THREE WHITE STARS
Three white stars shine out in evening summer twilight, those of
the Summer Triangle: Vega (of Lyra), Deneb (of Cygnus),
and Altair (of Aquila), stars so familiar, stars of no flashing color,
yet stars that have secrets and their own stories to tell.
High in the sky in early summer evenings is bright white Vega.
With a temperature of 9300 Celsius and a luminosity 54 times that
of the Sun, Vega -- a mere 25 light years away -- is the second
brightest star in the northern hemisphere. Vega is the
astronomer's touchstone. The original magnitude standard was
actually Polaris, set to visual mag
2.00. But rescaling and recalibrations brought the standard to
Vega, which now registers magnitude 0.03. With painstaking care,
astronomers of the 1960s measured the intensity of the light in its
spectrum against that shining from molten metal, so they could
relate the various magnitude scales to energy output in watts.
Once Vega was so calibrated, so were all the other stars. Though
there are many standards, there is but one Vega.
In the early 1980s, the Infrared Astronomical Satellite, IRAS,
found a cloud of warm dust around the star that suggested some kind
of disk that perhaps might be related to orbiting planets. There
is a hole in the disk. Do planets reside there? Have they used
the dust in their making, as did the planets of our Sun? Vega is
the paradigm of numerous "Vega-like" stars that include Fomalhaut of the Southern Fish and Denebola
in Leo. Vega's reputation as a
"standard," however, is a bit compromised by its metal content,
which is only a quarter that of the Sun. Vega also blows out a
wind that is seen to extend as much as 1.5 light years from the
star.
To the east of Vega lies Deneb, the luminary of Cygnus. Though not
as bright as Vega, it is still first magnitude. But it is also 100
times farther away! Deneb radiates 160,000 times the light of the
Sun. At Vega's distance, the star would shine 10,000 times more
brightly than it does now, with the light of a crescent Moon. With
a mass 20 to 25 times that of the Sun, it is
one of the most luminous white stars in the Galaxy, and sometime in
the astronomically near future it will surely explode. If it were
go to tonight, it would brighten 10,000 times and again shine like
the Moon, and we would thank our Galaxy for putting it so far from
us. If at Vega's distance, the explosion would damage the Earth's
atmosphere and place us all in jeopardy. At 25 light years!
Fortunately, no budding supernova is close to us. Even the bright
massive stars of Orion, half Deneb's
distance, are too far for any danger.
Then to the south, the southern anchor of the Summer Triangle,
Altair, shines with much less respect. Too bad, because, as one of
the faster spinning white stars of the sky, it is something of a
record-holder, rotating so quickly -- over 210 kilometers per
second at its equator -- that it is notably flattened at the poles,
this not a theoretician's idea, but actually observed! Contrast
that speed to the Sun, which spins at but 2 kilometers per second
and is a near-perfect sphere.
Amazing that we can know the spin rate a star, of a body that is a
near-point in the sky. Finding the rotation of the Sun is easy, as
we need look only at the march of spots across its surface. Cooler
stars also have starspots that cause detectable light variations,
allowing us to assess the rotation periods. However, white stars
like those of the Summer Triangle do not have spots. Their spins
are instead given away by their spectra. The famed Doppler shift
causes waves of all kinds to seem shorter if a body approaches,
longer if it recedes. The characters of stars are known by their
spectrum lines, gaps in their colored spectra caused by the
absorption of light by atoms in their outer layers. If a star
approaches, the absorptions shift to shorter wavelengths, and vice
versa, the shift easy to measure. Part of a spinning star is
coming at you, part going away, hence a spectral absorption is
stretched in both directions, and comes out fuzzy and broad. We
can therefore measure the rotation speed, rather a lower limit,
since we do not know how the axis is pitched. Altair's pole has to
be close to perpendicular to the line of sight for us to see such
mad turning. Vega, on the other hand seems to spin slowly. Its
rotation pole most likely points at us, which is consistent with
the face-on disk around it.
Even more amazing, how do we find Altair's shape? The star is too
far away for direct imaging. Instead we look at the star with a
pair of telescopes and observe how the light waves received by each
interfere with themselves. We can thereby mathematically
reconstruct the angular diameters. Sophisticated optical
"interferometers" can detect angles as small as a thousandth of a
second of arc.
Each of the stars is a guidepost to other stellar wonders. Just to
the northeast of Vega lies little Epsilon
Lyrae, the "double-double" in Lyra, a marvelous example of a
hierarchical double star, two pairs that go around each other. The
stars of the quartet are nearly identical and were almost certainly
born together as the parent birthcloud split and then each split
again. To the southeast lie Beta and Gamma Lyrae, Beta a mass-transferring close
binary. Between them sits the easiest-to-find of all planetary
nebulae, the Ring Nebula in Lyra. To the east of Deneb is stellar
history, 61 Cygni, an attractive double
star that was the first of any star to have its distance measured,
and at a distance of 17 light years is (counting doubles as one)
the 13th closest star to Earth. To the northeast is the North America Nebula, which on a dark
clear night can -- with difficulty -- be seen with binoculars, if
not with the naked eye. To the south, at the head of Cygnus, is Albireo, the orange and blue wide double
(my University's colors!)
Now back to Altair. It sits above a glorious part of the Milky
Way, the Scutum star cloud. Just as good,
Altair lies just north of Eta Aquilae,
which was actually the first-known Cepheid variable. Only by accident do we
call this famed class of pulsating stars, those that allow us to
tag the distances of galaxies, "Cepheids" -- they might have been
"Aquilids."
Marvelous stars are this trio, all seeming the same, all very
different, all with their own mysteries, as do all the other stars
of the heavens.
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the September/December 2002 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.