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Today we are going to talk about stars, as we often do, but we’re going to talk about a way that stars are classified. Stars can be classified in various different ways, but one of the easier ways, one of the more immediate ways to classify stars, is by their color. Taking a look at the late night sky for December, there are already different colors of stars visible. Some stars are more of a bluish-white, some stars are more of a bright white, some stars are more of a reddish or orange color, and this is one of the ways that stars can be broken up.

Taking a look at a definitely quite blue star to start with, we’re looking at Alnitak, the eastern star in Orion’s belt. Alnitak is a blue giant star, a big, bright, energetic, young star. Blue stars are always big, they’re always large stars, up to giants or supergiants. Blue stars generally can’t be small unless their the remains of a larger star. Blue stars are blue because they’re at a very, very high temperature, blue is the highest temperature that we see a star. Alnitak here is an O class star, which means its surface temperature is 33,000 Kelvin or more. You have to be that hot to generate the rich blue color. In Stellarium, the writing describing the star will be all in blue to match. More specifically, Alnitak is spectral type O9.7 Ib. The Ib is to do with its size, so it is a less luminous supergiant star.

All of the blue stars in Orion are giants, but when it comes to other colors size can vary more, you can have red dwarfs and red giants. For example, if we take a look at V1192 Orion, its color is kind of a yellowy-orange color, and it is spectral class K2 III. We will get back to K, it’s a cooler star, even cooler than our Sun, but the III marks it as a giant star, a normal giant star, rather than a less luminous supergiant star, which is what the Ib is telling us about Alnitak, that it’s a bigger star. These hot blue stars are quite rare, they’re among the brightest, but there’s not that many of them. As such, they’re not necessarily going to be the closest stars to us and proximity to us also makes a huge, huge difference in apparent brightness. Here’s another example, the opposite side of Orion’s Belt is Mintaka, also an O type star, but with a B type going around it. Alnilam in the middle is a B type star.

This is the next step down, B class stars. The writing in Stellarium is a different colour. It’s still blue, but the blue for Alnitak was a brighter, richer blue, whereas Alnilam has a slightly paler blue color. It’s still considered a blue star and it still looks blue, but it’s a B class star and its temperature range is just a little bit lower, from 10,000 degrees Kelvin up to 33,000 degrees Kelvin. 33,000 degrees Kelvin and beyond puts you in the O class type. These B class stars are also usually giants, Alnilam is Ia, which makes it a luminous supergiant. They do get smaller, like Sigma Orionid, it’s a B IV making it a subgiant. This is still a large star, it’s not a normal stellar mass star like our Sun, in the V group. Some components of the Rigel system are BV, which is more of a main sequence normal sized star. That’s about as small as a star that’s burning blue will get, unless it’s the remains of a larger star. These bright blue stars are very energetic and young, so you often see them in star forming regions which is one of the reasons that we’re seeing so many of them in Orion. Pulling back out from Orion, we’re going down from O and B class to A class. This is not really alphabetical order, it doesn’t obviously make sense, but this is the order of the set of letters with these meanings.

For an A class star, we have Sirius. Sirius is the brightest star that we can see in the sky, but it’s not as hot as the O and B stars. Its surface temperature is from 7,300 Kelvin up to 10,000 Kelvin. From 10,000 onward is B up to 33,000, and from 33,000 onwards is O. They’re all broken up according to temperature but their temperature also decides their color. Sirius is a kind of whitish-blue star. Zooming in it is really Sirius A we’re looking at. It is a binary star and next to it is Sirius B. It is a different type of star, it’s a DA, which is just a different kind of A. The D isn’t part of the same O, B, A, … classification, instead the D stands for dwarf because it is a smaller star orbiting a much larger star, the main Sirius, Sirius A. Sirius A is an A V star, a main sequence star like our Sun, where as Sirius B could be written A VII as a dwarf star. Sirius’s apparent magnitude is -1.46, that’s how bright it looks to us. Its absolute magnitude however, how bright it is, is just 1.43. If we take a look at a couple of these other stars, like Alnita, its apparent magnitude is 1.77, but its absolute magnitude is -4.99, practically -5. Alnitak, being this big O type star, is actually brighter, but it’s further away from us. It’s 700 light years away, give or take 100 light years or so. Sirius is much closer, just 8.6 light years away. Even though it appears brighter in our sky, it’s brighter to us, it’s not as bright as some bigger, hotter stars. A stars are pretty common and as we go down the list from hottest to coolest, we’re getting into more common stars.

If we turn around to the west, we’ve got Vega, another A V type star, a main sequence star. It’s also nice and bright, again partly because it’s quite close to us. Deneb as well is another A type star, much further away from us, but also a much brighter star. Deneb is actually a brighter star, it’s a higher luminosity, even than the O type star that we previously looked at. Deneb is a A Ia star, a luminous supergiant, but there’s a lot going on with Deneb. It is a double star, so there’s two stars involved, but it is a particularly bright star. We can see it clearly, even though it’s very far away from us, thousands of light years away. It’s one of the further stars away that we can easily see with our eyes, and that gets harder as the stars get dimmer.

We’re going from A now down to F. Again the class system doesn’t exactly follow the alphabet. We’re looking here for Procyon. It’s an F IV star. Again, the writing in Stellarium shows a difference, Sirius is kind of a whitish-blue as an A class star, whereas Procyon is more white-yellow as an F class star. Again, we’re getting a little bit cooler, it’s going to be less than 7,300 kelvin, down as far as 6,000. Even though this kind of star is more commonly visible, roughly a third of the stars that we see in the sky are F class, it is not necessarily the most common out there. This star is cooler, but still not as cool as our Sun. It’s a lot cooler than those other high ranking stars, but it is often smaller and fainter. Most of the stars that we can see with our eyes, they’re going to be in a higher spectral classes. Many of the brighter stars that we’re going to be familiar with, they’re either going to be higher on the spectral class like B stars, or they’re going to be giants that are lower on the spectra class but giant, for example Betelgeuse, lower on the spectral class down to M and Aldebrand a K class star. These stars are big, old, giant stars, and because they’re so large, they’re a bit more imposing in the sky, whereas we don’t see as many giants in the F and the G class.

The G class is next, and of course our Sun is a G class star. Looking down in Virgo, we have Vindemiatrix. Vindemiatrix is a G class star. It is a similar kind of star to our Sun, in that it’s G class, but there’s many differences. It’s a IIIb, so that makes it a normal giant or bright normal, rather than V, the main sequence size of our Sun. These colors don’t necessarily tell you the size, they only tell you the temperature. In the case of these G stars, less than 6,000 degrees Kelvin, down as far as just 5,300. This is a class of star that very rarely has giants. There are yellow giants, but they’re very rare. Polaris is another F type star and it’s an F Ib, so it is a supergiant F type star. For both the G and the F type stars these giants are less common.

For the last two classes, the real orange and red stars, our K and our M class, with examples here, Betelgeuse and Aldebaran, these types of stars are very, very common. Red or orange stars in the K class like Aldebaran go from 5,000 kelvin down to just 3,900 kelvin, and then red stars in the M class like Betelgeuse going from 3,900 down to 2,300, much cooler. These colors of stars are some of the most common, and we do see them as giants. Betelgeuse is a M Ia, a luminous supergiant red star and even Aldebaran is K III, a normal giant, not quite the super giants that we see in class I and class II. These red giants aren’t super common. Red dwarfs however, are the most common stars that we see, or don’t see unfortunately, in the universe. No red dwarfs are visible to the naked eye. Even if we push out into the countryside so that more stars are visible, if we randomly click around all of the stars visible to naked eye, we won’t get red dwarfs. We’ve got a B class and G class there, like our Sun, A class and O class, but none that are very small.

None of the stars that are dwarfs, smaller stars in the K and M class, are visible. We can see these classes of star, nut only when they are larger. Clicking through these stars shows how Stellarium changes the color of what’s written to match their spectral type. For red dwarfs, we really have to search for them. I’ll search for my favorite, Barnard’s star. It’s in Ophiuchus, so we’ll come up to morning time. By mid December we’re starting to see the Scorpion coming up in the morning, with Ophiuchus coming back out around the side of the Sun. If we take a look t Barnard’s Star, it’s an M V class star. This is what we would consider a red dwarf. Our Sun is a V class as well, so our own Sun is a yellow dwarf. Red dwarfs are very common, but they’re very faint, so they’re very difficult to see.

That is the classification, those are the colors of the stars. There’s only a few divisions of these spectral classifications. We’ve got our hottest O type stars like the ones either end of Orion’s Belt, we’ve got our second hottest B type stars like the one in the middle of Orion’s Belt. Then we keep going down to cooler stars like Sirius as an A class star, and we keep going again down to the F class stars like Procyon. All of these classes are still hotter than our Sun. We finally come around to G class stars like our own Sun and Vindemiatrix. Then we can continue down past the G class stars to the K class stars like Aldebaran and then down again to the M class stars like Betelgeuse. Those O class stars the biggest, the hottest and the rarest. The M class stars, some of them are very large, but most of them are very small, they’re the coolest and the faintest.

That is spectral classification, a few examples of the classes, the colors that are involved and the temperatures that are involved. I very much hope that you enjoy this video. If you did, please do like it. If you’re interested in this kind of topic, then please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.