Centauri of just 0.21 ly (15,000 ± 700 astronomical units [AU]), Proxima Centauri may be in orbit around Alpha Centauri, with an orbital period of the order of 500,000 years or more. For this reason, Proxima is sometimes referred to as Alpha Centauri C. Modern estimates, taking into account the small separation between and relative velocity of the stars, suggest that the chance of the observed alignment being a coincidence is roughly one in a million. Data from the Hipparcos satellite, combined with ground-based observations, is consistent with the hypothesis that the three stars are truly a bound system. If so, Proxima would currently be near apastron, the farthest point in its orbit from the Alpha Centauri system. More accurate measurement of the radial velocity is needed to confirm this hypothesis.

If Proxima was bound to the Alpha Centauri system during its formation, the stars would be likely to share the same elemental composition. The gravitational influence of Proxima may also have stirred up the Alpha Centauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions. Any terrestrial planets in the system may have been enriched by this material.

Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point of origin, such as in a star cluster. If it is determined that Proxima Centauri is not gravitationally bound to Alpha Centauri, then such a moving group would help explain their relatively close proximity.>>

[DavidLeodis]

In the information brought up through the Rigel Kentaurus link it states "Rigel Kentaurus is the 3rd brightest star in the sky". However, in the explanation to the APOD it states it is the "fourth bightest star in the night sky". I would be grateful if someone could please let me know which is correct. Thanks.

PS. There should be a Clark Kentaurus star (perhaps there is!).