Date: March 29, 2009
Title: A Magnitude of Confusion
Podcaster: Chris Marr
Organization: The Astronomical Society of Western Australia
http://aswa.info
Description: Everything about star magnitudes, from the name to the numbers, seems to confuse the newcomer. However, that doesn’t mean one shouldn’t try to explain them. In this podcast, magnitudes are explained in layman’s language, from the origin of the term to what they measure to how they can be useful, even to newcomers. It concludes by reassuring those who are still confused that it really isn’t that big a deal – do like I do and accept them as they are. You don’t have to fully understand them to make use of them.
Bio: Chris Marr was was raised in the UK, but now he and Viv live in the beautiful city of Perth, Western Australia. Astronomy has always fascinated him but it wasn’t until his 50th birthday that he decided to buy himself a telescope, and only then did he realise he had no idea what to do with it. So he joined his local astronomical society, quickly became the Editor of their newsletter, then Training Officer, and finally President – he loves to write, he loves to talk, and he loves the subject of astronomy, so he’s in heaven!
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by the AAS.
Transcript:
This is a brief podcast about star magnitudes.
Let’s start off with where we got the term ‘magnitudes’ from originally. In the 2nd century BC, a Greek astronomer called Hipparchus decided to classify the stars into six classes of brightness. The first stars that he could see each evening after the sun set, he said these were the first class stars – these were the brightest. Those that appeared soon after became known as the second class stars, and those that appeared soon after that, the third class, and so on right down to the sixth class stars. Now it isn’t as if they were specifically appearing in blocks of time, but they were appearing slowly and gradually as the evening wore on, and he decided that six classes of stars were all that we needed, and he decided when one particular class of stars would end and the next one would begin.
When he wrote his star catalogue, Hipparchus classified all the stars that he could see using this method, as first class down to sixth class. But when the catalogue was translated into Latin a few years later, for some reason the word ‘class’ that Hipparchus had used was translated as the word ‘magnitudo’, which was subsequently translated to the word ‘magnitude’, meaning size. When it comes to stars of course, size doesn’t necessarily have any relationship with brightness, and consequently even though the word ‘magnitude’ has been around for a long time in this context, it’s still the source of confusion, especially to new astronomers.
Nowadays we can measure the relative brightness of a star to the Nth degree, we can calculate it very accurately indeed, and in order to keep more or less to the path that Hipparchus set us on originally, we arranged it such that magnitudes are 2.512 times different. Therefore a magnitude 1 star is 2.512 times brighter than a magnitude 2 star. And a magnitude 2 start is 2.512 times brighter than a magnitude 3 star. And so on and so forth. This eventually gives us the extraordinarily interesting situation that a magnitude 1 star is exactly 100 times brighter than a magnitude 6 star, and I have a feeling that Hipparchus would have been very happy with that very tidy result.
However, this does lead to the situation where the greater the magnitude number, the dimmer the star, which sounds like a contradiction in terms and is another source of regular confusion for newcomers. But it is the case.
Also, there are objects therefore which are now brighter than first magnitude which therefore end up with a magnitude of zero or even a negative value. For example the Sun has a magnitude of -26.72, and the Moon, Venus and Jupiter etc also have negative magnitude numbers. This doesn’t help in the understanding of course, but it is the way things are and I don’t think it’s going to change.
The naked eye can detect stars as faint as sixth magnitude, or sixth class as Hipparchus called them. However the Hubble Space Telescope can detect stars as faint as magnitude 30, which is about 4 billion times fainter than anything we can see with the naked eye.
Of course all of these magnitude calculations that we’ve seen so far are based on what we can see from the planet Earth, but we know that light diminishes over distance so the further away a star is the dimmer it will look. So the magnitude as Hipparchus saw it is now known as the apparent magnitude of a star, but we can also work out an absolute magnitude. An absolute magnitude is how bright the object would be if viewed from a common distance, and the common distance we use is 10 parsecs, which is the equivalent of 32.6 light years. So how bright would the star be from 10 parsecs? That’s calculated and used as the absolute magnitude, which gives us a much more accurate picture of the star.
As I said, the Sun has a magnitude of -26.72. From the Earth, nothing is brighter than the Sun. It completely dominates our sky when it’s up. However from 10 parsecs away, the Sun would have a magnitude of only 4.9, making it a fifth magnitude star – nothing to write home about. To us the full Moon is very bright, with an apparent magnitude of around -12. However if you went to 10 parsecs, you wouldn’t be able to see the Moon at all, let alone have an idea of its magnitude. So it does depend on the distance from the object and how bright it appears at that distance.
If we look at an example of that, Sirius is just about the brightest star in the night sky, with an apparent magnitude of -1.44, so it’s very bright. But it’s only 8.6 light years from us. Deneb has an apparent magnitude of plus 1.25, so it’s a bit dimmer than Sirius, but it is about 1,600 light years away – a lot further away. If we look at the absolute magnitudes of those two, Sirius has an absolute magnitude of 1.5, but Deneb has an absolute magnitude of -7.2! So it would be enormously dominating if it was the same distance as Sirius is from us.
Nowadays, we can measure the magnitudes of planets, man-made satellites, comets, asteroids, nebulae and even deeper sky objects like globular clusters and galaxies.
So why is all this important? Well it probably isn’t, unless you’re a scientist into the ins and outs of this kind of stuff. If you’re an amateur astronomer, it helps to know how bright an object is compared to another object, to help you figure out exactly what you’re looking at when you look through a telescope. So that might be one reason you’d want to know the relative magnitudes of two different objects. Or if your particular telescope has a magnitude limit, then you’d do well to know what magnitude limit you’ve got, and also what objects are within your view. There’s no point trying to find something dim, like the Horsehead Nebula, on a very small telescope which doesn’t have that kind of magnitude definition. Other than that, it really doesn’t have a lot of uses. It’s really not essential to know or understand, so if you don’t, don’t let it stop you being an astronomer.
That just about wraps it up for magnitudes – a potted resume of what they’re about. There’s certainly lots of information on the Internet if you wish to look it up – go to your favourite search engine, type in “star magnitudes” and off you go.
My name’s Chris Marr. I’m with the Astronomical Society of Western Australia. I hope that was helpful.
End of podcast:
365 Days of Astronomy
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