So, You’re Still Interested In Astronomy? Sun’s galore – part 27 min read

Hello again. Last time out, we took a little peek at the InSight mission to Mars and the implications that might have on the search for life elsewhere.

This time, we’re looking at something much closer to home – and much further afield too. Yes, we’re going to remind ourselves of something we rely on for life, and how it compares to what else is out there.


Say hello to our Sun

What’s so special about our Sun?
For one thing, it’s unique as the only star we know of for sure that sheds its light on intelligent life. While we’ve found other planets out in the vast reaches of space, we’ve not yet confirmed life itself.
But apart from that, the Sun isn’t all that spectacular. It’s a G-type main sequence star (G2V to be exact) and is classed as a yellow dwarf. With a radius of 695,842 km (432,376 miles), a surface temperature of 5778 K, it accounts for 99.86 the total mass of our solar system. The sun is 4.6 billion years old and is about halfway through its stable life. (Good news for us).

Fun fact: The sun is in fact, white, and actually emits more photons in the green portion of the spectrum than any other

What kind of suns are there?
Even keeping things simple will blow your mind. Take a look at the picture showing all 7 main spectral classes of stars. Bear in mind, were talking of spectral classes (the color/heat they generate).
As you can see, stars range from class M (starting at a mere 2400 K) and range up to class O (that exceed 30,000 K). That means we get a variety of main sequence stars ranging from red, orange, yellow, white and through to blue.
When it comes to sizes, we have dwarf stars, sub dwarfs, main sequence stars, subgiants, normal giants, super giants and finally, hypergiants.


If our Sun isn’t all that spectacular, are there any that are smaller?
There certainly is. One of the smallest we know of is the aptly named OGLE-TR-122b, a red dwarf that is actually part of a binary stellar system. Here’s the thing, it’s only 167,000 km (103,000miles) in diameter. (At 69, 911 km – 43,441miles) Jupiter’s not that much smaller.
Here’s a comparison pic


Okay, let’s start going up in size. What’s a good example of a subgiant?
A well known subgiant,
Gamma Cassiopeiae, can easily be found in our night sky as it forms the center of the distinctive “W” asterism in the constellation of Cassiopeia.
Gamma Cassiopeiae is an eruptive variable star, whose apparent magnitude changes at irregular intervals.
What’s interesting is that she has reached a stage of her evolution where the hydrogen at the star’s core is exhausted. One day in the not too far distant future, she’ll grow into a fully-fledged giant.

And giants?
One of the most famous red giants is Arcturus. She can be found in t
he constellation of Boötes and is relatively close to us at only 36.7 light-years from the Sun.


Arcturus is more than100 times brighter than the sun and 25 times larger.
Fun fact: Even though Arcturus is huge in comparison to the sun, it’s a lot cooler.

So, what’s a supergiant?
Supergiants are among the most massive and most luminous stars with absolute visual magnitudes between −3 and −8 with temperatures spanning from about 3,500 K to over 20,000 K
Supergiant does not have a single concrete definition. The term giant star was first coined by Ejnar Hertzsprung, a Danish chemist and astronomer who developed a special diagram (see pic) showing the relationship between the absolute magnitudes or luminosities of stars versus their spectral classification or actual temperatures.


It became apparent that the majority of stars fell into two distinct regions of that diagram. One region, containing larger and more luminous stars of spectral types A to M, received the name giant. Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were significantly larger and more luminous than the bulk, and the term super-giant arose, quickly adopted as supergiant.

Is that right that smaller suns can become supergiants?
O type main-sequence stars and the most massive of the B type blue-white stars can become supergiants. Because of their extreme masses they have short lifespans of 30 million years down to a few hundred thousand years. They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges, and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly of old stars.
Supergiants develop when massive main-sequence stars run out of hydrogen in their cores. They then start to expand, just like lower-mass stars, but unlike lower-mass stars, they begin to fuse helium in the core smoothly and not long after exhausting their hydrogen. This means that they do not increase their luminosity as dramatically as lower-mass stars and they progress nearly horizontally across the HR diagram to become red supergiants. Also unlike lower-mass stars, red supergiants are massive enough to fuse elements heavier than helium, so they do not eject their atmospheres as planetary nebulae after a period of hydrogen and helium shell burning. Instead they continue to burn heavier elements in their cores until they collapse. They cannot lose enough mass to form a white dwarf, so will leave behind a neutron star or black hole remnant, usually after a core collapse supernova explosion. Some of the more massive stars cannot expand into a red supergiant. They burn too quickly and lose their outer layers too quickly, so they reach the blue supergiant stage, or perhaps yellow hypergiant, and then return to become hotter stars.

Once these massive stars leave the main sequence their atmospheres inflate and they are described as supergiants.
One of the brightest stars visible to the naked eye –
Rho Cassiopeiae – is a yellow hypergiant in the constellation of Cassiopeiae.


Rho Cassiopeiae is about 8,200 light-years (2,500 pc) from Earth, yet can still be seen by the naked eye as it is 500,000 times brighter than the Sun. On average it has an absolute magnitude of −9.5, making it visually one of the brightest stars known. Its diameter measures 450 times that of the Sun, approximately 630,000,000 kilometers, or about twice the size of the Earth’s orbit.

What’s the difference between a supergiant and a hyper giant?
 are frequently treated as a different category of star from supergiants, although in all important respects they are merely evolved, expanded, massive and luminous stars like supergiants, with particular additional properties of undergoing high mass-loss due to their extreme luminosities and instability.

What’s the hottest known sun?
The hottest known stars in the Universe are the blue hypergiants. These are stars with more than 100 times the mass of the Sun. One of the best known examples is Eta Carinae, located about 7,500 light-years from us. Eta Carinae could be as large as 180 times the radius of the Sun and its surface temperature as high as 36,000-40,000 Kelvin.
Just for comparison, 40,000 Kelvin is about 72,000 degrees F.


And the biggest?
This is where it gets tricky. Because of distance and intervening nebulae, it can be difficult to be precise about the largest star. However, UY Scuti, a
bright red supergiant and pulsating variable star in the constellation Scutum is a current and leading candidate, being the largest known star by radius. It is also one of the most luminous of its kind.
It has an estimated radius of 1,708 
solar radii (1.188×109 kilometers – that’s 7.94 astronomical units) and therefore a volume nearly 5 billion times that of the Sun. It is approximately 2.9 kiloparsecs (9,500 light-years) from Earth. If placed at the center of the Solar System, its photosphere would at least engulf the orbit of Jupiter.


So there you go. A fascinating glance into what’s in the night sky when we look up.

Next time, we’ll skip back to Mars, and see what the latest development there involve. See you then.

Andrew Weston
Latest posts by Andrew Weston (see all)
Andrew Weston

Andrew P. Weston is Royal Marine and Police veteran from the UK who now lives on the beautiful Greek island of Kos with his wife, Annette, and their growing family of rescue cats. An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society. When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for and Amazing Stories. He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea. If you would like to find out more, visit his blog or website at:

Leave a Reply

Your email address will not be published. Required fields are marked *

Subscribe To Our Newsletter

Join our mailing list to receive the latest news and updates from

You have Successfully Subscribed!