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We’ve all seen pictures of galaxies, often glorious looking objects with spectacular
colours and shapes.
However, because they are static images, it may make you wonder how they rotate.
The obvious first thought is that they spin around an axis, perhaps looking something
like this as they do so.
Would it surprise you to hear that this is totally wrong?
For a start, galaxies are not solid objects, but rather made up of millions to trillions
of stars.
Each star follows their own orbit, although often they are going in the same direction
at least.
But actually, one of the main reasons a spiral galaxy does not orbit like the first example
is that angular momentum would wind the arms up into tighter and tighter spirals.
We don’t see this, so something else is at play.
The leading theory is that these arms are caused by density waves, meaning a galaxy’s
rotation would in fact look like this.
Here, the arms stay in place, or at least move extremely slowly as stars and gas passes
through them.
Another theory is that shockwaves produced by supernova and stellar winds are the cause
of the arms.
These theories might not be mutually exclusive, and both processes may be at work here.
Density waves are potentially caused by the self-gravity of stars as they orbit after
they have been perturbed by another force.
As stars have a gravitational influence on each other, eventually a pattern forms where
their orbits meet around certain areas in the galaxy.
Very, very interestingly, density waves are even seen in certain parts of Saturn’s rings.
Here, you have a section of ring that appears like the inside of a tree trunk.
But this is not the case, in this region the ring is influenced by the gravity of small
shepherd moons tucked inside the rings.
What you are actually looking at is one very tightly wound ring, a bit like a LP record.
And just like with a galaxy, this arm stays the same shape, never getting tighter or looser.
Within these arms is where the majority of star formation takes place in a spiral galaxy.
The gas and dust within a galaxy clump up in these arms too, which is why you often
see star forming nebula dotting the arms.
The arms are also brighter not just because there are more stars here, but also because
all the hot, young, bright stars are found in these regions.
Because the hottest types of stars are so short lived, by the time they move away from
the arms, a lot of them will have already burned out.
Another interesting phenomenon to do with spiral galaxies is something caused by dark
matter.
In the early universe, when dark matter was more disperse, stars within galaxies that
are dominated by normal matter would orbit a lot slower around the outside of the galaxy
than towards the centre.
However, today stars are moving a lot faster near the edge of the galaxy thanks to the
influence of dark matter.
Dark matter is mysterious in that it cannot be seen or observed in any way other than
by its gravitational influence.
Dark matter seems to have clumped towards the centres of galaxies over time, making
stars orbit faster than they should around the edge of a galaxy if only gravity from
normal matter was accounted for.
Other theories also exist about why this may take place, but these would have to change
the way we currently understand physics.
Anyway, how do we know about this change in the way stars orbit galaxies?
Well of course, we can see back in time when we look at extremely distant objects.
Stars within galaxies billions of light years away seems to orbit slower than stars much
closer to us, and this effects scales depending on the distance to the galaxy.
Of course though, there are some rule breakers out there.
While most galaxies have stars that orbit like this, a few have been spotted that have
leading outer arms.
One example is NGC 4622, a bizarre galaxy that rotates in the direction the arms are
pointing.
This was hard to accept at first, until smaller, inner arms were also found.
This unusual galaxy has probably experienced a merger in its not too distance past, which
likely caused this phenomenon.
Another rule breaker is the Black Eye Galaxy, which has two counter rotating disks of gas
and dust.
The inner disk, where you can see all the dust lanes, rotates normally, whereas gas
in this outer disk, rotates the opposite direction.
Interestingly, stars in this outer region do not seem to also be orbiting retrograde,
meaning it is just the gas that does so.
It is believed that gas must still be getting fed into the galaxy from the intergalactic
medium, or that this galaxy also merged with another extremely gas rich galaxy.
The last thing I wanted to show you today is the other major category of galaxy, elliptical
galaxies.
Elliptical galaxies contain stars that are generally much older than those found in spiral
galaxies, because the dust lanes in these galaxies have been exhausted, meaning stellar
production has all but stopped.
In these kinds of galaxies, stars are very much independent from each other, following
their own rather elliptical orbits.
Sometimes galaxies can exhibit characteristics of both types of galaxies, these are known
as lenticular galaxies.
These galaxies have dust rings which haven’t fully been exhausted yet, and they have quite
a ghostly appearance.
Star formation does appear to be a factor in keeping spiral arms defined, as once the
stellar building blocks dry up, we’ve seen examples of spiral galaxies losing their definition.
Perhaps lenticular galaxies are the mid-way point between spiral galaxies and elliptical
galaxies.
So, there we have it, a look at how galaxies rotate!
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