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To astronomers, stars are as frustrating as they are fascinating. They
know that stars, like people, change dramatically from birth to death. Unlike
people, however, most stars take millions or billions of years to alter
their appearance. Because stars evolve so slowly, deducing the life story
of a star from observations spanning a few years is like constructing the
life story of a celebrity from a single sentence published in their autobiography.

Fortunately, a remarkable star called FG Sagittae seems almost human.
It is evolving rapidly, on a timescale of just a few decades, and has helped
scientists piece together the lives of ageing stars. Over the past 40 years,
FG Sagittae has changed colour from blue to yellow.

FG Sagittae takes its last name from the constellation it resides in,
Sagitta the Arrow. Sagitta is a tiny constellation lodged in the summer
Milky Way between the bright nearby stars, Vega and Altair. At a distance
of roughly 8000 light years, however, FG Sagittae lies much farther from
the Sun than either Vega or Altair.

The capital letters preceding the star’s constellation name mean the
star is variable. Astronomers discovered this in 1943, but they knew then
only that the star’s output of light was variable. At that time, they did
not know the star’s colour was also changing. Colour is a key property describing
any star, so FG Sagittae’s colour change hints at radical transformations
occurring in the star. Colour measures temperature: blue stars are hot,
yellow stars warm, and red stars cool. In 1955, when the star was blue,
FG Sagittae was about 12 000 K; today, as a yellow star, FG Sagittae is
only 5000 K.

Colour is so important that astronomers classify stars of different
colours into different spectral types. From hot and blue to red and cool,
the seven main spectral types are O, B, A, F, G, K and M. Stars of type
O and B are hot and blue; A stars are white; F stars are yellow-white; G
stars are yellow; K stars are orange; and M stars, the coolest of all, are
red. The Sun is a yellow G star.

In 1955, the first time that astronomers measured FG Sagittae’s spectral
type, the star was a B (blue) star. Even then, though, the temperature of
the star was dropping and its spectral type was changing: in the early
1960s, the star became spectral type A (white), and by the end of the decade,
the spectral type was F (yellow-white). The star continued to cool during
the 1970s, when it became a G star similar in colour and temperature to
the Sun. It remains a G star today.

FG Sagittae is cooling because it is expanding. In 1955, when the star
was blue, it was 10 times bigger than the Sun, and astronomers classified
it as a blue supergiant. Today, as a yellow supergiant, FG Sagittae is about
60 times the the size of the Sun. If it was put at the centre of our Solar
System, the star would almost reach Mercury.

Astronomers have two reasons for believing that FG Sagittae is expanding.
First, FG Sagittae pulsates like a human heart, rhythmically contracting
and expanding. The period of these pulsations – which depends on the size
of the star – has increased over the past 30 years. The pulsations are perfectly
normal. Many other yellowish supergiants also pulsate. For example, the
F-type supergiant Polaris, the North Star, is a pulsating star with a period
of four days. As Pola-ris expands and contracts, it periodically brightens
and dims.

When FG Sagittae cooled and became an A star, it started to pulsate.
Astronomers first detected small, periodic variations in its light during
the early 1960s. The period of these pulsations was then 15 days. As the
star cooled further, the pulsations continued, but the period of the pulsations
lengthened, indicating that the star was getting bigger. Today the pulsation
period is more than 100 days.

More evidence that astronomers also know FG Sagittae is expanding comes
from the fact that its total luminosity over all wavelengths has held steady
despite the star’s plummeting temperature. The luminosity of a star depends
on both temperature and radius. If one falls, the other must rise to compensate,
for the luminosity to stay constant. Through all the dramatic changes, FG
Sagittae has remained several thousand times more luminous than the Sun.
The combination of constant luminosity and declining temperature means that
the star must have expanded.

Although FG Sagittae’s total luminosity over all wavelengths has held
steady, its brightness at visible wavelengths has increased as the star
has gone from blue to yellow. In 1955, as a B-type star, FG Sagittae emitted
most of its radiation in the ultraviolet. Today, as a G star, FG Sagittae
gives off nearly all its radiation in the visible. Therefore, if we could
have watched FG Sagittae over the past several decades, we would not only
have seen it change colour, but we would also have seen it get brighter
– even though its total luminosity, over all wavelengths, has remained constant.

In fact, this apparent increase in brightness first attracted attention
to the star shortly after its discovery. Cuno Hoffmeister at the Sonneberg
Observatory in Germany discovered FG Sagittae in 1943. The star seemed to
be an irregular variable. Variable stars are common; astronomers have catalogued
thousands of them, and there seemed nothing unusual about FG Sagittae. But
when astronomers checked old photographic plates, they found that the star
had continuously got brighter since 1894, the first time it was photographed.
From 1894 to 1943, the star’s brightness had increased eight-fold, and the
star continued to brighten afterwards. Though it puzzled astronomers then,
the brightness increase is no longer mysterious: now that we know the star’s
colour has been changing, we know that the total luminosity was not itself
changing; what was changing was the wavelength where the star was emitting
the most energy.

The old data suggest that, if FG Sagittae was indeed constant in luminosity,
it was even hotter and bluer prior to 1955. Indeed, the star was probably
a very hot O star not long before the 1950s, pouring out lots of ultraviolet
radiation that the photographic plates failed to detect. The old data also
mean that FG Sagittae probably began its peculiar expansion and colour change
about a century ago.

If the star then had a total luminosity the same as it has today, and
its temperature was 50 000 K, it must have been smaller than the Sun. One
hundred years ago, FG Sagittae was probably a hot, bright, small star with
a radius half that of the Sun. Then some huge explosion hit the star, and
it rapidly expanded. Like any expanding gas, the star cooled: it moved into
spectral class B during or before the 1950s, into spectral class A in the
early 1960s, into spectral class F in the late 1960s, and finally into spectral
class G in the 1970s.

In 1955, Karl Henize, then at Mount Wilson Observatory in California,
turned up a major clue in the FG Sagittae mystery when he discovered a planetary
nebula surrounding the star. A planetary nebula is a beautiful bubble of
glowing gas that a star gives off near the end of its life. The Ring Nebula
in the constellation Lyra is the best example. Planetary nebulae have nothing
to do with planets; they get their misleading name because, like planets,
they look like discs through a telescope.

At the centre of a planetary nebula is a small but extremely hot star,
similar to what FG Sagittae was probably like 100 years ago. Astronomers
call this small, hot star a ‘planetary nebula central star’. The ultraviolet
radiation from the hot central star excites the atoms of the planetary nebula,
which then glows with the light these atoms emit.

Today, as a yellow G star, FG Sagittae is far too cool to make its planetary
nebula glow, for the star emits almost no ultraviolet radiation. Even during
the 1950s, when it was a B star, FG Sagittae was too cool to excite its
nebula. FG Sagittae must, therefore, have been much hotter prior to the
1950s, because its planetary nebula is glowing just as they normally do.
(Even if FG Sagittae stays cool, its past ultraviolet input will keep the
nebula aglow for hundreds of years to come.)

In fact, in all respects, FG Sagittae’s planetary nebula seems completely
normal – unlike the central star itself. The nebula measures a little more
than a light year across, which is typical. However, this figure is uncertain.
Astronomers know that the apparent diameter of the nebula is 36 arcseconds.
(One arcsecond is 1/3600 of a degree.) To convert this apparent size into
an actual size, astronomers must know how far away FG Sagittae is. The best
estimate is 8000 light years, making the planetary nebula 1.4 light years
across. But the estimate of the distance to FG Sagittae is not accurate
and could be out by several thousand light years. If FG Sagittae is further
away than 8000 light years, its planetary nebula must be bigger; if FG Sagittae
is nearer, the nebula is smaller.

Because planetary nebulae have been ejected by the central stars that
now make them glow, they must all be expanding. That of FG Sagittae is no
exception: it is growing at 34 kilometres per second. Because they all expand,
planetary nebulae eventually disperse into the interstellar medium. This
process takes only a few tens of thousands of years, so a planetary nebula
is a short-lived stage in the life of any star.

FG Sagittae’s planetary nebula is probably about 6000 years old, but
this number is uncertain. To obtain the age, we calculate how long gas travelling
at 34 kilometres per second would take to reach the present extent of the
nebula. That tells us when the star ejected the nebula. But because the
distance to the star is uncertain, so is the nebula’s size, and so, therefore,
is its age. If FG Sagittae is further than we think, then its planetary
nebula is bigger and its age older than 6000 years; if it is closer, then
the nebula is younger.

Whatever the exact age of the planetary nebula, it clearly formed long
before the bizarre changes now happening in FG Sagittae. Yet the planetary
nebula must hold clues to the central star’s strange behaviour, because
such a nebula forms only in the final stages of a star’s life. In fact,
astronomers now believe that the rapid expansion and cooling of FG Sagittae
are the last gasps of a dying star as it attempts to regain normal starhood.

Planetary nebulae arise from stars that were once similar to the Sun.
So long ago, FG Sagittae probably resembled the Sun: it was fusing hydrogen
into helium at its centre, creating light and heat. When its core ran out
of hydrogen, the star began to burn hydrogen in a shell outside the core.
The star grew bigger, brighter, and cooler, becoming a red M-type giant,
a fate that will befall our Sun in five billion years. The red giant’s luminosity
was 100 times that of the Sun today.

The star continued to fuse hydrogen into helium outside its core, but
the helium core itself soon ignited. Helium generates energy by creating
carbon and oxygen. When the core filled with carbon and oxygen and ran out
of helium, the helium began to fuse in a shell surrounding the core, and
the star got even bigger and brighter. It became a red supergiant 10 000
times brighter than the Sun. If the star was at the centre of the Solar
System, it would have touched Mars.

A star blow its top

This red supergiant was FG Sagittae just a few thousand years ago. But
red supergiants are unstable: they pulsate wildly. About 6000 years ago,
FG Sagittae kicked off its outer atmosphere, creating an expanding bubble
of gas – a planetary nebula. The ejection of the nebula exposed the star’s
small but hot core. The ultraviolet radiation from this core made the planetary
nebula glow. As a red supergiant, FG Sagittae had been big, red and cool;
now, as the central star of a young planetary nebula, FG Sagittae was small,
blue and hot.

FG Sagittae has probably spent most of the past 6000 years as a normal
central star governing a normal planetary nebula. Though in an advanced
stage of evolution, such a star is down but not yet out, for its nuclear
‘fires’ continue to smoulder. In particular, helium may keep burning in
a shell surrounding the carbon-oxygen star centre.

Helium-shell burning is unstable. Whether it occurs in a red supergiant
or a planetary nebula central star, helium-shell burning leads to a nuclear
runaway. The result is a gigantic explosion that astronomers call a helium-shell
flash. Helium-shell flashes may be responsible for kicking off a red supergiant’s
outer atmosphere and forming a planetary nebula. If a helium-shell flash
occurs in a planetary nebula central star, the star rapidly expands and
cools. Polish astronomer Bohdan Paczynski identified this phenomenon in
the 1970s as he was computing theoretical models of planetary nebula central
stars, and more recent investigations by Icko Iben at the University of
Illinois verify Paczynski’s calculations. This work indicates that the planetary
nebula central star suffering a helium-shell flash can cool to spectral
type G but will maintain a fairly constant total luminosity as it does so.
After it cools, the star will then contract and heat up, until it once again
becomes a normal, very hot planetary nebula central star.

Obviously, such a scenario explains the puzzling behaviour of FG Sagittae.
As a planetary nebula central star, FG Sagittae was burning helium in a
shell. Shortly before 1894, thishelium burning got out of control. The star
rapidly expanded and cooled, becoming a G supergiant. Someday it will heat
up and look like a normal central star again.

Helium burning may also account for some strange elements that suddenly
appeared on the surface of FG Sagittae in the late 1960s and early 1970s.
As helium fuses to carbon and oxygen, it releases a slow flux of neutrons.
These neutrons then collide with other nuclei and create heavy elements
such as yttrium, zirconium, lanthanum and cerium. In the early 1970s, G.
Edward Langer, Robert Kraft, and Kurt Anderson reported that such elements
began to appear in the spectrum. From 1969 to 1972, their abundances increased
steadily. Today, the star has far more of them on its surface than the Sun
does. FG Sagittae’s helium burning – either as a red supergiant or, more
recently, during the helium-shell flash – must have created them. The sudden
flash that has caused the star to expand then tossed these elements up to
the star’s surface, where astronomers can now observe them.

FG Sagittae is today a yellow supergiant. What will happen to it now?
During the past 15 years, FG Sagittae has stalled. It is the same colour
and spectral type today as it was then. This holding pattern may only be
a temporary pause, after which the star might resume its cooling. If so,
FG Sagittae would move into spectral type K and then to M. But it is more
likely that FG Sagittae is about to turn around and reverse itself. Recently,
in fact, some astronomers believe that they have found evidence that the
star is heating up. Other astronomers, however, dispute this. In any case,
Iben’s calculations, published in 1984, show that a planetary nebula central
star suffering a helium-shell flash will not cool to a temperature much
below 5000 K. Since 5000 K is FG Sagittae’s present temperature, the star
may already be as cool as it will get. Iben’s calculations indicate that
the model star remains at this temperature for decades or even centuries.
This may explain why FG Sagittae is holding steady at about 5000 K. If Iben
is right, FG Sagittae may not change much for a long time.

But eventually the star will turn around. Iben’s calculations show that
the model star moves out of spectral type G, back through F, A, B, and into
O. The star will thus change from yellow to blue. But the star’s total luminosity
will not change much, so the star will shrink as it heats up. FG Sagittae
– now a supergiant some 60 times the size of the Sun – will contract into
a sphere smaller than the Sun.

And then what? The planetary nebula surrounding FG Sagittae will continue
to expand, dispersing altogether in a few tens of thousands of years. All
that will be left is a small, hot star that has used up its nuclear fuel.
FG Sagittae will slowly fade and cool as its heat escapes into the cold
of space. It will become a white dwarf’ – a hot, dense object the size of
Earth, much fainter than the Sun. But long before that, FG Sagittae will
have given astronomers valuable insights into the lives of stars such as
our Sun – and the future that awaits them.

Ken Croswell write about astronomy and lives in California.