A few facts about the Milky Way.
The faint, luminous band of light that can be seen stretching across the sky on a clear night, away from town and city lights, is our galaxy. The Milky Way. The home of our sun and our solar system and of billions of stars. All the stars we can see in the sky with our naked eye are part of the milky way.
The Milky Way is just one of billions of other galaxies in the known universe and it belongs to what is known as 'the local group' - a cluster of galaxies in the immediate vicinity. It's the second largest galaxy in the local group, the Andromeda Galaxy or M31, being the largest.
Like the Andromeda Galaxy, the Milky Way is a huge spiral galaxy, but recently scientists have discovered that, unlike Andromeda, the Milky Way is a barred spiral. That is, it has a bar running across the central part. The bar was only discovered quite recently. It's existence was confirmed using the Spitzer Space Telescope in 2005.
Living on the planet Earth and being part of the galaxy, we cannot see it's spiral structure. This is a great shame. It would be wonderful to see the structure from 'outside' as it were. But astronomers have a good estimate of it's size and appearance using data from radio emissions given out from the galaxy.
The Milky Way is believed to be more than 13 billion years old which is virtually as old as the entire universe itself. It has a very large circumference of around 250 to 300 billion light years. (see Footnote for description of a "light year"). Whilst the galactic disk has a diameter of between 70,000 and 100,000 light years. The galaxy contains between 200 billion and 400 billion stars, in comparison with the Andromeda system which contains a trillion stars.
Even though Andromeda (M31) has more stars than the Milky Way they both have about the same overall mass (or density). This means that our own galaxy is more dense than M31.
It is thought that the spiral arms are wound fairly loosely around the galactic centre.
The distance of the sun from the centre of the galaxy is estimated at around 26,000 light years. This is closer than was at first thought - 36,000 light years being the previous estimate.
The centre of the galaxy, detected by radio emissions, contains a large mass known as Sagittarius A, which itself contains a supermassive Black Hole. This is in accordance with almost all massive spiral galaxies, most are homes to black holes at their centre.
Also in the centre, the galactic bar is around 27,000 light years across, and the bar itself is surrounded by a ring called the "5-kpc ring" If viewed from outside the galaxy, this ring would be visible as the brightest part of the galaxy.
At one time there were thought to be four spiral arms surrounding the central bulge, but now it is known that there are just two major arms; the Perseus arm and the Scutum-centaurus arm. The rest is made up of minor arms.
As with the vast majority of spirals, the Milky Way has a galactic halo made up chiefly of old stars and globular clusters of stars. There is also a river of galactic debris from a 'too close encounter' with the Sagittarius Dwarf Elliptical galaxy - the interaction with the Milky Way tearing the dwarf galaxy apart. In addition, a ring of debris was found from a galactic encounter with the Canis Major Dwarf galaxy and this stream of debris encircles the galactic disk.
A mysterious huge and diffuse structure in the galactic plain was discovered by the Sloan Digital Sky Survey. It spreads through a huge area about 5,000 times that of the full moon and rises almost perpendicular to the spiral disk. The current theory as to what this structure is suggests that it is a dwarf galaxy merging with the Milky Way. In other words, our galaxy is gobbling up a much smaller one.
Scientists think that in around 3 billion years the Milky Way will collide with the Andromeda Galaxy, as the two are moving towards each other. Also the outer edge of the Andromeda galaxy spreads out much further than was at first thought, and it may be possible that this applies to the Milky Way Galaxy too. So the question might be - are the two galaxies in some ways linked to each other? Only time will tell. More time than we will ever know.
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Footnote: A "light year" is a measure of how fast light travels in a year.
Light travels at 186,000 miles per second1
In a year that's 5.88 trillion miles
Note: Top picture of the Milky Way looking from Earth and an artist's impression of the Milky Way
en.wikipedia.org/wiki/Milky_Way
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