In 1907, the American astronomer Henrietta Leavitt reported a similar correlation for Cepheid variables, bright yellow stars that pulsate like a human heart, rhythmically expanding and contracting. The longer a Cepheid’s pulsation period, the greater its true brightness. So by measuring a Cepheid’s apparent brightness, astronomers can estimate its distance from the Earth. Last year, this method yielded a value for the Hubble constant which seemed to make the Universe younger than its oldest stars – and other researchers report a similar result in this week’s issue of Nature (vol 377, p 27).

Now Richard Stothers and Chao-wen Chin of NASA’s Goddard Institute for Space Studies in New York have found that the period of luminous blue variable stars is also related to their luminosity. Luminous blue variables, such as Eta Carinae, are the brightest stars known. They outshine the Sun a million times over and are more than a hundred times as bright as most Cepheids. This means they can be seen in very distant galaxies, whose distances astronomers must know in order to obtain better estimates of the Hubble constant.

Stothers and Chin examined seven luminous blue variables in four galaxies, including our own. The stars’ periods of variation ranged from 4 to 20 years, and those with short periods outshone long-period stars – the opposite of the relationship for Cepheids.

Stothers and Chin say that luminous blue variables do not flicker for the same reason that Cepheids do. Instead, they argue that instabilities in the stars’ outer atmospheres cause them to periodically eject matter into space. They have produced theoretical models of this process which reproduce the stars’ behaviour.

In future, astronomers will be able to use this relationship between period and luminosity to determine the distances of far-off blue variables. The only drawback is that these studies will be very time-consuming, because the stars’ periods are much longer than those of Cepheids.

The discovery will be reported in the 1 October issue of Astrophysical Journal Letters.

Philip Massey and Andrea Thompson of Kitt Peak National Observatory
in Tucson, Arizona, say that not only does VI Cygni No 12 outshine every
other star visible to the naked eye, but it is also brighter than some complete
galaxies. If a star this bright was at the distance of Alpha Centauri, the
nearest star to the Sun, it would appear several times brighter than the
full Moon.

VI Cygni No 12 is a blue ‘supergiant’ star with a surface temperature
of 13,000 K. It lies in a group of more than 100 hot blue stars 5700 light
years away. They are known as the Cygnus OB2 Association.

When astronomers measure the brightness of a star, they measure only
its ‘apparent’ brightness. To determine a star’s true or ‘intrinsic’ luminosity
they need to know how far away it is.

If VI Cygni No 12 really belongs to the Cygnus OB2 Association, then
it must be 5700 light years away as well. This would make the star extremely
luminous. But some astronomers believe that the star is closer than this,
and consequently less luminous. This is the source of the long-running debate
in astronomical circles.

Massey and Thompson observed more than 100 members of the Cygnus OB2
Association, including VI Cygni No 12 itself. Their study leads them to
believe that the star is indeed a member of the Cygnus OB2 Association.

As their first piece of evidence, the two astronomers noted that the
star lies among the association’s other stars. Also, they point to a spectrum
which they obtained of the star. This indicates that VI Cygni No 12 is extremely
luminous.

If Massey and Thompson are right, VI Cygni No 12 is about 15 times as
luminous as Rigel in the constellation of Orion. Rigel, a well-known blue
supergiant star, has a luminosity at visual wavelengths that is about 60,000
times that of the Sun.

Massey and Thompson say that there is some uncertainty in their calculations
of the luminosity of VI Cygni No 12. It arises because the star is surrounded
by a thick cloud of dust. This absorbs so much light that the star – which
is blue – appears red when observed from the Earth. In fact, if the star
had no dust around it, say the astronomers, it would be visible to the naked
eye.

The two astronomers estimate that the dust cloud absorbs about 10,000
times as much visible light as it lets through. But if they are wrong in
this, and have overestimated the absorption by the dust, they will also
have overestimated the star’s intrinsic luminosity.

Whatever the precise figure, though, the star is definitely brighter
than Rigel. Massey and Thompson believe that it is no coincidence that such
a luminous star has a thick cloud of dust surrounding it. Many luminious
stars lose mass, and Massey and Thompson speculate that the cloud surrounding
VI Cygni No 12 arose when the star lost mass many years ago.

The astronomers’ results will appear in a future issue of The Astronomical
Journal.

Brown dwarfs, if they exist, have so little mass – less than 8 per cent
of the Sun’s – that they never ignite the hydrogen fuel at their cores.
But for a few hundred million years after their birth, they shine faintly
by converting gravitational energy into heat. During this time, brown dwarfs
appear faint and red.

Brown dwarfs might be very common. Some astronomers believe they might
be the ‘dark matter’, which accounts for about 90 per cent of the mass of
the Universe.

Maria Teresa Ruiz and Marianne Takamiya of the University of Chile and
Miguel Roth of Las Campanas Observatory in Chile identified ESO 207-61 by
inspecting photographice plates of the sourthern sky which were sensitive
to red light. They then used the Cerro Tololo Inter-American Observatory
in Chile to obtain the star’s spectrum. It resembled the spectrum of another
faint star, called LHS 2924. The astronomers will report their discovery
early next year in Astrophysical Journal Letters.

LHS 2924 is also a possible brown dwarf. In 1983, American astronomers
discovered that the star is very cool and intrinsically faint, and declared
it a brown dwarf ‘candidate’. Ruiz and collealgues believe that because
ESO 207-61 resembles LHS 2924, it may itself be a brown dwarf star. The
astronomers’ report says that the star is very red, with a temperature of
only 2200 K.

According to Ruiz and his colleagues, the motion of ESO 207-61 through
space suggests that the star is young. Its motions matches that of a star
cluster called the Hyades, which is only 600 million years old. ESO 207-61
is not part of the Hyades, but its similar motion means that the star may
have been born at the same time as the Hyades. If so, ESO 207-61 is also
only 600 million years old – young enough not to have faded to invisibility
yet.

Ruiz and his colleagues admit that ESO 207-61 may not be a brown dwarf
at all, but a red dwarf star instead. Red dwarfs are the most common type
of star, accounting for 70 per cent of all stars in the Galaxy. They have
masses between 8 per cent and 60 per cent of the Sun’s and burn their hydrogen
fuel the way the sun does. Red dwarfs consume their fuel slowly, so they
are faint, red and cool.

Is ESO 207-61 a brown dwart or merely another red dwarf? A lot hinges
on the star’s distance, which no one knows. If astronomers knew the distance,
they would be able to determine the star’s intrinsic brightness. To be as
intrinsibally faint as LHS 2924, it would have to be 49 light years away.
So, if astronomers find that the distance is greater than this, it will
mean that the star’ intrinsic brightness must be greater, too, and ESO 207-61
may not be a brown dwarf either, but simply a very faint red dwarf.

Red dwarf stars are small and faint, typically with surface temperatures
of only a few thousand °Celsius. Many brighten unpredictably, sometimes
several times in a single day.

Astronomers believe that the outbursts on such ‘flare stars’ are caused
by flares similar to those seen regularly on the surface of the Sun. The
reason that a flare on a red dwarf appears to be so much more dramatic is
that the star itself is intrinsically faint. Often the flare outshines the
entire star.

CZ Cancri, which is in the constellation of Cancer, drew attention to
itself in 1976 when M. Lovas of Konkoly Observatory, Hungary, took two photographs
of the star in blue light, two hours apart. He found that the star had faded
from magnitude 13.5 to 18.0 between the two photographs.

Astronomers measure the brightness of stars in terms of magnitude. The
scale is logarithmic: an increase in magnitude of 5 corresponds roughly
to a decrease of 100 times in brightness. For this reason, the larger the
magnitude of a star, the fainter it is.

Lovas searched through prints of the Palomar Sky Survey taken in blue
light. This is a photographic survey of all objects in the northern sky
down to magnitude 20. But Lovas could find no image of CZ Cancri, indicating
that, normally, the star is fainter than magnitude 20. He concluded that
the star had ‘jumped’ at least 6.5 magnitudes, becoming at least 400 times
brighter than usual.

Now Schaefer in Greenbelt has obtained new, more sensitive, observations
of CZ Cancri. He has confirmed that the star is indeed a red dwarf: it is
very red in colour, and only one ten-thousandth as luminous as the Sun.

Schaefer finds that the star’s blue magnitude is now only 23.0. This
means that the flare Lovas saw in 1976 caused CZ Cancri to jump 9.5 magnitudes,
a 6000-fold increase in brightness. Such a huge outburst is a record for
any flare on a red dwarf. The previous champion was the star AF Piscium,
which brightened by 7.7 magnitudes, also in 1976.

Schaefer will be publishing his results in the journal Astrophysical
Journal Letters. He says that ‘superflares’ such as CZ Cancri’s could complicate
the search for the optical counterparts of gamma-ray bursts. These are mysterious
explosions of gamma radiation. Astronomers might mistake a superflare on
a previously unknown red dwarf for such an optical component.

Halton Arp, and Geoffrey Burbidge, of the University of California,
San Diego, argue that 3C 273 is interacting with a giant elliptical cloud
of hydrogen gas, known to be in the vicinity of the Virgo cluster. Astronomers
discovered the Virgo Cloud only last year. They think it is a dwarf galaxy
which is still in the process of forming (This Week, 9 September 1989).

Arp and Burbidge believe that 3C 273 is a member of the Virgo cluster,
even though it has a much larger red shift. Most astronomers believe that
the red shift of a galaxy or quasar is a measure of its distance: the greater
the red shift, the greater the distance.

A few astronomers, in particular Arp, disagree with this. Arp has identified
cases where galaxies which have low red shifts appear to be interacting
with quasars which have a high red shift. If they are interacting, Arp reasons,
both must be the same distance from us. Most astronomers dismiss Arp’s examples
as chance projections of a foreground galaxy against a distant quasar.

The quasar 3C 273 has a red shift that corresponds to a distance of
2000 million light years. The Virgo Cloud, on the other hand, is only 65
million light years away. If 3C 273 is interacting with the Virgo cloud,
say Arp and Burbidge, then the quasar must be part of the Virgo cluster
and must also be at a distance of only 65 million light years.

Arp and Burbidge have several reasons for believing that 3C 273 and
the Virgo cloud are related. The quasar is the nearest and brightest of
all quasars. They point out that 3C 273 is also the nearest bright object
beyond our Galaxy which is close on the sky to the Virgo cloud. The quasar
and cloud are only three-quarters of a degree apart, which corresponds to
850 000 light years at a distance of 65 million light years. By comparison,
the major axis of the Virgo cloud is 650 000 light years long.

Arp and Burbidge say that both the Virgo cloud and 3C 273 are unique
objects. Astronomers have catalogued thousands of quasars in the past 25
years, but 3C 273 is the brightest. Also, 3C 273 is shooting out a jet of
matter. Arp and Burbidge say this jet points along the long axis of the
cloud.

Commenting on the claim, Martin Rees, of the Institute of Astronomy,
Cambridge, said the association was ‘an amusing coincidence, no more’.

Arp and Burbidge will be publishing their results in Astrophysical Journal
Letters.