Roger Hanson: The little galaxies on our doorstep
Wednesday, 26 July 2017
Two of the closest galaxies to Earth are dwarf galaxies, the Large and Small Magellanic Clouds (the LMC and SMC respectively). At a distance of 200,000 light years they are tens times closer than our nearest big galaxy, the Andromeda Galaxy.
The Magellanic Clouds are only visible from the Southern Hemisphere - an experienced astronomer could pick them out as two cloudy smudges in the night sky. However, studies using some of the world's biggest telescopes show they are far more than interstellar (between the stars) gas clouds.
Europeans documented the Magellanic Clouds in the 15th century but they were know to Polynesians well before then. Maori referred to them as, Nga Patari-Kaihau or Te Reporepo. The name Magellanic Cloud came from the circumnavigation of the world (1519-1522) organised by Ferdinand Magellan.
The LMC and SMC are about 75,000 light years apart – light travelling at 300,000kms per second would take 75,000 years to travel between them – however they are linked by the Magellanic Bridge, a stream of hydrogen gas and stars. Although they are called 'clouds', the two galaxies comprise stars, clusters of stars and nebulae.
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The LMC alone has 20 billion stars. Nebulae are vast regions of interstellar dust, hydrogen and helium gas. Some of this gas was present prior to the stars being there, held in a vast cloud by gravitational attraction forces. The rest of the gas is material shed by its stars.
An astronomically significant part of the LMC is the Tarantula Nebula, an active area of star formation. In the central region is a stellar giant called R136a1 - the most massive star with the highest luminosity of any star known in the Universe.
A star 20 times the mass of the Sun is regarded as a massive star, but R136a1 is a colossal 300 times the mass of the Sun. Its luminosity, that is, the amount of energy it emits per second, is 10 million times that of our Sun. R136a1 radiates more energy in 5 seconds than the Sun does in one year – we aren't blinded by it because it is 200,000 light years away and shrouded somewhat by a canopy of interstellar material.
Edwin Hubble in the early 1920s was convinced that the Magellanic Clouds are orbiting our galaxy, however recent analysis shows that at 400 kilometres per second relative to our galaxy, they are moving too fast to be captured by the gravitational field of the Milky Way, so aren't orbiting us, they are passing by. In a few billion years they will have disappeared into deep space.
Observation using specialist telescopes in the late 1960s showed that there is an enormous streak of gas in the sky above us with the two Magellanic Clouds at its head. It is called the Magellanic Stream and is the result of the collision between radiation and particles emitted by our galaxy which strip material from the SMC and LMC producing this vast stream of debris.
A further interesting feature of the Magellanic Clouds is that they have a high population of blue giant stars. Blue giants are massive and very hot stars; they burn blue not yellow like our Sun.
This is a clue to the origins of the Magellanic Clouds. Galaxies like ours are huge, the Milky Way has a diameter of between about 100,000 to 150,000 light years. Just after the Big Bang all galaxies were small, ragged and full of hydrogen gas.
Over billions of years these small galaxies, gradually through gravitationally attraction, merged and became bigger and more structured, like our beautiful spiral galaxy, the Milky Way.
The first galaxies, soon after the Big Bang, had very high populations of blue giants which are massive stars made from the abundance of dense gas present in the then compact Universe – these early galaxies were just like the ragged, blue giant rich, hot Magellanic Clouds.
Far from from being insignificant wisps of cloud, the Magellanic Clouds turn out to be rare isolated left overs from the early Universe, regions that somehow managed to escape the clutches of other galaxies and in the case of the LMC, contain the most luminous star known.