How Galaxies Came To Be: Astronomers Explain Hubble Sequence

January 12, 2010, Royal Astronomical Society
The image shows some of the galaxies generated by the computer model. The yellow objects are most distant and therefore appear as they were 13 billion years ago, whilst those closer are seen as they looked more recently. Image: A. Benson (University of Durham), NASA / STScI

( -- For the first time, two astronomers have explained the diversity of galaxy shapes seen in the universe. The scientists, Dr Andrew Benson of the California Institute of Technology (Caltech) and Dr Nick Devereux of Embry-Riddle University in Arizona, tracked the evolution of galaxies over thirteen billion years from the early Universe to the present day. Their results appear in the journal Monthly Notices of the Royal Astronomical Society.

Galaxies are the collections of , planets, gas and dust that make up most of the visible component of the cosmos. The smallest have a few million and the largest as many as a million million (a trillion) stars.

American astronomer Edwin Hubble first developed a taxonomy for in the 1930s that has since become known as the ‘Hubble Sequence’. There are three basic shapes: spiral, where arms of material wind out in a disk from a small central bulge, barred spirals, where the arms wind out in a disk from a larger bar of material and elliptical, where the galaxy’s stars are distributed more evenly in a bulge without arms or disk. For comparison, the galaxy we live in, the Milky Way, has between two and four hundred thousand million stars and is classified as a barred spiral.

Explaining the Hubble Sequence is complex. The different types clearly result from different evolutionary paths but until now a detailed explanation has eluded scientists.

A figure illustrating the Hubble sequence. On the left are elliptical galaxies, with their shapes ranging from spherical (E0) to elongated (E7). Type S0 is intermediate between elliptical and spiral galaxies. The upper right line of objects stretch from Sa (tightly wound spiral) to Sc (loosely wound spiral). The lower right line shows the barred spirals that range from the tightly wound SBa to loosely wound SBc types. Image: Ville Koistinen

Benson and Devereux combined data from the infrared Two Micron All (2MASS) with their sophisticated GALFORM computer model to reproduce the evolutionary history of the Universe over thirteen billion years. To their surprise, their computations reproduced not only the different galaxy shapes but also their relative numbers.

“We were completely astonished that our model predicted both the abundance and diversity of galaxy types so precisely”, said Devereux. “It really boosts my confidence in the model”, added Benson.

The astronomers’ model is underpinned by and endorses the ‘Lambda Cold ’ model of the Universe. Here ‘Lambda’ is the mysterious ‘dark energy’ component believed to make up about 72% of the cosmos, with cold dark matter making up another 23%. Just 4% of the Universe consists of the familiar visible or ‘baryonic’ matter that makes up the stars and planets of which galaxies are comprised.

Galaxies are thought to be embedded in very large haloes of dark matter and Benson and Devereux believe these to be crucial to their evolution. Their model suggests that the number of mergers between these haloes and their galaxies drives the final outcome - elliptical galaxies result from multiple mergers whereas disk galaxies have seen none at all. Our Milky Way galaxy’s barred spiral shape suggests it has seen a complex evolutionary history, with only a few minor collisions and at least one episode where the inner disk collapsed to form the large central bar.

“These new findings set a clear direction for future research. Our goal now is to compare the model predictions with observations of more distant galaxies seen in images obtained with the Hubble and those of the soon to be launched James Webb Space Telescope (JWST)”, said Devereux.

Explore further: Hubble provides new evidence for dark matter around small galaxies

More information: A preprint of the MNRAS paper can be found at

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1 / 5 (2) Jan 12, 2010
"combined data from the infrared Two Micron All Sky Survey (2MASS) with their sophisticated GALFORM computer model"

What data precisely? Of course you're going to get an accurate description from your computer model if you basically do the simulation before the simulation.

And I forget, why do galaxies compress vertically and expand horizontally? Should not gravity be equal in all directions? Is this an effect of dark matter or standard gravitational theory?
1 / 5 (1) Jan 12, 2010
And I forget, why do galaxies compress vertically and expand horizontally? Should not gravity be equal in all directions? Is this an effect of dark matter or standard gravitational theory?

Further to that and perhaps this is covered elsewhere - but in that vein .. why is our universe (sun and planets) more flat than in the shape of a ball?
5 / 5 (2) Jan 12, 2010
Oh. Right. Because they're spinning. Angular momentum.
1 / 5 (1) Jan 12, 2010
@in7x - "compress vertically and expand horizontally" due to conservation of angular momentum
EDIT - I see I should have refreshed before replying
not rated yet Jan 12, 2010
Thanks in7x and barakn - that is what I get for asking a question without thinking of the construction of it.

Let us say we a looking at a model of our universe - the sun at the centre and the plants all in their orbits.

Earth depending on where we put it (is this an astrolabe?) goes from left to right in front of the sun and returns right to left behind.

Mars does as well, in its' own time ... why does it not go from top to bottom in the front and bottom to top in the back - sort of in a 90 degree angle?

Likewise with the other planets ... they (we) are all, more of less on the same plane. Is it gravity that moves them to this plane? or something else?
3 / 5 (2) Jan 12, 2010
If your question is what makes the planets align in pretty much the same plane, it is because the cloud of material that they coalesced out of was (and still is) rotating. If your question is what set this cloud of matter rotating to start with then I don't believe I have ever heard a satisfactory explanation.

jonnyboy ... the first part makes sense and I much prefer the second part, 'I just don't know', then something made up.

thank you :)
not rated yet Jan 12, 2010
If you consider that a solar system is built at roughly the same time. i.e. planets and star form out of the same cloud of matter then it would be reasonable that except for minor exceptions that all would revolve in the same plane.

Assuming that matter continues in strait line unless forced to change then all matter in a solar system would have its own original but different vector. then gravity would impart a new vector which would bring closer together but increase angular momentum.

Collisions would be more frequent on matter with an alternate angular vector than the majority and this would tend to bring the dissenters into line along the same or similar vectors.
4.7 / 5 (3) Jan 12, 2010
Mars does as well, in its' own time ... why does it not go from top to bottom in the front and bottom to top in the back - sort of in a 90 degree angle?

As soon as you have even a tiny preferetial plane the the following will happen: whenever a planet dips below the solar systems plane there will be a net gravitational force upwards (and a force downwards when it goes above that plane). Eventually everything will be in the same plane. (Pluto's inclined plane is one reason why some think that it is actually not native to this solar system)

Only PERFECTLY homogeneus, globular and non rotating initial configuration would render a system in which all planet planes aren't correlated (and not even that because such a system would not produce planets at all - it could create independently rotating nebulous 'shells', though). But such a starting configuration is astronomically unlikely and the gravity of the galaxy the system is in would break the symmetry soon, anyways.
Jan 13, 2010
This comment has been removed by a moderator.
1 / 5 (1) Jan 13, 2010
If your question is what makes the planets align in pretty much the same plane, it is because the cloud of material that they coalesced out of was (and still is) rotating.

jonnyboy - you did so well with that answer .... here is one that I posted before and got no response.

Hubble can 'see' back to the beginnings of galaxies ... 12.5 or 13 billion years ago. That os taken as the beginning of the universe.

But there must have been some time between the Big Bang and galaxy formation - is this not taken into account where judging the age of the universe?

The galaxies seem to be well formed, suns, planetary systems and all that - they are not nebulous clouds, so some time must have passed.

Is this included in estimates of age?

amd thanks antialias ... you have filled out some gaps in my understanding for me. :)
1 / 5 (3) Jan 13, 2010
The article title should say that 'scientists theorize' not that 'Scientists explain'. Explain is a dogmatic word in this utilization, and theorize is the correct context.

This is fundamentally important, as it is a subliminal situation about wording and context... and 'Explain= inescapable proven fact' (when real science is all theory and there are no facts, just excruciatingly regular occurring instances) in this context creates dogmatism in science, and that is very very BAD.
5 / 5 (2) Jan 13, 2010
OOPs.. I found an answer in (of all places) Physorg.

"we now know that the universe is 13.7 billion years old, that the first atoms formed 380,000 years after the Big Bang, that the first stars formed around 400 million years after the Big Bang,..."

link: http://www.physor...680.html

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