Feeds:
Posts
Comments

Posts Tagged ‘the Large Magellanic Cloud’

The clouds of Magellan

I am currently giving astronomy talks on a cruise around South America; by the time you read this the ship will be sailing up the Chilean coast after having been to Cape Horn, Ushuaia (the most southerly town in the World) and through the Beagle Channel. I was lucky enough to give talks on the same cruise in March last year, and jumped at the chance to do the same again.

The cruise on which I am currently giving astronomy talks. It left Buenos Aires on 2 February and will arrive in Santiago on 15 February.

The cruise on which I am currently giving astronomy talks. It left Buenos Aires on 2 February and will arrive in Santiago on 15 February.

One of the highlights of this cruise from an astronomy point of view is being able to see the Magellanic Clouds, two satellite galaxies of our Milky Way galaxy. The first time these were seen by Europeans was back in the 15th Century by Portuguese and Dutch sailors. They were initially known as “Cape Clouds”. They were noted in 1503-1504 by Amerigo Vespucci (after whom the “Americas” are named). When Ferdinand Magellan and his ships circumnavigated the Earth in 1519-1522 the sailors on the ships noticed these two “clouds” in the sky which were visible from southerly latitudes. They were described in some detail by Antonio Pagafetta, who sailed with Magellan and, as a consequence, have become known as the Magellanic Clouds.

The large and the small Magellanic clouds (LMC and SMC) are not visible to people living in Europe or North America, as they lie too far south in the sky. In astronomy, we specify the position of something in the sky by its Right Ascension and Declination. Declination is the celestial equivalent of latitude, and an object in the sky is either north or south of the celestial equator (which is just an imaginary line on the sky where the Earth’s equator would be projected if we imagine the Earth surrounded by the sphere of the sky – see the figure below).

All objects in the sky have celestial coordinates. The declination of an object is either north of south of the celestial equator, an extension of the Earth's equator out onto the celestial sphere

All objects in the sky have celestial coordinates. The declination of an object is either north of south of the celestial equator, an extension of the Earth’s equator out onto the celestial sphere

The SMC lies at a declination of about -73 degrees (73 degrees south of the celestial equator), and the LMC is at a declination of about -70 degrees. I say “about”, because they each cover a patch on the sky which extends for several degrees; these declinations are the objects’ approximate ‘centres’. What this means in practical terms is that you cannot see the LMC or SMC unless you are south of the equator. To be more specific, the furthest north you can be for the LMC to appear on the horizon is 20 degrees north, but practically you cannot see something unless it comes up about 10 degrees above the horizon; so you need to be south of about 10 degrees north of the equator to see the LMC. The same is true for the SMC, but it is a few degrees again further south in the sky.

By the time you get down to southern South America, you are far enough south that the Magellanic Clouds become circumpolar, meaning they never dip below the horizon (the Plough, or Big Dipper, is circumpolar to most people in Europe and North America).

 

The Small Magellanic Cloud

  

The Large Magellanic Cloud

 There are many things I could say about the Magellanic clouds, but I will briefly mention two things, and I will expand on each of them in future blogs.

  1. The last supernova (exploding star) to be visible to the naked eye occurred in the LMC in 1987 (SN1987A). Or, I should say, we saw it in 1987; it happened some 160,000 years ago because of the distance of the LMC. SN1987A is the most studied supernova in history, the last supernova to be visible to the naked eye before SN1987A was Kepler’s supernova, which was seen in 1604! We have learnt a great deal about supernovae by studying SN1987A.
  2. In 1912 Henrietta Leavitt made a very important discovery about some variable stars in the SMC. This discovery, which I will explain in more detail in a future blog, enabled us to measure the distances to very distant objects. It was used in 1923 by Edwin Hubble to show that the Andromeda nebula was, in fact, too far away to be within our Milky Way galaxy; the first ever proof that our Galaxy was not the entire Universe.

  

    Advertisements

    Read Full Post »