Hospitals & Asylums
Alpha
Centauri System: Our closest neighbor HA-12-3-05
Being visible to the naked eye,
Alpha Centauri has been known for centuries, if not millennia, although perhaps
not as a double star until the 1752 observation of the Abbé [Abbot] Nicholas
Louis de La Caille (1713-1762) from the Cape of Good Hope, the southernmost
point of Africa, where he was studying the stars of the southern hemisphere
with just an half-inch (8x) refractor. Dim Proxima, however, was not discovered
until 1915 by Robert
Thorburn Ayton Innes (1861-1933) of Edinburgh, Scotland who also was
observing from Cape Hope, probably with the 7-inch refractor at the Royal
Observatory. If our own Sun, Sol, were viewed
from the Alpha Centauri system, it would be located in Cassiopeia
near the border with Perseus and
about five degrees north of a double cluster near the nebula IC 1805/1848,
visible as a bright yellow star that would be almost as bright as Capella (Alpha
Aurigae) appears in Earth's night sky.
Sol's three closest stellar
neighbors are located in the southeastern corner of Constellation Centaurus,
the Centaur. Proxima Centauri (or Alpha Centauri C) is only 4.22 light-years
(ly) away (14:29:42.95-62:40:46.14, ICRS 2000.0) but is too dim to be seen with
the naked eye. The two bright stars, Alpha Centauri A and B
(14:39:36.5-62:50:02.3 and 14:39:35.1-60:50:13.8, ICRS 2000.0), are a little
farther away at about 4.36 ly, around 40,000,000,000,000 km. They form a close
binary that is separated "on average" by only about 24 times the
Earth-Sun distance -- 23.7 astronomical units (AUs) of an orbital semi-major
axis -- which is only slightly greater than the distance between Uranus and the
Sun ("Sol"). (See an animation of the orbits of Stars A and B
and their potentially habitable zones in this system, with a table of basic
orbital and physical characteristics.) Visible only from latitudes south of
about 25° the star we call Alpha Centauri lies 4.35 light-years from the Sun.
But it is actually a triple star system. The two brightest components Alpha
Centauri A and B form a binary. They orbit each other in 80 years with a mean
separation of 23 astronomical units (1 astronomical unit = 1 AU = distance
between the Sun and Earth). The third member of the system Alpha Centauri C
lies 13,000 AU from A and B, or 400 times the distance between the Sun and
Neptune. This is so far that it is not known whether Alpha Centauri C is really
bound to A and B, or if it will have left the system in some million years.
Alpha Centauri C lies measurably closer to us than the other two: It is only
4.22 light-years away, and it is the nearest individual star to the Sun.
Because of this proximity, Alpha Centauri C is also called Proxima (Centauri). Alpha
Centauri A is a yellow star with a spectral type of G2, exactly the same as the
Sun's. Therefore its temperature and color also match those of the Sun. Alpha
Centauri B is an orange star with a spectral type of K1. Whereas Alpha Centauri
A and B are stars like the Sun, Proxima is a dim red dwarf with a spectral type
of M5 - much fainter, cooler, and smaller than the Sun. Proxima is so faint
that astronomers did not discover it until 1915.
Alpha Centauri A, Rigil
Kentaurus ("Foot of the Centaur" in Arabic) is the fourth brightest
star in the night sky as well as the brightest star in
Constellation Centaurus. Like Sol, it is a yellow-orange main sequence
dwarf star of spectral and luminosity type G2 V. It has about 1.09 to 1.10
times Sol's mass and 1.23 its diameter (ESO; and
Demarque
et al, 1986), is about 52 to 60 percent brighter than Sol (ESO; and
Demarque
et al, 1986). It may may be only somewhat older than Sol at 4.85 billion
years in age (ESO), or
much older at around 7.6 (+/- around 10 percent) or 6.8 billion years if it
does not have a convective core (Guenther
and Demarque, 2000). Since Alpha Centauri A is very similar to our own Sun,
however, many speculate whether it might contain planets that harbor life.
According to Weigert
and Holman (1997), the distance from the star where an Earth-type planet
would be "comfortable" with liquid water is centered around 1.25 AUs
(1.2 to 1.3 AUs) -- about midway between the orbits of the Earth and Mars in the Solar System --
with an orbital period of 1.34 years using calculations based on Hart
(1979), but more recent calculations based on Kasting
et al (1993) allow for a wider "habitable zone." The distance
separating Alpha Centauri A from its companion star B averages 23.7 AUs
(semi-major axis of 17.59" with a HIPPARCOS distance estimate of 4.40
light-years). The stars swings between 11.4 and 36.0 AUs away in a highly
elliptical orbit (e= 0.519) that takes almost 80 (79.90) years to complete and
are inclined at an angle of 79.23° from the perspective of an observer on Earth
(see Dimitri
Pourbaix, 2000 in the new Sixth
Catalog of Orbits of Visual Binaries; Heintz Orbit
Table, 12/1997; and Worley
and Heintz, 1983). As viewed from a hypothetical planet around either star,
the brightness of the other increases as the two approach and decreases as they
recede. However, the variation in brightness is considered to be insignificant
for life on Earth-type planets around either star. At their closest approach,
Stars A and B are almost two AUs farther apart than the average orbital
distance of Saturn
around the Sun, while their widest separation is still about six AUs farther
the average orbital distance of Neptune. (See an
animation of the orbits
of Stars A and B and their potentially habitable zones in this system, with
a table of basic orbital and physical characteristics.)
Centauri B is a much dimmer
companion star is a main sequence, reddish-orange dwarf (K0-1 V). It appears to
have only 90.7 percent of Sol's mass, about 86.5 percent of its diameter, and
45 to 52 percent of its luminosity (ESO; and
Johnson
and Wright, 1983, page 681). Viewed from a planet at Earth's orbital
distance around Alpha Centauri A, this companion B star would provide more
light than the full Moon does on Earth as its brightest night sky object, but
the additional light at a distance greater than Saturn's orbital distance
in the Solar System would not be significant for the growth of Earth-type life.
According to Weigert
and Holman (1997), the distance from the star where an Earth-type planet
would be comfortable with liquid water is centered around 0.73 to 0.74 AU --
somewhat beyond the orbital distance of Venus in the Solar System
-- with an orbital period under an Earth year using calculations based on Hart
(1979), but more recent calculations based on Kasting
et al (1993) allow for a wider habitable zone. Useful catalogue numbers and
designations for Alpha Centauri B include: Alp or Alf Cen B, HR 5460, Gl 559 B,
Hip 71681, HD 128621, and LHS 51.
Proxima (Alpha Centauri C) is a
very cool and very dim, main sequence red dwarf (M5.5Ve) that appears to have
only 12.3 percent of Sol's mass and 14.5 percent of its diameter (ESO press
releases of 3/15/03
and 2/22/02;
and Doyle
and Butler, 1990, page 337). With a visual luminosity that has reportedly
varied between 0.000053 and 0.00012 of Sol's (based on a distance of 4.22
light-years)the star is as much as 19,000 times fainter than the Sun, and so if
it was placed at the location of our Sun from Earth, the disk of the star would
barely be visible. It is chromosperically active with a rotation period of 31.5
+/- 1.5 days and appears to be between five and six billion years old (Guinan and Morgan,
1996). The star is located roughly a fifth of a light-year from the AB
binary pair and, if gravitationally bound to it, may have an orbital period of
around half a million years. According to Anosova
et al (1994), however, its motion with respect to the AB pair is
hyperbolic. Accounting for infrared radiation, the distance from Proxima where
an Earth-type planet would be "comfortable" with liquid water is
around 0.02 to 0.06 AU (Endl et al, 2003, in pdf)
-- much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with
an orbital period of two to 16 days. Hence, the rotation of such a planet would
probably be tidally locked so that one side would be in perpetual daylight and
the other in darkness. Three star spots may have been observed recently with
the Hubble Space Telescope (Benedict
et al, 1998). Like many red dwarfs, Proxima is a "Flare Star"
that can brighten suddenly to many times its normal luminosity. Its flares can
roughly double the star's brightness and occur sporadically from hour to hour.
Moreover, more than one flare may be emitting at a time. From May to August
1995, several flares were observed with changes within a time-scale of weeks,
and archival data suggests that the star may have a long-term activity cycle (Guinan and Morgan,
1996). Its designated variable star name is V645 Centauri. Other useful
catalogue numbers for Proxima include: Gl 551, Hip 70890, and LHS 49. Using
data collected up to early 1994, astronomers using the Hubble Space Telescope
discerned a 77-day variation in the proper motion of Proxima (Benedict
et al, 1994). The astrometric perturbations found could be due to the
gravitational pull of a large planet with about 80 percent of Jupiter's mass at a 1994
separation from Proxima of about 0.17 AUs -- 17 percent of Earth's orbital
distance in the Solar System from the distance, or less than half Mercury's
orbital distance. The Hubble astrometry team calculated that the chance of a
false positive reading from their data -- same perturbations without a planet
-- to be around 25 percent.
In a binary system, a planet
must not be located too far away from its "home" star or its orbit
will be unstable. If that distance exceeds about one fifth of the closest approach
of the other star, then the gravitational pull of that second star can disrupt
the orbit of the planet. Recent numerical integrations, however, suggest that
stable planetary orbits exist: within three AUs (four AUs for retrograde
orbits) of either Alpha Centauri A or B in the plane of the binary's orbit;
only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for
planets circling both stars (Weigert
and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A
and B could hold four inner rocky planets like the Solar System: Mercury (0.4
AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AUs). Indeed, the AB system may
be more than twice (1.3 to 2.3 times) as enriched in elements heavier than
hydrogen ("high metallicity") than our own Solar System (Cayrel
de Strobel et al, 1991, page 297; Furenlid
and Meylan, 1984; and Flannery
and Ayres, 1978). Hence, either stars A or B could have one or two
"rocky" planets in orbital zones where liquid water is possible.
Useful star catalogue numbers and designations for Alpha Centauri A include:
Alp or Alf Cen A, HR 5459, Gl 559 A, Hip 71683, HD 128620, CP(D)-60 5483, SAO
252838, FK5 538, and LHS 50.
Comet
Swift-Tuttle orbits the sun once every 134 years and last visited our region of
the solar system in 1992. Its orbit stretches from near Neptune to inside the
orbit of Earth. The velocity of the
Comet of Swift Tuttle is estimated 60,000 m/sec, 60 km/sec, 3,600
km/min, 216,000 km/hr, 5,184,000 km/day, 1,892,200,000 km/yr. A light year
represents the 9,500,000,000,000 kilometers that light travels in one
year. Impact with the Comet of Swift Tuttle is estimated at 225,000
gigatons of TNT. The question that we
hope to answer is does the Comet of Swift-Tuttle reach the next solar system
similar to an electron in a double bond in organic chemistry? After 134 years traveling
at 1,892,200,000 the Comet of Swift Tuttle would only travel 253,554,800,000 km
in the entire round trip voyage, therefore on the basis of the facts presented
by the authors in the bibliography we must concur that the trajectory of comets
do not leave the Solar System.
Alpha Centauri is a special
place, because it may offer life conditions similar to our solar system. A star
must pass five tests before we can call it a promising place for terrestrial
life as we know it. Most stars in the Galaxy would fail. In the case of Alpha
Centauri, however, we see that Alpha Centauri A passes all five tests, Alpha
Centauri B passes either all but one, and only Proxima Centauri flunks out.
The first criterion is to ensure
a star's maturity and stability, which means it has to be on the main sequence.
Main-sequence stars fuse hydrogen into helium at their cores, generating light
and heat. Because hydrogen is so abundant in stars, most of them stay on the
main sequence a long time, giving life a chance to evolve. The Sun and all
three components of Alpha Centauri pass this test.
The second test is much tougher,
however, we want the star to have the right spectral type, because this
determines how much energy a star emits. The hotter stars - those with spectral
types O, B, A, and early F - are no good because they burn out fast and die
quickly. The cooler stars - those with spectral types M and late K - may not
produce enough energy to sustain life, for instance they may not permit the
existance of liquid water on their planets. Between the stars that are too hot
and those that are too cool, we find the stars that are just right. As our
existance proves, yellow G-type stars like the Sun can give rise to life. Late
(cool) F stars and early (hot) K stars may be fine too. Luckily, Alpha Centauri
A passes this test with bravour, as it is of the same class as our Sun. Alpha
Centauri B is a K1 star, so it is hotter and brighter than most K stars,
therefore it may pass this test or it may not. And the red dwarf Proxima
Centauri seems to be a hopeless case.
For the third test, a system
must demonstrate stable conditions. The star's brightness must not vary so much
that the star would alternately freeze and fry any life that does manage to
develop around it. But because Alpha Centauri A and B form a binary pair
there's a further issue. How much does the light received by the planets of one
star vary as the other star revolves around it ? During their 80-year orbit,
the separation between A and B changes from 11 AU to 35 AU. As viewed from the
planets of one star, the brightness of the other increases as the stars approach
and decreases as the stars recede. Fortunately, the variation is too small to
matter, and Alpha Centauri A and B pass this test. However, Proxima fails this
test, too. Like many red dwarfs it is a flare star, prone to outbursts that
cause its light to double or triple in just a few minutes.
The fourth condition concerns
the stars' ages. The Sun is about 4.6 billion years old, so on Earth life had
enough time to develop. A star must be old enough to give life a chance to
evolve. Remarkably, Alpha Centauri A and B are even older than the Sun, they
have an age of 5 to 6 billion years, therefore they pass this test with
glamour, too. Proxima, however, may be only a billion years or so old, then it
fails this test, too.
And the fifth and final test: Do
the stars have enough heavy elements - such as carbon, nitrogen, oxygen and
iron - that biological life needs ? Like most stars, the Sun is primarily
hydrogen and helium, but 2 percent of the Sun's weight is metals. (Astronomers
call all elements heavier than helium "metals".) Although 2 percent
may not sound a lot, it is enough to build rocky planets and to give rise to
us. And again, fortunately, Alpha Centauri A and B pass this test. They are
metal-rich stars.
Now to the final question. Do we
find at Alpha Centauri warm, rocky planets like Earth, full of liquid water ?
Unfortunately, we don't know yet whether Alpha Centauri even has planets or
not. What we know is that in a binary system the planets must not be too far
away from a particular star, or else their orbits become unstable. If the
distance exceeds about one fifth of the closest approach of the two stars then
the second member of the binary star fatally disturbes the orbit of the planet.
For the binary Alpha Centauri A and B, their closest approach is 11 AU, so the
limit for planetary orbits is at about 2 astronomical units. Comparing with our
system, we see that both Alpha Centauri A and B might hold four inner planets
like we have Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AU).
Therefore, both Alpha Centauri A and B might have one or two planets in the
life zone where liquid water is possible.
