Spiral galaxy is one of four main classes of galaxies, the other being Elliptical, Lenticular and Irregular. Unlike the other classes, there are two sub classes of spiral galaxies, they are :-
Spiral galaxies are the most common type of galaxies in the Universe. Our galaxy, the milky way is believed to be a spiral galaxy but no one has seen it, it is just calculated as being a spiral galaxy. The nearest largest galaxy to us, the Andromeda Galaxy is also a spiral galaxy. Photos of the Andromeda Galaxy show what a spiral galaxy looks like from above.
Spiral and elliptical galaxies have one thing in common, they both have Supermassive Black Hole but how the stars are arranged are different. In a spiral galaxy, the stars are in a disc shape. In a elliptical galaxy, the stars are not arranged and can orbit in elongated patterns and routes.
The Milky Way is about 100 Light Years, the Andromeda Galaxy is double that size but the largest Spiral Galaxy on record at the moment is NGC 6872 in the constellation of Pavo . The galaxy is 522,000 Light Years across, more or less five times the size of our galaxy. The largest galaxy is an elliptical galaxy, IC 1101 which is an impressive 3,913,800 light years.
Whilst some of the largest galaxies are elliptical, some ellipticals are also amongst the smallest, ellipticals don't just have the monopoly on largest.
We may see a spiral galaxy face on like we do with the Andromeda Galaxy or we may see them edge on like NGC 6814 in the constellation of Aquila. Alternatively, we can see them edge on as in NGC 2683 in the constellation of Lynx.
In the case of the Milky Way and our position in it, the quickest way out of the galaxy is north, upwards towards a spot located near Arcturus rather than towards the anti-centre El Nath. The Anti-centre is the point in the sky that is opposite the centre of the galaxy.
Barred spiral galaxies are galaxies where the centre is more like a bar than a sphere hence their name. When you see a galaxy's Morphological type, a small b in the type will indicate that it is a barred spiral.
Both have arms of stars coming from the centre of the galaxy outwards. The main difference is how the supermassive black hole or bulge is shaped at the centre of the galaxy. If the centre is more elongated than spherical then it is referred to as barred.
As you can see from the picture from Nova Celestia shows the different types of galaxies including the forking of galaxies between barred and non-barred. Our galaxy, the Milky Way galaxy is probably a Sa galaxy. As no one has ever seen the galaxy from outside, we can't be 100% sure but its most likely candidate.
As you can see from the picture above, stars are in arms stretching out from the centre of the galaxy. Our galaxy has arms that have been named after constellations. We are in the Orion arm. If you were to shrink the Milky Way down to the size of London, the City of London would the centre and Earth would be located in suburbs, in Barking for example.
If you look at this Google Map, you can see Barking on the right. The M25 orbital road would mark the boundary of the galaxy.
A spiral galaxy is flat unlike a elliptical which is round. All the stars orbit in a spiral in a disc shape. Given that the centre of our galaxy Sagittarius A* is in the southern hemisphere, it is safe to say we are in the upper part of the disk.
The below picture illustrates where we are in respect to the centre of the galaxy.
Stars in a spiral galaxy orbit round the centre of the galaxy. It would take 225 - 250 Million years to complete a complete orbit of the centre of the Milky Way galaxy. We have completed two galactic years since the explosion of animal diversity that kicked off in the Cambrian period. National Geographic dinosaurs have lived in less than a galactic year. Dinosaurs first appeared 230 million years ago Guardian therefore we have completed one galactic year since their arrival.
It is thought that spiral galaxy will evolve into elliptical galaxies as they grow older. Elliptical galaxies tend to have more older stars, redder stars compared to spiral galaxies with younger bluer stars. Space
It is thought that some six billion years ago, some spiral galaxies had peculiar shapes. Six billion years is before the creation of our Solar System which is thought to be about five billion years give or take a few million years. Those ugly ducklings evolved into the spiral galaxies of today. E.S.A.
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