Look up into a clear night sky, and you’ll see thousands of different stars dotting your
view.
While they all look like pinpricks of light from our viewpoint, to some of you keen observers,
you’ll notice that some of them look slightly different from others.
Some are bright, some are barely visible.
Some are white, and others are reddish or even blue.
How can that be?
Why are stars so different from each other?
I’m Alex McColgan, and you’re watching Astrum, and together we will learn all about
the various types of stars, and find out what makes them all so interesting.
Let’s start off by having a recap of where stars come from.
Although space is a vacuum, it’s not a true vacuum in that there are still tens to millions
of particles per cubic centimetre, like hydrogen or oxygen.
These particles make up what we call the interstellar medium.
The interstellar medium is surprisingly massive, making up 15% of the mass of our galaxy, the
Milky Way.
This is a lot, considering that the mass of the interstellar medium is compared to the
hundreds of billions of stars found in the Milky Way.
The interstellar medium exists throughout space, but it’s not evenly distributed.
The densest parts are called nebula, which can either be colourful regions of ionised
gas, or cold regions called molecular clouds.
Generally, these clouds are supported by their own internal pressure pushing out against
the inward pull of gravity, but every so often, these clouds collapse because of external
sources.
When the internal pressure of the cloud is overcome, this causes the particles within
them to clump together under their own gravity.
As the clump begins to build up, its density and thus gravity increases, causing more nearby
particles to get drawn in.
These particles begin spiralling inwards around the centre of mass until they eventually impact
the central object.
With the combination of the force of these impacts, and the increasing pressure inside
the object, it begins to warm up and spin.
This is what we call a protostar, or if you want to imagine it this way, a star in the
baby stage of its development.
Every star had these humble beginnings.
But what happens next decides the fate of the protostar.
The molecular cloud feeding into the protostar will eventually run out of material,
or the protostar will be ejected away, early in its protostar phase.
If this happens before the protostar reaches 0.08 solar masses, or 0.08 times the mass
of our Sun, the protostar will become a brown dwarf, or a failed star.
Should the molecular cloud feed enough material into the protostar that its mass amounts to
over 0.08 solar masses, the protostar will eventually become a main sequence star, or
in other words, it will reach the adulthood phase of being a star.
Why this 0.08 solar mass cut off?
It’s because stars with a mass above 0.08 solar masses are big enough for nuclear fusion
to take place in the stars core, where the pressure there will convert hydrogen to helium
as a power source.
At this mass, as long as there is hydrogen to convert, it is self-sustaining, needing
no external source to keep it hot.
Brown dwarves don’t reach the mass and pressure at the core needed for nuclear fusion to take
place, so they will eventually cool off.
Their cool and dim nature means that brown dwarves are pretty hard to see, there are
certainly none in our sky visible to the naked eye.
Their dim nature means it’s hard to put an estimate on how many there are, there could
be a lot more in existence than we currently know about.
Zooming in on Hubble’s picture of the Orion Nebula though shows quite a few brown dwarves
visible against the backdrop of the nebula they formed in.
The ones we know about are really interesting, but brown dwarves could have a whole video
to themselves so maybe I’ll save it for another time.
So, our protostar has reached 0.08 solar masses.
What happens next determines the type of main sequence star it will become.
When the molecular cloud eventually stops feeding material into the protostar, the protostar
begins to cool and contract.
This increases the pressure at the core until the pressure is so great that nuclear fusion
occurs, the reactions of which create an internal pressure which counteracts any further contracting.
Or in other words, the forces within the core of the star are in equilibrium.
At that point, the protostar hits the main sequence.
But what type of star will it become?
Let’s have a look at the spectral class chart.
I want to draw your attention to this band by here, this band indicates that these stars
are on the main sequence.
As you can see, stars generally belong within this curve.
The lowest mass stars are found to the bottom right of the curve, where the molecular cloud
ran out of material early on in the star’s protostar phase.
These stars are small and cool, in a category of stars called red dwarves.
As we go up the curve, it indicates that more mass was fed into the protostar as it developed.
And the more mass there is, the hotter the star.
As the temperature of the star increases, the spectral class shifts, moving from red,
to orange, to yellow, to white, to blue.
Not only does the spectral class change, but the brightness of the star also increases.
But if this is the case, why is this band so thick?
Surely it should be a much narrower band if the mass is always equal to the temperature
and luminosity.
Well that would probably be the case if all stars were the same age.
But throughout a star’s adult phase, it is still changing slightly.
As a star’s hydrogen gets diluted by all the helium it has now generated, it is more
difficult for the hydrogen to fuse together.
The equilibrium within the star shifts slightly, to a smaller, denser, hotter core to maintain
the fusion of hydrogen.
This means over time; the size and luminosity of the star increases slightly.
A good example of this is our Sun, as it used to be only 70% of the brightness it
is now.
So, going back to the chart, a star that may start here may shift to here over its lifetime.
But, as is often the case with anything, there are some outliers and exceptions to the rule.
On this diagram, you’ll notice this band here.
This is known as the “instability strip”, and is full of variable stars.
A variable star is a star whose brightness and diameter increases and decreases in regular
intervals, giving them a pulsating appearance.
Some take only hours to pulsate, some take weeks.
A breath-taking example of a variable star seen by the Hubble space telescope is RS Puppis.
It varies in brightness by a factor of 5 once every 40 or so days.
Because this star is surrounded by a beautiful dust structure, we can see the variable nature
of this star through a “light-echo”.
Light travels extremely fast, but it still takes a long time to get across astronomical
distances.
This means we can see the procession of light travelling away from the star, with some rings
being dark as the star was dimmer, and some rings being brighter as the star brightened!
Hubble imaged RS Puppis over a 5-week period, and you can actually see this light echo pulsing.
They exist along this strip here because at this mass, the internal structure of the star
is more prone to instability due to the ionisation of Helium in the star’s convective zone.
Again, perhaps a topic for another video as it gets pretty complicated.
Another variable to main sequence stars is what material was actually fed into them during
the protostar phase.
The majority of the interstellar medium is hydrogen, but there are other atoms out there
like helium, oxygen, and even metals like iron and gold.
If a star has more of an abundance of metals, which in the case of stellar structure is
anything above an atomic number of two, it will be redder on the spectrum, and metal-poor
stars will be bluer.
So, as you can see, there are lots of factors for main sequence stars to be different from
each other.
These are: their mass, how much material was fed into the star during its protostar phase?
Their age, have they only just become main sequence stars, or have they had time to increase
in brightness?
And their composition, are they rich or poor in metal?
Well I hope you learned something today about the main sequence of stars, and hope you’ll
stick around for the conclusion to this series with what happens to stars once they run out
of hydrogen fuel!
In that we’ll look at black holes, neutron stars, giant and super giants.
But can’t wait for next time?
Brilliant has an amazing astronomy course on what makes stars different which really
delves into the details and maths of star development.
And the best thing is, thanks to their iOS and android app, you can take this course and many others
on the go with you thanks to their offline content, so you aren’t bound by an internet
connection.
If you want to support Astrum and get unlimited access to all of Brilliant's in-depth math
and science courses, you can head over to brilliant.org/astrum/ to get 20% off their
annual Premium subscription.
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