M44 (although a cluster in itself, you get the idea!)
Stars are what primarily make up the visible components in galaxies, and galaxies are what primarily make up the universe. If it were not for stars, there would be no black holes, no planets, spectacular supernovae, star clusters or galaxies.
| The majority of stars in a galaxy are individual systems with no stellar partners and are usually separated by a few light years. This distance is dependant on the stars position in the galaxy. | In many areas, congregations of gravitationally bound stars group in clusters of many thousand or hundreds of thousands. | The spacing of stars decreases as they get closer to the galaxy centre, this forms star clouds. | From the outside looking in, a galaxy is an astonishing piece of art. The image below shows the combined light emitted by hundreds of billions of stars enormously swamped by the huge galaxy in which they live. |
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M44 (although a cluster in itself, you get the idea!) |
M67 | Scutum Star cloud | NGC 4414 |
Galaxies are the largest structures in the universe, varying from thousands of light years to 150,000 light years in diameter. The Milkyway galaxy we live in, is estimated to be around 100,000ly in diameter. NGC 4414 above, is estimated to have an optical diameter of 56,000ly. The Sun and of course our solar system lie at approximately 27,000ly from the centre of the Milkyway. Even with over 100 billion stars in our galaxy, we are still 24 trillion miles (4.2 light years) from the nearest star.
Below is a series of images showing the different categories of galaxy.
M100 spiral Galaxy M109 Barred Spiral (Bill keel©) M82 Irregular Galaxy
M86 Lenticular Galaxy M87 Elliptical Galaxy
The image below is known as the Hubble tuning fork diagram, and shows the different morphological types of all galaxies.
Elliptical Galaxies.
(Eo) – Spherically symmetric galaxy, appears round.
(E3) – As above but with more increasing ellipticity i.e. flatter
(E7) – The most extreme of the Elliptical galaxies, almost appearing as a disk (axial ratio 3:1).
An Elliptical galaxy contains very little gas and dust and is composed primarily of older population II stars, very little feature is seen in an elliptical galaxy. It is believed that elliptical galaxies may be triaxial, meaning that the stars within the galaxy follow only random motions throughout, and not predictably as in other types of galaxy. Elliptical galaxies are believed to inhabit up to 40% of all bright galaxies in dense regions, but as low as 10% in low density regions. The luminosity of this type of galaxy falls of smoothly with radius.
Spiral Galaxies
(S0) – Disk without spiral structure or prominent bar (also known as a Lenticular galaxy, contains very little gas and dust as do ellipticals, but follows the same exponential surface brightness law as spiral galaxies [see below])
(Sa) – Large central bulge, tight spiral arms (small pitch angle), low gas content, brightest of all spiral galaxies.
(Sb) – Smaller central bulge, larger pitch angle, higher gas content, dimmer than type Sa.
(Sc) – Smallest bulge, largest pitch angle, highest gas content, least luminous of all spiral galaxies.
Barred Spiral Galaxies.
(SBa) – Large central bulge, small pitch angle, low gas content, small visible central bar, (same luminosity rules apply to barred spirals as spirals).
(SBb) – Smaller bulge, larger pitch angle, higher gas content, more prominent central bar.
(SBc) – Smallest central bulge, largest pitch angle, highest gas content, very prominent central bar.
Both Spiral Galaxies and Barred spiral galaxies seem to be the most numerous amongst all other types of galaxy in low density regions, claiming a population of up to 80%, although dropping to only 10% in higher density regions. The luminosity of spiral galaxies falls of exponentially with radius, meaning that although the bright centres can be seen with the naked eye, the edges of the galaxy are only visible with long exposure imaging. Spiral Galaxies are very rich in gas and dust and have spiral arms composed mainly of younger population I stars. Spiral galaxies tend to have most of their visible material sitting within a disk rotating roughly in circles around the centre.
The other two galaxy types mentioned above are irregular (Irr) and peculiar (Pec). Both of these are as they sound and do not fit into any particular category, they do however have sub-categories.
Irregular galaxies.
An irregular galaxy is one in which no particular pattern can be discerned. Almost all irregular galaxies have low luminosity primarily due to the low number of stars (although still in millions) and abundance of gas and dust spread out over a large area. Many irregular galaxies are found to be old spiral or elliptical galaxies that have been gravitationally distorted by an encounter with another galaxy. Bars or part of spiral arms are often seen in irregular galaxies. As these galaxies tend to be found close to larger galaxies, intense star formation is often seen, making them an interesting target for amateur and professional astronomers. Our own galaxy has two such galaxies in orbit around it, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) both gravitationally distorted by our own Milky Way. Both are visible with the naked eye from southern latitudes.
Peculiar galaxies.
Peculiar galaxies can have a mixture of properties that make them ‘peculiar’. An ordinary spiral or elliptical galaxy can be classified as a peculiar galaxy for many reasons, these may include;
· A large infra-red emission which exceeds its visible counterpart in luminosity.
· Large radio emission emanating from the end of visible ‘jets’, whose sizes exceed many times that of the parent galaxy. (NGC 4486 is such an example).
· Intense star formation within the nucleus can create strong ultra-violet and infrared emission in this region.
· Intense and unexpected star formation within the disk (as seen in NGC 1569).
· Rings, tails and ovular shaped inner regions, all of which are seen in regions of galaxy interaction can be all classed as peculiar.
Many galaxies looked at in the very early Universe using the Hubble space telescope (HST) have shown a large percentage having peculiar and irregular traits. This may be good proof of an expanding Universe, knowing the Universe should have been much denser in the past and therefore the number of interacting galaxies should have been greatly increased.
Will we be able to visit other galaxies in the future?
If you read my page faster than light you will realise that we won't even be able to travel to the nearest stars in the next few thousand years, never mind galaxies. Below is a picture of M31 (The Andromeda galaxy), originally thought to be a nebula within the Milky way, Charles Messier never knew what he had stumbled on when he came across this beauty in Andromeda. In-fact it was the first Galaxy he discovered (although he had no idea what it actually was).
Every point in this picture is a star within our own Milkyway Galaxy. The Andromeda galaxy is around 110,000 ly in diameter which is a little bigger than our own galaxy. Although an incredible six hundred and sixty thousand trillion miles wide, it is in fact thirteen million two hundred thousand trillion miles away (2.5 million ly). Even though these distances may seem enormous, the Andromeda Galaxy is our closest galaxy (not including the SMC, LMC or Sagittarius dwarf galaxy), the furthest known galaxies are around 10 billion ly away (sixty billion trillion miles).
The best way to end a page like this is to leave you with a view of M51, the whirlpool galaxy as imaged by the HST. Probably one of the most viewed galaxies in the sky. The pink regions are called HII regions, or regions of active star formation. They are visible due to the ionisation of hydrogen caused by high energy UV photons.
