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With the James Webb Space Telescope nearing its launch, I felt like it would be a good
idea to have another look at some of the remarkable images of its predecessor, the Hubble Space
Telescope.
Hubble has been in space for 30 years now and has increased our understanding of the
universe drastically during this time.
A type of object Hubble has observed is galaxies.
Galaxies, as we’ve seen earlier in this series, come in all different shapes and sizes,
with star populations ranging from the millions to the trillions.
They are fascinating, beautiful objects, each containing its own story of how they formed
the way they did, let alone the tantalising question about the possibility of life somewhere
in them.
While we have only observed the tiniest fraction of the total number of galaxies in the observable
universe up close, did you know that Hubble has still seen galaxies all the way through
their evolution, from galaxies in the throes of star birth, or starburst, to galaxies that
are inert, anaemic or dying?
I’m Alex McColgan, and you’re watching Astrum, and today I wanted to look through
this spectrum of galaxy evolution, focusing today on the ghostly, beautiful remnants of
galaxies that are on their way out.
I hope by the end of this video to have earned your like and subscription.
When looking at a galaxy, the first indicator of the stage of its evolution is to look at
its shape.
It is generally believed that spiral and barred galaxies are some of the least evolved galaxies
in the universe, whereas elliptical galaxies are the most evolved.
That doesn’t necessarily mean spiral and bar galaxies are younger though, it’s just
that they have taken longer to evolve.
Here’s an example.
Number 68.
NGC 2336.
Galaxies with spirals consist of flat disks where stars mainly orbit in the same direction.
One of their defining characteristics are the arms that extend from the galaxy’s centre.
These arms can spiral out directly from the galaxies core, or sometimes the spirals come
from a bar.
They consist of a concentration of stars, gas and dust.
Galaxies that have an abundance of dust are likely not as far along the evolution process
as less dusty galaxies, as this dust has yet to be turned into new stars.
Should no outside influence accelerate the dust consumption, a galaxy like this one may
produce stars at a fairly slow but steady rate for billions of years yet.
How do we know it’s not producing stars very quickly?
Well, a distinct feature about this galaxy is the lack of red HII regions, or stellar
nurseries.
Instead, while star formation is clearly still taking place due to all the bright blue stars
you can see in the image, it will remain as a spiral galaxy for a lot longer than a galaxy
bright with HII regions.
Like Number 69.
NGC 972.
Wow, what a pretty galaxy.
It almost looks like its ablaze with all the activity happening around it.
When a galaxy is so red, we characterise it as a starburst galaxy, that is to say, star
formation is happening at an incredible rate.
These red regions are the HII regions the previous galaxy was missing an abundance of.
As starburst continues, it will quickly use up the gas and dust structures you see here,
converting it all to stars.
Also, notice the blue glow like a halo around the galaxy.
The blue hue comes from blue stars, the hottest and most massive star type.
This is another indicator of rapid star formation as these stars require huge amounts of matter
to fall into them during their formation.
Number 70.
M61.
Another incredible example of a starburst galaxy is M61.
M61 is a fantastic galaxy for observation because it’s angled almost exactly face-on
to us.
This means all the dust lanes and its red nebulae can be studied in detail, that’s
a lot harder to do with a galaxy that’s edge on.
Looking closely at the centre of the galaxy shows a neat symmetry with the dust lanes,
however the further out you zoom, the more you notice how it becomes less symmetrical.
This arm seems to split into three, whereas this one retains its structure.
Usually, an asymmetrical galaxy indicates that it is being perturbed in some way, either
from the gravitational influence of a nearby galaxy, or from an actual collision with one.
In this case, M61 is actually found 50 million light years away in the heart of the Virgo
Cluster, surrounded by over 1000 galaxies.
It’s this outside influence that likely causes starburst to happen and for a galaxy
to accelerate its evolution, as the outside tug of gravity causes the dust clouds within
the galaxy to collapse in on themselves, creating stars.
Number 71.
NGC 4921.
All galaxies will eventually exhaust their supply of gas and dust as they get converted
to stars, and then neutron stars or black holes.
A small percentage of gas and dust also gets consumed by the supermassive blackhole believed
to exist at the centre of every sizable galaxy.
What happens to a galaxy then?
Well, NGC 4921 is a good example of a galaxy about to transition into what is known as
a Lenticular Galaxy, however this one still has its spiral arms visible, so it remains
classified as a spiral galaxy for now.
It seems that star formation and dust lanes are key elements of keeping the shape of a
spiral galaxy’s arms, and when this process stops and the material runs out, the arms
seem to lose some of their shape and definition.
The colour of the galaxy also changes to a ghostly white or reddish white as the galaxy’s
blue star population burns out and disappears.
This is a fascinating galaxy, it’s really unusual to see a spiral galaxy so devoid of
colour.
The only colours visible really are the few remaining blue stars clinging to the dust
lanes.
Astronomers have described this galaxy as anaemic as star formation here must be incredibly
slow.
Number 72.
NGC 2217.
This is the first true Lenticular Galaxy I’ve shown you today, although NGC 2217 isn’t
totally done with star formation just yet.
Notice the blue stars around the outer edge of the galaxy.
However, in this instance, the dust here is almost completely exhausted, and the spiral
arms have lost their definition.
The bulge at the centre of the galaxy has a white and reddish hue, showing that only
older and less massive star types exist here, there are no blue stars visible to speak of.
Number 73.
PGC 10922.
Here’s another face-on galaxy for you.
This particular lenticular galaxy is further along its evolution again, most of the galaxy
here is completely devoid of gas and dust apart from towards the centre.
You’ll also notice how any arms that once existed have been totally wound up until they
are almost indistinguishable.
All that’s left really are old stars and a flat disk.
But even the disk will eventually lose its shape as the galaxy widens and evolves into
an elliptical galaxy.
Like what is happening with Number 74.
NGC 1947.
With this lenticular galaxy, we can still see the wispy remains of the spiral arms.
However, the stars in this galaxy have clearly already stopped orbiting along a flat disk,
and now simply orbit slowly and from all directions.
The very last stages of a lenticular galaxy look something like NGC 1533.
There’s no gas or dust noticeable, however you can just about see the remains of a bar
structure coming away from the core, indicating this was once a barred spiral galaxy.
Eventually, what you end up with is something like Number 76, Messier 105.
This is a true elliptical galaxy, a galaxy almost totally devoid of gas and dust.
This galaxy is so anaemic that only one Sun like star gets produced in a galaxy like this
every 10,000 years.
So, what you are left with is the vast majority of the stars found in this galaxy being older,
redder type stars.
If we really zoom in to this image, you can see the red sprinkling of stars, especially
as we move away from the brightness of the galaxy’s core.
This is last stage of a galaxies evolution that we can observe.
These stars will last for billions to trillions of years, and the universe is too young to
see what happens eventually to a galaxy like this, although it is thought that they will
simply become dimmer and dimmer as one by one the stars reach the ends of their lives.
So, there we have it, a snapshot of the evolution of galaxies.
While having an almost spooky vibe to them, I really like lenticular galaxies.
Their unstable nature produces some amazing patterns that you can’t find in any other
galaxy type, although it is somewhat sad to think too that as the universe ages, spiral
galaxies will become less and less common, until the only galaxy type remaining are the
elliptical galaxies.
Although we are long overdue for an upgrade to Hubble, it really is still a feat of engineering,
and it’s been really interesting to me to learn about the telescope itself.
MagellanTV has an amazing documentary called “Hubble Eye In The Sky” about the behind
of scenes of Hubble, how the mission team operates it and some of the obstacles it has
to overcome, for instance, the South Atlantic Anomaly which is really quite fascinating
and something I didn’t know about before.
MagellanTV has the most varied science content available anywhere, with over 3000 documentaries,
a lot of which is available in 4K, and you can watch them with no ads and from pretty
much any device.
If want to watch this Hubble documentary and more like it, you can use my link in the description
to try MagellanTV for free for a whole month, so you should definitely give it a go, I highly
recommend it.
Thanks for watching!
If you liked this video, be sure to have a look at the rest of the videos in this Hubble
playlist here.
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All the best and see you next time.