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How to Estimate Star Magnitude

Astronomy Olympiad Preparation
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  HOW TO ESTIMATE STAR MAGNITUDE Even to casual stargazers it’s pretty obvious that the stars are of differing brightness. Astronomers always like tocatalogue and classify objects in the sky, and the brightness of stars is no exception. Over 2,000 years ago, the Greekastronomer Hipparchus devised the system we use for this purpose, called the magnitude scale.In Hipparchus’s magnitude scale, the brightest stars were known as first magnitude and the faintest stars were sixthmagnitude. He gave a higher number to the faintest stars,which sounds a little topsy-turvy until you swap the word ‘magnitude’ for the word ‘class’. Looking at it this way you startto see them as ‘first class’ stars, ‘second class’ stars and so on as the stars get fainter, putting the scale into perspective. At the brighter end of the scale, magnitudes become a little awkward as some stars and other objects are brighter thanfirst magnitude. There are stars with zero magnitude–wrongly suggesting they have no brightness–and in cases wherethe stars are even brighter, they have a negative magnitude, as you can see in these examples:The Sun–27Full Moon–12Venus (at its brightest)–4.4 Arcturus–0.04Vega+0.03Polaris+1.99Pluto+13.9With telescopes and imaging equipmentlike CCD cameras, you can go way beyond the sixth-magnitude objects onHipparchus’s srcinal scale and capture objects like Pluto, which is far too dim to be seen with the naked eye. The HubbleSpace Telescope has managed to image objects as faint as magnitude +30. Don’t forget that the magnitude doesn’t tellyou how luminous an object really is in itself; it’s a measure of the apparent brightness of a star as seen from our vantagepoint here on Earth.The great thing about star magnitudes is that by getting to know the brightness of stars and carefully observing them,you’ll be able to estimate the magnitudes of other objects you see in the sky. This is handy when you see a meteor streaking across the sky, or a passing comet. You might also want to keeptrack of the changing brightness of a variablestar so that you can let others know how bright it is. Know your sky The first step in learning how to estimate magnitudes is to make sure you know what you’re looking at. Print out adetailed star chart showing star magnitudes near your chosen star from planetarium software such as Redshift, or fromone of the websites in ‘Find out more’ on page 80. You need to identify the field of view correctly. In our step-by-stepguide using Ursa Minor over the page, this may seem easy, but when you’re tackling fainter stars in star fields that youhaven’t observed much, it can take some time to find where you are.  When you’ve got to grips with what you’re looking at, use your chart printout to find two stars in the samefield of view asthe one you’re interested in, to compare it against. These ‘comparison stars’ need to be on either side of your target star in terms of brightness–one brighter and one fainter. You also need to make sure that both comparison stars are notvariable stars, which change in brightness. Now decide how bright your target star is when compared to the twocomparison stars. Is it halfway between them, or is it somewhere else in between, being of a more similar brightness toone than the other? Useyour own judgement and be consistent in how you make your comparisons.Once you’ve decided, it’s time to put a figure on the target star’s magnitude in comparison to your two other stars. Lookup the magnitudes of the comparison stars on your star chart.Knowing this will help you make a reasonable estimate of your target star’s magnitude. Let’s say the two comparison stars are of magnitudes +4.0 and +3.0. If the target star is two-thirds of the way from the faintest star to the brightest, this gives you an estimated magnitude of about +3.3. Don’t worry if you’re a little unsure; practice makes perfect. Give it a go Now let’s try a real example. High up at this time of year is the constellation of Draco, the Dragon. The stars you’ll needare marked on it in the image below. It’s a constellation meandering between the two bears, Ursa Major and Ursa Minor,and it has a distinct pattern of four stars marking its head, known as the Lozenge. We’re going to estimate the magnitudeof one of those stars, Grumium (Xi ( ξ ) Draconis). The comparison stars we’re going to use are Etamin (Gamma ( γ )Draconis) and Kuma (Nu (  ν ) Draconis), at magnitudes +2.2 and +5.0 respectively.  The constellation of Draco is high this time of year. Use the stars Gamma ( γ ) and Nu (  ν ) Draconis to estimate themagnitude of Xi ( ξ ) Observe carefully and try and work out where the brightness of Grumium lies between our two comparison stars. Youshould see that it’s roughly two-thirds of the way from Etamin towards Kuma, which will then give you a rough estimate for this star’s magnitude of about +4.0.With your newfound skill you’ll be able to work out the brightness of a hostof different objects, including variable stars.You’ll also be able to use binoculars or a telescope to estimate the brightness of really faint objects such as minor planetsor comets. Make sure you record your findings in a log book–you never know whatyou might find.


Jul 23, 2017
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