Barnard’s Star
Also known as
BD+04º3561a, GCTP 4098.00, GI 140-024, Gliese 699, HIP 87937, LFT 1385,
LHS 57, LTT 15309, Munich 15040, Proxima Ophiuchi, V2500 Ophiuchi,
Velox Barnardi, Vyssotsky 799
We come at last to a star that you may have a good chance of being familiar with. Barnard’s Star is after all, after the Alpha Centauri
triple star system, the second closest star to our own Sun. Yet even so,
despite its being less than six light years away, at magnitude 9.54 it remains
completely invisible to the naked eye.
Just think about that for a moment.
Imagine a universe in which every last star was a red dwarf. Looking up into
the night sky, one would see… nothing (unless there were other planets in your
solar system). And even then, a planet as large and distant as Jupiter would be
little more than the faintest of dim lights, scarcely (if at all) visible to the unaided
eye. There would not be all that much light from one’s own star to reflect. And
even to professional astronomers with huge telescopes, the distant galaxies
would be all but undetectable, and their existence perhaps even unsuspected. So
despite the fact that in reality the vast majority of stars are indeed red dwarfs,
we should all be thankful that there remains (in our universe, at least) that
minority of brilliant giants which so magnificently light up our sky.
The Night Sky in a Universe of Red Dwarfs
Let’s continue our little thought
experiment here a bit longer, and place our own Earth in such a system. If we
made no alteration to our planet’s current orbit, our new sun would shine in
the noontime sky with the brightness of approximately 100 full moons. That may
sound like a lot, but keep in mind that our actual Sun is as bright as 398,352
full moons! (In other words, almost like having 40,000 Barnard’s Stars in place
of just one.) At one AU, the Earth would be dark, frozen solid, and utterly
lifeless.
To maintain our planet’s temperature (or
at least one at which liquid water can exist for sustained periods on the
surface), we’d need to move in a bit closer, in fact quite a bit closer – to
about 6% of our current distance from the Sun (or a little less than 1/7th of
the way out to Mercury in our own solar system). One year in such an orbit would
be only 13 days long, assuming the planet was not tidally locked to the star,
causing one hemisphere to be in perpetual daylight and the other in everlasting
night ( but bringing the Moon along with us would likely prevent such a state
of affairs). Barnard’s Star has only 1/5th the diameter of the Sun, but that
decrease in size would be more than made up for by our planet’s closer distance
to it. In fact, in this alternate Earth’s sky, our red dwarf would appear to be
as wide as three suns! One would never see any total solar or lunar eclipses in
such a system, since the Moon would appear only 1/3rd as large as Barnard’s
Star from the Earth’s surface (and thus unable to cover the whole star), while
the umbra of our planet’s shadow would extend only 283,000 miles out (in
contrast to its current length of about 850,000 miles). So it would never cover
more than a small portion of the lunar disk at the distance of our satellite’s
orbit. (Keep in mind that the Earth’s umbra narrows to a point as one moves further
along it.) We would observe a lunar eclipse as a dark patch of shadow moving
across the face of the Moon’s surface, but not covering the entire disk. (To
make up for that deficiency, however, partial lunar eclipses would be 9 times
more common!)
But enough imagination; let’s return to
facts. Our subject is interesting enough, without our having to
move there. To begin with, Barnard’s Star turns out to be one of the oldest
stars in the entire universe, and certainly among the most ancient in the Milky
Way. We are exceptionally fortunate to have such a specimen from the Dawn of
Creation right next door, so to speak. But not having actually witnessed any of
its history, however, we must conjecture its story to date from clues contained
within its present characteristics.
First of all, its stellar class ensures us
that it has undergone relatively little change over its life so far – red
dwarfs (at least in theory) tend to stay “just the way they are” for
uncountable billions of years. Since it emerged from its primordial
protostellar nebula, Barnard’s Star has shone out with 4/10,000ths of the Sun’s
visual luminosity. There is some debate over whether it can be classed as a
flare star. Despite being perhaps the most observed red dwarf in existence,
only one such event has ever been recorded – on July 17th 1998. The flare
lasted about an hour, and boosted the star’s magnitude for that duration to about
8.9 (still far below naked eye visibility). Does a single event determine
classification? Astronomers can’t agree.
Another important clue to the past is the
star’s metallicity; that is, its percentage of elements heavier than hydrogen
and helium. Barnard’s Star’s metal content is only about 1/10th that of the
sun. This is strong evidence of two things: the star’s age, and its probable
origin in the galactic halo. The metallicity of a star tends to increase as a
function of how young it is. Current theory holds that the extremely early
universe (at about the time the very first stars were being formed) was utterly
devoid of heavy elements. The hypothesized first generation of suns apparently
consisted of supermassive bodies (far larger than anything in existence today),
composed entirely of the very lightest elements. These monsters rapidly went
through their hyper-fast life cycles, ending their brief existence in
galaxy-shattering supernovae which spewed out into interstellar space vast
quantities of heavy elements, such as carbon, iron, and oxygen, which had been
forged as the by-products of nuclear fusion in their unimaginably hot cores.
Successive stellar generations were formed out of the products of these first
stars, thus composed of greater and greater concentrations of elements needed
to build terrestrial planets (and us!).
So a good rule of thumb could be: all else
being equal, the lower the heavy element content, the older the star. In the
case of Barnard’s Star, we’re talking 12 billion years old. The universe itself
is only about 13.7 billion years old, so when looking at Barnard’s Star, we’re
essentially peering back in time to practically creation itself.
So what do we see, looking at this star?
First of all, it is physically quite typical for its spectral class (M4).
Barnard’s Star has the mass of approximately 150 Jupiters (i.e., 14% solar
mass), all contained within a diameter slightly less than twice that of
Jupiter. Its surface temperature is a respectable 3,170º Kelvin. It is
magnetically active, displaying signs of coronal X-ray activity and fairly
strong chromospheric ultraviolet emissions. As mentioned above, only a single
flare event has been noted in more than a century of intensive observation. One
quite interesting feature is its remarkably slow period of rotation of 130
days. This leads to the obvious question, where did all of its angular momentum
go? Although one would be tempted to assume it was taken up by a planetary
system, this does not appear to be the case with Barnard’s Star. Once again,
herein lies a tale.
Peter van de Kamp
In 1963, astronomer Peter van de Kamp announced he had discovered one to two Jupiter-sized planets about Barnard’s Star by
measuring minute wobbles in the star’s position over time. This claim rapidly
gained wide acceptance in the global scientific community, and was even the
cause of the world’s first serious attempt to engineer a means of interstellar
travel (the British Interplanetary Society’s Project Daedalus), with the goal
of reaching another planetary system within 50 years after launch.
Unfortunately, like so many of these early exoplanet “discoveries,” this one
also turned out in the end to be spurious. Ten years after the first
announcement, John L. Hershey traced van de Kamp’s findings to a systemic error
introduced into the data due to periodic cleaning and remounting of the lens in
the telescope used to observe the reputed wobbles. This interpretation was the
source of some very regrettable discord between former colleagues, and van de
Kamp never reconciled himself, either to the refutation of his work, or to the
astronomers who accepted such. He died in 1995 still convinced he had
discovered another solar system. But Hubble Space Telescope observations made
four years after his death definitively ruled out all possibility of any planet
about Barnard’s Star as large or larger than Neptune, a finding that was
subsequently refined to include any object significantly larger than the Earth
itself. In addition, there were found no signs of interplanetary dust around
the star, and no cold disk was observed. There still remains the remote possibility
of worlds the size of Mars, or perhaps even as large as Venus, but no plans
exist at this time to search for them, should they exist.
Edward Emerson Barnard
But we have yet to come to the most
remarkable fact of all about Barnard’s Star – the principle reason for its fame
other than its nearness. For until the quite recent discovery of hypervelocity
stars (confirmed in 2005), Barnard’s star was the fastest-known moving star in
the entire galaxy. Its velocity relative to our solar system is an eye-popping
87 miles per second, and its radial velocity toward the Sun is no less than 56
miles per second. In the time it likely took you to read that last sentence,
Barnard’s Star had decreased its distance from us by as much as two hundred
miles! This amazing rate of motion was discovered by American astronomer E.E.
Barnard (mentioned in the posting on Wolf 359) as far back as 1916 (and
hence the star’s name). Its apparent motion across our sky is nothing short of
fantastic, traversing fully one half the angular diameter of the Moon in a
typical person’s lifetime. Star atlases cannot even chart its position with a
single dot; it must be displayed as a line with various dates indicated along
its length. Barnard’s Star will continue to approach the solar system until the
year 9800 AD, at which point it will be only 3.75 light years from the Earth.
Yet even then it will still remain below naked eye visibility, topping out at
magnitude 8.5. After that closest approach, the relative motions of it and our
own Sun will cause the distance between them to increase.
The direction of motion and the star’s
velocity, along with other physical characteristics, indicate that Barnard’s
Star does not belong to the Milky Way’s spiral arms, but is a member of the
galactic halo. It just happens to be “passing through” at the moment, and lucky
we are to be living at this precise time of its being so close to ourselves.
The British Interplanetary Society's Proposed Daedalus Interstellar space probe,
as compared to the Saturn V Moon rocket
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