Antares outshines the brightness of the Sun
In science and technology, we are now fairly used to the idea of achieving things today that were unthinkable even a few years ago. This applies to astronomy too.
One of these is our new ability to make useful images of other stars. Stars other than the Sun are no longer inaccessible points of light. We are finding that other stars can be very different from our local, yellow dwarf star.
A good example of this is revealed by images taken by the Atacama Large Millimetre Array of the red supergiant Antares, a star in the constellation of Scorpius, which we can see this time of the year low in the southern sky.
ALMA is a radio telescope consisting of an array of 66 dish antennas, which function together as a radio camera. This is an international project in which Canada is a partner. It uses radio wave lengths of a few millimetres (your local FM stations use wavelengths in the region of three metres). At these wavelengths the instrument acts as a very sensitive thermal imager, seeing through the dust clouds that block visible light and infrared.
Millimetre wavelengths are strongly absorbed by our atmosphere, especially the water vapour in it, so ALMA is located on one the highest, driest places in the world, the Atacama Plateau in Chile.
Antares is a red supergiant star. It has about 12 times the mass of the Sun. The brightness of a star increases enormously as the mass increases. A star with 12 times the mass of the Sun will radiate energy at about 5,000 times the rate our Sun radiates energy.
This means that despite its having more fuel available, it will have a shorter life than the Sun. Our star will have a lifetime in the region of 10 billion years. Antares will run out of fuel after a lifetime of about 25 million years. If it has any planets, there is not much chance of life developing on any of them.
Antares is now close to the end of its life, and is running out of fuel.
When stars get old, they swell enormously into red giant stars. The Sun will do this. With its higher mass, Antares has swollen into a red supergiant star. When we start to run out of something, we generally become more frugal in the way we use it.
Paradoxically, stars do the reverse. They burn through their remaining fuel even faster. Antares is now shining about 80,000 times the brightness of the Sun. To sustain this, it is totally annihilating 320 billion tonnes of its material every second.
Antares has expanded to about 700 times the diameter of the Sun. Even with 12 times the solar mass, this means the star is about as close to being a very hot vacuum as one can get. Its gravitational hold on its outer layers is weak, and they are flowing off into space as a supersized solar wind.
This is where ALMA comes in; it has imaged the outer layers of Antares and revealed how they have become hugely swollen. For example, immediately above the yellow, shining “surface” of our Sun, there is a hot layer known as the chromosphere, maybe 2000 km thick. Antares has pushed its chromosphere out to a thickness of around
500 million kilometres. It is losing material into space at a horrendous rate.
This cannot last.
When the fuel runs out, the outward pressure will vanish and the star will collapse and then explode. This is likely to happen in the next few thousand years. For a few months, it will outshine everything in the sky other than the Sun and Moon.
-Antares means “Rival of Ares”, where Ares is the Greek name for the god of war. His Roman name is Mars. Both bodies appear as red lights in the sky. However, since stars twinkle and planets don’t, it is easy to see which is which.
Some time in the next few thousand years, Mars will no longer have any competition. It might worth keeping a weather eye on the southern sky.
----Before dawn, Jupiter and Saturn are close together in the south. Mars lies low in the southeast. Venus lies low in the sunrise glow. The Moon will be Full on the 4th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton.