What is a blue supergiant? (Part I)
Typically people think of blue being cold, and red being hot. In reality, when it comes to turning the tap, astrophysics doesn’t apply; blue stars are actually hotter than red stars! Looking at the visible region of the electromagnetic spectrum (i.e. the light we can detect with our eyes) red and blue are at opposite ends of the scale. You can think of the colours of the rainbow, which are the colours you get when shining white light through a prism (thanks Newton).
Red light has a longer wavelength than blue light, which also means that it has a lower frequency than light in the blue. The higher the frequency the little packets of light have (called photons) the more energy they carry. So there we have it, stars in the sky that appear bluer are in fact hotter!
So what is a blue supergiant? They are the hottest stars in the universe: between 12,000 and 50,000 degrees Celsius (the Sun is around 6,000 °C). They are also phenomenally large. If you were to look up at the night sky in winter and see the constellation Orion, you would notice the brightest star Rigel, Orion’s left foot. Or looking towards Cygnus, the swan in the Milky Way, you would see his tail Deneb. With a radius of 140 million km, Deneb would engulf Mercury and Venus, and Earth would be a very different place indeed! (Earth is around 150 million km from the Sun.)
“Sirius was rising in the east;
And, slow ascending one by one,
The kindling constellations shone.
Begirt with many a blazing star,
Stood the great giant Algebar,
Orion, hunter of the beast!
His sword hung gleaming by his side,
And, on his arm, the lion’s hide
Scattered across the midnight air
The golden radiance of its hair.”
Henry Wadsworth Longfellow
(‘Begirt’ and ‘Algebar’ are Arabic for Orion and Rigel)
Blue supergiants are short-lived, as they burn through the hydrogen in their core much more quickly than any other star. The flame that burns twice as bright burns half as long. Our Sun is a ripe old age of 4.6 billion years old (the universe is 13.7 billion years old, to put that into perspective). After another 5 billion years the hydrogen will run out. Yet a blue supergiant will die long before one billion years. In only a few million years all of the hydrogen will be fused into helium, and the elements beyond. The burning stops when it reaches iron, since fusing elements heavier than that would no longer be energy efficient. Soon after, the blue supergiant goes supernova, like SN 1987A. New stars are reborn out of the ashes (i.e. the dust and gas), and the cycle continues.
To see how blue supergiants compare to all the other stars, we can look at the Hertzsprung-Russell Diagram. It is a useful map that can show the lifecycles of many different kinds of stars in one single snapshot. The key components of the diagram are the temperature and luminosity (in other words brightness) of the star. You can see Rigel and Deneb right up at the top, which tells us that they are around a million times brighter than the Sun.
Blue supergiants and the distance ladder (Part II)
But what can we learn from blue supergiants? Astronomers want to work out accurate distances to objects (e.g. stars and galaxies) in the universe. The more accurately we know these distances, the better we can determine Hubble’s constant. This is the number that tells us how fast the universe is expanding, and we still can’t decide on a value. Hubble’s constant relates to the universe in more ways than one: we can work out when the universe came into existence; and how much dark matter and dark energy the universe is really made up of.
At the moment Hubble’s constant is somewhere between 67 and 73 km/s/Mpc (a Megaparsec is the same as 3.26 million light years). This means that at a distance of one Mpc (a bit further away than Andromeda – which can be seen with the naked eye), a galaxy would be travelling away from us at about 70 km/s.
The more accurately we know the distances to stars in our local environment, the better we can work out distances to objects even further away (as this video excellently explains). Standard candles, used to determine distance, tend to be either supernovae or a type of star called a Cepheid. But blue supergiants are perfectly capable of being used as a standard candle too. They are extremely bright, allowing us to observe them up to 10 Mpc away with the telescopes available to us now. By adding blue supergiants into the picture we have a new and independent way to determine distance to nearby galaxies. As we improve the distances to galaxies further away, we ultimately improve upon the value of Hubble’s constant.
But how do we work out the distance? Blue supergiants evolve very rapidly, and because of this their brightness and mass doesn’t really change over a short space of time. Rolf Kudritzki plotted the magnitude (i.e. brightness) and gravity (related to mass) of 24 blue supergiants in the Sculptor Galaxy. He found that a straight line can be fit to the data, showing a relation between brightness and mass. Since the Sculptor Galaxy is very close, we have a good idea of its distance already. Because of this, we can fix this relation and then apply it to blue supergiants in other galaxies to work out their distance. (I did this for my Master thesis, for which I worked out the distance to Barnard’s Galaxy – feel free to ask me questions!)
Currently, work is being done to improve upon this relation using the Large Magellanic Cloud, another nearby galaxy for which the distance is well known. This will improve the distance calculations made using blue supergiants in the future. Stars (especially the big ones) are the most fundamental objects in the universe; they process the elements needed to create life as we know it, and provide the radiation needed to sustain that life so that we may be sitting here today pondering about the meaning of it all.