Les Nuages de Magellan

 

 

The Large Magellanic Cloud, LMC

[LMC, UKS]

Right Ascension

5 : 23.6 (h:m)

Declination

-69 : 45 (deg:m)

Distance

179.0 (kly)

Visual Brightness

0.1 (mag)

Apparent Dimension

650x550 (arc min)

Known pre-historically on the Southern hemisphere. Mentioned 964 A.D. by Al Sufi. Discovered by Magellan 1519.

The Large Magellanic Cloud, together with its apparent neighbor and relative, the Small Magellanic Cloud , are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way for the naked eye. They were certainly known since the earliest times by the ancient southerners, but these people produced little documents which are still preserved. The first preserved mention of the Large Magellanic Cloud was by Persian astronomer Al Sufi , who in 964 A.D., in his Book of Fixed Stars calls it Al Bakr , the White Ox, of the southern Arabs, and points out that while invisible from Northern Arabia and Baghdad, this object is visible from the Strait of Babd al Mandab, at 12deg 15' Northern latitude. Eventually, it was Magellan and his discovery expedition who brought them to our knowledge in 1519.

Both Magellanic Clouds are irregular dwarf galaxies orbiting our Milky Way galaxy, and thus are members of our Local Group of galaxies. The Large Magellanic Cloud, at its distance of 179,000 light years, was longly considered the nearest external galaxy, until in 1994, the Sagittarius Dwarf Elliptical Galaxy was discovered at only about 80,000 light years. (Our current distance value takes into account the corrected Cepheid distance scale based on the Hipparcos satellite data published in early 1997.)

Although a small irregular galaxy, the LMC is full of interesting objects including diffuse nebulae (especially the Tarantula Nebula, NGC 2070 , a giant H II region), globular and open clusters, planetary nebulae, and more.

On February 24, 1987, supernova 1987A occurred in the Large Magellanic Cloud, which was the nearest observed supernova since Keplers, which occured before the invention of the telescope. Supernova 1987A, peculiar and of type II, was one of the most interesting objects for the astrophysicists in the 1980s (some even say of this century).

 

Introduction and History of the Magellanic Clouds

The Clouds of Magellan borrowed their name from the navigator portuguais who was the first with launching his ships in a voyage of circumnavigation (turn of the world in boat) between 1519 and 1522.  Fernand of Magellan (1480-1520) set sail towards Cape Horn when its sailors saw - the first in Europe two species of fragments of the Milky Way deriving in a portion from the sky in addition deprived of any interest.  Most significant of these misty tablecloths is from now on known under the name of Grand Cloud of Magellan;  the other under that of Small Cloud of Magellan.  Formerly, they were called Nubecula Major and Nubecula Minor.  Part of the Large Cloud of Magellan belongs to the constellation of Mensa, but it is especially in the Dorado constellation.  This constellation décousue was defined in 1603 by Johann Bayer starting from the accounts which the sailors told who had followed the wake of Magellan.  Bayer gave also Greek letters to brilliant stars - as Alpha Centauri or Crucis Beta, for example.  In most of the northern hemisphere, the Clouds of Magellan remain constantly below the horizon.  In the south of the equator, they are easy to see with the naked eye if one moves away from the light of the cities.  In the south of Australia, they are almost always above the horizon, whatever the season.  The two Clouds of Magellan are undoubtedly connected one to the other and the Milky Way by gravity.  The Clouds of Magellan have an animated history.  Today, they tear mutually and disperse their stars and their gases.  One day, our galaxy will receive their remains.  But the time scales are so long that we never feel these shocks. 

The Clouds of Magellan constitute the example nearest to irregular galaxy.  In fact satellite galaxies of the Milky Way and have masses of 2 billion and 10 billion solar masses.  The Large Cloud of Magellan, located between the constellation of Mensa and the Doradus constellation, has a structure besides bars some with a pretence of spiral arm on a side, but the gravitational interaction of our Galaxy probably prevented from developing a regular structure.  Sometimes, the irregular galaxies result quite simply from a collision between two galaxies.  The Large Cloud of Magellan, who has a diameter of approximately 40000 light-years and whose size corresponds to the third or half of our own galaxy, is located at a distance of 178000 light-years. 

It is connected to its neighbor, the Small Cloud of Magellan, by a "bridge", thin hydrogen line which is common for them.  The Large cloud of Magellan contains a great number of supergéantes extraordinarily brilliant whose luminosity reaches until a million times that of the Sun. 

A little history now:  were these galaxies of clouds always qualified?  Which are the men who observed them for the first time?  Such are the questions that sometimes many people are posed among the amateurs of astronomy or astrophysics and to which we propose to bring some answers or rather, some assumptions.  Evidence attests that in the past, the Clouds of Magellan would have been brought closer.  He may be even that five hundred million years behind, the Large Cloud of Magellan almost came very close to Milky Way, approaching 70000 light-years of his center.

The Large Cloud of Magellan is, apart from the Milky Way, the widest object which one can see in the sky:  it extends indeed on more than 50 square degrees whereas, by way of comparison, the galaxy of Andromède M 31 occupies a surface much smaller.  The large Cloud is the galaxy nearest to us and its extent is explained only by its proximity because, intrinsically, it is a small galaxy.  So diameter 5 kiloparsecs, either the sixth of that of our Galaxy - and its mass, we said it - 10 billion times that of the Sun, or twentieth among that of our Galaxy - are definitely lower than those of the typical spiral galaxies.  Photography attached shows the real colors of the Cloud and, although not covering it entirely, makes it possible to distinguish from them the elements the most significant oldest stars, those which have colors active of the yellow to the green, are distributed on all the surface of the image.  Many is concentrated in this famous bar which, at first sight, seems the element dominating of this galaxy.  The youngest stars, of blue color, are distributed that and there or form small insulated clusters.  Often, of the reddish filamentous areas are associated for them.  They are ionized gas areas whose color comes from the emission of the red line H I of ionized hydrogen.  Largest of these areas, the nebula of the Tarantula, is located above the left end of the bar. 

The image below is a photograph of  the Tarantula Nebula :

[NGC 2070, AAT]

A SHORT EXERCISE

MEASUREMENT OF DISTANCE OF A CEPHEID.

            We search how far we are from the cepheid               in the constellation Virgo.

            By definition the point brilliance of star is this star’s luminosity that we receive from the Earth, per unit of area :

                        Q = L/A          with :

-          Q point brilliance (W/m²)

-          L luminosity (W)

-          A the area of the sphere whose radius is d distance of the cepheid (m²)

Then   Q=L/4πd². For a cepheid we use an average equivalents of L and Q :

                        Qav = Lav/4πd²

So to calculate d, we just have to determine Qav and Lav.

A.Determination of Lav.

            From 8 photographs of the cepheid            with a « normal » star taken some days apart, we have got the brilliance Bc and Bn of the cepheid and the normal star respectively, in function of time :

Day

6 th May

8 th May

10 th May

11 th May

14 th May

15 th May

18 th May

21 st May

Bc/Bn

1%

33%

61%

44%

1%

25%

56%

22%

            We have at our disposal the diagram, established by Henrietta Leavitt in 1908 from cepheids of Magellanical Clouds, which gives us average luminosity of a cepheid in function of its period T.

This graph is in a log-log scale !!

1°) Determine the period T of the brilliance Bc of the cepheid, noting that the one of the normal star is approximately constant.

2°) From the period T, determine the maximum and minimum average equivalents of the average luminosity Lav, Lav max and Lav min , in solar units (L¤) then in watts (W).

Recall : 1L¤ = 5,7.10^25W.

B.Determination of Qav.

            Brilliance B is proportional to point brilliance, that’s why Qc/Qn = Bc/Bn.The average point brilliance Qav of the cepheid is equal to the average light flow of a « normal » star which would give as much luminous energy. Luminous energy identifies with a luminous power multiplied by a duration ; it is proportional to the area limited by the graph : luminous power L = f(t) and the x-axis. Q is proportional to L, so we search Qc/Qn so that the two areas, the one limited by Qc/Qn and the one limited by Qav/Qn, are equal.

1°) Plot the graph Qc/Qn = f(t) considreing 6th May as t = 0.

2°) With this graph determine Qav/Qn.

3°) We have taken the normal star as a reference. Calculate Qav in case its point brilliance is equivalent to Qn = 2,28.10^-12 W/m².

C.Calculation of d.

1°) From the relationship Qav = Lav/2πd², calculate d.

2°) From the equivalents Lav max and Lav min , calculate d max and d min.

3°) Convert the three equivalents of d into light years.

Recall : 1ly = 9,4.10^15 m.

4°) How accurate is the calculation of d ? How many significant figure(s) is(are) required to express d ?